Electrophilic allenes participate in polar Diels–Alder reactions: study of the reactivity and site-, regio- and stereoselectivity from a molecular electron density theory perspective

Abstract

The reactivity and selectivities in the polar Diels–Alder (DA) reactions of two 1-sulfonyl allenes with two cyclic dienes have been studied within the Molecular Electron Density Theory (MEDT) at the M06-2X/6-311G(d,p) computational level. DFT-based reactivity indices characterize sulfonyl allenes as strong electrophiles and cyclic dienes as strong nucleophiles participating in polar reactions. The polar DA reaction of methylsulphonyl allene with cyclopentadiene presents a low activation enthalpy of 11.3 kcal mol⁻¹, and is strongly exothermic, with a reaction enthalpy of −39.9 kcal mol⁻¹. This DA reaction is completely site selective and exhibits some endo stereoselectivity. The polar DA reaction with 2-methoxy-cyclopentadiene presents a lower activation enthalpy of 4.1 kcal mol⁻¹ and is completely para regioselective. The more favorable transition state structures (TSs) show an asynchronous C–C single bond formation controlled by the most electrophilic C2 carbon of the sulphonyl allenes. The analysis of the ground state of global electron density transfer (GEDT) at TSs points out the polar character of these DA reactions. A relative interacting atomic energy (RIAE) analysis reveals that while the sulfonyl allene framework is stabilized at TSs via GEDT, the diene framework is destabilized. The higher stabilization of the former enables the reduction in activation energies. The strong stabilization of the intra-atomic energies of the sulphur atom of the sulphonyl group is the main atomic-level electronic factor responsible for the acceleration of these polar DA reactions. The presented MEDT study will contribute to the design of further reactions involving allenes, providing deeper insight into the structure and reactivity, even through the analysis of the reagents in their ground state.