Competing Reaction Pathways in the Cycloaddition of 3-(Dimethylamino)-2H-azirines with Ketenes

Article abstract of DOI:10.1021/jo01303a006

Addition of diphenylketene (2a) to the azirines 1a,b affords the isomeric cycloadducts 3a,b and 4a,b as well as the acyclic addition products 5a,b in variable amounts.In the reaction with 1a, the 3-benzazepine 6 is also formed and can be hydrolyzed to give 8 via 7 or trapped to furnish the Diels-Alder adduct 10.The 2-vinylazirine 1c and 2a yield the pyrrolinone 4c as the sole product.These compounds as well as the corresponding products 16 and 17 from 1b and tert-butylcyanoketene (2b) all arise from cleavage of what was the original 1,2 bond in the azirine precursor with subsequent ring closure, hydrogen migration or intramolecular S_E reaction with a phenyl substituent and the cation of the intermediate zwitterion 21.On the other hand, the 2,2-dimethylazirine 1a and the cyanoketene 2b afford the ketenimine 12 resulting from complete opening of the original 1,3 bond of the azirine.Thiolysis of 12 to give the urea 14 can be understood in terms of attack of hydrogen sulfide on a small equilibrium concentration of the intermediate zwitterion 11.