PUB - Publikationen an der Universität Bielefeld

Photolysis of chromium-carbene complexes 1 in the presence of olefins and dienes yields cyclobutanones 4 and 5 with varying stereoselectivity. Under oxidative conditions lactones 6, 7, 8, and 10 are obtained as byproducts. Thermal reactions of ketenes 3 with olefins and dienes yield the same cyclobutanones with usually equal selectivities. In the case of 2,3-dihydro-1,4-dioxin as olefin, diene 9 is obtained as a byproduct. Cycloaddition reactions of 1a or 3a with 1,2-dimethylenecyclopentane (11) yield the spirocyclic cyclobutanones 12 and 13. The symmetric ketone 16 and the ketene dimer 17 are formed as additional products upon cycloaddition of ketene 3b to tetraethoxyethylene. Novel alpha-(butylthio)cyclobutanones 4j, k, l, m and 5k, l are formed by cycloaddition reactions of the newly prepared compounds 1c and 3c. The ketene dimer 20 is formed by thermal reactions of 3a with olefins. Unusually low stereoselectivities are observed in the case of photolytic reactions of 1c with dienes.