Rearrangement of lithiated S-alkyl O,O-dialkyl thiophosphates: Scope and stereochemistry of the thiophosphate–mercaptophosphonate rearrangement

Abstract

S-Alkyl O,O-dialkyl thiophosphates are prepared by alkylation of the triethylammonium salt of O,O-diisopropyl thiophosphoric acid. S-Benzyl thiophosphate was metallated at temperatures of ≥−45 °C by trityllithium and LiTMP (lithium 2,2,6,6-tetramethylpiperidide) and S-alkyl thiophosphates only by LiTMP to give dipole-stabilised carbanions which rearrange to α-mercaptophosphonates in yields of up to 45%. Metallation occurs with a high primary kinetic isotope effect (k_H/k_D up to ≈50). When the lithium (R)-N-isopropyl-1-phenylethylamide was used to induce the isomerisation of S-pentyl thiophosphate an α-mercaptophosphonate with an ee of 22% was isolated. (R)-S-[1-D₁]hexyl O,O-diisopropyl thiophosphate was rearranged to a dextrorotary α-mercapto-[1-D₁]hexylphosphonate, whose (R)-configuration was determined by chemical correlation. The thiophosphate–mercaptophosphonate rearrangement proceeds with retention of configuration.