Cobalt enolates

The chiral cyclic cobalt enolate 2 reacts with acetone to produce the aldol adduct 3 which arises from attack of the electrophile from the least hindered side of the enolate, i.e., away from the phosphane13. None of the other possible diastereomers was detected. 

Stoichiometric, nucleophilic addition of alkylcobalt compounds to carbonyl compounds were reviewed in 1996.374 This chapter focuses on recent progress in the reactions of cobalt enolates with carbonyls and cobalt-catalyzed coupling reactions. 

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... 

The high levels of, sy -diastereoselectivity suggest aldolization through a closed Zimmerman-Traxler-type transition structure via intermediacy of the Z-enolate. When the transformation is performed using PhSiDj, a single deuterium is incorporated at the /3-position of the product as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization or reversible aldol addition. The stereochemistry of the deuterated product was established by single crystal neutron diffraction analysis (Scheme 44). 

Isoxazolines are related to 1,2-dioxines in that they are also prone to reductive ring opening. Ishikawa and colleagues found that A-benzylisoxazolines form 2-acylaziridines by SET reduction/ring opening and recyclization of the resulting aminyl radical to the cobalt enolate [430]. 

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