Cumulene Rearrangements: Ketene-Ketene, Isocyanate-Isocyanate, Thioketene-Ketene, Imidoylketene-Ketenimine, and Ketene-Allene Rearrangements

Interconversions between α-oxoketenes, imidoylketenes and α-oxoketenimines, thioacylketenes and acylthioketenes, vinylketenes and acylallenes, isocyanates, and thioacylisocyanates and acylisothiocyanates take place by means of 1,3-shifts of substituents, which are facilitated by electron-rich migrating groups, especially those containing lone pairs on the migrating atoms. A bonding interaction between the lone pair orbital and the LUMO of the cumulene moiety stabilizes the transition state and can even make it become an intermediate. The 1,3-shifts can also be said to be pseudopericyclic reactions. The 1,3-migration of aryl groups is accelerated by electron- donating substituents in the phenyl ring. In general, for like substituents, migratory aptitudes decrease in the series α-oxoketene > imidoylketenes > acylallene > vinylketene.