Ni(NHC) Catalyzed Rearrangement of 1-Acyl-2-vinylcyclopropanes: Tackling a Mechanistic Puzzle by Combined Experimental and Computational Studies

The Ni(NHC) catalyzed rearrangement of 1-acyl-2-vinylcyclopropanes to the corresponding 4-acyl-cyclopent-1-enes is highly promising for the synthesis of keto-functionalized annelated bi- and tricyclic subunits of natural products. Therefore, we investigated the catalytic activity of Ni(NHC) complexes in the rearrangement of 1-acyl-2-vinylcyclopropanes with different ring sizes and substitution patterns. Surprising effects regarding substrate scope and stereoselectivity of the Ni(NHC) catalyzed vinylcyclopropane-cyclopentene rearrangement were observed. Only vinylcyclopropanes with 1-methyl, 1-phenyl, 1,2-dialkyl or 2-phenyl-substitution at the vinyl moiety could be rearranged successfully. Moreover, an endo-configuration on the cyclopropane ring was required for successful rearrangement. By treatment of the vinylcyclopropanes with Rh catalysts or Lewis acids, the involvement of Lewis acid catalysis could be ruled out. In order to understand these experimental results and to rationalize the reactivity of the Ni(NHC) complexes computational studies were performed, which provided insights into mechanistic details.