The strain-promoted alkyne-nitrone and alkyne-nitrile oxide cycloaddition reactions: A theoretical study

Abstract

Density functional theory (DFT) calculations were performed to study the mechanism of bioorthogonal strain-promoted dibenzocyclooctyne (DIBO)-nitrone and DIBO-nitrile oxide cycloaddition (CA) reactions at B3LYP/6-31G(d,p), M06-2X/6-31G(d,p) and B3LYP-D3/6-31G(d,p) levels of theory. The potential energy surface analysis, synchronicity indices and global electron density transfer (GEDT) calculations at the transition states (TSs) show that these cycloadditions take place through a nearly synchronous mechanism with non-polar character. The strain and electronic effects on the reactivity of DIBO with nitrone and nitrile oxide were studied using a distortion/interaction transition state model and compared with CA reactions of these dipoles with acetylene as a strain-free reaction. The electron-reorganizations along these CA reactions have been studied using the topological analysis of the electron localization function (ELF) at the B3LYP/6-31G(d,p) level. The ELF topological patterns indicate a one-step two-stage mechanism for both reactions in which the formation of CO single bonds takes place after the formation of CC bonds.