Catalysts of the type [Rhfchiral diphosphine)]+ convert 4-substituted 4-pentenals into the corresponding 3-substituted cyclopentanones with generally high turnover numbers and frequencies at 25°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.Intramolecular hydroacylation is one of the more attractive catalytic processes because it converts a pentenal to a cyclic ketone (eq 1). The process is unusual in that the events leading to the product appear to involve scission of the acyl-hydrogen bond to give a hydrido-acyl intermediate followed by the addition of the two fragments to the double bond. With rhodium-based catalysts, 5-membered ring products are almost always produced.The discovery of Tsuji1 that Wilkinson's catalyst, [Rh(PPh3)3-Cl], was capable of decarbonylating aldehydes to form the alkane and the carbonylated complex, trans-[Rh(PPh3)2(CO)Cl], suggested that oxidative addition across the acyl-hydrogen bond was a step in the decarbonylation. Such a hydrido-acyl intermediate could, in principle, hydroacylate a double bond, provided that the rate of hydroacylation was faster than the decarbonylation rate.The first example of intramolecular hydroacylation of 4-pentenals was reported by Sakai,2 who used Wilkinson's catalyst.Stoichiometric amounts of [RhCPPhs^Cl] and the pentenal were used, and only a 30% yield of the desired cyclopentanone was obtained. The remainder of the material was composed of decarbonylation products. Subsequently, Miller3 showed that the [RhfPPhs^Cl] complex could act as a catalyst under ethylene pressure, and Larock4 explored the catalysis with a variety of substrates and with catalysts incorporating differently substituted unidentate triarylphosphines. In neither study were more than a few turnovers observed before all of the catalyst was converted

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