M Gaudon
email: gaudon@icmcb-bordeaux.cnrs.fr

B Basly

Y Fauque

J Majimel

M H Delville

Thermochromic Phase Transition on CuMo 0.9 W 0.1 O 4 @SiO 2 Core-Shell Particles

The thermochromic phase transition of CuMo 1-x W x O 4 oxide was delayed by the deposit of SiO 2 shells. The phase transition temperature was investigated by optical reflectivity versus temperature. The effect of the shell thickness on the transition temperature is established. The cyclability of the phenomenon is also discussed.

Introduction

A thermochromic substance changes its color when subjected to temperature variation in a certain range. Besides the materials based on organic molecules such as spiropyran and ethylene derivatives, 1 or liquid crystals, 2 widely applied as thermo-indicators and thermometers, there has been an increasing interest in applied research for the thermochromic properties of inorganic materials, as used in optical switching elements 3 and smart windows. 4,5 However, very little attention has been paid to the thermochromism which takes place in the very few ceramics that are able to show this interesting phenomenon. 6,7 We have shown in recent studies that, at ambient pressure, CuMo 0.9 W 0.1 O 4 oxide changes its color because it crystallizes in two polymorphs, that is, a high-temperature-low-pressure form (R form with green coloration) and a low-temperaturehigh-pressure one (γ form with reddish brown coloration). [8][9][10] The γfR phase transition occurs by warming at 85 °C, and the reverse one occurs by cooling at about -20 °C. For some of the desired industrial applications, such a thermochromism should occur over 150 °C and be reversible above room temperature; that is, the phase transition temperatures should be increased. The first-order phase transition between the two structures is associated with a variation of about 10% of the unit cell volume at the transition. 11 The first report on the stabilization of the γ-CuMoO 4 phase with tungsten doping was reported by Wiesmann et al.; 12 the first study on the optical properties of such compounds was made by Rodriguez et al. 13 Silica coatings on core particles have proved to be efficient for modifying the core intrinsic properties. As examples, a size dependence of the melting temperature of silicaencapsulated gold nanoparticles 14 and a modification of the blocking temperatures (T B ) between the FePt and FePt@SiO 2 nanoparticles 15 have been observed.

In the present work, we show that it is possible to delay and therefore to control the transition temperatures by the deposit of SiO 2 shells around the γ-CuMo 0.9 W 0.1 O 4 cores. Indeed, one can imagine that constraints will take place because of the shell during the volume expansion associated with the γfR phase transition. Hence, the high-pressure

V TEOS )N part [(F SiO 2 M TEOS ) ⁄ (M SiO 2 F TEOS )]V SO 2 (1)
with N part ) m CuMo 0.9 W 0.1 O 4 ⁄ ( 4 ⁄ 3πRr 3 F CuMo 0.9 W 0.1 O 4 ) and V SiO 2 ) [4 ⁄ 3π ( r + e shell ) 3 -4 ⁄ 3πr 3 ] where F SiO 2 , V SiO 2 , and M SiO 2 are the density volume and molecular weight of SiO 2 and F TEOS , V TEOS , and M TEOS are the density volume and molecular weight of TEOS. N part is the number of particles.

Additionally, the TEOS was added in several small fractions under mild stirring, one fraction a day, in order to limit the probability of its self-hydrolysis-condensation. Under such conditions, despite a rough control of the particles' surface area, secondary nucleation could essentially be prevented. 18 However, ammonia usually used for the Sto ¨ber process induced powder damage, as already mentioned. A set of organic amines (CH 3 ) n NH 3-n was checked in terms of long-term stability of the powder in such media, and the results showed that trimethyl amine was the best compromise for the encapsulation (no observed powder degradation during the time required for the coating).

Transmission electronic microscopy (TEM) was performed on TECNAI F20 equipment with a field emissive gun, operating at 200 kV and with a point resolution of 0.24 nm. Samples were prepared by dissolving a few milligrams of powder in ethanol or directly depositing one drop of the alkoxide solution on a Formvar/ carbon copper grid.

Visible spectra in the 400-800 nm range between room temperature and 200 °C are recorded on a setup equipped with a photomultiplicator fiber. In both cases, the white light source was a halogen lamp and the heating/cooling rate was settled at 10 K min -1 and controlled with a standard tubular furnace.

Results and Discussion

The control of encapsulation by a silica shell is easier to perform when a low polydispersity of the particles can be achieved, which was not the case for high-temperature synthesized material. The very high sensitivity of γ-CuMo 0.9 W 0.1 O 4 powder to acidic or basic media, leading respectively to a superficial amorphization (Figure S1, Supporting Information) or a partial dissolution (Figure S2, Supporting Information) of the oxide, prevented the use of strong electrostatic repulsions to ensure its dispersion. Attempts were therefore performed in alcoholic media (EtOH), which is the further reaction solvent for silica encapsulation. However, even after the powder suspensions were ultrasonicated, the granulometry curves still showed a bimodal distribution (Figure 1a).

Even if only a small minority of big agglomerates is still present (in number), their cumulated surface is of the same order as the surface of the dispersed particles of the first mode, and the volume of the powder is mainly constituted of the agglomerates. To solve this problem, granulometry sorting was performed: about 95% of the powder was let for sedimentation and was thrown away (Figure 1b). Thus, dispersion with a unique mode centered at about 0.2 μm could confidently be achieved. The synthesis of CuMo 0.9 -W 0.1 O 4 @SiO 2 particles is based on the growth of the silica shell through the TEOS hydrolysis/condensation process derived from the modified Sto ¨ber method. 16,17 To coat the particles, the overall amount of added TEOS was set to comply with the desired silica shell thickness, according to eq 1 (see Experimental Section), and assuming an average size of 200 nm for the particles. The addition of TEOS in several fractions under mild stirring, in the presence of trimethyl amine over one to several days, limited the probability of nucleation of a secondary population.

TEM images show that both individual particles and agglomerates are coated. Very few silica spheres issued from secondary nucleation were also detected (Figure S3, Supporting Information). Nevertheless, there is an "average" effect which shows that the experimental thickness of the individual coated particles is directly correlated with the theoretical one, as shown in Figures 2 and3, the theoretical thickness being calculated considering that all of the particles are spherical with about a 200 nm diameter. (low-temperature) form should be stabilized, and the γfR transition temperature should be increased.

Experimental Section

The starting γ-CuMo 0.9 W 0.1 O 4 powder is produced by a conventional solid-state route at 700 °C under air followed by a mechanical grinding. The powder is then dispersed and coated with silica shells using a sol-gel process; the optimization of these two steps is discussed in the Results and Discussion section. 16,17 To obtain the core-shell particles, the amount of tetraethylorthosilicate (TEOS) was determined according to eq 1:

The slope in Figure 3, is characteristic of a slight overestimation of the overall particle surface area.

The effect of the SiO 2 coating on the transition temperatures between the two allotropic forms was observed by diffuse reflectivity measurement versus a temperature between room temperature and 300 °C.

Then, plotting the maximal reflectivity versus temperature gave S-type curves, which are reported in Figure 5; the optical contrasts between the low-and high-temperature forms are also determined.

The inflection point of the S curve (representing a mixture of 50% of each phase) can already be extracted. Nevertheless, for more accuracy, a correction in order to obtain standard S Boltzmann curves such as those reported in Figure 6 was performed. The latter represents the transition width (here defined as the temperature width between the state with 10% R and 90% γ phase and that with 10% γ and 90% R phase) was also extracted.

The coating by a silica shell allows tuning of the transition temperature toward higher values. In the illustrated example (shell thickness of 18 nm), the transition temperature is nearly double that of the raw-core oxide (from 85 to 165 °C). Moreover, the transition width, which is around 80 °C for the raw-core oxide, goes up to 100 °C for the core@shell powder. The large transition width for the raw powder seems to be a consequence of the large granulometric distribution; the enlargement of this width with the shell deposit may hence be due, along the same lines, to a significant distribution of the core/shell size ratio.

The dependence of the transition temperature on the shell thickness for γ-CuMo 0.9 W 0.1 O 4 @SiO 2 composites is compiled in Figure 7. The coating of the molybdates efficiently leads to an increase of the γfR transition temperature with a width of transition that is rather constant (double arrow). The variation can be described by an asymptotic growth law, which is characteristic of an attenuation of the shell thickness effect on the transition temperature.

Eventually, the cyclability of the synthesized core@shell particle and, especially, the resistance of the shell during the γfR transition were studied. TEM images and chemical cartographies obtained with coupled EDX analyses clearly show that the silica shell can severely crack during the transition because of the volume extension of the cores (see Figure 8). For a right perception of this phenomenon of the silica layer cracking, TEM micrographs, on a sample with a large core/shell diameter ratio and a 60 nm thickness silica layer, are presented.

A more accurate analysis shows that, actually, the shells crack when the core has overmicrometric dimensions. Consequently, during cycling (the materials were heated and cooled successively, one cycle being a complete γfRfγ sequence), a significant deterioration of the shell takes place, with cracks appearing particularly during the first γfR transition. Therefore, during the following cycles, the shell's influence on the temperature was less efficient.

In Figure 9, the variation of the γfR transition (determined by reflectivity versus temperature) of an 18-nm-thick γ-CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell was compared in its first and third cycles.

It is clear that the deposited shell induces a large increase of the transition temperature from 85 °C for raw cores up to 165 °C for the first cycle; however, when successive cycles are applied, a significant decrease of this transition temperature is observed. As an example, the third cycling already shows a temperature decrease down to 135 °C. Furthermore, the transition width also tends to diminish with cycling. From these results, we can infer that the silica shell deposited on the bigger core cracks during the first transition, leading to a first overestimation of the transition temperature of the overall powder, which tends to an equilibrium value after a couple of cycles.

To conclude, it was clearly shown here that it is possible to increase a first-order phase transition by coating the material with a silica shell. Here, constraints occurring during the transition are thus enhanced because the shell opposes a resistance to the high-volume dilatation associated with the phase transition. Hence, the high-pressure-low-temperature form is stabilized and the phase transition temperature is increased. It is possible to tune the transition temperature within a relatively large range through a shell thickness control. Beyond a critical ratio of the core diameter/shell thickness, the shell can crack during the first transition, which exhibits a strong effect only on the first transition cycle (80 °C increase instead of 50 °C for the following ones for an 18 nm shell thickness). Nevertheless, even if smaller, this shift of the transition temperature with such a small thickness of silica, could be improved for bigger ones and find new potential applications.

Supporting Information Available: Figures S1, S2, andS3. This material is available free of charge via the Internet at http://pubs.acs.org.

( 16 )

 16 Sto ¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (17) Mornet, S.; Elissade, C.; Hornebeck, V.; Bidault, O.; Duguet, E.; Brisson, A.; Maglione, M. Chem. Mater. 2005, 17, 4530. (18) Chen, S.-L.; Dong, P.; Yang, G.-H.; Yang, J.-J. J. Colloid Interface Sci. 1996, 180, 237.

Figure 1 .

 1 Figure 1. Granulometry distributions: (a) various presentations of the same dispersion, (b) comparison of this dispersion obtained with and without an additive gravimetric sorting.

Figure 2 .

 2 Figure 2. TEM pictures of the core@shell particles, with (a) 10 nm, (b) 30 nm, and (c) 50 nm silica shell thicknesses.

Figure 3 .

 3 Figure 3. Variation of the experimental silica shell thickness of the CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell vs the theoretical one.

Figure 4 .

 4 Figure 4. Variation of the diffuse reflectivity curve of one characteristic CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell vs temperature (silica thickness: 18 nm).

Figure 5 .

 5 Figure5. Maximal reflectivity of raw CuMo 0.9 W 0.1 O 4 and of CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell (18 nm thickness) vs temperature.

Figure 6 .

 6 Figure 6. Variation of the R-form percentage vs temperature for γ-CuMo 0.9 W 0.1 O 4 (2) and γ-CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell particles (18 nm) (9).

Figure 7 .

 7 Figure 7. Evolution of the γfR transition temperature for various γ-CuMo 0.9 W 0.1 O 4 @SiO 2 versus silica thickness.

Figure 8 .

 8 Figure 8. TEM images and chemical cartographies obtained with coupled EDX analyses for CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell obtained after the first γfR transition.

Figure 9 .

 9 Figure9. Variation of the R-form percentage vs temperature for γ-CuMo 0.9 W 0.1 O 4 (2) and for γ-CuMo 0.9 W 0.1 O 4 @SiO 2 core-shell, during the first cycle (9) and during the third cycle (().


Martine Largeron

Maurice-Bernard Fleury

Oxidative Deamination of Benzylamine by Electrogenerated Quinonoid Systems as Mimics of Amine Oxidoreductases Cofactors

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Oxidative Deamination of Benzylamine by Electrogenerated Quinonoid

Systems as Mimics of Amine Oxidoreductases Cofactors.

_______________________________________________________________________________

Introduction

Copper amine oxidases and bacterial amine dehydrogenases are enzymes containing a covalently bound quinone cofactor. These enzymes catalyze the oxidative deamination of a primary amine to produce the corresponding aldehyde and ammonia, concomitant with a two-electron reduction of dioxygen to hydrogen peroxide. 1 For instance, the active carbonyl cofactor used by mammalian copper amine oxidases is the quinone form (TPQ) of an active site 2,4,5trihydroxyphenylalanine (TOPA) residue. 2 Likewise, the bacterial methylamine dehydrogenase has been shown to contain an active site tryptophan tryptophylquinone (TTQ). It is also well known that pyrroloquinoline quinone (PQQ) behaves as a catalyst of amine oxidation, although PQQ is now recognized not to be the cofactor of amine oxidases. 3 The design of artificial catalysts that begin to approach the reactivity of amine oxidoreductases cofactors is a challenging problem, that could provide further insight into the function of the quinonoid cofactors in amine oxidation. Several synthetic models of naturally occurring quinones have been developed and model reactions of amine oxidoreductases were examined. [4][5][6][7][8][9] In particular, substantial informations have been obtained through catalytic aerobic deamination of benzylamine mediated by model cofactors, providing relevant insight into the enzyme catalysis. All the ionic mechanisms proposed so far involve a carbinolamine intermediate (Scheme 1). The most popular pathway is akin to the transamination reaction of pyridoxal phosphate with amino-acids. This involves a tautomerization of the benzyliminoquinone species to the product Schiff base, which is then hydrolysed to yield the aldehyde product and the aminophenol. The latter is autoxidized to the iminoquinone species, which is in turn hydrolysed to the quinone and ammonia. The alternative is an addition-elimination mechanism proposed first for PQQ, in which the carbinolamine is directly converted into the quinol and benzylimine without generating aminophenol intermediate. 5 The quinol is then autoxidized to regenerate the quinone.

The observation of catalytic turnover did not permit a distinction between transamination and addition-elimination mechanisms. Furthermore, attempts to distinguish under anaerobic singleturnover between these two mechanisms through product analysis, (aminophenol from transamination or quinol from addition-elimination), were thwarted by the occurrence of redox interchange reactions that scrambled the initial cofactor reduction product. 7 Nonetheless, in the case of the TPQ and TTQ-catalyzed amine oxidation reaction, solutions were found to circumvent these problems allowing to demonstrate a preferred transamination mechanism. 10,11 In the course of our search for new neuroprotective agents, we recently reported the one-pot electrochemical syntheses of two novel classes of derivatives exhibiting potent neuroprotective activities close to that of standard -tocopherol. 12,13 The key step consisted of the reactivity of the highly electrophilic electrogenerated 3,4-quinone 1 ox (Chart 1) with amino-alcohols. Depending on the nature of the condensed amino-alcohols, attack at the 2-position led to 3-substituted-1,4benzoxazin-8-one derivatives, while attack at the 3-position gave substituted alkylaminophenols. 14,15 The C(3) reactivity of the electrogenerated 3,4-quinone 1 ox was further extended to amines. At the outset, we believed that 3,4-quinone 1 ox efficiently catalysed the oxidative deamination of benzylamine, in deaerated CH 3 OH, in a way reminiscent of that reported for PQQ, TTQ and TPQ model cofactors.

This paper describes our efforts to provide an unambiguous conclusion based on an electrochemical approach. Through controlled potential electrolysis, whose advantages derive from the possibility of realizing selective oxidation by varying the potential applied to the working anode, we will endeavor to demonstrate that 3,4-quinone 1 ox is incapable of deaminating benzylamine, while 3,4-iminoquinone species 3 ox and 4 ox act as a new type of efficient cofactor for the autorecycling oxidation of benzylamine.

Results and Discussion

Reaction of the electrogenerated 3,4-quinone 1 ox with benzylamine. The cyclic voltammogram of compound 1 red (2mM), in deaerated CH 3 OH containing LiClO 4 as the supporting electrolyte and an excess of benzylamine (100 mM), at a dropping mercury electrode, showed an oxidation peak Pa 1 , due to a diffusion controlled two-electron process, at -140 mV vs SCE, the sweep rate v being 0.5 Vs -1 . As can be seen in Figure 1 (curve a), a cathodic peak Pc 1 appeared on the reverse sweep, at -190 mV vs SCE, illustrating the partial reversibility of the twoelectron transfer that could be assigned to the 3,4-quinol 1 red / 3,4-quinone 1 ox redox couple.

However, the redox potential E' 0 could not be accurately evaluated, under our experimental conditions, as the system (Pa 1 , Pc 1 ) did not fulfill all the diagnostic criteria required for a reversible process, at least when v was ≤ 500 Vs -1 : the ratio of the height of Pa 1 over that of Pc 1 never reached unity (i Pc 1 /i Pa 1 ~ 0.8) and the value of E Pa 1 -E Pc 1 (E being the peak potential) was found to be higher than 30 mV. Note that, in the reverse sweep, a second reduction peak was recorded at a more negative potential (-1720 mV vs SCE), due to the irreversible two-electron reduction of the carbonyl group of the benzophenone skeleton. 16 From the results of preparative electrolyses, two different anodic behaviors could be distinguished depending on the selected potential.

When the controlled potential E of the mercury pool was fixed at -180 mV vs SCE, i.e. at a potential for which the quinol 1 red could be oxidized exclusive of the reaction products 2 red -4 red (Chart 2), a coulometric value of 2.1 ± 0.1 was found for the number of electrons (n) involved in the oxidation of one molecule of 1 red . As the electrolysis proceeded, a decrease in the Pa 1 and Pc 1

intensities was observed, while a new system of peaks (Pa 2 , Pc 2 ) developed at a 100 mV more positive potential, due to a partially reversible two-electron transfer. The lack of peak Pc 1 in the voltammogram of the oxidized solution clearly indicated that the electrochemical oxidation of 1 red was followed by a chemical reaction involving the produced 3,4-quinone 1 ox . The height of Pa 2 was found to be roughly identical to that of the initial peak Pa 1 , and the voltammogram of the oxidized solution exhibited a well defined cathodic peak Pc 3 at -1 300 mV vs SCE (Figure 1, curve c). This corresponded to the reduction of N-benzylidenebenzylamine (PhCH=NCH 2 Ph), as further confirmed after recording the cyclic voltammogram of an authentical sample. The progress of the electrolysis was simultaneously followed by monitoring the UV-vis absorption spectrum.

After applying the potential E, no spectral changes were observed : the UV-vis spectrum shown by the oxidized solution was nearly identical with that of the monoanionic form of the quinol 1 red

with  max at 348 nm, suggesting a similarity in their chromophoric groups. The preparative electrolysis was stopped after consumption of 2.1 Faraday (F). mol -1 , when a negligible value of the current was recorded, allowing the isolation of two benzylaminophenols 2 red (16% yield) and 3 red (5% yield) as the minor products, along with aminophenol 4 red (62% yield) as the major product (See experimental section). The structure of products 2 red -4 red justified, in the cyclic voltammogram of the oxidized solution, the presence of the system of peaks (Pa 2 , Pc 2 ) that could be assigned to the oxidation/reduction of the aminophenol/iminoquinone redox couples. These structures were also consistent with the analogy existing between the UV-vis absorption spectrum of the oxidized solution and that of the starting quinol 1 red .

At that point it could be deduced that, when the controlled potential E of the mercury working electrode was fixed at -180 mV vs SCE, i.e. at a potential for which the quinol 1 red could be electrochemically oxidized whereas the reaction products 2 red -4 red remained nonoxidizable, no catalytic behavior was observed. Accordingly, PhCH=NCH 2 Ph (quantified in terms of benzaldehyde phenylhydrazone obtained upon workup of the oxidized solution with phenylhydrazine) was isolated in equimolar amount based on 1 red initial concentration.

These results demonstrated that, under our experimental conditions, the electrogenerated quinone 1 ox was incapable of deaminating benzylamine, whatever the nature of the mechanism (path A or path B in Scheme 1). Consequently, the primary identification of 1 ox as an efficient catalyst for benzylamine oxidation proved to be unfounded.

In contrast, when E was fixed at + 100 mV vs SCE, i.e. at a potential following the peak Pa 2 for which the reaction products 2 red -4 red could be oxidized to the iminoquinone forms 2 ox -4 ox , a different anodic behavior was observed. After the consumption of 2F.mol -1 , the system of peaks (Pa 2 , Pc 2 ) remained unchanged over a long time, a result in agreement with a steady-state catalytic behavior, while a significant increase in the cathodic peak Pc 3 intensity was observed (Figure 2). As benzylamine was present in large amount compared with that of the quinonoid forms, these results supported the suggestion that benzylamine suffered catalytic oxidation. The turnover yield was firstly determined in terms of the benzaldehyde phenylhydrazone which was isolated in 2600% turnover yield, based on 1 red initial concentration (Table 1). In order to quantify more accurately the products of benzylamine anodic oxidation, these were isolated after subsequent electrochemical reduction of the exhaustively oxidized solution conducted at a potential fixed at -1 600 mV vs SCE, i.e. at a potential immediately following the peak Pc 4 . The results of the exhaustive cathodic electrolyses are summarized in Table 2. They indicated that the main product of benzylamine oxidation was PhCH=NCH 2 Ph, whereas PhCHO was present as a minor product.

As earlier reported, 17 dibenzylamine 5 resulted from a two-electron reduction process, while dimers 6 and 7, isolated as meso and (D,L) forms, arised from a one-electron reduction mechanism.

The formation of imidazolidine 8 could be reasonably regarded as proceeding through a tandem two-electron oxidation-cyclisation reaction of a dimer form, during isolation or upon column chromatography. (Scheme 2). Interesting enough, the isolation of mixed dimer 9 constituted a proof of the presence, in the exhaustively oxidized solution, of benzaldehyde as the minor product.

Mixed dimer 9 resulted obviously from the condensation reaction of radicals formed consecutively to the one-electron reduction of both PhCHO and PhCH=NCH 2 Ph. 17 Therefore, we were encouraged to think that one of the iminoquinone systems 2 ox -4 ox could be effective in benzylamine oxidative deamination. Consequently, in order to clarify the substantial role of each iminoquinone form in supporting the catalytic efficiency, the electrochemical oxidation of compounds 2 red -4 red was separately performed, in deaerated CH 3 OH, in the presence of benzylamine excess. With 2 red as the starting material, the catalytic process ceased after 16 turnovers, and the yield of benzaldehyde phenylhydrazone did not exceed 800% (Table 1). As a consequence, 2 ox could not be considered as responsible for the potent catalytic efficiency (2600% turnover yield) in deaminating benzylamine. So, we focused our attention on the catalytic potential of iminoquinone derivatives 3 ox and 4 ox which proved to be equivalent. In the first procedure, the anodic electrolysis was allowed to go to completion. As the electrolysis proceeded, the system of peaks (Pa 2 , Pc 2 ) remained unchanged for a long time, consistent with a steady-state catalytic behavior, while the cathodic peak Pc 3 continuously increased. Finally, a coulometric value of 70 ± 1 was found for n, and benzaldehyde phenylhydrazone was isolated in 3200% turnover yield (Table 1).

Reaction

In the second procedure, the anodic electrolysis was spontaneously stopped after consumption of 2F.mol -1 . Then, 3,4-aminophenol 4 red was isolated in 70% yield along with 6% of the recovered starting material 3 red , a result in agreement with a transamination mechanism which implied the 3,4-iminoquinone species 4 ox as the efficient catalyst in deaminating benzylamine.

The hypothesis that 4 ox might play a crucial role in the oxidative deamination of benzylamine was further substantiated when considering the specific behavior of 4 red , under the same experimental conditions. 2F.mol -1 , the catalytic efficiency was identical to that previously described from 3 red . The results of these experiments are collected in Tables 1 and2.

Reaction

Mechanism of benzylamine oxidation.

On the basis of the results given by the controlled potential electrolyses, we can formulate the following assumptions about the mechanism of benzylamine oxidative deamination.

Electrogenerated 3,4-iminoquinone species 3 ox and 4 ox turned out to be efficient catalysts in deaminating benzylamine, producing 3200% turnover yield. The fact that, starting from 3 red , 4 red was isolated in 70% yield after a single turnover (2F.mol -1 ), clearly demonstrated that the oxidative catalytic deamination of benzylamine proceeded via a transamination mechanism. This involved proton abstraction and the subsequent electron flow from the -carbon to the quinone moiety, to produce the Schiff base intermediate 3' ox , the driving force being the tendency to restore aromaticity within the 3,4-iminoquinone ring. The proposed mechanism (Scheme 3) is the analog of path B in Scheme 1 which is the accepted mechanism for catalytic deamination by quinonoid cofactors.

Although the structure of our model compounds markedly differs from that of naturally occurring quinones, the reaction displays two features that are most often associated with enzymatic systems: a) the reactions seems to be enhanced through the participation of neighboring substituents, here, the benzoyl and 2-hydroxyl substituents, as they prevented the competing formation of Michael adducts. b) the presence of an active hydroxyl proton at the 2-position very probably is an essential component of the catalytic activity. It can be remembered that similar active hydrogens (pyrrole proton 1-NH of PQQ and hydroxyl proton 2-OH of TPQ) were found to be essential for optimal catalytic activity. 4,7 In this connection, further analogs development would be useful to confirm the substantial role of both the benzoyl and 2-hydroxyl groups in enhancing the reactivity of the iminoquinone ring.

Concluding remarks

In this paper, the catalytic efficiency of a new type of quinonoid cofactors was investigated towards the oxidation of benzylamine. Contrary to previous studies on related redox cofactors, in our work the unstable quinonoid species were electrogenerated. Under strictly anaerobic conditions, the use of controlled potential electrolysis enabled us to modulate the oxidative strength by a simple variation of the potential applied to the working anode. On this basis, we have established that electrogenerated 3,4-quinone 1 ox was incapable of deaminating benzylamine, while electrogenerated 3,4-iminoquinone species 3 ox and 4 ox behaved as effective catalysts. Under our experimental conditions, the reaction efficiency reached 64 turnovers.

Finally, using 3 red as the starting material, we were led to conclude that the deamination mechanism of benzylamine was akin to the transamination reaction reported earlier for quinone cofactors of amine oxidoreductases. We are currently conducting experiments to explore the ability of our model system 4 ox to catalyze the oxidative deamination of unactivated aliphatic primary amines.

Experimental Section

Chemicals were commercial products of the highest available purity and were used as supplied. All apparatus, cells and electrodes were identical with those described previously. Catalytic oxidation of benzylamine from electrogenerated 3,4-quinone 1 ox . Method B. A solution of 2,3,4-trihydroxybenzophenone 1 red (115 mg, 0.5 mmol), LiClO 4 (5.30 g, 50 mmol) and benzylamine (2.67 g, 25 mmol) in CH 3 OH (250 mL) was oxidized under N 2 , at room temperature, at a mercury pool working electrode (E = + 100 mV vs SCE). After exhaustive oxidation (55F.mol - 1 ), phenylhydrazine (2.70g, 25 mmol) and 100 mL of a molar acetic buffered aqueous solution of pH ~ 3.5 were added to the reaction mixture. The resulting hydroalcoholic solution was concentrated to remove CH 3 OH and allowed to come to room temperature. The solid was collected by filtration, washed with H 2 O, and dried in a vacuum dessicator. Benzaldehyde phenylhydrazone was then isolated as a colorless solid in 53% yield, based on PhCH 2 NH 2 initial concentration. mp 156-158°C (recristallized from toluene). Spectroscopic data of benzaldehyde phenylhydrazone have been reported earlier. 19,20 Method C. A solution of 2,3,4-trihydroxybenzophenone 1 red (115 mg, 0.5 mmol), LiClO 4 (5.3 g, 50 mmol) and benzylamine (2.67 g, 25 mmol) in CH 3 OH (250 mL) was oxidized using method B.

After exhaustive oxidation (55F.mol -1 ), the resulting methanolic solution was immediately reduced after the potential of the mercury pool was switched to -1 600 mV vs SCE. After exhaustive cathodic electrolysis, the solution was poured into H 2 O (100 mL). The resulting hydroalcoholic solution was concentrated to remove CH 3 OH and extracted with CH 2 Cl 2 (100 mL). After the extract was dried over MgSO 4 , evaporation of the solvent gave a pale yellow oil from which pure dimer product 6 (meso form) was isolated after washing with ethanol in 12% yield, based on PhCH 2 NH 2 initial concentration. The resulting ethanolic layer was concentrated to remove ethanol leading to a crude product which was purified by flash column chromatography (SiO

  At the same time, a new absorption band at 250 nm markedly increased, its height being much higher than the absorption band at 348 nm. The electrolysis proceeded further until a coulometric value of 55 ± 1 was found for n. It could be noted that a) during the last part of the anodic electrolysis (n > 40) a new cathodic peak Pc 4 developed at a more negative potential (-1 550 mV vs SCE), that could be assigned to the reduction of PhCHO, as corroborated after recording the cyclic voltammogram of an authentical sample ; b) the voltammogram given by the exhaustively oxidized solution no longer exhibited the pair of peaks (Pa 2 , Pc 2 ).

of the electrogenerated 3 , 4 -

 34 benzyliminoquinone 3 ox with benzylamine. The cyclic voltammogram of 3 red (2mM), in deaerated CH 3 OH containing LiClO 4 as the supporting electrolyte and an excess of benzylamine (100 mM), at a dropping mercury electrode, exhibited an oxidation peak Pa 1 at -70 mV vs SCE, in agreement with the two-electron oxidation of 3 red to 3 ox .The partial reversibility of the two electron transfer was illustrated by the appearance of a cathodic peak Pc 1 , on the reverse sweep, at -120 mV vs SCE.When E was fixed at + 100 mV vs SCE, a decrease in the Pa 1 and Pc 1 intensities was observed simultaneously, while a new system of peaks (Pa 2 , Pc 2 ) developed at a 30 mV more positive potential, due to a partially reversible two-electron transfer. Simultaneously, the cathodic peak Pc 3 characteristic of the reduction of PhCH=NCH 2 Ph developed at -1 300 mV vs SCE.

of the electrogenerated 3 , 4 -

 34 iminoquinone 4 ox with benzylamine. The cyclic voltammogram of 4 red (2mM), in deaerated CH 3 OH containing LiClO 4 as the supporting electrolyte and an excess of benzylamine (100 mM), at a dropping mercury electrode, showed an oxidation peak Pa at -40 mV vs SCE, characteristic of the two-electron oxidation of 4 red to 4 ox .When E was fixed at + 100 mV vs SCE, the intensity of the anodic peak at -40 mV vs SCE remained unchanged, in agreement with a steady-state catalytic behavior. When the electrolysis was stopped after consumption of 2F.mol -1 , 76% of the starting material 4 red was recovered without evidence of other products. When the controlled potential electrolysis was pursued beyond

Table 2 .E

 2 Scheme 3

Figure 1 .

 1 Figure 1. Progress of the cyclic voltammogram in the course of anodic controlled potential electrolysis of 1 red (2mM) at a mercury working electrode (E = -180 mV vs SCE), in deaerated CH 3 OH containing LiClO 4 (200 mM) and benzylamine (100 mM). Curve a ( ___ ), before electrolysis; curve b (---), after consumption of 1 Faraday per mole of substrate; curve c (-. -), after exhaustive electrolysis (2.1 F.mol -1 ). Arrowheads indicate the direction of the potential sweep; v = 0.5 Vs -1 . The vertical arrow indicates the initial potential point.

Figure 2 .

 2 Figure 2. Progress of the cyclic voltammogram in the course of anodic controlled potential electrolysis of 1 red (0.4mM) at a mercury working electrode (E = + 100 mV vs SCE), in deaerated CH 3 OH containing LiClO 4 (40 mM) and benzylamine (20 mM). Curve a ( ___ ), before electrolysis; curve b

Table of Contents Graphic _______________________________________________________________________________ Oxidative Deamination of Benzylamine by Electrogenerated Quinonoid Systems as Mimics of Amine Oxidoreductases Cofactors

			OH	O
		RN	Ph
	Martine Largeron and Maurice-Bernard Fleury*	O	3 ox : R = CH 2 Ph
			4

ox : R = H

  18

Preparation of 4-benzylamino-2,3-dihydroxybenzophenone 2 red , 3-benzyl-amino-2,4- dihydroxybenzophenone 3 red and 3-amino-2,4-dihydroxybenzophenone 4 red from electrochemical oxidation of 1 red in the presence of benzylamine.

	Method A. A solution of
	2,3,4-trihydroxybenzophenone 1 red (115 mg, 0.5 mmol), LiClO 4 (5.30 g, 50 mmol) and
	benzylamine (2.67 g, 25 mmol) in CH 3 OH (250 mL) was oxidized under N 2 , at room temperature,
	at a mercury pool working electrode (E = -180 mV vs SCE). After exhaustive oxidation (2F.mol -
	2 red (recrystallized from ether) mp 124-126°C; 1 H NMR (300 MHz, DMSO-d 6 )  4.45 (d, 2H, J =
	5 Hz), 6.10 (d, 1H, J = 9 Hz), 6.85 (d, 1H, J = 9 Hz), 6.90 (t, 1H, J = 5 Hz), 7.20-7.35 (m, 5H),
	7.40-7.55 (m, 5H), 8.95 (s, br, 1H), 12.70 (s, br, 1H); 13 C NMR (75MHz, DMSO-d 6 )  46.2, 103.1,
	110.0, 127.8, 128.0, 128.3, 129.3, 129.5, 131.9, 130.4, 139.2, 140.8, 146.0, 151.5, 197.0. MS DCI
	m/z 320 (MH + ). Anal. Calcd for C 20 H 17 NO 3 C, 75.23; H, 5.33; N, 4.39. Found : C, 74.97; H,
	5.37; N, 4.36.
	3 red (recrystallized from ether) mp 149-151°C dec; 1 H NMR (300 MHz, DMSO-d 6 ) 4.50 (d, 2H,
	J = 5 Hz), 6.40 (d, 1H, J = 9 Hz), 6.85 (d, 1H, J = 9 Hz), 7.20-7.35 (m, 5H), 7.50-7.65 (m, 5H ),
	12.80 (s, br, 1H); 13 C NMR (75MHz, DMSO-d 6 )  50.1, 108.8, 113.1, 125.1, 126.9, 127.6, 128.5,
	129.2, 129.4, 129.7, 132.4, 139.2 and 142.6, 155.3, 155.7, 201.0. MS DCI m/z 320 (MH + ). Anal.
	Calcd for C 20 H 17 NO 3 C, 75.23; H, 5.33; N, 4.39. Found : C, 74.93; H, 5.35; N, 4.37.
	4 red (recrystallized from CH 2 Cl 2 ) mp 186-188°C dec; Spectroscopic data for product 4 red have
	been reported earlier. 15

1 ), i.e. when a negligible value of the current was recorded, the methanolic solution was poured into a molar citric acid buffered aqueous solution of pH  3 (100 mL). The resulting hydroalcoholic solution was concentrated to remove CH 3 OH and extracted with ether (100 mL). After the extract was dried over MgSO 4 , evaporation of the solvent gave a dark yellow residue from which aminophenols 2 red , 3 red and 4 red were isolated in 16%, 5% and 62% yields, respectively, by flash column chromatography [SiO 2 , toluene-acetone gradient from (100:0) to (95:5)].

Catalytic oxidation of benzylamine by electrogenerated 3,4-benzyliminoquinone 3 ox

  . The above described method A, replacing 1 red by 3 red , with E = + 100 mV vs SCE and n = 2 afforded, after flash column chromatography [SiO 2 , toluene-acetone 90:10], compound 4 red in 70% yield, along with 6% of the recovered starting material 3 red .

	7.10-7.40 (m, 20H); 13 C NMR (75 MHz, CDCl 3 )  51.3, 68.3, 126.7, 126.9, 127.9, 128.0, 128.3, Chart 1 Chart 2 Scheme 1 140.6, 141.1. MS DCI m/z 393 (MH + ). 3-Benzyl-2,4,5-triphenylimidazolidine 8 : colorless oil. 1 H NMR (600 MHz, CDCl 3 ) 2.05 (1H, Scheme 2
	broad s, NH, D 2 O exchanged), 3.65 (2H, AB, J = 16 Hz, CH 2 -N), 3.80 (1H, d, J = 8 Hz, H-4), 4.42 HO
	(1H, d, J = 8 Hz, H-5), 4.88 (1H, s, H-2), 6.95 (2H, broad d, J = 8 Hz, ortho Ph-3), 7.20 (2H, broad d, J = 8 Hz, ortho Ph-5), 7.25 to 7.40 (12H, m, aromatic proton), 7.45 (2H, broad d, J = 8 Hz, ortho The above described method B, replacing 1 red by 3 red , allowed, after consumption of 70F.mol -1 , to the isolation of benzaldehyde phenylhydrazone in 64% yield, based on the NH CO 2 H HO 2 C NH Ph HO OH O Ph PhCH=NH O 2 HO OH O HO Ph O Ph N Ph
	Ph-4), 7.70 (2H, broad d, J = 8 Hz, ortho Ph-2); 13 C NMR (150 MHz, CDCl 3 )  53.5 (CH 2 -N), 69.8 (C-5), 75.6 (C-4), 81.3 (C-2), 127.2 to 130.4 (CH, aromatic carbon), 136.3, 140.5 and 142.7 (C Q , aromatic). HRMS (LC TOF, reserpine as the lock mass) m/z 391.2185 (MH + ) Calcd for C 28 H 27 N 2 391.2174. PhCH 2 NH 2 initial concentration. Catalytic oxidation of benzylamine by electrogenerated 3,4-iminoquinone 4 ox . The above described method A, replacing 1 red by 4 red , with E = + 100 mV vs SCE and n = 2, led, after flash column chromatography [SiO 2 , toluene-acetone 90:10], to the recovered starting material 4 red in N O O CO 2 H PQQ N H HO N H Ph quinol e , H + e , H + H 2 O 2 path A O O TTQ 1 red 2 red O PhCH 2 NH 2 N Ph N Ph H H 2 O Ph OH Ph H N Ph
	The relative stereochemistry of the three asymmetric centres was determined through a 2D O HO HO NOESY experiment (mixing time = 250 ms). NOE cross peaks were observed between H-2 and H-76% yield. O OH O O Ph H N OH O Ph Ph H 2 N OH O O HO OH Ph Ph Ph N H O O e , H +
	4, ortho Ph-5, on the one hand, while correlations were obtained between H-5 and ortho Ph-4, ortho NH 3 O Ph-2, on the other. The carbon type (methyl, methylene, methide or quaternary) was determined by HN HO O 3 red 4 red quinone carbinolamine benzyliminoquinone Ph N H Ph Ph N H Ph Ph H N Ph
	DEPT experiments.	TPQ	path B	1 ox
	N-benzyl-1,2-diphenyl-aminoethanol 9 : white solid recrystallized from ethanol mp 156-158 °C.
	1 H NMR (300 MHz, CDCl 3 ) 1.70 (s, 1H), 3.00 (s, 1H), 3.55 (d, 1H, J = 13 Hz), 3.75 (d, 1H, J = H 2 O
	2 , toluene-acetone gradient (100:0) 200 mL, (95:5) 200 mL, (90:10) 200 mL. Dibenzylamine 5, dimer 7 (D,L form), imidazolidine 8 and mixed dimer 9 were then isolated in 11%, 10%, 5% and 2% yields, 13 Hz), 3.95 (d, 1H, J = 13 Hz), 4.85 (d, 1H, J = 6 Hz), 7.10-7.35 (m, 15H); 13 C NMR (75MHz, N Ph H 2 O PhCH=O H 2 N H 2 O 2 O 2 HN CDCl 3 )  51.0, 67.8, 76.7, 126.8, 126.9, 127.6, 127.7, 127.8, 128.0, 128.2, 128.3, 128.4, 139.1 HO HO O 139.8 and 140.3. MS DCI m/z 304 (MH + ). Anal. Calcd for C 21 H 21 NO C, 83.17; H, 6.93; N, 4.62. quinone quinol
	respectively. Found : C, 82.87; H, 6.97; N, 4.60. iminoquinone	aminophenol	product Schiff	base
	Dibenzylamine 5 : colorless oil. Spectroscopic data of dibenzylamine have been reported earlier. 21 Catalytic oxidation of benzylamine by electrogenerated 4,3-benzyliminoquinone 2 ox . The
	N,N'-dibenzyl-1,2-diphenyl-1,2-diaminoethane 6 (meso form) : white solid recrystallized from ethanol mp 149-151°C. 1 H NMR (300 MHz, CDCl 3 ) 1.70 (s, br, 2H), 3.30 (d, 2H, J = 14 Hz), above described method A, replacing 1 red by 2 red , with E = + 100 mV vs SCE and n = 2, led, after
	3.55 (d, 2H, J = 14 Hz), 3.75 (s, 2H), 7.00-7.35 (m, 20H); 13 C NMR (75MHz, CDCl 3 )  50.9, 67.1, flash column chromatography [SiO 2 , toluene-acetone 98:2], to the recovered starting material in
	126.6, 127.6, 127.8, 128.1, 128.3, 128.5, 140.2, 140.7. MS DCI m/z 393 (MH + ). Anal. Calcd for 75% yield.
	C 28 H 28 N 2 C, 85.71; H, 7.14; N, 7.14. Found : C, 85.77; H, 7.19; N, 7.11. The above described method B, replacing 1 red by 2 red , allowed, after consumption of
	N,N'-dibenzyl-1,2-diphenyl-1,2-diaminoethane 7 (D, L form) : colorless oil. 1 H NMR (300 18F.mol -1 , to the isolation of benzaldehyde phenylhydrazone in 16% yield, based on PhCH 2 NH 2
	MHz, CDCl 3 ) 2.40 (s, br, 2H), 3.50 (d, 2H, J = 13 Hz), 3.75 (d, 2H, J = 13 Hz), 3.80 (s, 2H), initial concentration.

The above described method B, replacing 1 red by 3 red , allowed, after consumption of 70F.mol -1 , to the isolation of benzaldehyde phenylhydrazone in 64% yield, based on PhCH 2 NH 2 initial concentration.

The above described method C, replacing 1 red by 4 red , afforded dibenzylamine 5 (13% yield), dimer compounds 6 (12% yield, meso form) and 7 (10% yield, D,L form), imidazolidine 8 (4% yield) and mixed dimer 9 (2% yield).

Acknowledgements : The authors are indebted to Marc Vuilhorgne for HRMS and stereochemical attribution of compound 8.