Hydrogen donors conjugate reduction

Sodium borohydride (160) was found to serve as a hydrogen donor in the asymmetric reduction of the presence of an a,pi-unsaturated ester or amide 162 catalyzed by a cobalt-Semicorrin 161 complex, which gave the corresponding saturated carbonyl compound 163 with 94-97% ee [93]. The [i-hydrogen in the products was confirmed to come from sodium borohydride, indicating the formation of a metal enolate intermediate via conjugate addition of cobalt-hydride species (Scheme 2.17). 

The dissociation of an acidic residue in water is essentially a downhill proton transfer between the donor and the solvent, and in the case of a large value for ApfCa, the reaction can be as fast as a barrier-less reaction [35]. To achieve the maximal rate of the reaction, the proton-donor atom must establish a hydrogen bond with the acceptor, and the reaction then takes place along this bond. As the rate of the reaction can be as fast as the hydroxyl vibration frequency, - 60-150 fs [36, 37], it is implicit that, for a fast reaction, the hydrogen bond must be established before the proton transfer event. In the case of proton transfer to water, pre-orientation of a few water molecules is required to stabilize the ejected proton and the conjugated base from which the proton was released. Accordingly, reduction in the availability of free water molecules will reduce the probability of the proton acceptor configurations, and this effect will appear as a reduction in the rate of dissociation. 

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