Ligand-Controlled Regiodivergent Ring Expansion of Benzosilacyclobutenes with Alkynes en Route to Axially Chiral Silacyclohexenyl Arenes.

Abstract

A ligand-controlled regiodivergent and enantioselective ring expansion of benzosilacyclobutenes with internal naphthyl alkynes has been achieved by adjusting the ligand cavity size. The ligand (S)-8H-binaphthyl phosphoramidite, featuring small methyl groups on its arms, provides a spacious cavity that favors sterically demanding Si-Csp3 ring expansion, predominantly yielding axially chiral (S)-1-silacyclohexenyl arenes. In contrast, the ligand (R)-spiro phosphoramidite, with bulky t-Bu groups on its arms, offers a compact cavity that facilitates less sterically demanding Si-Csp2 ring expansion, leading primarily to axially chiral (S)-2-silacyclohexenyl arenes. Density functional theory calculations delineate distinct mechanistic pathways for each ring expansion route and elucidate their regio- and enantioselectivity.