Regenerable Hantzsch ester

Scheme 11.18 Asymmetric transfer hydrogenation with regenerable Hantzsch ester 39.

The reversible formation of a N,N-dibenzyl iminium intermediate, which is reduced by hydride capture from the Hantzsch ester 1 was proposed. Subsequent hydrolysis regenerates catalyst 2 and releases the saturated aldehyde. The transition state A has been suggested for the hydride transfer. An example of the asymmetric version of this reaction was also realized, by using a chiral imidazolidi-none catalyst (the McMillan imidazolidinium salt 3 [13]) (see Scheme 11.4). 

Zhou et al. have successfully developed an efficient method for regeneration of the Hantzsch ester (378) from the Hantzsch pyridine (379), with... 

Ru complexes as catalysts, under hydrogen gas. A catalytic amount of the Hantzsch ester (378), regenerated in situ, has been employed in the chiral phosphoric acid (382) promoted biomimetic asymmetric hydrogenation of benzoxazinones (380) to dihydrobenzoxazinones (381) with up to 99% ee (Scheme 100). ... 

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