Ligand-Controlled Regiodivergent\nRing Expansion of\nBenzosilacyclobutenes with Alkynes <i>en</i> Route to Axially\nChiral Silacyclohexenyl Arenes

A ligand-controlled regiodivergent and enantioselective\nring expansion\nof benzosilacyclobutenes with internal naphthyl alkynes has been achieved\nby adjusting the ligand cavity size. The ligand (<i>S</i>)-8<i>H</i>-binaphthyl phosphoramidite, featuring small\nmethyl groups on its arms, provides a spacious cavity that favors\nsterically demanding Si–Csp<sup>3</sup> ring expansion, predominantly\nyielding axially chiral (<i>S</i>)-1-silacyclohexenyl arenes.\nIn contrast, the ligand (<i>R</i>)-spiro phosphoramidite,\nwith bulky <i>t</i>-Bu groups on its arms, offers a compact\ncavity that facilitates less sterically demanding Si–Csp<sup>2</sup> ring expansion, leading primarily to axially chiral (<i>S</i>)-2-silacyclohexenyl arenes. Density functional theory\ncalculations delineate distinct mechanistic pathways for each ring\nexpansion route and elucidate their regio- and enantioselectivity.