Arene Oxidation with Malonoyl Peroxides

Abstract

Malonoyl peroxide 7, prepared in a single step from the commercially available diacid, is an effective reagent for the oxidation of aromatics. Reaction of an arene with peroxide 7 at room temperature leads to the corresponding protected phenol which can be unmasked by aminolysis. An ionic mechanism consistent with the experimental findings and supported by isotopic labeling, Hammett analysis, EPR investigations and reactivity profile studies is proposed.The oxidation and functionalization of hydrocarbons is a central facet of the chemical industry for the production of hightonnage commodities and the preparation of high-value pharmaceuticals, agrochemicals and fine chemicals. Therefore, methods for selective oxidation of C-H bonds are of great importance.1 Phenols represent a key class of oxidized hydrocarbon.2 While there are a small number of reports in which arenes are oxidized to the respective phenols using peroxides and strong acids as additives, 3 the oxidation of aromatic C-H bonds still presents a synthetic challenge specifically with respect to avoiding over oxidation. Scheme 1. C-H oxidation using phthaloyl peroxide. 2A recent report from the laboratories of Houk and Siegel described a metal-free oxidation of aromatic carbon-hydrogen bonds which was proposed to proceed through an intriguing reverse-rebound mechanism (Scheme 1). 4Reaction of mesitylene 2 with 1.3 equiv. of phthaloyl peroxide 1 in hexafluoroisopropanol (HFIP) followed by basic solvolysis gave the phenol 3 (97%). The method outlined in Scheme 1 represents a significant advance in arene oxidation. The proposed mechanistic pathway for the transformation suggested homolytic fission of the weak oxygen-oxygen bond leading to diradical 4. Addition of this radical to the arene gives 5 which though H-atom abstraction provides the observed product 6. Ester hydrolysis leads to the phenol 3 (97%, 2 steps). The procedure has wide functional group tolerance and arene over oxidation did not prove problematic. We believed two fundamental opportunities existed for development of this procedure: Firstly, phthaloyl peroxide 1 is known to be very shock sensitive and explodes violently when heated, representing a significant hazard. 5,6 Secondly, the proposed reverse-rebound mechanism leading to 6 was based upon theoretical studies. Provision of experimental evidence to support this pathway would be of great importance to the understanding and development of this procedure. In recent years we have been interested in the chemistry of cyclic diacylperoxides and have shown that malonoyl peroxide 7, 7 and related derivatives, 8 are effective for the syndihydroxylation of alkenes.9 This reagent provides significant advantages over phthaloyl peroxide 1 within the syndihydroxylation reaction in terms of yield, selectivity, reaction rate, substrate scope and operating temperature.10,11 Given our experience in understanding the mechanism of reactions involving the peroxide 7, 12 together with the specific advantages provided in alkene dihydroxylation we el...

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