2-Isothiocyanatobenzyltriphenylphosphonium bromides – new type of functionalized heterocumulenes suitable for synthesis of indole derivatives

Abstract

Reaction of 2-bromomethylphenyl isothiocyanates with triphenylphosphine afforded triphenyl-(2-isothiocyanatobenzyl)phosphonium salts. Their reaction with bases liberated the corresponding carbanions which on intramolecular addition to the N=C=S group gave substituted 3-triphenylphosphoniumindolyl-2-thiolates. These compounds did not react in the Wittig reaction but underwent electrophilic reactions on the sulfur atom or on the indol nitrogen atom. According to a detailed analysis of 1H, 13C, 31P NMR and mass spectra, the indole derivatives exist predominantly in the betaine form. X-Ray diffraction analysis of the unsubstituted betaine, 3-triphenylphosphoniumindolyl-2-thiolate, agrees well with the spectral results.