Borylative cyclisation of diynes using BCl3 and borocations

Abstract

The borylative cyclisation of 1,2-dialkynyl benzenes with BCl₃ leads to dibenzopentalenes (via intramolecular S_EAr) or benzofulvenes (via chloride addition) depending on substituents, with stabilised vinyl cation intermediates (e.g. with a p-MeO-C₆H₄-group) favouring the latter. The use of borocations leads to more selective dibenzopentalene formation, while other diyne frameworks undergo intramolecular S_EAr selectively even with p-MeO groups.