Insight into the Rh-catalyzed cyclization of dissymmetrically racemic (±)-3,4-disubstituted 4-pentenal: regio-, diastereo-, enantioselectivity, and kinetic resolution

Rh-Catalyzed cyclization was applied for the kinetic resolution of dissymmetrical (±)-3-(1-methylvinyl)-4-phenylpent-4-enal. The cyclization by a chiral neutral Rh[(S)-BINAP]Cl afforded a mixture of 3,4-cis-4-methylcyclopentanone (>95% ee, 19% yield), 3,4-trans-4-methylcyclopentanone, and 3,4-cis-4-phenylcyclopentanone (>95% ee, 21% yield). The cyclization by a cationic Rh[(R)-BINAP]ClO4 afforded a mixture of 3,4-trans-4-methylcyclopentanone (>95% ee, 36% yield), 3,4-cis-4-methyl-, and 3,4-trans-4-phenylcyclopentanone, accompanied with recovery of the starting material (−)-4-pentenal (54% ee, 34% yield), and by a cationic Rh[(S)-BINAP]ClO4 afforded the corresponding enantiomers. The course of the cyclization and the stereochemistry of products are discussed based on the plausible acyl–hydride rhodium intermediates.