ON THE DIRECT METALATION OF TERTIARY PHENYLPHOSPHINES

Abstract

We have envisaged a route towards ortho-phosphinobenzoic acids via direct ortho-metalation of tertiary phenylphosphines. The direct metalation of tertiary phenylphosphines with BuLi/KOt-Bu or BuLi/TMEDA proves to be an a-selective process. After carbonation mixtures of carboxylic acids are obtained. With BuLi/KOt-Bu/THF the reaction occurs preferably at the meta and para positions. With BuLi/TMEDA/hexane all three possible positions are metalated. Introduction of an auxiliary methoxy group leads to a higher selectivity. Blocking of all meta and para positions with methoxy groups leads to metalation in the ortho position. The rate of metalation at the ortho position is relatively slow. Observed side reactions are: substitution of phenyl groups, α-metalation, and substitution of a methoxy group.

Key Words:

• metalation
• butyllithium
• phosphinobenzoic acid
• phosphine
• carbonation
• substitution