Ligand Effects in the Rhodium(II)-Catalyzed Reactions of α-Diazoamides. Oxindole Formation is Promoted by the Use of Rhodium(II) Perfluorocarboxamide Catalysts

Article abstract of DOI:10.1021/jo00088a028

An improved procedure for the preparation of ethyl 2-diazomalonyl chloride was developed which involves the reaction of ethyl diazoacetate with triphosgene.Using this diazo acid chloride, it was possible to prepare a variety of diazoamides from substituted amines.The rhodium(II)-catalyzed decomposition of these diazoamides was studied in order to probe the chemoselectivity of the carbenoid intermediates in intramolecular insertion reactions.Rhodium(II) acetate decomposition of N-benzyl-2-diazo-N-phenylmalonamic acid ethyl ester resulted in intramolecular C-H insertion to give ethyl 1,4-diphenyl-2-oxoazetidine-3-carboxylate.By changing the catalyst ligand to trifluoroacetamide, β-lactam formation was completely suppressed in favor of the aromatic C-H insertion which produces an oxindole as the only detectable product.The competition between aliphatic and aromatic carbon-hydrogen insertion of 2-diazo-N-isobutyl-N-phenylmalonamic acid ethyl ester provides another example of ligand effectiveness in controlling chemoselectivity in dirhodium(II)-catalyzed metal carbene reactions.Thus, treatment of the N-isobutyldiazoanilide with rhodium(II) acetate results in exclusive aliphatic C-H insertion giving 4,4-dimethyl-2-oxo-1-phenylpyrrolidine-3-carboxylic acid ethyl ester, while the perfluorobutyramide ligand promotes oxindole formation by aromatic C-H insertion.Several other rhodium(II)-catalyzed reactions were studied and were found to be highly catalyst dependent, rhodium(II) perfluorocarboxamides promoting aromatic C-H insertion, and hence oxindole formation, over O-H insertion, cyclization onto adjacent triple bonds, or cyclization to generate 1,3-dipolar intermediates.