Ketenes and Other Cumulenes as Reactive Intermediates

Cumulenes are a highly varied class of compounds, including such species as ketenes, allenes, ketenimines, and isocyanates, as well as analogues where carbon is replaced by silicon or germanium, oxygen is replaced by sulfur or selenium, and nitrogen by phosphorus or arsenic. The unique structural characteristics of cumulenes lead to their distinctive patterns of reactivity, and the investigations of their structures and properties have enhanced their utility. Silylcyclobutenones prepared by new methodology from dimethoxycyclobutenedione are readily converted to the corresponding stabilized 2-silylvinylketenes, and these give efficient [4 + 2] cycloadditions with tetracyanoethylene (TCNE) and also with imines. There is increasing interest in cumulenes, including the preparation and identification of new members of this family, with new ways of catalyzing their reactivity, and the development of stereoselective processes.