Theoretical exploration of siloxy carbenes: photogeneration and [2+1] photocyclization mechanisms

Carbenes are highly reactive intermediates central to various organic transformations, particularly within photochemistry. This study investigates siloxy carbenes generated from acyl silanes via a 1,2-silyl shift, focusing on their generation and reactivity in excited states, using the multiconfiguration perturbation theory (CASPT2//CASSCF/PCM). Our findings reveal that the presence of an aryl group conjugated with the carbonyl moiety substantially lowers the excitation energy of the singlet ¹nπ* state, enabling the 1,2-Brook rearrangement to proceed directly on the singlet hypersurface. This direct pathway, mediated by singlet S_ΣP(σ¹π¹) and S₀(σ²π⁰) carbenes, bypasses the need for intersystem crossing (ISC) to the triplet ³nπ* state, which is the rate-determining step in the stepwise triplet pathway involving a triplet T_ΣP(σ¹π¹) carbene, thereby enhancing reaction rates and stereoselectivity by preventing undesired bond rotations. This contribution deepens the understanding of siloxy carbene reactivity and lays the groundwork for their future applications.