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

• Published in: Physical chemistry chemical physics : PCCP 2024-11, Vol.26 (44), p.281-2818
• Main Authors: Li, Jianhao, Jiang, Xiaohan, Liang, Zixi, Tang, Chao, Ma, Lishuang, Lin, Xufeng, Liu, Xinmei, Chen, Xuebo
• Format: Article
• Language: English
• Subjects: Carbenes; Carbonyls; Hyperspaces; Perturbation theory; Photochemistry; Stereoselectivity
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d4cp03416c

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 1 nπ* state, enabling the 1,2-Brook rearrangement to proceed directly on the singlet hypersurface. This direct pathway, mediated by singlet S ΣP (σ 1 π 1 ) and S 0 (σ 2 π 0 ) carbenes, bypasses the need for intersystem crossing (ISC) to the triplet 3 nπ* state, which is the rate-determining step in the stepwise triplet pathway involving a triplet T ΣP (σ 1 π 1 ) 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. Regulatory mechanisms for the photogeneration of singlet/triplet siloxy carbenes and their involvement in [2+1] photocyclizations have been thoroughly studied at the CASPT2//CASSCF/PCM level of theory.