Enantioselective Synthesis of Spirosilabicyclohexenes by Asymmetric Dual Ring Expansion of Spirosilabicyclobutane with Alkynes

Abstract

Silaspiranes have attracted particular attention due to their chiral spiro‐silicon center, which serves as an ideal carbon isostere and can endow spiro‐analogs with distinct properties. Distinct from previously reported cyclization or cycloaddition strategies to form 5/5‐silaspiranes, we report herein the asymmetric dual ring expansion of spirosilabicyclobutanes with alkynes to synthesize axially chiral spirosilabicyclohexenes bearing a novel 6/6‐silaspirane framework. DFT (density functional theory) calculations provide the deep insight into the origin of the high enantioselectivity controlled by the sterically demanding binaphthyl phosphoramidite ligand. Preliminary studies of chiroptical properties indicate that one of the spirosilabicyclohexene analogs exhibit fluorescence emission, Cotton effects and CPL (circularly polarized luminescence) activity.

Enantioselective Synthesis of 6/5-Spirosilafluorenes by Asymmetric Ring Expansion of 4/5-Spirosilafluorenes with Alkynes

“…Recently, we developed a rhodium-catalyzed asymmetric dual ring expansion of spirosilabicyclobutanes with alkynes to synthesize C 1 -or C 2 -symmetric 6/6-spirosilabicyclohexenes. 10 The sterically demanding binaphthyl phosphoramidite ligands played a key role in constructing the stereogenic silicon with high enantioselectivity. We envisioned that such sterically demanding ligands might also find good applicability in the ring expansion of other silacyclobutane-containing silaspiranes.…”

“…In our previous work on the asymmetric construction of 6/ 6-spirosilabicyclohexenes via dual ring expansion of 4/4spirosilabicyclobutanes, DFT calculations suggested that oxidative addition was reversible and migration insertion was the enantioselectivity-determining step. 10 We hypothesized that such a model could also apply to the ring expansion of 1monosubstituted 4/5-spirosilafluorenes developed in this work (Scheme 6). The transition-state models of TS-R and TS-S were proposed using 1a and phenylacetylene.…”