ASYMMETRIC CATALYSIS - ASYMMETRIC CATALYTIC INTRAMOLECULAR HYDROACYLATION OF 4-PENTENALS USING CHIRAL RHODIUM DIPHOSPHINE CATALYSTS

Catalysts of the type [Rh(chiral diphosphine)](+) convert 4-substituted 4-pentenals into the corresponding 3-substituted cyclopentanones with generally high turnover numbers and frequencies at 25 degrees C. The enantioselectivities of various substituted 4-pentenals with two chiral diphosphines have been explored. It was found that with the binap catalyst, almost complete enantioselectivity is observed for 4-pentenal substrates bearing 4-substituted tertiary substituents and for ester groups. Ketonic substituents give very high enantioselectivities. The mechanism of intramolecular hydroacylation has been explored, and it is suggested that an important consideration for obtaining high turnover frequencies is related to the acyl-alkyl reductive elimination mechanism which is inferred to occur by a process similar to ester hydrolysis. The origin of the enantioselection is discussed in terms of the interactions between the phenyl groups of the phosphine and the substituent of the pentenal.