Electrochemical Preparation and Some Reactions of Alkoxy Triphenylphosphonium Ions.

Abstract

The formation of an alkoxy triphenylphosphonium ion by anodic oxidation of Ph3 P in the presence of an alcohol was reinvestigated. When a CH2CL2 solution of Ph3P, Ph3P+H·ClO-4, and an alcohol was subjected to constantcurrent electrolysis in an undivided cell equipped with a graphite anode and a Pt cathode, the 31P-NMR spectra of the resulting electrolyte showed that alkoxy triphenylphosphonium perchlorates (2) were formed in good to fair yields from primary and secondary aliphatic alcohols, while allylic and benzylic alcohols were transformed to the correspinding alkyl phosphonium ions, and in the case of tertiary aliphatic alcohols, no formation of the corresponding alkoxy or alkyl phosphonium ions was recognized at all. The isolation of 2 thus formed was achieved in good yields by a simple procedure. For the electrolysis, Ph3P+H·BF-4 could be utilized insted of the perchlorate salt, giving an alkoxy triphenylphosphonium tetrafluoroborate (3) from primary and secondary aliphatic alcohols. The reaction of the alkoxy phosphonium ions prepared from β- and α-cholestanol with various nucleophiles such as Bu4N+·X- (X=Br, Cl, F, N3, SCN), PhSH, and PhOH was examined. The results indicated that the reaction site of the phosphonium ions is dictated by the identity of the nucleophile. A soft nucleophile was apt to attack at the α-carbon, giving the corresponding SN2 reaction product in a good yield, while a hard one tended to react at the phosphorus of the phosphonium ion, leading to the regeneration of the cholestanol.