| The rhodium(II) acetate catalyzed behavior of o- ((propenyloxy)-methyl) -;The carbenoid intermediate derived by the treatment of several 1-diazobutanediones with rhodium (II) acetate undergoes ready trans-annular cyclization onto the neighboring keto group to give five-membered ring carbonyl ylides. The observed regioselectivity can be nicely accommodated 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. The formation of a mixture of products in this case suggests that entropic factors have sufficiently retarded the rate of intramolecular cyclization so as to allow the bimolecular reaction with benzene to occur.;The intricacies of the intramolecular 1,3-dipolar cycloaddition of cyclic carbonyl ylides have been probed using a variety of ylide precursors. Tethers of three or four methylene units, which ultimately lead to five and six membered rings fused to the oxabicyclic backbone, readily enter into intramolecular cycloaddition, while longer and shorter tethers were reluctant to do so.;A series of olefinic keto-... |
