A Bartasyte
email: ausrine.bartasyte@gmail.com

O Chaix-Pluchery

J Kreisel

C Jimenez

F Weiss

A Abrutis

Z Saltyte

M Boudard

Investigation of thickness-dependent stress in PbTiO 3 thin films

X-ray diffraction (XRD) and Raman spectroscopy were used to investigate stress dependence on thickness in PbTiO 3 (PTO) films grown by pulsed liquid injection Metalorganic Chemical Vapor Deposition on a LaAlO 3 (001) substrate (LAO). Films on sapphire substrate (R-plane) were used as polycrystalline film reference. Epitaxial PTO films with a dominant c-domain structure are grown on LAO substrate whereas the films on sapphire are polycrystalline. A those determined by XRD. Both techniques also indicate that thinner films are more stressed and residual stresses are partially relaxed with increasing thickness. Moreover, a-domains are more stressed than c-domains. The two components of the large A 1 (2TO) and A 1 (3TO) Raman modes have been associated with a-and c-domains and their intensity ratio clearly correlated with the volume fraction of a-domains.

detailed investigation of the PTO/LAO film microstructure by XRD gives evidence of PTO twinning. Both techniques reveal that PTO films are under tensile in-plane stresses. The study of the film thickness dependence of microstrains, grain size, volume fraction of a-domains as well as surface morphology of PTO/LAO films indicates that these parameters are clearly correlated. A change in the relaxation mechanism between 65 and 125 nm of film thickness has been evidenced. A c parameter gradient occurs throughout the film depth; it originates in stress relaxation due to a thickness increase. Raman spectra of PTO films allowed in-plane residual stress values to be estimated from the Raman shifts and are in good agreement with

I. Introduction

Thin films of PbTiO 3 (PTO) and other related ferroelectric materials offer key advantages for a variety of applications in non-volatile ferroelectric random access memories, microelectronics, and sensors technologies 1 . The physical properties of ferroelectric thin layers are substantially different from those of bulk ferroelectrics, due to substrate -film interactions, grain size effects, defects. 2,3 Film stresses can significantly change mechanical, optical and electrical properties, domain structure formation, and the nature of phase transition in ferroelectric films, 4,5 which in turn has a potential influence on the reliability of devices. Therefore, the investigation of residual stress helps in predicting physical properties for ferroelectric film -based devices. Optical fluorescence, 6 X-ray diffraction (XRD), 7 wafer curvature measurements, 8 cantilever beam deflection, 9 laser reflectance, 10 and Raman spectroscopy [11][12][13][14][15][16][17] have been used for stress analysis in ferroelectric materials. Among them, XRD and Raman spectroscopy are most popular, as they are powerful tools for the nondestructive investigation of structure. Raman spectroscopy is complementary to XRD methods as it is a local probe and sensitive to even subtle changes of symmetry. Moreover, Raman modes are strongly influenced by mechanical deformation of the sample such as hydrostatic pressure or stress. [18][19][20][21] Films are submitted to substantial stresses during growth and subsequent cooling process from deposition temperature to room temperature (RT). At the growth temperature, stresses are mainly misfit stresses (also called epitaxial stresses) which result from lattice mismatch between the film and substrate. Intrinsic stresses related to the growth technique are negligible in Metalorganic Chemical Vapor Deposition (MOCVD). During the cooling process, additional stresses may develop in the film, either due to thermal stress arising from different film and substrate thermal expansion coefficients or to stress related to the structural phase transition. Stresses in PbTiO 3 thin films are mainly relaxed by dislocations and by twins forming at and below the cubic to tetragonal phase transition. The distribution and volume fraction of twins depend on the substrate nature and film thickness 22 and on the cooling rate from the deposition temperature. 23 Even though epitaxial films often have physical properties superior to those of polycrystalline films (i.e. in terms of polarization), only a few papers on stress evaluation by Raman spectroscopy are known in such films; 15,16 most reports concern stress evaluation in polycrystalline films on sapphire or Si substrates. [11][12][13][14] Different theoretical studies analyse the stress state 24 and the possible mechanisms for stress relaxation 23 in epitaxial PTO thin films.

Here we report a systematic study of stress and domain structure evolution as a function of film thickness for PTO films by using XRD, Raman spectroscopy and atomic force microscopy (AFM).

II. Experimental details

A. Film deposition

Film depositions were carried out in a vertical hot wall pulsed injection MOCVD reactor described elsewhere. [25][START_REF] Sénateur | Advances in Inorganic Films and Coatings[END_REF][START_REF] Sénateur | Innovative processing of films and nanocrystalline powders[END_REF] The injector injects microdroplets (a few microliters) of an organic solution containing a dissolved mixture of metalorganic precursors. After flash evaporation of the microdroplets, the resulting vapour mixture is transported by an Ar + O 2 gas flow towards the heated substrate. The injector consists of a computer-driven high-speed precision electromagnetic valve operating under a repetitive pulsing regime. The deposition conditions were optimized (see Table 1) in order to obtain a pure and high quality PTO perovskite phase on two different single crystalline substrates: LaAlO 3 (LAO) with the surface cut parallel to a (00l) plane (indices refer to the pseudocubic setting, a=3.791 Å) [START_REF] Hayward | [END_REF] and sapphire with the surface cut parallel to a rhombohedral R-plane (a=3.5 Å) 29 . Corresponding misfit strains between tetragonal PTO and LAO (SAPH) substrate at RT are -2.8% (-11.4 %) and -9.6 % (-18.7 %) with the PTO a-axis a t and c-axis c t , respectively (bulk PTO lattice parameters: a t =3.899 Å and c t =4.153 Å) 30 . Such considerations using PTO bulk parameters suggest that PTO films are under compressive in-plane stress for both Al 2 O 3 and LAO substrates.

Standard film characterisation by scanning electron microscopy (SEM) and XRD shows a high quality single phase film. More details concerning the deposition technique and standard sample characterisation can be found in Ref. 31.

Films of different thickness were deposited on LAO at 650°C and then slowly cooled at a rate of 2 ˚/min; the thicknesses correspond roughly to 30, 65, 125, 250 and 460 nm as measured from film cross-sectional micrographs obtained by SEM. Naturally cooled (10-20 ˚/min) PTO films deposited on LAO and SAPH, 250 nm of thickness, were also analysed.

B. Film analysis

The surface morphology of the different films was examined by AFM using a Digital Instruments Multimode Scanning Probe Microscope, in the tapping mode. The film texture and microstructure were studied by XRD in Schulz and Bragg-Brentano geometries using a SIEMENS D5000 4-circle diffractometer with monochromatic CuK a radiation (l = 0.15418 nm). An analysis of the microstructure of epitaxial films was carried out through standard w-, cand f-scans and pole figures. The proportion of c-and a-domains was quantified as the percentage of a-domains (A, %) in the films and was calculated by peak integration of the (102) reflection in f-scans corresponding to the different domains:

A = 2( c= 58°I (102) + c= 62°I (102) ) 2( c= 58°I
(102)

+ c= 62°I (102) ) + c= 27°I (102) (1)
where I(102) is the peak area in f-scans measured at different c angles, corresponding to aaxis and c-axis oriented crystallites (i.e. a-and c-domains). Lorentz-polarization and absorption corrections were not applied.

Raman spectra were collected using a Jobin Yvon/Horiba LabRam spectrometer equipped with a liquid nitrogen cooled charge coupled device detector. Experiments were conducted in the micro-Raman mode at RT in a backscattering geometry. The 514.5 nm line of an Ar + ion laser was focused to a spot size smaller than 1 mm. Spectra from different experiments are calibrated using Si spectra at RT.

III. Results

A. X-ray diffraction

Typical q/2q XRD patterns of PTO films deposited on LAO (PTO/LAO) and SAPH substrate (PTO/SAPH) are shown in Fig. 1a and Fig. 1b, respectively. The XRD pattern of the 250 nm thick PTO/LAO film is characteristic of the PTO tetragonal structure with a high degree of preferred orientation consisting only of intense (00l) and very weak (h00) reflections (Fig. 1 a). This indicates a dominant c-domain structure containing a small fraction of a-domains. PTO/SAPH films are polycrystalline with a slightly (101) preferred orientation (Fig. 1b).

The full width at half maximum (FWHM) value of the rocking curve (w-scan) measured on the (001) PTO/LAO reflection varies between 0.3° and 1.0° in the 30 -460 nm range of film thickness as shown in Table 2. As expected, the 001 texture is more pronounced in thinner films. The f-scans performed from the (102) reflection of a-and c-domains and of the substrate confirm a cube on cube epitaxial growth of PTO/LAO. Taking into account the influence of twinning and instrumental resolution (0.1° in w-scan, 0.5° in f-scan) on FWHM values given in Table 2, the PTO/LAO films can be considered as rather well-oriented films.

peak splitting in (102) pole figures: therefore, the c-axis (a-axis) of the tetragonal unit cell in c-domains (a-domains) is tilted by a small angle from the film-substrate plane. More details about texture and domain structure are given in Ref. 31. An evaluation of the volume fraction of twinned a-domains according to Eq. ( 1) is given in Table 2 and Fig. 2 for PTO/LAO films of different thickness. 2). The lowest c value is observed for the thinnest film and gradually increases with increasing film thickness without reaching the bulk c value (Fig. 3). Compressive stress in the [00l] direction, dominant in the substrate/c-domains interface region, could explain such a difference with bulk values.

However, thin films are usually considered as subjected to biaxial stresses in the film plane whereas the normal direction is stress-free. Therefore, the c parameter decrease in c-domains should rather arise from biaxial tensile stress in the film plane although a compressive stress is expected from lattice mismatch.

In the case of the tetragonal c-axis oriented thin film on cubic substrate, the film is under biaxial stress s b in the basal xy plane and z direction is stress free. Therefore, the principal stress components are equal s xx =s yy = s b and there are no shear stress/strain components. For symmetry reasons, the basal strain is also biaxial e xx =e yy =e b . Considering the tensor relationship between stresses and strains expressed with Hooke's law, the mechanical equilibrium relation is obtained:

b s b s 0 Ê Ë Á Á Á Á ˆ ¯ ˜ ˜ ˜ ˜ = 11 C 12 C 13 C 12 C 11 C 13 C 13 C 13 C 33 C Ê Ë Á Á Á Á ˆ ¯ ˜ ˜ ˜ ˜ b e b e zz e Ê Ë Á Á Á Á ˆ ¯ ˜ ˜ ˜ ˜ (2)
where C ij are the terms of the elastic stiffness matrix. Then, the biaxial stress s b can be expressed as a function of the strain in the z direction (e zz ) from Eq. ( 2) as:

b s = - ( 11 C + 12 C ) 33 C 2 13 C + 13 C Ê Ë Á Á ˆ ¯ ˜ ˜ zz e (3)
the strain e zz being calculated from the film and bulk PTO c lattice parameters.

The residual stress values in PTO/LAO films estimated from Eq. (3) (C 11 =237 GPa, C 33 =60 GPa, C 12 =90 GPa and C 13 =70 GPa) 32 are given in Table 3. The positive stress values s b suggest that the films are under tensile stress in the film plane although a compressive stress is expected from lattice mismatch. Residual stress gradually relaxes from 1 to 0.4 GPa with increasing film thickness from 30 to 460 nm.

Reflection profiles of the 250 and 460 nm films present a marked asymmetry on the low d side which we attribute to a c parameter gradient throughout the film depth. The c values, smaller near the substrate-film interface than at the free surface of the film, are directly related to a stress gradient throughout the film depth due to relaxation. As a consequence, all films are expected to present a similar c parameter at a given distance from the film substrate.

A detailed analysis of lattice parameters in all films is in progress and results will be reported in a forthcoming paper.

A line broadening may originate in strain and size effects. For instance, the FWHM observed for the 125 nm film is almost three times larger than the instrumental resolution (0.17 ˚2q evaluated from the substrate line profile). The individual contributions from strain and size to the (00l) reflection line broadening were estimated using both the Williamson-Hall analysis (WH) 33 and the Halder-Wagner approximation (HW). 34 The deconvolution of the instrumental contribution and profile fitting using a pseudo-Voigt function approximation were carried out using the program WinplotR. [START_REF] Roisnel | Proc. 7 th Eur. Powder Diffraction Conference (EPDIC 7)[END_REF] Calculations were performed using the five (00l) reflections and results are given in Fig. 4. Both methods give the same tendency: the average grain size increases with film thickness, and is close to the thickness value in thinner film (Fig. 4a). Values obtained by the WH analysis are in the 20-135 nm range against 18-75 nm when using the HW approximation. Microstrains first decrease with film thickness reaching a minimum value (0.8 % in WH and 1.8 % in HW analysis) between a thickness of 65 and 125 nm and then increase again (up to 1.4 % in WH and 2.9 % in HW analysis) (Fig. 4b). The same evolution of microstrains was reported by de Keijser et al in PTO/SrTiO 3 films. [START_REF] De Keijser | [END_REF] Microstrain values are estimated as rather high values and comparable to reported ones [START_REF] De Keijser | [END_REF] . Both analyses converge and give evidence that the line broadening is mainly due to strain effect but grain size effect cannot be neglected especially in thinner films.

B. Film morphology

The surface morphology of PTO/SAPH and PTO/LAO films was observed by AFM. As shown in Fig. 5 and Fig. 6, a different morphology is observed depending on the cooling rate of the films. The films obtained after natural cooling process (10-20 °/min) have a very smooth surface and present a tweed pattern morphology characteristic of a twinning relaxation mechanism with perpendicular lines of a-domains and larger rectangular zones of c-domains (Fig. 5a). A slow cooling rate initiates dislocations, reducing the formation of twinned a-domains. For example, in the 250 nm PTO/LAO film, the fraction of a-crystallites decreases from 29 % to 14.5 % when the cooling rate is reduced to 2 °/min. Then, stress relaxation in slowly cooled films takes place through the formation of more dislocations. The surface morphology of such films consists mainly of spherical grains (250 and 460 nm in Fig. 6). This morphology is typical of films deposited by MOCVD which grow by an island growth mechanism with high growth rates. Such a morphology was also observed for polycrystalline PTO films on SAPH as shown in Fig. 5b.

The evolution of film surface morphology of slowly cooled PTO/LAO layers as a function of film thickness is given in Fig. 6. The film surface is progressively affected by twinning up to 125 nm of thickness. A further increase in the film thickness results in a surface composed of spherical grains.

C. Raman spectroscopy

Effect of polarization and scattering geometry

Polarized Raman spectra given in Figs. 7a and 7b confirm that PTO/LAO and PTO/SAPH films consist of a well crystallized single PTO tetragonal phase. The spectra were recorded on the film surface, i.e. the direction of the incident and scattered laser beam is normal to the substrate plane. As expected from the C 4v point group selection rules, 37 only E(TO) modes are observed in the PTO/LAO spectrum recorded in crossed polarization configuration (VH) (Fig. 7a). The spectrum measured in parallel polarization configuration (VV) contains a mixture of A 1 (TO), A 1 (LO), E(TO) and E(LO) modes instead of the only allowed A 1 (TO) modes; this is related to the existence of domains leading to laser depolarization. Considering the thin film orientation and Raman selection rules, it follows that E(TO) and A 1 (TO) modes, respectively observed in VH and VV configurations, come from a-domains.

In the case of PTO/SAPH, A 1 and E modes are observed whatever the polarization configuration but the A 1 and E(LO) intensities decrease in VH geometry in comparison with VV configuration and they do not completely disappear (Fig. 7b). This indicates that films on SAPH are rather polycrystalline with only a minor preferred orientation.

A second scattering geometry with the incident and scattered laser beams parallel to the substrate plane was used ("cross section spectra" in the following): the spectra recorded in this geometry in VH (VV) configuration consist also of E(TO) (A 1 (TO)) modes which come from c-and a-domains. Considering that the c-domain fraction is at least 80 % whatever the film thickness, we can assume that these modes come mainly from c-domains.

Effect of thickness

E(TO) modes and residual stress

The measurements of PTO/LAO polarized Raman spectra also permit us to isolate the low-frequency E(1TO) soft mode from the very intense LAO A 1g soft mode (123 cm -1 ) which entirely covers it in spectra recorded without polarization selection.

In 250 nm thick PTO/SAPH and PTO/LAO films, the E(1TO) soft mode is observed well below that observed in bulk PTO (89.5 cm -1 ), at roughly 83 cm -1 in the case of SAPH substrate and at 87 cm -1 in the case of LAO substrate. The fact that all E(TO) modes,

including the E(3TO) hard mode, also shift towards low-wavenumbers in each spectrum (Table 4) suggests that PTO films are under tensile stress whatever the SAPH or LAO substrate, in agreement with XRD results.

As reported in earlier studies, 18 the E(1TO) soft mode is pressure sensitive and, as a consequence, it shifts in wavenumber as a function of increasing pressure as follows:

w[E(TO)] = w 0 [E(TO)] + (dw[E(TO)] /dP)P (4)
where w 0 [E(1TO)] = 89.5 cm -1 is the soft mode wavenumber at zero pressure and room temperature, and dw[ E(1TO)] /dP = -5.8 ± 0.2 cm -1 GPa -1 is the pressure coefficient. 18 Although the biaxial strain of thin films cannot be directly compared with the effect of hydrostatic pressure, the band shift offers an evolution of the residual stress in compressively strained thin films. Let us however parenthesize that Eq. ( 4) is used in the literature regardless of the stress state (compressive and tensile) 11,12,14,16,38 and this although the pressure coefficient is experimentally known only for compression. As a matter of fact, it is difficult to anticipate the tensile pressure coefficient for an anharmonic soft mode from compression data. Furthermore, the soft mode should shift to lower frequencies by the size effect. 39 As a consequence of this we chose to estimate the stress also from the shift of E(3TO) which is a real (harmonic) hard mode in the sense that its pressure coefficient is positive (dw[E(3TO)]/dP = +7.1 ± 1 cm -1 GPa - 1 ) 18 . We expect that this mode offers a more reliable estimation of the stress. Stress values obtained from the E(1TO) soft mode and E(3TO) hard mode by using Eq. ( 4) are given in Table 3. This equation is reasonably usable in the case of PTO/LAO films as reported in Ref.

40.

Corresponding stress values in PTO/SAPH and PTO/LAO calculated from Eq. ( 4) are closed to 1.1 and 0.4 GPa using E(1TO) and to 0.3 and 0.6 GPa using E(3TO), respectively.

As exposed above, we believe that the values derived from the E(3TO) hard mode are more reliable, and thus the stress values of PTO/SAPH films are rather close to 0.3 GPa. The latter is confirmed by the fact that the measured c lattice parameter of films on LAO and SAPH are almost the same (4.119±0.005 Å and 4.114±0.005 Å, respectively). The stress values related to 50-950 nm thick polycrystalline films were reported by different authors to be in the range of 2.6-0.9 GPa. 11,12,14,38,41,42 3). However, let us emphasize again that stresses are not only dependent on the lattice mismatch but are highly sensitive to the deposition method and experimental conditions.

The Raman spectra of 250 nm thick PTO/LAO films recorded for a-and c-domains using both scattering geometries are given in Fig. 8a. It appears that the deviation from the single crystal wavenumbers of E(1TO) as well as that of the other E(TO) modes is more important in a-domains than in c-domains. Fig. 8b and8c show the evolution of the profile of the E(1TO) soft mode measured in both types of domains as a function of PTO/LAO film thickness; the Raman shifts of E(1TO), E(2TO) and E(3TO) are given in Table 4. The deviation from the bulk values, just as the stress, increases as thickness decreases. Corresponding stress values estimated from Eq. ( 4) using E(1TO) and E(3TO), when possible, are given in Table 3.

Indeed, the E(3TO) mode is partially masked by the intense A 1 (3TO) PTO and E g LAO modes in the thinner films in the case of a-domains. In Fig. 9 we compare stress values measured by XRD with values obtained by the hard E(3TO); it can be seen that there is a good agreement between both techniques, which further validates the reliability of the E(3TO) mode.

Moreover, as mentioned above for a 250 nm thick film, a-domains are more stressed than cdomains: stress values in c-domains estimated from the E(3TO) Raman shift are in the range of 0.7-0.5 GPa against 1.2-0.8 GPa in a-domains as thickness increases from 125 to 460 nm.

Such a difference can be explained by a larger lattice mismatch with the substrate in adomains and by the fact that they form thin sheet zones in a matrix of c-domains, 22,43 which in turn generates a supplementary strain at c/a domain interfaces. Similar results were obtained by Lee et al. in PTO/MgO epitaxial films 16 despite a reverse mismatch between PTO and MgO lattice parameters as compared with those of PTO and LAO.

As observed in Fig. 8c, a second weak component is also observed in E(1TO) (shown with an asterisk) as well as in E(2TO) in a-domains in films thinner than 460 nm. Unlike the main component, it shifts towards higher wavenumbers when the film thickness decreases.

Although our Raman study does not allow a clear interpretation of this observation, we suggest that this might be related to stress or optical effects; complementary work is in progress to check this hypothesis.

PTO/LAO depolarized Raman spectra of high-wavenumber modes as a function of film thickness are given in Fig. 10. All modes are systematically shifted towards higher wavenumbers in comparison with those in VH spectra. This shift can be explained by a change in relative intensities between each E(TO) mode and its corresponding oblique modes. 44 This high complexity of such depolarized spectra implies that one should record VH spectra in order to obtain well defined E(TO) mode profiles. Just as for the B 1 +E mode, it is pointed at 290.1 cm -1 whatever the film thickness (Fig. 10). Note that it is a degenerate silent mode; however, the polarized Raman spectra in Fig. 7a indicate that the E mode can be entirely isolated from the B 1 mode in VH geometry, as previously observed by Lee et al. 16

Domains and A 1 (TO) modes

The second A 1 (1TO) soft mode in PTO/LAO has not been accurately observed due to the too close LAO soft mode. A 1 (2TO) and A 1 (3TO) modes appear as very large bands split into two components corresponding to a-and c-domains in VV spectra recorded on film surface, labeled a and c in Fig. 10, whose intensity ratio varies with film thickness. As already mentioned, A 1 (TO) modes arise from a-domains in spectra collected on the film surface and the presence of c-domain A 1 (TO) modes results from depolarization. The separation of modes coming from both a-and c-domains is not easy, as the a-domain fraction is quite small, thus c-domain A 1 (TO) modes may have higher intensity than a-domain modes. Nevertheless, A 1 (TO) modes coming from different domains can be separated because the intensity of A 1 (TO) modes related to c-domains highly increases in spectra recorded on the cross section.

Also these modes can be assigned to a-and c-domains from other observations. First, a-domains are more stressed than c-domains which results in lower wavenumbers for a-component. Second, the intensity ratio between a-and c-components is correlated with the volume fraction of a-domains as attested in Fig. 10; it follows the same thickness dependence, a-being the more intense component in the 125 nm thick film spectrum (Fig. 10). In a recent study of Pb(Zr,Ti)O 3 /MgO epitaxial films, 45 a remarkable correlation is found between the A 1 (1TO) mode intensity and the c-domains volume fraction, thus providing a simple probe for domain characterization. However, although the A 1 (TO) modes come from both a-and c-domains according to Raman selection rules, the authors in Ref. 45 The origin of the tensile residual stress is not only the mismatch between the film and the substrate. Other possible origins of tensile stress are related to phase transition and to difference in film and substrate thermal expansion coefficients. Thermal stresses are rather small in PTO/LAO films, thus the high residual stress values, especially in thinner films, cannot be explained if the stress related to the phase transition is assumed to be completely relaxed (or to be stress free). To conclude, residual stress measured at RT is the sum of all stresses developed during the complex process of film preparation, and it cannot be associated to only one origin of stress.

Complementary XRD and TEM measurements are in progress. They are essential for a more complete study of lattice parameters, and then of stresses, and `of twinning in PTO films as a function of film thickness.

Tables

Fig. 3

 3 Fig.3shows the evolution of the (002) reflection line as a function of film thickness. The analysis of the line profile indicates an important line broadening in all films and an increase of the corresponding d 002 values due to an increase with thickness of the c-parameter in cdomains determined from the five (00l) reflections (Table2). The lowest c value is observed

  have not considered the a-domain contribution to the A 1 (TO) mode intensity. IV. Discussion and concluding remarks XRD and Raman spectroscopy were used to investigate the stress dependence on thickness in PbTiO 3 films grown by pulsed liquid injection MOCVD on LaAlO 3 (001) substrate. Both techniques reveal that the films are under tensile stress in the film plane although a compressive stress is expected from mismatch with the substrate. Residual stress values obtained from the Raman shifts of the E(3TO) hard mode are in good agreement with the values calculated from the c parameters measured by XRD in c-domains. They are strongly thickness dependent: thinner films are more stressed and residual stresses are partially relaxed with increasing film thickness. However, very thin films are affected by compressive misfit stress unfavouring a-domain formation, whereas they are under high tensile residual stress; this results in high microstrains in the film. An increase of thickness up to 125 nm leads the film to be less substrate dependent which allows some tensile stress relaxation by twinning (a-domain formation). As a consequence, microstrains decrease and adomain fraction increases modifying the film morphology. A further increase of film thickness up to 460 nm leads to a change in the dominant relaxation mechanism from twinning to dislocation formation; the film surface is then composed of spherical grains and the a-domain fraction starts to decrease. The increase of dislocation concentration results in higher disorder and thus, in increasing microstrains. Residual stress values also indicate that a-domains are more stressed than c-domains.

Figure captions Fig. 1 :Fig. 2 :

 captions12 Figure captions

Fig. 3 :

 3 Fig. 3: Film thickness evolution of the (002) reflection line in the XRD pattern of PTO/LAO films. The d 002 value of PTO single crystal is indicated as a dashed line.

Fig. 4 :

 4 Fig. 4: Film thickness evolution of (a) average grain size and (b) microstrains in c-domains along the [001] direction as obtained by Williamson-Hall and Halder-Wagner analyses.

Fig. 5 :

 5 Fig. 5: AFM images (2x2 mm 2 ) of 250 nm thick (a) PTO/LAO and (b) PTO/SAPH films. Cooling rate after deposition was 10-20 o /min.

Fig. 6 :

 6 Fig. 6: AFM images (1x1 mm 2 ) of PTO/LAO films of various thicknesses (tilted images are given in the insets for 250 and 460 nm thick films). Average roughness values (Ra) are indicated for each film thickness. Cooling rate after deposition was 2 o /min.

Fig. 7 :

 7 Fig. 7: Polarized Raman spectra of 250 nm thick (a) PTO/LAO and (b) PTO/SAPH films recorded on film surface in parallel (VV) and crossed (VH) polarization configurations.

Fig. 8 :

 8 Fig. 8: (a) Comparison of Raman spectra measured on film surface (a-domains) and on cross section (c-domains) in VH polarization configuration. The wavenumbers of E(1TO), E(2TO) and E(3TO) modes in PTO single crystal are shown as dashed lines. The film thickness evolution of the E(1TO) soft mode recorded on cross section and on film surface in VH configuration is given in (b) and (c), respectively.

Fig. 9 :

 9 Fig. 9: Film thickness evolution of the residual stress values in c-domains calculated from c lattice parameters (XRD) and from Raman shifts of E(3TO) Raman mode.

Fig. 10 :

 10 Fig. 10: Film thickness dependence of PTO/LAO depolarized Raman spectra. The evolution of the intensity ratio between a-and c-components of the split A 1 (3TO) mode as a function of a-domain volume fraction in PTO/LAO is given in the inset.

Fig. 1

 1 Fig. 1

Fig. 2

 2 Fig. 2

Fig. 3 Fig. 4

 34 Fig. 3

Fig. 5

 5 Fig. 5

Fig. 6

 6 Fig. 6

Fig. 7

 7 Fig. 7

Fig. 8

 8 Fig. 8

Fig. 9

 9 Fig. 9

Fig. 10

 10 Fig. 10

  Dobal et al. reported a higher stress value (1.5 GPa) for the 250 nm PTO/SAPH film 14 compared with our measured value (0.3 GPa). The stress was measured as high as 2 GPa in epitaxial 250 nm thick PTO/MgO films.16 Stress values estimated in our films are rather lower than the literature values but we note that literature studies use only the

disputable E(1TO) soft mode (which we have also calculated for comparison with the literature values, Table

Table 1 :

 1 General conditions of PbTiO 3 deposition by PI-MOCVD

	Substrates	LaAlO 3 (001); Sapphire (R-plane)
	Substrate temperature, °C	650
	Evaporation temperature, °C	280
	Transport gas	Ar + O 2
	Total gas flow rate, l/h	60
	Oxygen flow rate, l/h	24
	Pressure, Torr	10
	Precursors	Pb(thd) 2 ; Ti(thd) 2 (O i Pr) 2	a
	Solvent	toluene
	Solution concentration (total), mol/l	0.04
	Injection frequency, Hz	2
	Growth rate, nm/h	~ 1160
	Thickness, nm	30-460
	Cooling rate	2°/min until 400 °C
	a thd -2,2,6,6-tetramethyl-3,5-heptanedionate, O i Pr -i-propoxide

Table 2 :

 2 XRD characterisation of PTO/LAO films of various thicknesses

	Film thickness (nm)	460	250	125	65	30
	Volume fraction of a-domains (%) 4.7	14.5	19.4	9.0	-a
	FWHM of (001) w-scan (°) b	1.0	0.9	0.8	0.7	0.3
	FWHM of (102) f-scan (°) b	1.9	2.0	2.2	1.7	1.8
	c-lattice parameter (Å)	4.130 4.119 4.107 4.094 4.093
	(±0.005 Å) in c-domains
	a Undetermined due to very low peak intensities

b c-domains

Table 3 :

 3 Residual stress values determined from the c lattice parameters in c-domains obtained by XRD and from the Raman shift of E(1TO) and E(3TO) modes in PTO/LAO films of different thicknesses.

	Film thickness (nm)		30	65	125	250	460
		XRD	1.02 ± 0.15 1.00 ± 0.14 0.79 ± 0.11 0.58 ± 0.08 0.40 ± 0.06
	Residual stress value in	E(1TO) 1.10 ± 0.35 0.81 ± 0.17	-	0.43 ± 0.09 0.36 ± 0.04
	c-domains (GPa)
		E(3TO) 1.30 ± 0.21 0.88 ± 0.14 0.70 ± 0.11 0.59 ± 0.10 0.45 ± 0.07
	Residual stress value in	E(1TO) 1.97 ± 0.34 1.71 ± 0.17 0.64 ±0.09 0.60 ± 0.09 0.53 ± 0.04
	a-domains (GPa)	E(3TO)	-	-	1.15 ± 0.16 0.95 ± 0.16 0.76 ± 0.12

Table 4 :

 4 E(1TO), E(2TO) and E(3TO) phonon wavenumbers in PTO single crystal, 250 nm thick PTO/SAPH and PTO/LAO films of different thicknesses. Both PTO single crystal and PTO/LAO thin films have the same ferroelectric 180º and 90º domain structure. The a-and cdomain values were obtained from film surface and cross-sectional spectra recorded in VH polarization configuration, respectively.

		Thickness	w (E(1TO))	w (E(2TO))	w (E(3TO))
		(nm)	(cm -1 )	(cm -1 )	(cm -1 )
	Bulk PTO	-	89.5	219.9		506.2
	PTO/SAPH	250	83.1	207.8		504.3
			a-domains	c-domains a-domains	c-domains	a-domains	c-domains
	PTO/LAO	460	86.4	87.4	210.5	214.6	500.8	503.0
	PTO/LAO	250	86	87.0	208.8	212.4	499.4	502.0
	PTO/LAO	125	85.8	-	206.5	-	498	501.2
	PTO/LAO	65	79.6	84.8	204.3	208.6	-	499.9
	PTO/LAO	30	78.1	83.1	204.3	204.3	-	496.9

Acknowledgments: This work has been financially supported by the European network of excellence FAME and EGIDE.


Gabriel M Vieira

Artur V Granato

Elena V Gusevskaya

Eduardo N Dos Santos

Pierre H Dixneuf

Cédric Fischmeister

Christian Bruneau
email: christian.bruneau@univ-rennes1.fr

Tandem hydroformylation/isomerization/hydrogenation of bio-derived 1arylbutadienes for the regioselective synthesis of branched aldehydes

Keywords: tandem catalysis, hydroformylation, fragrance 1. Introduction

Highlights

 Synthesis of 1-arylbutadienes from bio-renewable 2-propenylbenzenes  Tandem hydroformylation/isomerization/hydrogenation of 1-arylbutadienes  Branched aldehydes useful as fragrances in high selectivity

Hydroformylation (oxo synthesis) is an efficient atom economical reaction, widely employed in industry, both in commodity and fine chemicals sector. [START_REF] Borner | Hydroformylation: Fundamentals, Processes, and Applications in Organic Synthesis[END_REF] Formally, hydroformylation corresponds to the addition of a formyl group and a hydride to a C-C double bond (C=C), as represented in Scheme 1 for an alphaolefin. The actual mechanism involves the combination of the olefin, carbon monoxide and hydrogen in the coordination sphere of a hydridocarbonyl transition metal complex. Although discovered long ago by Otto Rölen [START_REF] Rölen | [END_REF], this reaction still offers practical and scientific challenges because the selectivity control is highly dependent on the reactants, the catalytic system, and the reaction conditions.

Insert Scheme 1

State-of-art catalytic systems are predominately formed by a rhodium(I) precursor and phosphorus(III) ligands, and the design of new ligands has allowed to develop highly selective systems for specific applications. [3] For alpha-olefins, the linear product is preferentially formed for steric reasons, but high linear selectivity is only achieved by the employment of chelating phosphorus ligands with large bite angles. [4] Obtaining branched aldehydes in high selectivity when a methylidene group is in the alpha position of the C=C is a far more complex task. We developed a catalytic system capable of producing the branched aldehyde in selectivity higher than 65% from naturally occurring 2propenylbenzenes based on a rhodium(I) pre-catalyst and the commercially available 1,3-bis(diphenylphosphino)propane (DPPP) as ancillary ligand. [5] More recently, by the employment of specially designed ligands, this regioselectivity was surpassed. [6][7][8][9][10][11][12][13] When the terminal C=C bond has an electron-withdrawing group, a phenyl ring or a conjugated C=C in the alpha position, the tendency to form branched products is higher. In this context, the hydroformylation of conjugated dienes opens opportunities for the synthesis of branched products, provided the selectivity can be controlled. The hydroformylation of 1,3-butadiene, isoprene and piperylene is useful for the synthesis of commodity products. [14][15][16][17][18] Also, 1,3-dienes moieties often occur in natural products, and their hydroformylation leads to aldehydes of interest for the fragrance industry. [19][20][21] However, hydroformylation of this kind of substrates is particularly difficult to achieve. Special reactions conditions, such as atypically high pressure, temperature or high ligand concentrations, are required to get reasonable reaction rates. Although there are more possibilities for the positioning of the formyl group, the primarily formed aldehydes in the hydroformylation of 1,3-dienes are usually the ones represented in Scheme 2 [22].

Insert Scheme 2

Subsequent C=C isomerization to form alpha,beta-unsaturated aldehydes followed by their hydrogenation are often observed, which leads to the problems of selectivity control. It is thus surprising that Nozaki et al. [23] were able to develop a system that allowed a regio and enantioselective hydroformylation of 1-phenylbutadiene and other dienes, although under rather harsh pressure conditions (100 bar of CO/H2), by the employment of (R)-2-J o u r n a l P r e -p r o o f (diphenylphosphino)-1,1-binaphthalen-2-yl(S)-1,1-binaphthalene-2,2-diyl phosphite ((R,S)-BINAPHOS) as a ligand. Years later, Landis et al. [22] employed chiral diazaphospholane ligands and developed a very efficient protocol that allows the regio and enantioselective hydroformylation of 1,3-dienes under much milder conditions. Although very efficient, these systems involve noncommercially available ligands whose synthesis is quite sophisticated. We considered that it would be worth to develop catalytic systems for the selective hydroformylation of conjugated dienes based on standard commercial ligands that could be readily implemented by industry. Thus, in this paper we describe a protocol to convert naturally occurring 2-propenylbenzenes [24], namely estragole, eugenol and safrole, to the corresponding 1-arylbutadienes, which were subsequently hydroformylated to produce in high selectivity novel branched aldehydes that can be useful for Food & Flavor industry.

Experimental

The commercially acquired chemicals were used without an additional treatment except indicated cases. Estragole (98%), eugenol (99%), palladium acetate (99.9%), triphenylphosphine (99%), 1,2-bis(diphenylphosphino)ethane (98%), 1,3-bis(diphenylphosphino)propane (97%) and 9,9-dimethyl-4,5bis(diphenylphosphino)xanthene (98%) were purchased from Acros Organics. Safrole (97%), cesium carbonate (99%), tris(2,4-di-tert-butylphenyl) phosphite (98%) were purchased from Merck and dichloro(2-isopropoxyphenylmethylene) (tricyclohexylphosphine)ruthenium(II) (HGII) was supplied by Umicore. Tribenzylphosphine and dicarbonyl(2,4-pentanedionato)rhodium(I) (99%) were purchased from Alfa Aesar. Allyl chloride (98%) was purchased from Acros Organics, distilled under argon and stored over 3 Å molecular sieves. Dichloromethane, acetonitrile and toluene were dried using MBraun solvent purification system.

Products were analyzed and characterized by GC and GC-MS using Shimadzu GC 2014 and QP2010-PLUS instrument (70 eV), respectively, both equipped with apolar column (Equity-5, 30 x 0.25x 0.25). NMR spectra were recorded on a 400 MHz Avance III spectrometer from Bruker. Chemical shifts are reported in ppm vs TMS using residual signals from deuterated solvents as internal reference.

Preparation of the 1-arylbutadienes

1-Arylbutadienes were prepared from naturally occurring 2-propenylbenzenes, namely, estragole (1a), eugenol (1b) and safrole (1c), using a previously reported route (Scheme 3). [25] Insert Scheme 3 Synthesis of 3-arylallyl chlorides (2a-c) (adapted from [25])

For the synthesis of compound 2a, a Schlenk tube was loaded under argon with 1.0 g (0.0067 mol) of estragole (1a), 0.084 g (0.00013 mol) of 2 nd generation Hoveyda-Grubbs (HGII), 3.1 g (0.040 mol) of allyl chloride and 20 mL of J o u r n a l P r e -p r o o f dichloromethane. The mixture was refluxed for 15 h. After evaporation of the solvent and the residual allyl chloride, the products were purified by column chromatography on silica gel using heptane/2% ethyl acetate mixture as eluent. An analogous protocol was employed to synthesize compounds 2b and 2c, but for the former a mixture heptane/20% ethyl acetate was employed in the purification. The isolated yields are summarized in Table 1, the side products, if formed, were not characterized. The purified products were analyzed by 1 H and 13 C NMR, as well as GC-MS and the data are in Supporting Information S2.

Synthesis of 1-arylbutadienes (3a-c) (adapted from [25])

For the synthesis of compound 3a, a Schlenk tube was loaded under argon with 0.8 g (0.0041 mol) of compound 2a, 0.046 g (0.000205 mol) of palladium acetate, 0.107 g (0.00041 mol) of triphenylphosphine, 1.59 g (0.0049 mol) of cesium carbonate and 25 mL of acetonitrile. This reaction mixture was stirred under reflux for 17 h. The resulting mixture was centrifuged at 6000 rpm for 5 min and the supernatant was collected. The residual solid was washed with cold acetone, the mixture was centrifuged at 6000 rpm for 5 min and the supernatant was collected. The supernatants were combined and the solvents were evaporated. The products were purified by column chromatography on silica gel using heptane/2% ethyl acetate mixture as eluent. An analogous protocol was employed to synthesize compounds 3b and 3c, but for the former a mixture heptane/10% ethyl acetate was employed in the purification. . These products have already been described in the literature. [26,27] The isolated yields are summarized in Table 1, the side products, if formed, were not characterized. The purified products were analyzed by 1 H and 13 C NMR, as well as GC-MS and the data are in Supporting Information S3-S4.

Insert Table 1

Hydroformylation of 1-arylbutadienes (3a-c)

The 1-arylbutadiene (3a) and phosphorus ligand were added in a 1.5 mL glass vial containing a magnetic stirrer. A solution of [Rh(acac)(CO)2] in toluene was prepared in a Schlenk tube under argon atmosphere and the proper amount (0.005 mol) of Rh per 1.0 mol of starting material of catalyst solution was transferred to the glass vial containing the diene and closed with a cap containing a rubber septum pierced with a small needle. The vial was set in a stainless steel bomb, which was closed and purged with a CO/H2 (1:1) mixture. The bomb was then pressurized with 40 bar of this gas mixture and kept under magnetic stirring in an oil bath at the desired temperature (60-100 o C) for 2 to 48 h. The bomb was cooled and carefully vented in a hood and then opened to air. After solvent stripping, chloroform-d 1 was added and the crude products were analyzed by 1 H NMR. The major products were separated by column chromatography on silica gel using heptane/ 2% ethyl acetate mixture as eluent. An analogous protocol was employed in the hydroformylation of compounds 3b and 3c, but for the former a mixture heptane/20% ethyl acetate was employed in the purification of the main products. The purified products were analyzed by 1 H and 13 C NMR, as well as GC-MS and the data are in Supporting Information S4-S6.

J o u r n a l P r e -p r o o f

Quantitative analysis of the hydroformylation products

The quantification of the products was made by 1 H-NMR of the crude reaction mixture. Depending on the catalytic system, up to 14 peaks were observed in the aldehyde region of the spectra ( 9.40 -10.30 ppm), but only four of them were predominant. These major signals were attributed to compounds 4a, 5a, 6a and 7a. To the best of our knowledge, compounds 4a and 5a have not been reported before. They were isolated from the reaction solutions and fully characterized (see spectroscopic data below). Aldehydes 6a [28] and 7a [29] were previously described so that their aldehyde signals were attributed based on the literature data. The remaining peaks in the aldehyde region in the NMR spectra ( 9.40 -10.30 ppm) of the crude product were added together and are reported in the Tables as "Others" (other aldehydes). The quantification was based on the intensities of the signals at 3.75 -3.90 ppm attributed to the methyl hydrogens of the methoxy groups present in all the products (100% of the area = 3 H). The mass balance was checked by the comparison of the sum integration of the aldehyde peaks (1H) and the methoxy peaks (3 H).

Results and Discussion

Hydroformylation of 1-arylbutadienes

Estragole is the most available compound of the series and for this reason, the diene 3a derived from estragole was chosen as the substrate for hydroformylation in the initial studies. (Acetylacetonato)bis(carbonyl)rhodium(I), [Rh(acac)(CO)2], a widely used Rh(I) pre-catalyst in hydroformylation was used in combination with various phosphorus(III) ancillary ligands. Under the hydroformylation conditions, the precatalyst [Rh(acac)(CO)2] is regarded to readily form in situ the 16-electron active catalyst [RhH(L)m(CO)n], in which m+n = 3 and L is a phosphorus atom of the ancillary ligand. [30] We employed representative phosphorus(III) ligands of the class of monophosphines (PPh3 and PBn3), monophosphite (TBDP) and chelating diphosphines with different bite angles (DPPE, DPPP, XANTPHOS), whose structures are depicted in Chart 1.

Insert Chart 1

The major products were 4a, 5a, 6a and 7a (Scheme 4). They differ from the generic ones in Scheme 2 because some of them are formed by consecutive reactions after hydroformylation, such as isomerization and hydrogenation. While linear aldehydes 6a and 7a can be formed directly by the hydroformylation of the terminal C=C in 3a, the formation of 4a should involve a further double bond migration that can occur either in the catalytic cycle or through a fast consecutive isomerization process of the -unsaturated branched aldehyde, which, nevertheless, was not observed by the analytical techniques employed. The hydrogenation of 4a gives the saturated branched aldehyde 5a.

J o u r n a l P r e -p r o o f

Insert Scheme 4

The first attempts for the hydroformylation of 3a were made employing the standard PPh3 as ancillary ligand at 100 o C, 40 bar -CO: H2 = 1, P/Rh = 10, 24 h, but the substrate conversion was very low. Under harsher conditions (100 o C, 80 bar -CO: H2 = 1, P/Rh = 20, 48 h) it was possible to obtain a higher conversion (92%), but the selectivity for individual aldehydes was very poor and other ligands were tested.

The results for the ligand screening in the hydroformylation of 3a are presented in Table 2. Employing the bulkier and more basic PBn3 (as compared to PPh3) as ancillary ligand, the substrate was completely converted after 24 h to give the saturated aldehyde 5a as a major product with 56% selectivity (Table 2, entry 1). TBDP is a bulky phosphite known to be an efficient promoter in the rhodium-catalyzed hydroformylation of olefins. [31] Employing this compound as ancillary ligand under the same conditions also allowed a complete substrate conversion within 24 h; however, the reaction was poorly selective providing a complex mixture of other aldehydes (Table 2, entry 2). As the data previously reported in the literature suggested that chelating ligands could give better results in the hydroformylation of conjugated dienes as compared to monodentated ligands [15] , a set of chelating diphosphines with different bite angles was tested in the hydroformylation of 3a. The ligands employed and the corresponding bite angles were: DPPE (85 o ), DPPP (91 o ), and XANTPHOS (108 o ). [32] In the presence of DPPE, the ligand with the lowest bite angle in the series, not only a full conversion was obtained after 24 h, but also a high selectivity (91%) for the saturated aldehyde 5a (Table 2, entry 3). Thus, this system is very efficient for the tandem isomerization/hydroformylation/C=C hydrogenation, as observed previously by Behr et al. [18] employing isoprene as substrate. Nevertheless, 1-arylbutadiene gives the branched aldehyde, opposed to isoprene, which yields the terminal aldehyde. This is remarkable finding from the practical point of view, considering that such kind of branched aldehydes are difficult to obtain in high selectivity by the direct hydroformylation of alphaolefins. [6] Insert Table 2 DPPP is a ligand with similar electronic features as DPPE, but with a larger bite angle. Although both systems presented the same trends, the selectivity of hydroformylation was lower in the system with DPPP (Table 2, cf. entries 4 and 3). [15] XANTPHOS is a bidentated ligand, well known to promote the rhodium-catalyzed hydroformylation of alpha-olefins with a high regioselectivity for linear aldehydes. [4] Thus, a higher selectivity for aldehydes 6a and 7a would be expected at the hydroformylation of 3a in the presence of XANTPHOS, but it was not actually observed (Table 2, entry 5). The branched aldehydes 4a and 5a were the major reaction products, with the combined selectivity being similar to that in the system promoted by DPPP (Table 2, cf. entries 4 and 5). It is noteworthy that there is a lower tendency to hydrogenate the -unsaturated aldehyde 4a by the XANTPHOS-promoted system, since even after 24 h, 37% of this aldehyde is present among the products.

J o u r n a l P r e -p r o o f

Considering that DPPE as ancillary ligand (L) gave the best results, further studies concerning the reaction conditions was carried out with this ligand and they are presented in Table 3. In entries 1-3, the effect of L/Rh molar ratio can be observed. The systems with L/Rh= 2.5 (entry 1) and 5.0 (entry 2) showed nearly the same performance, but at a higher ratio (L/Rh= 10, entry 3) the contribution of linear aldehydes 6a and 7a in the product distribution increased. The effect of the reaction temperature can be evaluated by comparing the results presented in entries 2, 4, and 5 (Table 3). Although at 80 o C the conversion was completed after 24 h, a significant amount of the branched aldehyde remained in the nonhydrogenated form (4a). This effect was even more evident in the reaction at 60 o C, in which aldehyde 4a was a predominant product, thus clearly showing that higher temperature favored the hydrogenation step.

Insert Table 3 Although toluene is presently considered an acceptable solvent, there are some environmental issues assigned to this compound [33]. In a further study, we decided to test other solvents, with higher sustainability rankings than toluene, as a reaction medium for the hydroformylation of 3a. p-Cymene has been successfully employed by us in hydroformylation [34] and metathesis. [35] For the system with p-cymene, the catalytic results were similar to those obtained in toluene (Table 3, cf. entries 2 and 6). Anisole has a much better sustainability rank than toluene and even than p-cymene, [33] and we have recently found it was an excellent solvent for hydroformylation [34]. Although the selectivity for aldehyde 5a in the reaction in anisole was slightly lower than in toluene, it was still reasonably high (85% vs. 91%, cf. entries 2 and 7 in Table 3). Thus, the more sustainable solvents p-cymene and anisole can be recommended as possible alternatives to toluene for the tandem hydroformylation/hydrogenation of conjugated diene 3a aiming at obtaining saturated aldehyde 5a with high selectivity.

In order to determine the reaction evolvement, a kinetic follow-up was performed and the results are presented in Figure 1. Under these reaction conditions, the substrate 3a is completely converted into products within 4 h. The nearly straight line for the substrate consumption up to 100% conversion showed an independence of the reaction rate on the substrate concentration, thus suggesting saturation kinetics. After 2 h, the predominant product was the unsaturated aldehyde 4a, along with minor amounts of aldehydes 6a and 5a. Then, the concentration of 4a started decreasing due to its conversion into 5a, whereas the concentration of 6a continued to increase until 4h, and then decreased. The slight decrease of 6a until 24 h is probably due to the C=C double bond isomerization to form other unsaturated aldehydes. Only after 24 h the formation of product 7a increased, likely due to the hydrogenation of ,unsaturated linear aldehydes rather than 6a directly. The variation in 5a percentage from 24 to 48 h (less than 4%) is within the experimental reproducibility, although a decrease due to formation of minor amounts of nonaldehyde derivatives cannot be ruled out. The -unsaturated aldehyde 4a' isomer of 4a was not observed, but it would be the logic precursor of 4a, firstly formed by the Markovnikov-type hydroformylation of the terminal double bond, followed by fast isomerization. Nevertheless, the direct formation of 4a as a J o u r n a l P r e -p r o o f primary product resulting from 1,3-H migration in the coordination sphere of the metal within the catalytic cycle cannot be ruled out. [16] Insert Figure 1 The substrate scope for the tandem hydrofomylation/hydrogenation process was extended to other 1-arylbutadienes derived from the natural products, eugenol (3b) and safrole (3c), being the results presented in Table 4.

Insert Table 4 The products, 4a-c and 5a-c obtained from these substrates are new compounds and the corresponding spectroscopic data are presented in the experimental section. As the diene double bonds of 3a-3c are conjugated to the aromatic ring, an influence of the ring substituents on the reaction output could be expected, being the more influent substituent at the para position. Nevertheless, the effect of the substituent of the aromatic ring was not so marked after 24 h for the systems promoted by DPPE (Table 4, entries 1-3). The high regioselectivity for the branched product (4 + 5) was about the same for all three substrates. The difference in reactivity was more evident for the systems promoted by XANTPHOS. The hydroformylation rate was significantly lower for substrate 3b as compared to the other two as only 80% conversion was reached after 24 h, whereas the product distributions were comparable (Table 4, entries 4, 5 and 6). Again, it was obvious that the XANTPHOS-promoted system was less efficient to promote the hydrogenation of 4 into 5, as 37-45% of 4 remained after 24 h. At 60 o C (Table 4, entries 7 and 8) unsaturated aldehyde 4a accounted for 76% of the products at 62% conversion. Interestingly enough, if the reaction was prolonged to 48 h (entry 8) the conversion rose to 89%, but no important change on product distribution occurred, which indicated that consecutive hydrogenation processes are not significant under these conditions. The unsaturated aldehyde 4a was thus obtained in 66% yield. It is noteworthy that the catalytic reaction can be driven selectively towards the unsaturated aldehyde 4a or the saturated aldehyde 5a, simply by changing the reaction temperature and the auxiliary phosphorous ligand, DPPE or XANTPHOS, respectively.

Regarding to catalyst recovery and reuse, some of us published recently a methodology for the hydroformylation of 2-propenylbenzenes in which the catalyst is supported in an aqueous phase by means of the water-soluble sulfonated version of DPPE, inter alia. [36] The catalyst is recovered by decantation of the aqueous phase after hydroformylation. We believe that this system could be adapted for the 1-arylbutadienes 3a-c, and this should be a matter for further studies.

The results are fully consistent with the reaction paths shown in Scheme 5, based on Nozaki's proposal [23], which is supported by spectroscopic evidences. Although there would be other possibilities, the dominant interaction between 1-arylbutadienes 3 and the active rhodium species (LnRhH) occurs through the terminal C-C double bond. In Path A, the hydrogen is transferred to carbon in position 3 at the chain, leading to an Rh-alkyl species A, which, after carbonylation and hydrogenolysis, yields aldehyde 6. Alternatively, path B, in which the hydrogen is transferred to carbon in position 4 at the chain, leads to J o u r n a l P r e -p r o o f the formation of a much more stable  3 -allyl intermediate B, which seems to be the resting state for rhodium species and would explaining the observed saturation kinetics. The  3 -allyl intermediate B does not undergo directly carbonylation/hydrogenolysis, [15,16,19,23] but it is in equilibrium with  1allyl intermediate C, which can suffer carbonylation/hydrogenolysis. The primarily formed products can be further transformed by C=C isomerization (migration) and hydrogenation. Under hydroformylation conditions catalyzed by rhodium, hydrogenation is only a major path when the C=C double bond is conjugated with the aldehydic C=O, and thus the efficient hydrogenation occurring for the formation of 5 in high yield should be preceded by the formation of 4. Likewise, formation of 7 should also be preceded by the isomerization of 6.

Insert Scheme 5

Although, the  3 -allyl (B) - 1 -allyl (C) equilibrium is widely favored towards the former, it can be shifted towards C by the enhancement of the steric encumbrance on rhodium, and this is the key point to explain the ligand effect.

Monodentated phosphorus ligands can promote the  3 -allyl- 1 -allyl shift by increase in their concentration or their binding ability. Increasing the concentration of certain ligands such as PPh3 may suffice in some cases, [16] but apparently the presence the conjugated phenyl ring in B makes it more stable, so that other ligands are needed. PBn3, a more basic and bulkier ligand than PPh3, acted better as a promoter in the hydroformylation of 3a, probably, due to the shift of the  3 - 1 equilibrium towards C. Employing TBDP as ancillary ligand, which is also bulky and binds strongly rhodium species, the same promoting effect on the transformation of  3 -intermediate B into  1 -intermediate C was observed. However, this ligand yields a more acidic rhodium intermediate, which is prone to promote the C-C double bond isomerization and less prone to perform its hydrogenation. As a result, a complex mixture of isomeric aldehydes with low individual selectivity for each compound was formed when TBDP was employed as ligand.

Bidentated phosphines, such as DPPE, DPPP and XANTPHOS form more stable Rh species containing two coordinated phosphorus(III) due to the chelate effect. This forces the  3 - 1 equilibrium to  1 species (intermediate C, Scheme 5) and for that reason, diphosphines are more efficient for the hydroformylation of this kind of dienes than the corresponding monodentated ligands. DPPE is more efficient than DPPP because it forms more stable 5-membered ring chelates than the 6-membered ring of DPPP. [37] .XANTPHOS has a wider bite angle and is conformationally more rigid than DPPE, resulting in a more encumbered environment around the Rh atom and favoring the formation of linear products in hydroformylation. [32] Thus, it is remarkable that the regioselectivity for the branched products (4 + 5) is comparable for both DPPE and XANTPHOS, since the latter is expected to favor Path A (less encumbered) [4] This finding reinforces the hypothesis that the regioselectivity in the hydroformylation of 1phenylbutadienes is biased by the formation of stable  3 -allyl B, so that Path B becomes a dominant reaction path, even in the presence of ligands know for directing hydroformilation for linear products.

J o u r n a l P r e -p r o o f

Although -unsaturated aldehyde 4' (Scheme 5) has not been detected in our work by the analytic technique used, its intermediate formation cannot be ruled out. On the other hand, the aldehydes similar to 4' were reported as major reaction products at the hydroformylation of related dienes with other catalytic systems and under different reaction conditions (in particular, lower temperatures in the range of 30-40 o C). [22,23] We suggest that in our systems, either the fast isomerization of -unsaturated aldehyde 4' into -unsaturated aldehyde 4 biased by the favorable conjugation of the C=C and C=O bonds is occurring, or the double bond migration starting from intermediate C to give directly aldehyde 4 is operating, as previously suggested. [16] Either way, under appropriate conditions, the -unsaturated aldehyde 4 can undergo hydrogenation in tandem to give branched aldehyde 5 in excellent yields. DPPE is the most efficient ancillary for this purpose. With XANTPHOS, the tandem hydrogenation is less efficient, probably, because the coordination of the trisubstituted C-C double bond in 4 to the rhodium species is more difficult due to the greater steric encumbrance caused by this ligand. On the other hand, under appropriate temperature conditions, the use of the XANTPHOS-promoted system offers the possibility to obtain selectively another useful product, the -unsaturated aldehyde 4a.

Conclusions

Bio-sourced 2-propenylbenzene derivatives have been transformed into 4-aryl-2-methylbutanals and 4-aryl-2-methylbutenals via a sequence of catalytic transformations involving ruthenium-catalyzed cross metathesis, palladiumcatalyzed dehydrochlorination and rhodium-catalyzed hydroformylation. The hydroformylation of the synthesized 1,3-dienes employing rhodium/monophosphorus ligands was slow and the selectivity was poor. Only by the employment of chelating diphosphines as ancillary ligands at relatively high concentration were the activity and selectivity increased. The selectivity was also highly dependent on the ligands. At 100 o C, consecutive C-C double bond isomerization and hydrogenation took place, resulting in a useful tandem process. Employing XANTPHOS as ancillary ligand, the C-C double bond hydrogenation was not efficient, allowing the synthesis of the intermediate enals 4a-c in good yields. Employing DPPE as ancillary ligand at 100 o C, a clean tandem sequence took place: although this system was efficient to reduce the C-C double bond of the enals, the C-O double bond remained untouched, leading to saturated branched aldehydes 5a-c in very good selectivity. These branched aldehydes are difficult to obtain through other routes and are potentially interesting for food and flavor industry as 5a and 5c are homologous to the commercial fragrances canthoxal and helional, respectively. Noteworthy is that these transformations could be carried out in green solvents such as anisole.

CRediT author statement

  Gabriel M. Vieira (investigation), Artur V. Granato(investigation), Elena V. Gusevskaya (writing -Review & Editing), Eduardo N. dos Santos (writing-Original Draft, writing -Review & Editing, supervision), Pierre H. Dixneuf J o u r n a l P r e -p r o o f

a

  Conditions: 3a -1.0 mol.L -1 , [Rh(acac)(CO)2] -0.5 mol%; ligand -2.5 mol%, solvent -toluene, 100 ºC, 40 bar (CO:H2 = 1), 24 h. b Determined by 1 HNMR, zero value means not observed by the technique. c Other products, mainly other aldehydes. d Ligand -5.0 mol%. J o u r n a l P r e -p r o o f

J o u r n a l P r e -p r o o f Chart 1 Figure 1 :

 11 Figure 1: Kinetic follow-up for the hydroformylation of 3a promoted by Rh/DPPE. Conditions: [3a] 0 = 1.0 mol.L -1 ; [Rh(acac)(CO) 2 ] , 0.5 mol%; DPPE, 2.5 mol%; toluene; 100ºC; 40 bar (CO:H 2 = 1); 24 h. b determined by 1 HNMR. Ot: Other products.

Table 1 -

 1 Synthesis of 3-arylallyl chlorides (2a-c) and 1-arylbutadienes (3a-c) -1.0 g, allyl chloride (6 equiv.), HGII (2 mol%), solvent -dichloromethane (20 mL), 40 ºC, 15 h. -0.5-1.0 g, Pd(OAc) 2 (5 mol%), PPh 3 (10 mol%), Cs 2 CO 3 (1.2 equiv.), solventacetonitrile, 90 ºC, 17 h.

	Starting material	Conversion (%)	Isolated Yield (%)
		Synthesis of 2a-c a
	1a	95	81 (2a)
	1b	85	71 (2b)
	1c	90	62 (2c)
		Synthesis of 3a-c b
	2a	100	62 (3a)
	2b	100	47 (3b)
	2c	100	62 (3c)

a Conditions: 1a-c b Conditions: 2a-c

Table 2 -

 2 Hydroformylation of 3a in the presence of different ligands a

	Entry	Ligand	Conversion b (%)	4a	Product distribution (%) 5a 6a 7a	Others c
	1 d	PBn3	100	0	56	1	6	37
	2 d	TBDP	100	0	21	0	3	76
	3	DPPE	100	0	91	3	0	6
	4	DPPP	100	2	73	7	2	16
	5	XANTPHOS	100	37	41	10	0	12

Table 3 -

 3 Hydroformylation of 3a using DPPE as ancillary under different reaction conditions a Conditions: 3a -1.0 mol.L -1 ; [Rh(acac)(CO)2] -0.5 mol%; solvent -toluene, 40 bar (CO:H2 = 1), 24 h. b Determined by 1 HNMR, zero value means not observed by the technique. c Other products, mainly other aldehydes.

	Entry	Ligand (mol %)	T ( o C)	Conversion b (%)	4a	Product distribution b (%) 5a 6a 7a Others c
	1	1.25	100	100	0	87	3	0	9
	2	2.5	100	100	0	91	3	0	6
	3	5.0	100	100	0	83	5	6	6
	4	2.5	80	100	13	73	7	0	7
	5	2.5	60	91	54	35	3	0	8
	6 d	2.5	100	100	0	88	4	3	5
	7 e	2.5	100	100	0	85	6	3	6

a d Solvent -p-cymene. e Solvent -anisole.

Table 4 -

 4 Hydroformylation of 1-arylbutadienes a Conditions: substrate -1.0 mol.L -1 , [Rh(acac)(CO)2] -0.5 mol%, ligand -2.5 mol%, solventtoluene, 40 bar (CO:H2 = 1), 24 h. b Determined by 1 HNMR, zero value means not observed by the technique. c Other products, mainly, other aldehydes. d 60 o C. e 48h.

	Entry	Ligand	Substrate	Conversion b %		Product distribution b (%) Others c
	1	DPPE	3a	100	0	91	3	0	6
	2	DPPE	3b	99	4	86	7	3	0
	3	DPPE	3c	100	0	92	5	3	0
	4	XANTPHOS	3a	100	37	41	10	0	12
	5	XANTPHOS	3b	80	45	42	13	0	0
	6	XANTPHOS	3c	100	45	46	9	0	0
	7 d	XANTPHOS	3a	62	76	3	12	0	9
	8 d,e	XANTPHOS	3a	89	74	2	13	0	11

4(a-c) 5(a-c) 6(a-c) 7(a-c)

a

Acknowledgements

The authors thank CAPES-COFECUB (project PHC 884-17-France; 883/2017-Brazil), INCT-Catalysis, and CNPq for financial support. Umicore is thanked for supplying Hoveyda-Grubbs catalyst M72 (C627).

Declaration of interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.