SELECTIVE REDUCTIVE DIMERIZATION OF PHENYLACETALDEHYDE TO 2,4-DIPHENYLBUTANAL CATALYZED BY NOVEL DIRHODIUM COMPLEXES

The selective dimerization of phenylacetaldehyde (1) into 2,4-diphenyl butanal (3) is catalysed by novel ortho-metallated dirhodium(II) compo unds (I-III), in dry pyridine and under a pressure of carbon monoxide and hydrogen. Among the by products, two other coupling products, 2,4- diphenylbutan-1-ol (4) and 2,4-diphenylbut-2-enal (5), as well as phen ethyl alcohol (6) are present but in low amounts. A comparison with sy stems in which phosphines are added to dirhodium tetraacetate complex has been realised. When triphenylphosphine is used in a blank experime nt, only 2,4-diphenylbut-2-enal (5) is produced with 38% yield and no aldehyde 2,4-diphenylbutanal (3) is formed. We have also checked that aldehyde 3 does not result from the hydrogenation of enal 5. This repo rt illustrates the first application of ortho-metallated dirhodium(II) compounds in organic catalysis.