Electrophile Induced Addition Reactions of Bis-phosphonio-isophosphindolide Cations

Bis-triphenylphosphonio-isophosphindolide cations 1 react with triflic acid to give C-protonated products 4, 5 which show an enhanced reactivity to undergo addition of H2O or MeOH to the cyclic π-system. This reaction is the first step of the hydrolytic decomposition of the heterocycle which was monitored by NMR spectroscopic studies. No evidence for direct P-protonation or P-alkylation (which had been postulated in a previous study) was obtained. Addition of H2O or H2S to the cyclic π-system of 1 is also promoted by oxidizing agents such as I3− or sulfur and affords novel zwitterionic (thio)-phosphinates 11, 13.