WILKINSON Carbonylation decarbonylation

WILKINSON Carbonylation decarbonylation catalyst Rh catalyst for carbonylation, decarbonylation, oxygenation, benzyl cleavage... 

Rhodium is used in a variety of catalysts. In particular, the Wilkinson complex RhCl(PPhv)3 as a rhodium catalyst is well known, and hydrogenations, hydro-metalations, carbonylations, decarbonylations, isomerizations and cyclizations proceed selectively [34,63]. 

Methyl deoxypodocarpate 127 (Scheme 1) 129) represents a simple problem since the ketone 132 is well-known and readily available from Hagemann s ester in three steps. The problem of geminal alkylation of this ketone stems from its existence as an EjZ mixture of ring fusion isomers. Recognizing that decarbonylation of aldehydes occurs readily with Wilkinson s catalyst creates a structural equivalence of an acetaldehyde chain and a methyl group as in 128. This simple relationship immediately establishes several options, a simple one uses a thioacetal such as 129 as a synthon for the aldehyde. The presence of a carbonyl group three carbons away... 

In contrast to a number of studies on the homogeneous hydrogenation of carbon-carbon multiple bonds [25], there had been few papers about hydrogenation of simple ketones before Schrock and Osborn [26] reported in 1970 a catalytic activity of cationic rhodium complexes with relatively basic phosphines as ligands. In fact, the Wilkinson s rhodium(I) complex usually lacks activity towards hydrogenation of carbonyl groups, and rather catalyzes decarbonylation of aldehydes. The catalytic cycle of the hydrogenation of ketones proposed by Schrock and Osborn is depicted in Scheme 3. 

Aldehydes undergo a decarbonylation reaction by action of a transition metal complex [17]. For example, benzaldehyde was decarbonylated with Wilkinson complex to furnish benzene along with a rhodium carbonyl complex 47 (Scheme 7.14) [17e,f]. Oxidative addition of the aldehydic C-H bond to rhodium, migratory deinsertion of CO, and reductive elimination operate in sequence for the decarbonylation reaction. Decanal was also decarbonylated to furnish a mixture of nonane and nonene, which were produced via the alkylpalladium intermediate 48 (Scheme 7.15) [17d]. 

Wilkinson s catalyst is useful for effecting intramolecular decarbonylation of aldehydes to hydrocarbons with retention of configuration (Walborsky and Allen, 1971). Decarbonylation of disubstituted cyclopropenones by iron, nickel, or cobalt carbonyls results in the formation of alkynes (Bird and Hudec, 1959). Tetraphenylethylene was obtained in 68% yield by reaction of diphenylketene with cobalt carbonyl (Hong et ai, 1968). 

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