An improved synthesis of fused 1,2,3-benzothiadiphospholes and a proposed reaction pathway

A fresh look has been taken at the reaction of PCl3 with thioanisole 1b and AlCl3 that gives, after treatment with water, the title compounds cis-2b in 38% yield together with small amounts of the isomeric cis-3b (2%). The course of this reaction has been studied by 31P-NMR spectroscopy. A multistep pathway, governed by the formation of several AlCl3 complexes with sulfur and phosphorus containing intermediates, has been proposed. The crucial step of this reaction is very reasonably an intramolecular electrocyclic ring closure of a diphosphane intermediate. From this plausible mechanism, an improved procedure that gives only the cis-2b isomer in 42% yield has been realized. In addition, an alternative synthesis using p-thiocresol that gives compounds cis-2b and cis-3b in a ratio of about 2:1 has been effected. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8:551–556, 1997