Alkenes cycloaddition reactions with ketenimines

Thermal and photochemical cycloaddition reactions of 27r-electron species represent an important synthetic approach to four-membered rings. The reactions summarized in this section include 2 + 2 cycloaddition reactions of thioketones, thioketenes, isothiocyanates, sulfenes and iminosulfenes with alkenes, allenes, ketenes, ketenimines and alkynes. 

There are only few reports of [2 + 2] cycloadditions with ketenimines. " In most cases, reactions occur across the C=C bond. Only ynamines and polyfluorinated alkenes cycloadd to the C=N bond. 

The ketenimine (93) reacts only at the C=N bond to give pyrroles and pyrrolines with methylene migration (equations 102,103). Carbon (hoxide codimerizes smoothly with methylenecyclopropanes to yield lactones with alkene isomerization (equations 104, lOS). The Pd -catalyzed rearrangement of y-butyrolactone (94) to butenolide (95) indicates that this CO2 insertion reaction is reversible. Hiis is fur-ter supported by the observation that lactone (94) can serve as a TMM precursor in the codimerization with aldehydes and electron-deficient alkenes (equations 106, 107). No cycloaddition to ketenes or simple ketone carbonyls has ever been reported. 

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