Rhodium, chlorotris catalyst decarbonylation

Aldehydes, both aliphatic and aromatic, can be decarbonylated by heating with chlorotris(triphenylphosphine)rhodium or other catalysts such as palladium. The compound RhCl(Ph3P)3 is often called Wilkinson s catalyst.In an older reaction, aliphatic (but not aromatic) aldehydes are decarbonylated by heating with di-tert-peroxide or other peroxides, usually in a solution containing a hydrogen donor, such as a thiol. The reaction has also been initiated with light, and thermally (without an initiator) by heating at 500°C. 

Not all decarbonylations give the expected product, and the relative yields are influenced by the rhodium catalyst employed. Chlorotris(triphenylphosphme)rhodium(I) rapidly and selectively decarbonylated the en /o-norbomene isomer shown in equation (4) to form a tricycline product, but [Rh(dppp)2]Cl additionally forms an isomeric alkene. In the case of the corresponding exo-isomer (equation 5), [RhCl(PPh3)3] is the less selective catalyst. It was also found that [Rh(dppp)2]Cl was a superior catalyst... 

Recently Blum reported that chlorotris(triphenylphosphine) rhodium (XI) is an active catalyst for the decarbonylation of aroyl halides and showed several examples (2). But in this case too, the real catalyst seems to be chlorocarbonylbis(triphenylphosphine)rhodium (XII), which is formed in situ from XI by the stoichiometric reaction with acyl halides. Formation of alkyl halides by decarbonylation of acyl halides can be carried out by the Hunsdiecker reaction, but the reaction is unsatisfactory when applied to aroyl halides. Therefore, the decarbonylation reaction of aroyl halides by the rhodium complex is a new and useful means of introducing halogen onto the aromatic ring. 

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