Asymmetric Catalysis. Asymmetric Catalytic Intramolecular Hydroacylation of 4-Pentenals Using Chiral Rhodium Diphosphine Catalysts

Abstract

Catalysts of the type [Rhfchiral diphosphine)]+ convert 4-substituted 4-pentenals into the corresponding
\n3-substituted cyclopentanones with generally high turnover numbers and frequencies at 25 °C. The enantioselectivities
\nof various substituted 4-pentenals with two chiral diphosphines have been explored. It was found that with the binap
\ncatalyst, almost complete enantioselectivity is observed for 4-pentenal substrates bearing 4-substituted tertiary substituents
\nand for ester groups. Ketonic substituents give very high enantioselectivities. The mechanism of intramolecular
\nhydroacylation has been explored, and it is suggested that an important consideration for obtaining high turnover
\nfrequencies is related to the acyl-alkyl reductive elimination mechanism which is inferred to occur by a process similar
\nto ester hydrolysis. The origin of the enantioselection is discussed in terms of the interactions between the phenyl groups
\nof the phosphine and the substituent of the pentenal