Understanding the reactivity and activation barrier origins in the cycloaddition of triplet pnictinidenes with phenylacetylene

Abstract

This study investigates the mechanistic details of the [1 + 2] cycloaddition of phenylacetylene (Ar′CCH;) with triplet monomeric pnictinidenes ([G15-Rea]³; G15 = Group 15 elements). The calculated results consistently support the greater stability of the triplet state ([G15-Rea]³) over the corresponding singlet state ([G15-Rea]¹). Furthermore, a gradual increase in singlet–triplet splitting is observed down the Group 15 series, highlighting the enhanced preference for triplet ground states among the heavier pnictogens. The origin of this trend is attributable to the relativistic inert s-pair effect and the concomitant non-hybridization effect. Theoretical results indicate that the reaction proceeds via the pathway: [G15-Rea]³ + Ar′CCH → [G15-TS]³ → [G15-Int]³ → [G15-T/S] → [G15-Prod]¹, while activation strain model analysis reveals that the activation barrier is primarily governed by the deformation energy of Ar′CCH. More specifically, the present findings demonstrate that the atomic radius of the Group 15 center critically influences the activation barrier of the [1 + 2] cycloaddition, such that increasing the size of the G15 element in [G15-Rea]³ leads to higher barriers for reaction with Ar′CCH. Overall, this study provides a qualitative rationalization of the reaction energetics and evaluates the feasibility of the proposed experimental observations.