RHODIUM(II)-CATALYZED CYCLIZATION REACTIONS OF ALKYNYL-SUBSTITUTED ALPHA-DIAZO KETONES

Treatment of several omicron-alkynl-substituted alpha-diazoacetophenone derivatives with rhodium (II) carboxylates results in the formation of substituted indenones. The simplest mechanism accounting for the results involves addition of a rhodium-stabilized carbenoid onto the acetylenic pi-bond to generate a vinyl carbenoid directly of possibly a highly strained cyclopropene derivative. The vinyl carbenoid was found to undergo addition to a neighboring alkene tethered on the side chain to give an indenyl-substituted bicyclo[3.1.0]hexane derivative. A number of related systems were examined so as to probe the scope and generality of the process. Treatment of omicron-(6,8-nonadien-1-ynyl)-alpha-diazoacetophenone with rhodium (II) mandelate afforded a cyclopent[g]azulenone derivative. The formation of this compound involves a divinylcyclopropane intermediate that readily undergoes a Cope rearrangement under the reaction conditions. The rhodium-catalyzed reaction of 2-(6-oxo-1-hepty-nyl)-alpha-diazoacetophenone in the presence of N-phenylmaleimide afforded a mixture of two cycloadducts. One of the products can be accounted for in terms of a cyclization of the vinyl carbenoid onto the oxygen atom of the neighboring carbonyl group to give a resonance-stabilized dipole. 1,3-Dipolar cycloaddition of the carbonyl ylide across the activated pi-bond of N-phenylmaleimide affords the observed product. The second cycloadduct is formed by a 1,2-hydrogen shift of the initially produced vinyl carbenoid. The resulting diene undergoes a subsequent Diels-Alder reactions with N-phenylmaleimide.