Mechanistic insights into nickel- and gold-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds: a DFT study

Abstract

Density functional theory (DFT) calculations were performed to gain an in-depth mechanistic understanding of Ni(0)- and Au(I)-catalyzed diastereoselective [4 + 2 + 1] cycloadditions between dienynes and diazo compounds. The computational results indicate that, for both Ni(0)- and Au(I)-catalysis, transition-metal catalysts are more readily available to activate diazo compounds to form transition-metal carbene intermediates prior to the activation of the dienynes. Subsequently, the insertion of the alkynyl moiety of the dienynes with the generated transition-metal carbene intermediates can afford the metallacyclobutene intermediates. Afterwards, the transition-metal-dependent transformation of the obtained metallacycle intermediate was revealed. For the Ni(0)-catalyzed reaction, the subsequent intramolecular migratory insertion, followed by cyclization, is the favored route for the formation of the divinylcyclopropane intermediate. However, for the Au(I)-catalyzed reaction, the subsequent ring-opening of the obtained metallacycle occurs, affording the vinyl Au-carbene intermediate, from which an analogous divinylcyclopropane can be obtained via an intramolecular cyclopropanation. The generated divinylcyclopropane intermediates can furnish the corresponding products via a [3,3]-sigmatropic rearrangement for both reactions. The stereoselectivity of the Ni(0)-catalyzed reaction is mainly controlled by minimizing the steric hindrance in the denitrogenation, migratory insertion, and [3,3]-sigmatropic rearrangement steps. Meanwhile, for the Au(I)-catalyzed reaction, the C–H⋯O H-bonding interactions in the ring-opening of the metallacyclobutene intermediate and the steric hindrance effects in the subsequent cyclopropanization and [3,3]-sigmatropic rearrangement steps collectively determine the experimentally observed stereoselectivity.