Stereochemistry and Mechanistic Insight in the [2k+2i+2i] Annulations of Ketenes and Imines

The stereochemistry and mechanistic insight in the annulations of one ketene molecule with two imine molecules ([2k+2i+2i] annulation) are studied by using six-membered 3,4-dihydroisoquinoline as an imine probe. A concerted hetero-Diels–Alder cycloaddition mechanism is proposed to explain the stereochemical outcomes. In most cases, the zwitterionic 2-aza-1,3-butadiene-type intermediates, generated from ketenes and imines, undergo endo hetero-Diels–Alder cycloaddition with the second imine molecule. For ketenes with electron-donating substituents, (2,4)-cis-(4,5)-cis-[2k+2i+2i] annuladducts formed stereospecifically, while, for ketenes with electron-accepting substituents, (2,4)-cis-(4,5)-trans-[2k+2i+2i] annuladducts are generated stereospecifically. The [2k+2i+2i] annulations of aryloxyketenes and 3,4-dihydroisoquinoline give stereodivergent products due to the occurrence of the stepwise nucleophilic annulation. However, in the [2k+2i+2i] annulations of seven-membered cyclic imine dibenzo­[b,f]­[1,4]­oxazepine, the zwitterionic aza-butadiene-type intermediates exclusively undergo exo hetero-Diels–Alder cycloadditions with another molecule of imine to yield (2,4)-trans-(4,5)-trans-[2k+2i+2i] annuladducts stereospecifically, regardless of the ketene substituents. The mechanistic model not only discloses the nature of the [2k+2i+2i] annulations, but also can be used to explain and predict the stereochemistry of the [2k+2i+2i] annuladducts from different ketenes and imines.