Chemical Manganese Dioxide (CMD), an Efficient Activated Manganese Dioxide. Application to Oxidation of Benzylic and Allylic Alcohols

Abstract

Oxidation of benzylic and allylic alcohols with chemical manganese dioxide smoothly proceeded under mild reaction conditions to give the corresponding aldehydes and ketones, respectively, in high yields. It is well-known that activated manganese dioxide (MnO2) is a useful reagent both for selective oxidation of benzylic and allylic alcohols to aldehydes and ketones, respectively, and for dehydrogenation of heterocycles to heteroaromatics. 1 Although several methods for preparation of activated MnO2 have been reported, 2 preparations are very tedious and sometimes the oxidation efficiency lacks reproducibility. Commercially available activated MnO 2 can also be used, but again its activity varies widely. We have already reported that chemical manganese dioxide (CMD), 3 produced for dry battery manufacture, can be efficiently used for oxidation of some allylic alcohols 4 and for dehydrogenation of heterocycles such as thiazolines, 5 2,3- dihydrofurans, 6 3-pyrrolines, 7 and 2-pyrrolines. 8 Further investigations along this line have revealed that CMD is widely applicable to the selective oxidation of benzylic and allylic alcohols 1 to aldehydes and ketones 2, respectively, as shown in Scheme 1.