Aldol intramolecular cobalt-catalyzed

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

Morken and co-workers have reported the highly enantioselective version of this reaction, albeit with low efficacy in the aldol-type coupling [8d, e]. Unfortunately, we obtain low enantioselectivity ee 2-4%) using chiral rhodium complexes under our reaction conditions. An intramolecular adaptation has led to new opportunities in cobalt-catalyzed carbocyclizations, wherein the use of PhSiHs was essential for smooth ring formation (Eq. 4) [9]. The identical products were also formed by a combination of [Rh(COD)2]OTf/(p-CE3Ph)3P and molecular hydrogen [10]. 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

Recently, extensive efforts have been made in the development of domino conjugate reduction/inter and intramolecular aldol reaction catalyzed by various metal complexes based on rhodium, iridium, cobalt, indium, nickel, and copper [22]. In 1999, Morken and Taylor [23] reported a rhodium-catalyzed conjugative reductive/aldol reaction between aldehyde and acrylate using QjMeSiH as reducing agent with a maximum diastereoselectivity of 23 1. 

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