Spiro-aziridine and bislactam formation from bisketene-imine cycloadditions

1,2-Bisketenes 6 react with imines PhCH=NAr, (E)-2, forming spiro-aziridines 7. DFT computations indicate that this occurs by nucleophilic attack of the imine on the carbonyl carbon of the more reactive arylketene moiety, followed by cyclization, and not by prior cyclization of the 1,2-bisketene forming a carbene lactone intermediate. Computations also indicate that the previously studied bisketene 10 from benzocyclobutadiene 9 is 4.0 kcal/mol less stable than carbene lactone 12 that would result from cyclization but that the failure to observe 12 results from a lower barrier for 10 to instead revert to 9. 1,2-, 1,3-, and 1,4-Bisketenylbenzenes 16, 19, and 22 react with imines forming bis(beta- lactams), with a preference for formation of mixtures of trans, trans chiral (+/-) and achiral diastereomeric products.