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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
Published on: November 29, 2018
Deciphering Carbene Reactivity: Ultrafast Spectroscopy and Computational Insights into Photochemical Heterocycle
Yinjiao Zhao1, Yang Cheng1, Shu-Lin Zhang1
1Key Laboratory of Applied Surface and Colloid Chemistry of Ministry of Education, Shaanxi Provincial Key Laboratory of New Concept Sensors and Molecular Materials, School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi'an 710119, China.
Abstract:
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.
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