Organic reactions with rhodium carbonyl cluster catalysts

Abstract

Rhodium carbonyl clusters have been found to catalyze two types of reactions: (1) Carbon-hydrogen bond activation of aromatic compounds and (2) Cross-hydrocarbonylation of acetylenes and olefins.(1) Under pressure of carbon monoxide (20 to 30 atm) and at higher temperatures (180 to 220 °C), rhodium carbonyl clusters Rh4(CO)12 and Rh6(CO)16 catalyze the addition of a C-H bond of benzene to unsaturated compounds such as diphenylketene, isocyanates and acetylenes to give phenylated compounds such as diphenylacetophenone, benzamides and phenylated olefins, respectively. Five-membered heterocycles such as furan, thiophene, and AT-methylpyrrole also react similarly with diphenylacetylene to give 2-furyl-, 2-thienyl- and 2-(N-methylpyrrolyl)-olefins, respectively.Among these, the reactivity of furan is the highest and is about 200 times that of benzene. In the reactions of arene with olefins, aryl-substituted olefins are obtained instead of simple addition products. For example, the reaction of benzene with ethylene gives styrene directly. The hydrogen generated in these reactions is used for the hydrocarbonylation or the hydrogenation of the starting olefins, yielding ketones or saturated compounds. The reaction of furan with olefins gives furyl olefins similarly. In the reactions of secondary amines such as indole, carbazole, and diphenylamine, the N-H bond participates in preference to the C-H bond, giving Nvinyl compounds. Regioselectivities in the reactions of substituted benzenes suggest the protophilic nature in the stage of C-H bond activation. The following mechanism is proposed: (1) initial oxidative addition of an aromatic C-H bond to the rhodium catalyst to give aryl- and hydride-rhodium species, (2) insertion of unsaturated compounds into the aryl-rhodium bond, and (3) reductive elimination with the hydride or β-hydrogen elimination affording the corresponding aryl-substituted compounds.(2) The cross-hydrocarbonylations of acetylenes and ethylene with carbon monoxide are found to give 5-ethyl-2(5H)-furanones when hydrogendonating solvents are used, and to give α, β-unsaturated ethyl ketones when molecular hydrogen is introduced into the reaction system. Since it is consistent with the regioselectivity in the reactions of unsymmetrical acetylenes, a β-acylvinyl rhodium intermediate is proposed as a common key intermediate for these two products. The convenient syntheses of 5-alkoxy2(5H)-furanones from acetylenes, carbon monoxide and alcohols are attained by adding a basic alkali metal salt such as NaOAc or Na2CO3 as a cocatalyst.