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