InSitu Allylic Oxidations With Collins Reagent

Abstract

Chromium trioxide-pyridine complex (Collins Reagent) oxidizes alcohols to aldehydes and ketones in high yield, and has been particularly effective in oxidizing acid-sensitive alcohols. A great improvement in the utility of the reagent was reported by Ratcliffe and Rodehorst who made the complex in situ in methylene chloride – thus avoiding the problems involved in isolating crystalline complex. The primary problems include keeping the crystalline complex dry; avoiding fires during addition of CrO₃ to pyridine; and having to make the complex in a separate step in the first place. Dauben, Lorber, and Fullerton found that the isolated complex is extremely effective in allylic oxidations, i.e. in oxidizing olefins to enones. Shaw and Sherry also were able to oxidize alkynes to acetylenic ketones in low yields with the isolated complex. In unpublished experiments, the in situ Ratcliffe procedure was tried for allylic oxidations – but with poor yields. Shaw and Sherry reported similar problems with the in situ reagent when preparing acetylenic ketones