Organocatalytic transfer hydrogenations

Organocatalytic transfer hydrogenation of enals has been discussed as a route for synthesis of some lepidopteran pheromones. 

This process was published in 2004 and constitutes the first metal-free organocatalytic transfer hydrogenation of a,f)-unsaturated aldehy-... 

Scheme 19. Organocatalytic transfer hydrogenation of different a,(1-unsaturated...

Other examples of asymmetric reactions involving organocatalysts are (i) Diels-Alder reactions, (ii) Michael reactions, (iii) Mannich reactions and (iv) Shi epoxida-tion and organocatalytic transfer hydrogenation. 

The organocatalytic transfer hydrogenation of cyclic enones has been also successfully achieved employing the imidazolidinone 12 [24], catalyst that also promotes enantioselective Diels-Alder reactions with cyclic enones [25]. The structure of the dihydropyridine reagent seems to have an important effect on the selectivity of the process since better enantioselectivities are observed when increasing the steric hindrance of the ester moiety 13 (Scheme 2.6). The reduction, which is performed with substoichiometric amounts of imidazolidinone 12 in ether at 0°C,... 

Rueping M, Antonchick AR, Theissmann T (2006) A Highly Enantioselective Br0nsted Acid Catalyzed Cascade Reaction Organocatalytic Transfer Hydrogenation of Quinolines and Their Application in the Synthesis of Alkaloids. Angew Chem Int Ed 45 3683... 

Later, the first enantioselective organocatalytic transfer hydrogenation involving cyclic enones was reported by MacMillan and co-workers following an operationally simple and rapid protocol that allowed access to chiral / -substituted... 

Ouellet, S.G., Walji, A.M., and MacMillan, D.W.C. (2007) Enantioselective organocatalytic transfer hydrogenation reactions using Hantzsch esters. Acc. Chem. Res., 40, 1327-1339. 

BOTH (Enantioselective Organocatalytic Transfer Hydrogenation) The group led by MacMillan showed that the combination of an imidazolidine catalyst 3 and a Hantzsch ester 4a is a powerful and versatile reducing agent (Scheme 32.2). They were able to selectively reduce 1,4-unsaturated enals and enones with yields up to 95% and ees up to 97%. 

Tuttle JB, Ouellet SG, MacMillan DWC. Organocatalytic transfer hydrogenation of cyclic enones. J. Am. Chem. Soc. 2006 128 12662-12663. 

Rueping M, Antonchick AP, Theissmann T. A highly enantioselective Brpnsted acid catalyzed cascade reaction organocatalytic transfer hydrogenation of quinolines and their application in the synthesis of alkaloids. Angew. Chem. Int. Ed. 2006 45 3683-3686. 

Scheme 2.11 Dynamic kinetic resolution of aldehydes via organocatalytic transfer hydrogenation...

---