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 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

Rhodium‐Catalyzed Enantioselective Conjugate Silyl Transfer: 1,4‐Addition of Silyl Boronic Esters to Cyclic Enones and Lactones

“…[21] and (S)-7 [22] was assigned unambiguously by comparison with reported data, while those of (S)-8 and (S)-10 were assigned on the basis of similar HPLC characteristics on a chiral stationary phase and the sign of the optical rotation. [22] We generally used catalyst and ligand loadings of 5.0 mol % in these reactions.…”

“…[22] We generally used catalyst and ligand loadings of 5.0 mol % in these reactions. We reduced the amount of [((S)-binap)Rh(cod)]ClO 4 and (S)-binap to 1.0 mol % for the thus far best reaction 1!…”

An Unprecedented Rhodium‐Catalyzed Asymmetric Intermolecular Hydroacylation Reaction with Salicylaldehydes

“…Later this approach was extended by Sakai, Bosnich and Morehead, who achieved high enantioselectivities with other substrates. [8][9][10] Tanaka, Sakai and Suemune developed the asymmetric intramolecular hydroacylation reaction further into efficient desymmetrizations of symmetric dienals. [11] The intermolecular hydroacylation reaction has been studied much less than its intramolecular counterpart.…”

Development of Novel Cyclizations <i>via</i> Rhodacycle Intermediate and Its Application to Synthetic Organic Chemistry

“…2) After that, this hydroacylation of 4-alkenals has been expanded to a catalytic process 3,4) and asymmetric reaction. [5][6][7][8][9][10][11][12][13][14][15][16][17][18] The proposed reaction mechanism is shown in Chart 1. Initially, oxidative addition of the C-H bond of the aldehyde moiety to the rhodium complex occurs to form acylrhodium intermediate i followed by insertion of an alkene moiety to the Rh-H bond to give the 6-membered rhodacycle intermediate ii.…”