Halogenation of benzylic and allylic alcohols with PPh₃/halogenating agent

Abstract

A novel, mild and high yielding synthetic method for the halogenation of benzylic and allylic alcohols using a combination of triphenylphosphine (PPh₃) and a halogenating agent, such as Cl₃CCONH₂, Br₃CCO₂Et and Br₃CCOCBrc₃, is disclosed. The halogenation of benzylic and allylic alcohols utilizing a combination of PPh₃ and a selected halogenating agent furnished the corresponding halides in high yield under mild conditions within short reaction time. Primary benzylic and allylic alcohols appeared to be reactive alcohols for transformation to the corresponding halides via SN2 mechanism without the formation of by-products. For secondary benzylic and allylic alcohols, the desired halides were attained in good yield except for terminal allylic alcohols. Secondary and tertiary terminal allylic alcohols mainly proceeded the desired chlorides via SN2 and E₂ mechanisms, respectively. On the other hand, the bromination of those alcohols could be performed via SN2' over E₂ pathway