Understanding the high reactivity of carbonyl compounds towards nucleophilic carbenoid intermediates generated from carbene isocyanides

The high reactivity of carbonyl compounds towards the carbenoid intermediate cis-IN, generated in situ by the addition of methyl isocyanide to dimethyl acetylenedicarboxylate (DMAD), has been investigated at the MPWB1K/6-311G(d,p) computational level by using Molecular Electron-Density Theory (MEDT). This multicomponent (MC) reaction is a domino process that comprises two sequential reactions: (i) the formation of a nucleophilic carbenoid intermediate trans-IN; and (ii) the nucleophilic attack of cis-IN on the carbonyl compound, resulting in the formation of the final 2-iminofuran derivative. The present MEDT study establishes that the high nucleophilic character and the electronic structure of the carbenoid intermediate, cis-IN, together with the specific approach mode of the carbonyl CO double bond during the nucleophilic attack of the sp² hybridised carbenoid C4 center of cis-IN on the carbonyl C5 carbon of acetone, enables the formation of the C4-C5 single bond with a very low activation enthalpy, 3.3 kcal mol^-1, without any external electrophilic activation of the carbonyl group, and the subsequent ring closure through the downhill formation of the C-O single bond. The Bonding Evolution Theory (BET) study for the formation of the 2-iminofuran allows characterisation of the mechanism as a [2n + 2n] cycloaddition, ruling out the proposed 1,3-dipolar cycloaddition mechanism.