Mechanistic elucidation of ketenimine-butynoate cycloaddition reaction: role of biradical intermediates in isotopomeric purity of benzyl (1,1a,6,6a-13C4)-6-methyl anthranilic ester

Thermal Diels-Alder reaction of vinyl ketenimines with 13C-labeled 2-butynoate, efficiently prepared from 13C4-acetoacetate via the enol triflate, rendered at 160 °C the corresponding methylanthranilic ester isotopomerically pure. However, when the reaction was run at 100 °C, 7% of isotope exchange between C6a and C5 was observed. This result is explained through a base-like catalyzed isomerization of the alkyne to the corresponding allene. Combined DFT and coupled-cluster computations show the feasibility of this mechanistic proposal. Better agreement between DFT and coupled-cluster activation and reaction energies is observed for the meta-GGA MPWB1K functional as compared to previous B3LYP results.