Deciphering Carbene Reactivity: Ultrafast Spectroscopy and Computational Insights into Photochemical Heterocycle Formation.

Carbenes are key intermediates in photochemical heterocycle synthesis. Diazo compounds and acylsilanes have been widely used to generate carbenes for heterocycle synthesis upon photoexcitation, yet the mechanistic origins of solvent- and substituent-controlled reactivity remain elusive. Here, we combine femtosecond transient absorption spectroscopy with density functional theory to decipher the excited-state dynamics of a diazo compound (C1) and two olefin-functionalized acylsilanes (C2 and C3). We reveal that the product distribution is dictated by the spin state and solvation of carbene: C1 undergoes triplet-state cyclization in acetonitrile but singlet-carbene-driven Wolff rearrangement in ethanol. In contrast, altering side-chain variation of acylsilanes redirects reactivity: C2 forms a cyclopropane derivative through cycloaddition, whereas C3 triggers a retro-Michael/Michael cascade, yielding pyran products. This study establishes types of solvents and structural tuning as effective levers to control carbene reactivity, providing general design principles for green, catalyst-free synthesis of valuable heterocyclic scaffolds.