Rational Design and Reaction Mechanism Study of the Photochemical Rearrangement of Fulvene Derivatives.

Photochemistry is considered one of the most efficient and reproducible techniques in organic synthesis. Recently, List and co-workers reported an efficient UV light triggered photochemical synthesis of spiro[2,4]heptadiene from fulvenes with different substituents ( Angew. Chem., Int. Ed. 2023, 62, e202303119); however, the mechanistic details remain unclear, and the intermediates have not been characterized. To facilitate the applications of this novel photochemical reaction, we theoretically designed a series of fulvene derivatives with different parent molecular skeletons for analyzing the substitution effects, and two of the representative fulvenes were synthesized for investigating the reaction mechanisms by employing time-resolved transient absorption spectroscopy (TA) experiments. It has been found that instead of density functional theory, the second-order n-electron valence state perturbation theory is necessary to acquire reliable theoretical characterization of the fulvenes examined. Our designed fulvenes were found to undergo the photorearrangement cyclopropanation reaction on the basis of photoproduct analysis. The intermediate species involved in the intramolecular hydrogen atom transfer and cyclization processes within the photorearrangement reaction were characterized by TA spectroscopy, and the full reaction pathways were proposed. Our work not only reveals the detailed mechanism of this photorearrangement reaction but also demonstrates the significance of appropriate theoretical methods for rational molecular design.