Phosphorus in organic synthesis. The tanigawa reaction revisited as a method for converting alcohols to tertiary amines

Abstract

Contrary to earlier observations, N-methyl- N-phenylaminotriphenylphosphonium iodide 1 (the Murahashi reagent) does not react with alcohols and primary or secondary amines under mild conditions (80°) to give secondary or tertiary amines. However the reaction can be successfully performed at higher temperatures. A mechanistic scheme implying initial amine exchange between 1 and the added amine leading to the N,N-dimethyl analog 5 is suggested. An improved synthesis where the reactive intermediate 5 is formed in situ from triphenylphosphine, carbon tetrachloride and HNR 3R 4 is described.