A Mechanistic Study of the Rhodium-Catalyzed Cyclization of 4-Hexenals. Reactions of Deuterio-4-hexenals

Article abstract of DOI:10.1021/ja00538a021

In independent experiments, four carbons in the 4-pentenal skeleton have been labeled with deuterium or methyl and the fate of each label has been determined as the pentenal was transformed into a cyclopentanone derivative by RhCl(PPh3)3 (1) at 24-26 deg C.The catalyst converted 4-hexenal to 2-methylcyclopentanone (2) in CHCl3 and C6H6.Approximately equivalent amounts of hydrocarbon decarbonylation products and RhCl(CO)(PPh3)2 were also formed. 3-Methyl-4-pentenal was isomerized to 3-methylcyclopentanone by 1. 4-Hexenal possessing deuterium at C-2 was isomerized to 2 which contained deuterium at C-5. trans-4-Hexenal-1-d was cyclized to 2-3-d and 2-2-d in 9:1 ratio when the reaction was carried to a low conversion.The deuterium in the 2-3-d product was found to be cis to the C-2 CH3 group. cis-4-Hexenal-1-d was isomerized by 1 to afford 2-3-d possessing deuterium trans to the C-2 CH3 group.NMR analyses of these products were assisted by the synthesis and characterization of 2-cis-2,3-d₂ by treatment of 2-methylcyclopent-2-en-1-one with D2 and 1.The 2-cis-2,3-d₂ could be converted to a 1:1 mixture of 2-3-d diastereomers on treatment with HCl in MeOH/H2O.The result demonstrated that the cyclization of 4-hexenal-1-d occurred by a syn addition of the C-D bond to the olefinic bond to generate 2-3-d.The presence of C2H4 in reactin mixtures of 1 and 4-hexenal-1-d resulted in the formation of substantial 2-d₀ and C2H3D.The deuterium locations in the 1-pentene, 2-pentene, and ethylcyclopropane decarbonylation products derived from reaction of 4-hexenal-1-d with 1 were determined.The results were interpreted in terms of a hydroacylation mechanism involving an acylrhodium(III) hydride complex and organometallic intermediates derived therefrom.The hydroacylation and decarbonylation products appear to be generated via common intermediates.