Elsevier

Tetrahedron

Volume 62, Issue 52, 25 December 2006, Pages 12326-12333
Tetrahedron

Efficient and selective oxidation of methyl substituted cycloalkanes by heterogeneous methyltrioxorhenium–hydrogen peroxide systems

https://doi.org/10.1016/j.tet.2006.10.013Get rights and content

Abstract

Polymer-supported methyltrioxorhenium (MTO) systems are efficient catalysts for the oxidative functionalisation of cyclohexane and cyclopentane derivatives with H2O2 as oxygen donor. Using poly(4-vinyl)pyridine and poly(4-vinyl)pyridine-N-oxide as MTO supports, cycloalkanol, cycloalkanediol, cycloalkanone and ω-hydroxy methyl ketone derivatives were obtained in different yields depending on the experimental conditions. Interestingly, cycloalkane dimers were selectively recovered in acceptable to good yields when the oxidation was performed with polystyrene-microencapsulated MTO catalyst. The EPR investigation suggests that the homolytic cleavage of the CH3–Re bond with formation of CH3radical dot radicals occurs inside the polystyrene capsule, indicating a possible role of methyl radical in the cycloalkane dimerisation pathway.

Introduction

In the last years increasing attention was directed to the use of methyltrioxorhenium (CH3ReO3, MTO)1, 1(a), 1(b) in oxidative reactions, in conjunction with hydrogen peroxide (H2O2) as oxygen donor, due to the excellent catalytic properties showed by this system.2, 2(a), 2(b), 2(c), 2(d), 2(e), 2(f) Among the various MTO catalysed reactions, the oxidation of hydrocarbons to alcohols and ketones is of relevant interest because of industrial and environmental concerns.3, 3(a), 3(b), 4, 4(a), 4(b) This reaction proceeds through the formation of the monoperoxo [MeRe(O)2O2] (A) and the bis-peroxo [MeReO(O2)2] (B) η2-rhenium complexes that have been isolated and fully characterised by single-crystal X-ray analysis (Fig. 1).5 In molecular solvents, the complex A was found to be more reactive than B, while the opposite occurs for reactions driven in ionic liquids.6, 6(a), 6(b) Theoretical and computational studies have been performed to elucidate the geometrical features of the transition state involved in this oxidation. Specifically, the oxygen transfer from complexes A and/or B to the substrate was always described by a concerted process requiring a butterfly-like transition state similar to that previously suggested for cyclic organic peroxides such as dimethyldioxirane (DMDO).7, 7(a), 7(b), 8, 8(a), 8(b) Heterogeneous rhenium catalysts behave in a similar way:9, 9(a), 9(b), 9(c) for example, the same A and B complexes were intermediates in the epoxidation of alkenes with H2O2, catalysed by MTO supported on silica tethered with polyethers.9c The heterogenation of MTO on polymeric supports is an important tool because it allows an easier recovery of the catalyst, decreases the toxicity of reaction wastes and sometimes improves the reactivity.10 The heterogeneous systems used in the present paper have been prepared either by heterogenation of MTO on easily available polymers bearing nitrogen atoms as anchorage sites, such as poly(4-vinylpyridine) (PVP) and poly(4-vinylpyridine)-N-oxide (PVPN), [2% or 25% cross-linked with divinylbenzene (PVP-2/MTO I, PVP-25/MTO II and PVPN-2/MTO III, respectively; Fig. 2)]11, 11(a), 11(b) or by physical microencapsulation on polystyrene of both MTO or its adduct with 2-aminomethyl pyridine (PS/MTO IV and PS/MTO-L V, respectively; Fig. 2).12

These systems showed high catalytic activity and selectivity in the oxidation of aromatic derivatives,13 pyrrolidines,14 alkenes and terpenes,15, 15(a), 15(b) including the oxygen atom insertion into the C–H sigma bond of hydrocarbons, both in molecular solvents16 and ionic liquids.17 As a result of our ongoing studies, herein we report on the oxidative derivatisation of different cycloalkane derivatives, namely stereoisomeric cis- and trans-1,2-dimethylcyclohexanes 1 and 2, methylcyclohexane 3 and cis-1,2-dimethylcyclopentane 4, with heterogeneous MTO and H2O2 in tert-butanol (t-BuOH). A different reaction pathway was observed depending on the catalyst used for the transformation. Alcohols or products obtained from further oxidation of alcohols, including ring-opened ω-hydroxy methyl ketones were obtained with poly(4-vinylpyridine) catalysts IIII. The oxidation of the same substrates with the PS/MTO IV catalytic system afforded, unexpectedly, cycloalkane dimers in appreciable amounts (10, 14 and 19, Scheme 1, Scheme 2, Scheme 3, Scheme 4). The Electron Paramagnetic Resonance (EPR) investigation showed that the homolytic cleavage of the Re–CH3 bond preferentially occurs inside the polystyrene capsule, thus suggesting a possible role of the methyl radical in the cycloalkane dimerisation pathway. To the best of our knowledge this is the first example in the literature dealing with the presence of a radical pathway in oxidation reactions with MTO and H2O2. Noteworthy, the effect of the polystyrene microcapsule environment on the reactivity of MTO appears to be finely tuned by the presence of nitrogen containing ligand bonded to rhenium atom and able to change its stereoelectronic properties. As an example, cycloalkane dimers were not recovered when the oxidation was repeated in the presence of the catalytic system PS/MTO-L V, obtained by the microencapsulation of previously formed complex between MTO and 2-aminomethyl pyridine15a (Fig. 2).

Section snippets

Results and discussion

The results of the oxidation of stereoisomeric cis- and trans-1,2-dimethylcyclohexanes 1 and 2, methylcyclohexane 3 and cis-1,2-dimethylcyclopentane 4 in t-BuOH with H2O2, catalysed by heterogeneous MTO-based catalysts, are reported in Scheme 1, Scheme 2, Scheme 3, Scheme 4 and Table 1, Table 2, Table 3. Oxidations with MTO under similar homogeneous conditions were performed as references. In the absence of catalyst, less than 5% conversion of substrates took place under otherwise identical

Conclusions

MTO and polymer-supported MTO systems IV are shown to be efficient catalysts for the oxidative functionalisation of cyclohexane and cyclopentane derivatives with H2O2 as oxygen atom donor. Different reaction pathways were observed depending on the nature of the polymeric support. In the case of catalysts IIII, obtained by heterogenation of MTO on poly(4-vinyl)pyridine and poly(4-vinylpyridine)-N-oxide resins, the reaction proceeded through a concerted oxygen insertion from the intermediate

General remarks

All commercial products were of the highest available grade and were used as such. NMR spectra were recorded on a Bruker (AC 200 MHz). When necessary, chromatographic purification was performed on columns packed with silica gel, 230–400 mesh, for flash technique. In order to evaluate the scaling-up of our catalytic procedure, the oxidation of cis-1,2-dimethylcyclohexane 1 with catalyst I (PVP-2/MTO) was repeated on the scale of 10 mmol of substrate, and working under the same experimental

Acknowledgements

Italian MIUR (PRIN-COFIN 2003) is acknowledged for financial support. Prof. Roberto Scotti (University of Milano-Bicocca, Milano, Italy) is acknowledged for helpful discussion about EPR investigation.

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