Abstract
The carbenoid intermediate derived by the treatment of several 1-diazobutanediones with rhodium(II) acetate undergoes ready transannular cyclization onto the neighboring keto group to give five-membered ring carbonyl ylides. The dipole derived from ethyl 4-diazo-2-methyl-3-oxobutyrate was found to undergo a rapid proton transfer, producing 5-ethoxy-4-methyl-3-(2H)-furnone. When the position adjacent to the diazo carbonyl group is blocked with two substituent groups, however, smooth 1,3-dipolar cycloaddition occurs. The observed regioselectivity can be nicely accomodated in terms of frontier molecular orbital (FMO) theory. A type II FMO interaction is involved since carbonyl ylides possess one of the smallest HOMO-LUMO energy gaps of common 1,3-dipoles. The rhodium(II)-catalyzed reaction of 1-diazo-6-phenyl-2,6-hexanedione afforded a mixture of products. In addition to the expected cycloadduct, a product derived from the bimolecular addition of the rhodium carbenoid to benzene was obtained
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