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Oxidation of meso- and d,l-hydrobenzoin by peroxomolybdenum complexes: A mechanistic investigation
Jouwuzl of Molecular
Catalysis,
78 (1993)
159-168
159
Elsevier Science Publishers B.V., Amsterdam M3077
Oxidation of meso- and d,L-hydrobenzoin by peroxomolybdenum complexes: a mechanistic investigation Sandro
Campestrini,
Fulvio Di Furia*
and Fabiola
Novello
Cen&o CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Universita’ di Padova, Via Marzolo I, 35131 Padua (Italy)
(Received March 31, 1992; accepted June 30, 1992)
Abstract The stoichiometry and the kinetic behavior of the oxidation of two isomeric glycols, rneso- and d,Z-hydrobenzoin, by a neutral (MoO~HMPT; HMPT = hexamethylphosphorictriamide) and by an anionic ([MoO,PICO]-Bu,N + ; PICO = picolinate N-oxido ligand) peroxomolybdenum complex have been examined. In all cases the reaction leads to the formation of two products, the a-diketone benzil and benzaldehyde, the latter deriving from the oxidative cleavage of the carbon-carbon bond of the glycols. The product distribution is greatly dependent on the nature of both the substrates and the oxidants. In particular, MoOsHMPT affords almost exclusively benzaldehyde with both isomers. By contrast, [ MoO,PICO] -Bu,N +, upon reaction with meso-hydrobenzoin, gives mainly benzil, whereas with the d&isomer benzaldehyde is the major product. Based also on kinetic evidence, it has been proposed that the two products derive from two different intermediates. In particular, benzil is formed from a ground-state complex where the glycol is coordinated to the oxidant as a monodentate ligand, while benzaldehyde originates from a complex in which the substrate is present as a chelating ligand. The occurrence of autooxidative processes when the reactions are carried out under air is considered strong evidence of the homolytic nature of the reactions that should take place within the coordination sphere of the metal.
Introduction The synthetic scope of anionic peroxomolybdenum and peroxotungsten derivatives as oxidants of the alcoholic function is well established [l-4]. Recently we reported on the efficiency of species such as 1, and in particular of lb, whose reactivity is cu. loo-fold larger than that of la in the oxidation of primary and secondary alcohols [5]. These reagents, which are soluble up to 1 M in nonpolar solvents such as dichloroethane (DCE) owing to the presence of the lipophilic cation, are able to oxidize primary alcohols to aldehydes without any overoxidation to carboxylic acids. Such a transformation proceeds with good chemical yields also when the substrates, together with the alcoholic function, contain other *Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
la
lb
oxidizable (double and triple bonds) or labile (oxirane ring) groups [6]. An additional, synthetically attractive, feature of lb is its ability to oxidize diols and glycols. As an example, by reacting a stoichiometric amount of lb with meso-hydrobenzoin, fair yields of the cY-diketone, benzil, are obtained. Thus the product deriving from the oxidative cleavage of the carbon-carbon bond, i.e., benzaldehyde, is formed only in a relatively small amount (= 15%), at variance with the behavior of other metal oxidants such as Co(III), Pb(IV) and Ce(IV) derivatives [7]. In an attempt to ascertain the source of the chemoselectivity observed, we started some time ago a mechanistic investigation of the behavior of molybdenum peroxo complexes such as 1 in the oxidation of simple alcohols. Preliminary reports on our study have already appeared [ 8, 91. In this paper we present in detail our results for the oxidation of two model substrates, i.e., meso- and d,l-hydrobenzoin. The oxidation of glycols by a variety of metal derivatives has been examined extensively from a mechanistic point of view by several authors [lo-131, thus making possible a comparison of our data with those available in the literature. As an example, both homolytic and heterolytic mechanistic pathways may be encountered [ 14-l 61. Moreover, the glycol oxidation may be a bimolecular reaction, or it may involve as a prerequisite the formation of a ground-state complex between the oxidant and the glycols, which, in turn, may act as monodentate or bidentate ligands. The data collected allow us to suggest that the oxidation of meso- and d,lhydrobenzoin by lb is a homolytic process taking place within the coordination sphere of molybdenum.
Experimental Materiak MoO(O&HMPT and [MoO(O&PICO]-Bu,N + were obtained and purified ([O,,] =2[MoO(O&HMPT] or 2[MoO(O&PICO], > 95%, iodometric titre) following published procedures [5, 171. Meso-hydrobenzoin and 2,6-di-tbutylcresol, high-purity products (Aldrich), were used without further purification. d,l-Hydrobenzoin was obtained by reduction with LiAlH, of the
161
corresponding trichloroacetic diester following a published procedure [ 18 1. ~,LY’GZO-isobutyronitrile was purified by crystallization from acetonitrile solution. The picolinate N-oxido tetrabutylammonium salt was prepared by mixing picolinic N-oxido acid with a stoichiometric amount of tetrabutylammonium hydroxide (1.5 M solution in water). The solvent was then removed by distillation under vacuum. The remaining material was dissolved in CHzClz and dried over MgSO,. After removal of the solvent, the organic salt was obtained. 1,2-Dichloroethane (DCE) was purified by distillation over P2%
Kinetic measurements All kinetic measurements were carried out under argon pressure, after repeated freeze-vacuum-ultrasound cycles aimed at minimizing the presence of dioxygen, in a thermostatic bath with a temperature control better than +0.05 “C. Aliquots of the reaction mixture were stripped out at various times and their content of active oxygen determined by iodometry. Benzil and benzaldehyde were analyzed, after quenching of the residual active oxygen by added PPh,, by HPLC (isobutyrophenone as internal standard) on a 25 cm RP 18 column equipped with a UV-Vis detector (Waters, 480) at 300 run. The response factor of each product versus the internal standard was calculated separately.
Results
and discussion
The stoichiometry of the oxidation of the two model substrates, mesoand c&l-hydrobenzoin (2a and 2b), by lb and by the neutral peroxomolybdenum complex MoO,HMPT, 3, has been determined by using a 2.5-fold excess of the substrate over the active oxygen of the oxidant.
meso-hydrobenzoin
(2a)
dl-hydrobenzoin
(2b)
3
The results of this study carried out under air are collected in Table 1. It may be noticed that in all cases two products are observed, i.e., the cY-diketone benzil together with benzaldehyde, the latter deriving from the oxidative cleavage of the carbon-carbon bond of the glycols. However, some important differences between the two substrates and also between the two oxidants may be observed. In particular, benzil is the major product of the oxidation by the anionic complex lb in the case of meso-hydrobenzoin, whereas in the case of the &?-isomer, the amount of benzaldehyde produced
162 TABLE
1
Yields and product ratios in the oxidation in DCE at 60 “C under air atmosphere Entry
Substrate
Oxidant
of 2a and 2b (0.025
Yield”
M) by lb and 3 (0.005
M)
[Be&l] /[ benzaldehyde]
(%)
lb 2 3' 4 5 6
2a 2a 2a 2a 2b 2b
lb lb lb 3' lb 3
325 300 350 200 120 260
1.6 1.4 7.5 0.01 0.2 0.002
aThe reaction mixtures were analyzed after complete disappearance of the oxidant and the yields calculated according to the expression: yield (%) = (2 x moles benzil+lx moles benzaldehyde/moles active oxygen) x 100. ‘At 50 "C. ‘In the presence of picolinate N-oxido tetrabutylammonium (0.03 M).
largely exceeds that of benzil. In addition, with both substrates, the neutral peroxo complex 3 affords benzaldehyde almost exclusively, benzil being present only in a trace amount. This finding is further experimental evidence of the superior ability of anionic molybdenum peroxo complexes such as lb, as compared with neutral ones, to oxidize the alcoholic function. Also worthy of notice is the result in entry 3, which indicates that the addition of the salt of the ligand of lb, i.e., the tetrabutylammonium picolinate Noxido, significantly enhances the production of benzil. In fact, the benzil/ benzaldehyde ratio in the oxidation of meso-hydrobenzoin by lb is cu. 1.5 in the absence of added ligand, whereas in the presence of the ligand, this ratio is increased by a factor of cu. 5. This has important mechanistic implications, which will be discussed later on in this paper. At any rate, the most striking feature of the data presented in Table 1 is the more-than-stoichiometric formation of both products. Based on the active oxygen content of the oxidant, yields larger than 300% are observed. Such a finding points to involvement of dioxygen in the reaction, thus providing the first evidence of the occurrence of carbon-centered radical species, which are well known to add dioxygen in a very fast, almost diffusioncontrolled, reaction [ 19 1. On one hand, the detection of autooxidative processes may be considered a most valuable clue for the understanding of the mechanism of glycol oxidation by lb. On the other hand, such processes increase the complexity of the reaction under investigation. As a consequence, the kinetic data reported in this paper were obtained under argon atmosphere, after careful degassing procedures. Under these conditions, as shown in Fig. 1, the yields of oxidized products, benzil and benzaldehyde, are almost quantitative, thus demonstrating that the involvement of dioxygen has been minimized.
163
T
0.008
.-o 5
0.006
‘. s
0.004
E ?-
0.002
0 0
40
20
60
80
100
1
time,m
G L ; : :
oxygen]
0
[active
+
[benzil]
A
[benzaldehyde]
0.006-
0.004-
A 0.002-
0
20
60
40
8.0
100
time-m J?ig. 1. Disappearance of the active oxygen and appearance of the products as a function of the time in the oxidation of 2a (0.05 IQ by lb (0.005 M) in DCE at 60 “C: (A) under air; (B) under argon atmosphere.
The different product distributions observed for the two isomer-k substrates, which had purposely been selected to be as close as possible in terms of their chemical reactivities, may be rationalized on the basis of their different attitudes toward acting as bidentate ligands, a feature that is well documented in the literature [20]. In particular, it is known that c&l-hydrobenzoin is a better chelating agent than the meso-isomer, mainly for steric reasons. As a consequence, benzaldehyde should be formed through a mechanistic pathway that involves association of the glycol to the metal to form an intermediate in which both alcoholic functions of the substrate are coordinated to molybdenum. Taking into account that lb is a coordinatively saturated complex where such a coordination implies the complete removal of the picolinate N-oxido ligand, whereas 3 already has a free coordination site and a second one that is occupied by the neutral, easily removable ligand HMPT, the experimental finding that the neutral peroxomolybdenum complex affords benzaldehyde almost exclusively is also nicely rationalized. Thus a general scheme for benzaldehyde formation would be:
164
+
lb
i-l
OH
Scheme
OH
1.
The processes outlined in Scheme 1 also account for the inhibitory effect of added ligand on benzaldehyde formation that should be due to the reversion to the left of the equilibrium (1) leading to the formation of the intermediate. Since the effect of such a reversion is increased production of benzil, it is also reasonable that the a-diketone is formed by a different mechanistic path. On the other hand, information on such a path is not provided by the data already presented. In particular, it should be ascertained whether or not the benzil production involves an intermediate which, from what has been concluded above, must be different from the intermediate leading to benzaldehyde. Some useful hints on this aspect are provided by the kinetic data collected in Table 2 and shown graphically in Pigs. 2 and 3. These data were obtained using an excess of the substrate over the oxidant and by measuring the initial rates of production of the two substrates. At the same time, it was observed that plots of log[O],, versus time are linear up to 50-60% reaction, indicating that the kinetic order of the oxidant is one in both processes. This is also confirmed by entries 11 and 12 of Table 2, where it can be seen that the initial rates of both reactions depend linearly on the active oxygen concentration. Prom the data of Table 2 the activation parameters for both benzil and benzaldehyde formation can be obtained. These are reported in note d of the table. Particularly telling are the AS+ values, which are relatively small. It should be recalled that in the oxidation of thioethers by lb in the same solvent, a clean bimolecular reaction, a AS+ value of = 120 J K -’ mol -I was measured [21]. Thus further evidence of associative processes in glycol oxidation is obtained. Entry 16 of Table 2 confirms the inhibitory effect of added ligand on benzaldehyde formation already noticed in stoichiometric experiments. The comparison of the data for benzaldehyde formation with those for benzil formation provides further evidence that we are dealing with two different reactions, characterized, inter c&a, by different activation parameters. In the reaction leading to benzaldehyde, saturation, Michaelis-Mententype behavior is observed, consistent with the proposal made above of the formation of an intermediate (Pig. 2). The saturation is achieved at relatively
165 TABLE 2 Initial rates for the oxidation Entry
(M)
(M>
ROVX lo7 (M s-i)
Ross x 1 or (M s-l)
1.25 2.5 5.0 10.0 2.5 2.5 2.5 2.5 2.5 2.5
5.0 5.0 5.0 5.0 2.9 1.3 5.0 5.0 5.0 5.0
4.25 8.75 17.5 35.0 5.51 2.79 1.25 3.50 24.3 7.75
1.75 3.75 7.00 8.00 2.61 1.36 0.95 2.00 9.25 1.75
[2a]
7 8 9 10 11 12 13” 14b 15’, d 16’
of 2a by lb in DCE at 60 “C under argon atmosphere
X
10’
[lb]x103
“At 40 “C. ‘At 50 “C. ‘At 70 “C. dFrom entries 8, 13, 14 and 15 the following activation parameters may be estimated: for bensil formation AH+ = 88 kJ mol-’ and AS = - 29 J K-’ mol-‘; for benzaldehyde formation AH+ =68 kJ mol-’ and AS’ = - 70 J K-’ mol-‘. eIn the presence of picolinate N-oxido tetrabutylammonium (0.025 M). ‘Relative to the benzil formation. 8Relative to the benzaldehyde formation.
&OOe-7
8
I 6 u
-
600e-7-
4.00e-7-
0 00
0 02
0 04
0 06
0 08
010
01
[Hydrobenzoin], M Fig. 2. Initial rates (M s-i) of benzaldehyde formation as a function of the initial substrate concentration in the oxidation of 2a by lb in DCE at 60 “C under argon atmosphere.
low substrate-oxidant ratios, as expected for glycol acting as a chelating agent. By contrast, the rate law for benzil formation is a second-order equation, first order in the substrate concentration and first order in the oxidant concentration for the entire concentration range explored (Fig. 3). As a consequence, we are left with two alternatives: either the benzil formation is a bimolecular process, or it involves the formation of an intermediate, which however must be present at a low, steady-state con-
0 00
002
004
006
[Hydrobenzoin],
008
0 IO
012
M
Fig. 3. Initial rates (M s-l) of benzil formation as a function of the initial substrate concentration in the oxidation of 2a by lb in DCE at 60 “C under argon atmosphere.
centration. Apart from the small AP value found, there is other indirect evidence that tends to support the latter possibility. We have already demonstrated on the basis of dioxygen interception that the reaction leading to benzil and that forming benzaldehyde are homolytic processes where, presumably, electron-transfer steps from the substrate to the oxidant are involved, leading to carbon-centered radicals. Such electron transfers are more likely to take place within the coordination sphere of the metal, rather than being outer-sphere processes. These, in fact, are not so frequently encountered in reactions involving metal complexes. The second, and even more stringent, piece of evidence favoring the formation of an intermediate is the experimental observation that benzoin, i.e., the hydroxyketone resulting from the oxidation of the first hydroxy group of meso-hydrobenzoin, is detected only in minute amounts in the reaction mixtures. This finding must be considered in light of the fact that direct control experiments have indicated that the rate of oxidation of benzoin to benzil is comparable with that of oxidation of mesohydrobenzoin to benzil. Therefore, neglecting the very unlikely possibility that the two OH groups of meso-hydrobenzoin are oxidized simultaneously, a plausible rationale for this apparently contradictory finding is that benzom is oxidized to benzil within the coordination sphere of the metal. Thus the general scheme for the oxidation of the glycol would be that shown in Scheme 2. Therefore, the substrate would act as a monodentate ligand whose association constant to the metal is low enough to prevent saturation, at least in the concentration interval explored. Through a keto-enol-type tautomeric equilibrium the initially formed benzoin remains coordinated to the metal via its residual alcoholic function, which then undergoes oxidation to the ketone. As far as the intimate mechanism of the homolytic oxidation is concerned, it is clear that at this stage any speculation would be rather hazardous. Although we did obtain evidence that organic radical species ketyl radicals - are formed in benzil production, so that we may assume
167
0 : I ‘MO’
o”/ 0
1
o.. ‘0
I
0
Scheme
2.
that in these reactions the substrates behave as one-electron reductants, it remains to be ascertained whether the electron is transferred to the peroxo group or to the metal, thus giving rise to a MO(V) intermediate. It is worthy of mention that in these reactions we have directly confirmed that added radical initiators (cx,(Y’-CUO-isobutyronitrile) or radical traps (2,6-di-t-butylcresol) have very little effect, or none, on the rates. This would indicate that the leakage of organic radicals outside the coordination sphere of the metal is an almost negligible event, suggesting that dioxygen interception may also involve the coordinated radical. Finally, the similarity between the processes shown in Scheme 2 and those conceivable for the oxidation of alcohols (which, of course, may coordinate to the peroxometal complexes only as monodentate ligand) is rather evident. Along this line, the lack of autooxidative processes in oxidation of alcohols [8 ] might be related to the much shorter lifetime of a simple ketyl radical as compared with the ketyl radical resulting from the oxidation of the first hydroxyl group in the two glycols examined here.
Conclusions It has been shown that oxidation of glycols by peroxomolybdenum complexes requires coordination of the substrates to the metal. When the glycol acts as monodentate ligand, a-diketone is formed, whereas when it acts as a chelating species, benzaldehyde is the product. Both processes are homolytic in nature, as revealed by the occurrence of autooxidative reactions. In spite of the high selectivity observed in oxidation of alcohols, the data presented here suggest that also in this reaction a radical pathway may be operating.
168
Acknowledgements This research was carried out in the frame of ‘Progetto Finalizzato Chimica Fine II’ of CNR. Financial support by MURST is also gratefully acknowledged.
References 1 2 3 4 5
6 7 8 9 10 11
12 13
14 15 16 17 18 19 20 21
S. E. Jacobson, P. A. Muccigrosso and F. Mares, J. Org. Chxm., 44 (1979) 921. B. M. Trost and Y. Masuyama, Tetrahedron Lett., 25 (1984) 173. Y. Masuyama, M. Takahashi and Y. Kurusu, Tetrahedron L&t., 25 (1984) 4417. 0. Bortolini, V. Conte, F. Di Furia and G. Modena, J. Org. Chum., 51 (1986) 2661. 0. BortoIini, S. Campestrini, F. Di Furia, G. Modena and G. VaIle, J. Org. Chem., 52 (1987) 5467. 0. Bortolim, S. Campestrini, F. Di Furia and G. Modena, J. Org. Cha., 55 (1990) 3658. W. S. Trahanovski, J. R. Gilmore and P. C. Heaton, J. Org. C/r&m., 38 (1973) 760 and references therein. S. Campestrini, F. Di Furia, G. Modena and F. NoveIIo, in L. I. Simandi (ed.), Dioxygen Activation and Homogeneous Catalytic Oxidation, Elsevier, Amsterdam, 1991, p. 375. S. Campestrini, F. Di Furia, G. Modena and F. Novello, in M. Graziani and C.N.R. Rao (eds.), Advances in Catalyst Design, World Scientific, Singapore, 1991, p. 333. F. P. Greenspan and H. M. Woodburn, J. Am. Chem. Sot., 76 (1954) 6345. G. Mino, S. Kaizerman and E. Rasmussen, J. Am. Chem. SOL, 81 (1959) 1494. H. L. Hintz and D. C. Johnson, J. Org. Chem., 32 (1967) 556. I. Bhatia and K. K. Banerji, J. Chem. Sot., Perkin Trans. 2, (1983) 1577. W. S. Trahanovsky, L. H. Young and M. H. Biorman, J. Org. Chem., 34 (1969) 869. E. S. Huyser and L. G. Rose, J. Org. Cb., 37 (1972) 849. E. S. Huyser and L. G. Rose, J. Org. Chem., 37 (1972) 851. H. Mimoun, I. Seree De Roth and L. Sajus, Tetrahedron, 26 (1970) 37. G. Berti and F. Bottari, J. Org. Chem., 25 (1960) 1286. R. A. Sheldon and J. K. Kochi, Metal Catalyzed Oxidation of Organic Compounds, Academic Press, New York, 1981, Chap. 2 and references therein. C. A. Bunton, in K. B. Wiberg (ed.), Oxidation in Organic Chemistry, Academic Press, New York, 1965, Chap. 6. 0. Bortolini, S. Campestrini, V. Conte, F. Di Furia and G. Modena, J. Org. Chem., 53 (1988) 5721.
Catalysis,
78 (1993)
159-168
159
Elsevier Science Publishers B.V., Amsterdam M3077
Oxidation of meso- and d,L-hydrobenzoin by peroxomolybdenum complexes: a mechanistic investigation Sandro
Campestrini,
Fulvio Di Furia*
and Fabiola
Novello
Cen&o CNR di Studio sui Meccanismi di Reazioni Organiche, Dipartimento di Chimica Organica, Universita’ di Padova, Via Marzolo I, 35131 Padua (Italy)
(Received March 31, 1992; accepted June 30, 1992)
Abstract The stoichiometry and the kinetic behavior of the oxidation of two isomeric glycols, rneso- and d,Z-hydrobenzoin, by a neutral (MoO~HMPT; HMPT = hexamethylphosphorictriamide) and by an anionic ([MoO,PICO]-Bu,N + ; PICO = picolinate N-oxido ligand) peroxomolybdenum complex have been examined. In all cases the reaction leads to the formation of two products, the a-diketone benzil and benzaldehyde, the latter deriving from the oxidative cleavage of the carbon-carbon bond of the glycols. The product distribution is greatly dependent on the nature of both the substrates and the oxidants. In particular, MoOsHMPT affords almost exclusively benzaldehyde with both isomers. By contrast, [ MoO,PICO] -Bu,N +, upon reaction with meso-hydrobenzoin, gives mainly benzil, whereas with the d&isomer benzaldehyde is the major product. Based also on kinetic evidence, it has been proposed that the two products derive from two different intermediates. In particular, benzil is formed from a ground-state complex where the glycol is coordinated to the oxidant as a monodentate ligand, while benzaldehyde originates from a complex in which the substrate is present as a chelating ligand. The occurrence of autooxidative processes when the reactions are carried out under air is considered strong evidence of the homolytic nature of the reactions that should take place within the coordination sphere of the metal.
Introduction The synthetic scope of anionic peroxomolybdenum and peroxotungsten derivatives as oxidants of the alcoholic function is well established [l-4]. Recently we reported on the efficiency of species such as 1, and in particular of lb, whose reactivity is cu. loo-fold larger than that of la in the oxidation of primary and secondary alcohols [5]. These reagents, which are soluble up to 1 M in nonpolar solvents such as dichloroethane (DCE) owing to the presence of the lipophilic cation, are able to oxidize primary alcohols to aldehydes without any overoxidation to carboxylic acids. Such a transformation proceeds with good chemical yields also when the substrates, together with the alcoholic function, contain other *Author to whom correspondence should be addressed.
0304-5102/93/$06.00
0 1993 - Elsevier Science Publishers B.V. All rights reserved
la
lb
oxidizable (double and triple bonds) or labile (oxirane ring) groups [6]. An additional, synthetically attractive, feature of lb is its ability to oxidize diols and glycols. As an example, by reacting a stoichiometric amount of lb with meso-hydrobenzoin, fair yields of the cY-diketone, benzil, are obtained. Thus the product deriving from the oxidative cleavage of the carbon-carbon bond, i.e., benzaldehyde, is formed only in a relatively small amount (= 15%), at variance with the behavior of other metal oxidants such as Co(III), Pb(IV) and Ce(IV) derivatives [7]. In an attempt to ascertain the source of the chemoselectivity observed, we started some time ago a mechanistic investigation of the behavior of molybdenum peroxo complexes such as 1 in the oxidation of simple alcohols. Preliminary reports on our study have already appeared [ 8, 91. In this paper we present in detail our results for the oxidation of two model substrates, i.e., meso- and d,l-hydrobenzoin. The oxidation of glycols by a variety of metal derivatives has been examined extensively from a mechanistic point of view by several authors [lo-131, thus making possible a comparison of our data with those available in the literature. As an example, both homolytic and heterolytic mechanistic pathways may be encountered [ 14-l 61. Moreover, the glycol oxidation may be a bimolecular reaction, or it may involve as a prerequisite the formation of a ground-state complex between the oxidant and the glycols, which, in turn, may act as monodentate or bidentate ligands. The data collected allow us to suggest that the oxidation of meso- and d,lhydrobenzoin by lb is a homolytic process taking place within the coordination sphere of molybdenum.
Experimental Materiak MoO(O&HMPT and [MoO(O&PICO]-Bu,N + were obtained and purified ([O,,] =2[MoO(O&HMPT] or 2[MoO(O&PICO], > 95%, iodometric titre) following published procedures [5, 171. Meso-hydrobenzoin and 2,6-di-tbutylcresol, high-purity products (Aldrich), were used without further purification. d,l-Hydrobenzoin was obtained by reduction with LiAlH, of the
161
corresponding trichloroacetic diester following a published procedure [ 18 1. ~,LY’GZO-isobutyronitrile was purified by crystallization from acetonitrile solution. The picolinate N-oxido tetrabutylammonium salt was prepared by mixing picolinic N-oxido acid with a stoichiometric amount of tetrabutylammonium hydroxide (1.5 M solution in water). The solvent was then removed by distillation under vacuum. The remaining material was dissolved in CHzClz and dried over MgSO,. After removal of the solvent, the organic salt was obtained. 1,2-Dichloroethane (DCE) was purified by distillation over P2%
Kinetic measurements All kinetic measurements were carried out under argon pressure, after repeated freeze-vacuum-ultrasound cycles aimed at minimizing the presence of dioxygen, in a thermostatic bath with a temperature control better than +0.05 “C. Aliquots of the reaction mixture were stripped out at various times and their content of active oxygen determined by iodometry. Benzil and benzaldehyde were analyzed, after quenching of the residual active oxygen by added PPh,, by HPLC (isobutyrophenone as internal standard) on a 25 cm RP 18 column equipped with a UV-Vis detector (Waters, 480) at 300 run. The response factor of each product versus the internal standard was calculated separately.
Results
and discussion
The stoichiometry of the oxidation of the two model substrates, mesoand c&l-hydrobenzoin (2a and 2b), by lb and by the neutral peroxomolybdenum complex MoO,HMPT, 3, has been determined by using a 2.5-fold excess of the substrate over the active oxygen of the oxidant.
meso-hydrobenzoin
(2a)
dl-hydrobenzoin
(2b)
3
The results of this study carried out under air are collected in Table 1. It may be noticed that in all cases two products are observed, i.e., the cY-diketone benzil together with benzaldehyde, the latter deriving from the oxidative cleavage of the carbon-carbon bond of the glycols. However, some important differences between the two substrates and also between the two oxidants may be observed. In particular, benzil is the major product of the oxidation by the anionic complex lb in the case of meso-hydrobenzoin, whereas in the case of the &?-isomer, the amount of benzaldehyde produced
162 TABLE
1
Yields and product ratios in the oxidation in DCE at 60 “C under air atmosphere Entry
Substrate
Oxidant
of 2a and 2b (0.025
Yield”
M) by lb and 3 (0.005
M)
[Be&l] /[ benzaldehyde]
(%)
lb 2 3' 4 5 6
2a 2a 2a 2a 2b 2b
lb lb lb 3' lb 3
325 300 350 200 120 260
1.6 1.4 7.5 0.01 0.2 0.002
aThe reaction mixtures were analyzed after complete disappearance of the oxidant and the yields calculated according to the expression: yield (%) = (2 x moles benzil+lx moles benzaldehyde/moles active oxygen) x 100. ‘At 50 "C. ‘In the presence of picolinate N-oxido tetrabutylammonium (0.03 M).
largely exceeds that of benzil. In addition, with both substrates, the neutral peroxo complex 3 affords benzaldehyde almost exclusively, benzil being present only in a trace amount. This finding is further experimental evidence of the superior ability of anionic molybdenum peroxo complexes such as lb, as compared with neutral ones, to oxidize the alcoholic function. Also worthy of notice is the result in entry 3, which indicates that the addition of the salt of the ligand of lb, i.e., the tetrabutylammonium picolinate Noxido, significantly enhances the production of benzil. In fact, the benzil/ benzaldehyde ratio in the oxidation of meso-hydrobenzoin by lb is cu. 1.5 in the absence of added ligand, whereas in the presence of the ligand, this ratio is increased by a factor of cu. 5. This has important mechanistic implications, which will be discussed later on in this paper. At any rate, the most striking feature of the data presented in Table 1 is the more-than-stoichiometric formation of both products. Based on the active oxygen content of the oxidant, yields larger than 300% are observed. Such a finding points to involvement of dioxygen in the reaction, thus providing the first evidence of the occurrence of carbon-centered radical species, which are well known to add dioxygen in a very fast, almost diffusioncontrolled, reaction [ 19 1. On one hand, the detection of autooxidative processes may be considered a most valuable clue for the understanding of the mechanism of glycol oxidation by lb. On the other hand, such processes increase the complexity of the reaction under investigation. As a consequence, the kinetic data reported in this paper were obtained under argon atmosphere, after careful degassing procedures. Under these conditions, as shown in Fig. 1, the yields of oxidized products, benzil and benzaldehyde, are almost quantitative, thus demonstrating that the involvement of dioxygen has been minimized.
163
T
0.008
.-o 5
0.006
‘. s
0.004
E ?-
0.002
0 0
40
20
60
80
100
1
time,m
G L ; : :
oxygen]
0
[active
+
[benzil]
A
[benzaldehyde]
0.006-
0.004-
A 0.002-
0
20
60
40
8.0
100
time-m J?ig. 1. Disappearance of the active oxygen and appearance of the products as a function of the time in the oxidation of 2a (0.05 IQ by lb (0.005 M) in DCE at 60 “C: (A) under air; (B) under argon atmosphere.
The different product distributions observed for the two isomer-k substrates, which had purposely been selected to be as close as possible in terms of their chemical reactivities, may be rationalized on the basis of their different attitudes toward acting as bidentate ligands, a feature that is well documented in the literature [20]. In particular, it is known that c&l-hydrobenzoin is a better chelating agent than the meso-isomer, mainly for steric reasons. As a consequence, benzaldehyde should be formed through a mechanistic pathway that involves association of the glycol to the metal to form an intermediate in which both alcoholic functions of the substrate are coordinated to molybdenum. Taking into account that lb is a coordinatively saturated complex where such a coordination implies the complete removal of the picolinate N-oxido ligand, whereas 3 already has a free coordination site and a second one that is occupied by the neutral, easily removable ligand HMPT, the experimental finding that the neutral peroxomolybdenum complex affords benzaldehyde almost exclusively is also nicely rationalized. Thus a general scheme for benzaldehyde formation would be:
164
+
lb
i-l
OH
Scheme
OH
1.
The processes outlined in Scheme 1 also account for the inhibitory effect of added ligand on benzaldehyde formation that should be due to the reversion to the left of the equilibrium (1) leading to the formation of the intermediate. Since the effect of such a reversion is increased production of benzil, it is also reasonable that the a-diketone is formed by a different mechanistic path. On the other hand, information on such a path is not provided by the data already presented. In particular, it should be ascertained whether or not the benzil production involves an intermediate which, from what has been concluded above, must be different from the intermediate leading to benzaldehyde. Some useful hints on this aspect are provided by the kinetic data collected in Table 2 and shown graphically in Pigs. 2 and 3. These data were obtained using an excess of the substrate over the oxidant and by measuring the initial rates of production of the two substrates. At the same time, it was observed that plots of log[O],, versus time are linear up to 50-60% reaction, indicating that the kinetic order of the oxidant is one in both processes. This is also confirmed by entries 11 and 12 of Table 2, where it can be seen that the initial rates of both reactions depend linearly on the active oxygen concentration. Prom the data of Table 2 the activation parameters for both benzil and benzaldehyde formation can be obtained. These are reported in note d of the table. Particularly telling are the AS+ values, which are relatively small. It should be recalled that in the oxidation of thioethers by lb in the same solvent, a clean bimolecular reaction, a AS+ value of = 120 J K -’ mol -I was measured [21]. Thus further evidence of associative processes in glycol oxidation is obtained. Entry 16 of Table 2 confirms the inhibitory effect of added ligand on benzaldehyde formation already noticed in stoichiometric experiments. The comparison of the data for benzaldehyde formation with those for benzil formation provides further evidence that we are dealing with two different reactions, characterized, inter c&a, by different activation parameters. In the reaction leading to benzaldehyde, saturation, Michaelis-Mententype behavior is observed, consistent with the proposal made above of the formation of an intermediate (Pig. 2). The saturation is achieved at relatively
165 TABLE 2 Initial rates for the oxidation Entry
(M)
(M>
ROVX lo7 (M s-i)
Ross x 1 or (M s-l)
1.25 2.5 5.0 10.0 2.5 2.5 2.5 2.5 2.5 2.5
5.0 5.0 5.0 5.0 2.9 1.3 5.0 5.0 5.0 5.0
4.25 8.75 17.5 35.0 5.51 2.79 1.25 3.50 24.3 7.75
1.75 3.75 7.00 8.00 2.61 1.36 0.95 2.00 9.25 1.75
[2a]
7 8 9 10 11 12 13” 14b 15’, d 16’
of 2a by lb in DCE at 60 “C under argon atmosphere
X
10’
[lb]x103
“At 40 “C. ‘At 50 “C. ‘At 70 “C. dFrom entries 8, 13, 14 and 15 the following activation parameters may be estimated: for bensil formation AH+ = 88 kJ mol-’ and AS = - 29 J K-’ mol-‘; for benzaldehyde formation AH+ =68 kJ mol-’ and AS’ = - 70 J K-’ mol-‘. eIn the presence of picolinate N-oxido tetrabutylammonium (0.025 M). ‘Relative to the benzil formation. 8Relative to the benzaldehyde formation.
&OOe-7
8
I 6 u
-
600e-7-
4.00e-7-
0 00
0 02
0 04
0 06
0 08
010
01
[Hydrobenzoin], M Fig. 2. Initial rates (M s-i) of benzaldehyde formation as a function of the initial substrate concentration in the oxidation of 2a by lb in DCE at 60 “C under argon atmosphere.
low substrate-oxidant ratios, as expected for glycol acting as a chelating agent. By contrast, the rate law for benzil formation is a second-order equation, first order in the substrate concentration and first order in the oxidant concentration for the entire concentration range explored (Fig. 3). As a consequence, we are left with two alternatives: either the benzil formation is a bimolecular process, or it involves the formation of an intermediate, which however must be present at a low, steady-state con-
0 00
002
004
006
[Hydrobenzoin],
008
0 IO
012
M
Fig. 3. Initial rates (M s-l) of benzil formation as a function of the initial substrate concentration in the oxidation of 2a by lb in DCE at 60 “C under argon atmosphere.
centration. Apart from the small AP value found, there is other indirect evidence that tends to support the latter possibility. We have already demonstrated on the basis of dioxygen interception that the reaction leading to benzil and that forming benzaldehyde are homolytic processes where, presumably, electron-transfer steps from the substrate to the oxidant are involved, leading to carbon-centered radicals. Such electron transfers are more likely to take place within the coordination sphere of the metal, rather than being outer-sphere processes. These, in fact, are not so frequently encountered in reactions involving metal complexes. The second, and even more stringent, piece of evidence favoring the formation of an intermediate is the experimental observation that benzoin, i.e., the hydroxyketone resulting from the oxidation of the first hydroxy group of meso-hydrobenzoin, is detected only in minute amounts in the reaction mixtures. This finding must be considered in light of the fact that direct control experiments have indicated that the rate of oxidation of benzoin to benzil is comparable with that of oxidation of mesohydrobenzoin to benzil. Therefore, neglecting the very unlikely possibility that the two OH groups of meso-hydrobenzoin are oxidized simultaneously, a plausible rationale for this apparently contradictory finding is that benzom is oxidized to benzil within the coordination sphere of the metal. Thus the general scheme for the oxidation of the glycol would be that shown in Scheme 2. Therefore, the substrate would act as a monodentate ligand whose association constant to the metal is low enough to prevent saturation, at least in the concentration interval explored. Through a keto-enol-type tautomeric equilibrium the initially formed benzoin remains coordinated to the metal via its residual alcoholic function, which then undergoes oxidation to the ketone. As far as the intimate mechanism of the homolytic oxidation is concerned, it is clear that at this stage any speculation would be rather hazardous. Although we did obtain evidence that organic radical species ketyl radicals - are formed in benzil production, so that we may assume
167
0 : I ‘MO’
o”/ 0
1
o.. ‘0
I
0
Scheme
2.
that in these reactions the substrates behave as one-electron reductants, it remains to be ascertained whether the electron is transferred to the peroxo group or to the metal, thus giving rise to a MO(V) intermediate. It is worthy of mention that in these reactions we have directly confirmed that added radical initiators (cx,(Y’-CUO-isobutyronitrile) or radical traps (2,6-di-t-butylcresol) have very little effect, or none, on the rates. This would indicate that the leakage of organic radicals outside the coordination sphere of the metal is an almost negligible event, suggesting that dioxygen interception may also involve the coordinated radical. Finally, the similarity between the processes shown in Scheme 2 and those conceivable for the oxidation of alcohols (which, of course, may coordinate to the peroxometal complexes only as monodentate ligand) is rather evident. Along this line, the lack of autooxidative processes in oxidation of alcohols [8 ] might be related to the much shorter lifetime of a simple ketyl radical as compared with the ketyl radical resulting from the oxidation of the first hydroxyl group in the two glycols examined here.
Conclusions It has been shown that oxidation of glycols by peroxomolybdenum complexes requires coordination of the substrates to the metal. When the glycol acts as monodentate ligand, a-diketone is formed, whereas when it acts as a chelating species, benzaldehyde is the product. Both processes are homolytic in nature, as revealed by the occurrence of autooxidative reactions. In spite of the high selectivity observed in oxidation of alcohols, the data presented here suggest that also in this reaction a radical pathway may be operating.
168
Acknowledgements This research was carried out in the frame of ‘Progetto Finalizzato Chimica Fine II’ of CNR. Financial support by MURST is also gratefully acknowledged.
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