Direct formation of λ5-phospholes from trialkyl phosphites and dimethyl acetylenedicarboxylate. Alkoxyphosphonium ylides as reactive intermediates and stable products

Abstract

Trialkyl phosphites and dimethyl acetylenedicarboxylate react in toluene below –10 °C to form high yields of tetramethyl 1,1,1-trialkoxy-1λ⁵-phosphole-2,3,4,5-tetracarboxylates (3). These phospholes can either rearrange thermally to stable dialkoxyphosphonium ylides (4) or be converted by addition of hydrogen bromide to give stable tetramethyl 1-alkoxy-1-oxo-1H-1λ⁵-phosphole-2,3,4,5-tetra-carboxylates (5). The unstable trialkoxy [methoxycarbonyl-(1,2,3-trismethoxycarbonylcyclopropenyl)-methylene]phosphorane precursors of (3) can be formed almost quantitatively by carrying out the reaction in toluene below –50 °C.