Baeyer-Villiger oxidation of ketones by peroxomolybdenum complexes: A revisit

Journal of Molecular

Catalysis, 79 (1993) 13-19

Elsevier Science Publishers

13

B.V., Amsterdam

MOOS

Baeyer-Villiger oxidation complexes: a revisit

of ketones

by peroxomolybdenum

Sandro Campestrini and Fulvio Di Furia* Centro CNR di Studio sui Meccanismi Organica, Universita’di 49)831297

Padova,

di Reazioni

Via Marzolo

Organiche, Dipartimento

di Chimica

1, 35131 Padua (Italy); tel. and fax. (+39-

(Received May 2,1992; accepted August 10,1992)

Abstract The Baeyer-Villiger oxidation of cyclopentanone to 8-valerolactone by a peroxomolybdenum complex, MOO (0) 2 (dipic), under catalytic (hydrogen peroxide) conditions was reported in the literature some years ago. Such a reaction might have both synthetic and mechanistic significance, being the only available example of nucleophilic reactivity of Group VI peroxometal complexes. However, the results of a reinvestigation of the reaction presented in this paper indicate that the oxidation is carried out by hydrogen peroxide and that the role of MOO (0) 2(dipic ) as well that of other molybdenum derivatives is simply to catalyze the reaction owing to their strong acid nature. Key words: Baeyer-Villiger

oxidation;

ketones; molybdenum;

peroxomolybdenum

complexes

Introduction The analogy between the behavior of groups IV-VI peroxometal complexes e.g., Ti, V, MO and W derivatives and of simple peroxides such as hydrogen peroxide, alkyl hydroperoxides and inorganic or organic peracids in the oxidation of nucleophilic substrates, is well established [ 11. The reaction proceeds for both classes of oxidants by electrophilic oxygen transfer from the peroxide to the substrate [lb]. Typical examples are the stereospecific epoxidation of olefins and allylic alcohols [ 21 and the oxidation of thioethers [ lb,3]. More recently it has also been shown that groups IV-VI peroxometal complexes, namely vanadium [4] and molybdenum [5] derivatives, may act as one-electron acceptors thus giving rise to radical chain oxidation reactions which are well known in the chemistry of simple peroxides. A third type of oxidative behavior is exhibited by simple peroxides i.e., the oxidation of electrophilic substrates such as electron-poor olefins [ 61 and ketones [ 71. In particular, the oxidation of ketones to esters by Caro’s acid was first observed by Baeyer and Villiger in 1899 [ 81. For this kind of reactivity the parallelism with group VI peroxometal complexes is much less documented as it is based only on the observation that a peroxomolybdenum complex containing the dipicol*Author to whom correspondence 0304-5102/93/$06.00

should be addressed.

0 1993 - Elsevier Science Publishers

B.V. All rights reserved.

14

S. Campestrini and F. Di FurialJ. Mol. Catal. 79 (1993) 13-19

inato ligand, MOO (0,) (dipic ) , 1 can oxidize cyclopentanone and cyclohexanone to the corresponding lactones [ 91. The proposed mechanism for such a reaction, which is carried out under catalytic conditions by using hydrogen peroxide to restore the peroxocomplex is shown in Scheme 1.

+

Scheme 1.

The intermediate 2 in the scheme bears much resemblance to those isolated after the addition of group VIII peroxometal complexes to ketones, e.g., 3 [lo].

/R\o/C\(3H, PhhP

3

By contrast, it has never been detected in the oxidation by group VI peroxoderivatives. The group VIII peroxometal derivatives according to the definition given by Sheldon and Kochi are “nucleophilic” peroxocomplexes whereas those of group VI are “electrophilic” [la]. Some years ago we became involved in the chemistry of anionic peroxomolybdenum complexes, such as 4, that are very selective oxidants of the alcoholic function [ 5,11,12].

4a

4b

Owing to their anionic nature we reasoned that species such as 4a and 4b

S. Campestrini

and F. Di Furia/J. Mol. Catal. 79 (1993) 13-19

15

should have been ideal candidates for displaying a nucleophilic reactivity similar to that of 1. Unfortunately, any attempt to have complexes 4a and 4b reacting with electrophilic centers, including cyclic ketones failed. A subsequent 170 NMR investigation revealed that in such complexes the negative charge is mainly located on the ligand [ 131. On this basis, the unique ability of 1 to oxidize ketones became difficult to rationalize. Therefore, a reinvestigation of the reaction of 1 with the substrate, cyclopentanone as example, was deemed worthwhile. In this paper we present the results of this reinvestigation which strongly indicates that the oxidant in the Baeyer-Villiger oxidation of cyclopentanone is not 1 but simply hydrogen peroxide. In this process 1 does play a role by acting as an acid catalyst. Experimental Materials

MoO(0,) (dipic), 1, [MoO(O,),(pic)]-(Bu),N+, 4a, and MoO(0,) ,HMPT, 5, were prepared and purified ( [ Oact. ] > 95%) iodometric titre ) by published procedures [ 11,14,15]. Reagent grade MO ( CO)6 (Pierce Inorganits, B.V.) and H,O, (70% w/v, Peroxid Chemie) were used without further purification. Spectrophotometric grade acetonitrile (Fluka) was used as the solvent. Cyclopentanone, b-valerolactone, 1,2-dichloro-benzene, acetic acid, picolinic acid, pyridine-2,6-dicarboxylic acid and tetrabutylammonium hydroxide were all commercially available, high purity products (Aldrich or Janssen ) used as received. Procedures

In a typical run, 0.45 mmol of catalyst (carboxylic acid and/or the molybdenum-peroxocomplex) were added to a CH,CN solution (5 ml) containing 6.8 mmol of cyclopentanone, 2.7 mmol of 1,2dichlorobenzene (internal standard) and 10.3 mmol of H202, in a glass reactor in a thermostatic bath maintained at the appropriate temperature. Aliquots of the reaction mixture were withdrawn after various time intervals and analyzed for hydrogen peroxide content by iodometry. Cyclopentanone and &valerolactone were analyzed by GLC, after quenching of residual hydrogen peroxide by addition of (Ph),P, on a FFAP 3% on Chromosorb W AW DMCS column (2.5 m.). The response factor of each compound versus the internal standard used was calculated separately. Results and discussion

The oxidation of the substrate cyclopentanone by the three molybdenum peroxocomplexes, e.g., 1,4a and the neutral derivative MoO,HMPT, 5, either

S. Campestrini

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and F. Di Furia/J. Mol. Catal. 79 (1993) 13-19

in the absence or the presence of a slight excess of hydrogen peroxide in acetonitrile at 70°C has been examined by measuring both the consumption of the oxidant and the 6-valerolactone formed. The pertinent results are collected in Table 1. A control experiment, entry 1, confirms that in the absence of the peroxocomplexes, hydrogen peroxide alone does not oxidize the ketone. The data of Table 1 also indicate that, in the absence of hydrogen peroxide, none of the three complexes can carry out the oxidation reaction. In particular, after 24 h, 1 is almost quantitatively recovered and, in consequence, not even traces of &valerolactone are observed. By contrast, when the reaction is carried out under the catalytic conditions, identical to those adopted by Mares [9], by using hydrogen peroxide as the oxidant in the presence of the three metal species, the oxidation, does proceed albeit at a different rate. The best results are obtained when 1 is used, entry 3, but 5 also allows the formation of an ample amount of the ketone. Of the three peroxocomplexes, anionic 4a provides the poorest results, owing to extensive decomposition of hydrogen peroxide. Such decomposition is also in evidence for the other two peroxomolybdenum derivatives 1 and 5. It may also be noticed that the yields of &valerolactone, based on converted cyclopentanone, are rather low, ranging from 38 to 54%. The disappearance of the ketone by reactions which do not lead to lactone is a drawback encountered in similar systems [ 7,9]. Therefore no effort has been made to identify the byproducts formed under the conditions adopted in our investigation. Moreover, also the lactone may undergo further transformation, e.g., to hydroxycarboxylic acid [ 7,9]. The data of Table 1 clearly indicate that the presence of molybdenum peroxoderivatives is necessary for the oxidation of the substrate by hydrogen peroxide, but they also strongly suggest that the oxidizing agent in solution is not the peroxocomplex itself, because it is unable to oxidize cyclopentanone to &valerolactone under stoichiometric conditions. TABLE 1 Oxidation of 1.4 M cyclopentanone by peroxomolybdenum absence of HsO,, in CHsCN, at 70” C Entry

[ Peroxocomplex]

o

[H202]o

Time

(MI

(h)

2.2

24 24 6” 24 6” 24 7”

type

concentration (MI

1 4a 4a

0.09 0.09 0.09 0.09

2.2

5 5

0.09 0.09

2.2

1

2.2

“Time at which the S-valerolactone concentration bBased on cyclopentanone converted.

complexes in the presence and in the

Cyclopentanone conversion (970)

Hz02 conversion (%I

&Valerolactone yield (%Jb

0 0

0 0

0

65 0 5 0 39

82

54 0 40 0 38

100 76

reaches its maximum value.

0

S. Campestrini

and F. Di FuriajJ. Mol. Catal. 79 (1993) 13-19

17

Thus, we are faced with the problem of explaining the catalytic effect exerted by the molybdenum species in the oxidation of cyclopentanone by hydrogen peroxide. With this as our aim, we carried out a series of experiments whose results are collected in Table 2. A first hypothesis considered was the formation, in situ, of an organic peracid resulting from the displacement of the carboxylic acids, pyridine 2,6-dicarboxylic and picolinic, respectively, present as ligands in the coordination sphere of 1 and 4a,and subsequent addition of hydrogen peroxide [ 161. It must be noticed that such a rationale does not apply to 5 which has no ligands capable of forming percarboxylic acids. The data of Table 2 tend to rule out significant involvement of percarboxylic acids in the formation of b-valerolactone. In fact, the oxidation reaction in the presence of pyridine 2,6-dicarboxylic acid is much slower than in the presence of 1.Moreover, very similar results, i.e. a slow reaction, are observed when picolinic acid and even acetic acid are added. Although we cannot exclude that some peracid is formed under the conditions adopted, it appears that this is not the real oxidizing agent in solution. A key to understanding the process is provided by the other data of Table 2. In fact, it is observed that the addition of a molybdenum derivative, such as MO (CO),, entries 9 and 12, which is rapidly oxidized to MO (VI) under the reaction conditions [ 171, markedly enhances the oxidation rates which become comparable with those observed when 1 is the catalyst. In this case H,O, is, of course, consumed not only by the oxidation of the ketone but also in the process of oxidation of MO (0) to MO (VI). Even more telling are the results obtained when a strong acid such as sulphuric acid is added either in TABLE 2 Oxidation of 1.4 M cyclopentanone by 2.2 M H,OP, in the presence of 0.09 M carboxylic acids and various catalysts 0.09 M, in CH,CN, at 70” C Entry Carboxylic acid

Catalyst

Time” (h)

8 9

10 11 12 13 14 15c

pyridine 2,6-dicarboxylic acid 55 pyridine 2,6-dicarboxylic acid MO (CO )a 8 picolinic acid 69 acetic acid 69 acetic acid Mo(CO), 7 acetic acid H,SO, 7 HZSO( 4 H,SO, 5

“See note (a), Table 1. bSee note (b), Table 1. ‘In dioxane solution.

S-Valerolactone Cyclopentanone H,O, conversion yield conversion (%o) (%lb (%) 72

62

42

78 72 83 38 68 68 58

79

58 21 58 47 56 54 33

78 63 94 100 96 93

18

S. Campestrini

and F. Di Furia/J. Mol. Catal. 79 (1993) 13-19

the presence or the absence of acetic acid, entries 13 and 14; also under these conditions remarkable enhancement of the reaction rates is observed. These data, while ruling out a major role for peracetic acid, unambigously indicate that the oxidizing agent in these latter experiments, and very likely in all the other cases where &valerolactone is formed, is hydrogen peroxide itself and that the reaction investigated is an acid catalyzed process. Interestingly, the catalytic effect by H,SO, is comparable with that exerted by molybdenum derivatives. This is not at all surprising as it has been already reported that peroxomolybdenum species are rather strong acids [lb]. The acid catalysis likely involves protonation of the carbonyl oxygen of the ketone thus facilitating nucleophilic attack by hydrogen peroxide. The oxidation of cyclic ketones to lactones, namely cyclobutanone to y-butyrolactone, by hydrogen peroxide and of aliphatic ketones have been previously reported in the literature [ 18,191. 0

*

i

+

H,O+

Scheme 2.

The data collected do not allow a careful comparison of the catalytic ability of the three molybdenum derivatives examined to be made. It appears, however, that the nature of their coordination sphere should play a role, and this is worth further investigation. Although the mechanistic proposal of Scheme 2 is the simplest which fits all the experimental data available, other possibilities can be considered. The formation of a peroxycarboximidic acid by reaction of H,O, with acetonitrile [ 201 is ruled out by the data obtained in dioxane, entry 15 of Table 2, which shows that in such a solvent the results are almost identical to those obtained in acetonitrile.

Conclusions

We have provided strong evidence that peroxomolybdenum complexes, including the anionic ones, are not able to act as nucleophilic oxidants of ketones. This could have been expected on the basis of their strong electrophilic character. Therefore, as pointed out in previous papers, the analogy between simple peroxides and peroxometal complexes, although shown in several instances, cannot be considered, at least up to now, as being of general significance. At the same time, the mechanistic investigation presented here has indicated the synthetic scope of molybdenum derivatives which catalyze the Baeyer-Villiger oxidation. Owing to the relevance of such a reaction, which for many substrates require rather forcing conditions, further investigation of the

S. Campestrini

and F. Di Furia/J. Mol. Caral. 79 (1993) 13-19

19

mechanism of the catalytic effect, also extended to other transition metals, is in order.

Acknowledgements This research was carried out within the framework of Progetto Finalizzato Chimica Fine II of CNR. Financial support by MURST is also gratefully acknowledged.

References 1

6 7 8 9

10 11 12 13 14 15 16 17

18 19 20

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