Investigation of the configurational stability of lithiated phosphine oxides using the Hoffmann test: X-ray structures of (2S*,3S*,4R *)-2-(N,N -dibenzylamino)-4-diphenylphosphinoyl-1-phenylpentan-3-ol and (2S*,4S*)-2-(N, N -dibenzylamino)-4-diphenylphosphinoyl-1-phenylpentan-3-one

Abstract

The Hoffmann test (reaction of a racemic organolithium with a phenylalanine-derived aldehyde) is used to show that lithiated diphenylphosphine oxides are not configurationally stable in THF at -78 °C (usual reaction conditions) on the timescale of their rate of reaction with the aldehyde. The test is carried out by reacting lithiated ethyldiphenylphosphine oxide with a phenylalanine-derived aldehyde and because all four diastereoisomeric alcohols are obtained, it is necessary to determine the relative stereochemistry of the products. This is done using a combination of synthesis and X-ray crystallography of (2S*,3S*,4R*)-2-(N,N-diben zylamino)-4-diphenylphosphinoyl-1-phenylpentan-3-ol and (2S*,4S*)-2-(N,N-dibenzylamino)-2-d iphenylphosphinoyl-1-phenylpentan-3-one.