Ligand-Controlled Regiodivergent Ring Expansion of Benzosilacyclobutenes with Alkynes <i>en</i> 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 (<i>S</i>)-8<i>H</i>-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 (<i>S</i>)-1-silacyclohexenyl arenes. In contrast, the ligand (<i>R</i>)-spiro phosphoramidite, with bulky <i>t</i>-Bu groups on its arms, offers a compact cavity that facilitates less sterically demanding Si-Csp² ring expansion, leading primarily to axially chiral (<i>S</i>)-2-silacyclohexenyl arenes. Density functional theory calculations delineate distinct mechanistic pathways for each ring expansion route and elucidate their regio- and enantioselectivity.