Computational Analysis of Diastereoselectivity and Carbene Reactivity in Pt- and Au-Catalyzed 1,5-Enyne Cycloisomerization to Bicyclo[3.1.0]hexane

Bicyclo[3.1.0]hexane is a structural motif of the bioactive, naturally occurring cryptotrione and could be realized via a Pt- or Au-catalyzed reaction on 1,5-enyne with good control of the diastereoselectivity. In this work, we perform a density functional theory (DFT) study that provides mechanistic insight into this intriguing reaction and discloses the origin of the diastereoselectivity and reactivity. Distortion/interaction analyses and computational models reveal that the diastereoselective cyclization of the [Pt]-catalyzed beta-enyne favors a transition state with stronger hydrogen bonding, CH<middle dot><middle dot><middle dot>pi interactions, and less steric repulsion. The degree of back donation of the platinum carbene determines the activation barrier of the rate-determining hydride migration step. In the diastereoselective transition states of the [Au]-catalyzed reaction of alpha-enyne, the degree of out-of-plane distortion of the alkenyl moiety and the bending of the alkynyl group determine the preference. DFT calculations provided insight into transition states and intermediates that are difficult to detect experimentally, revealing structural factors that control the selectivity and reactivity.