Enantioselective hydrogen atom abstraction

Additional examples of reagent-based enantioselection involve the enantioselective hydrogen atom abstraction by chiral amine-boryl [37] or silanethiyl radicals... 

Enantioselective hydrogen atom abstraction by chiral amine boryl radical 83 allows for kinetic resolution of racemic ester 84. In the specific example illustrated in Eq. (23), the enantioselectivity of the residual enantiomer of the substrate reached 74% (/ ,/ ). It is postulated that a transition state as shown in 86 could account for the asymmetric hydrogen atom abstraction. 

The enantioselectivity of the elimination process is controlled by the dia-stereoselectivity of the hydrogen atom abstraction and is rationalized by minimization of steric interactions between the sulfoxide oxygen atom and the cyclohexane ring. 

Due to the extensively represented oxidative behaviour of the carbenium ions as hydride abstractor or one-electron oxidant [157], attempts were made to employ the carbocations as reagents. Recently the enantioselective outcome in a hydride transfer reaction was reported [158, 159]. The abstraction of the exo hydrogen atoms from the tricarbonyliron complex 57 resulted in a yield up to 70% and enantiose-lectivity of 53% (Scheme 62) [158]. 

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