Enantioselective organocatalytic transfer

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%. 

An enantioselective organocatalytic reductive amination has been achieved using Hantzsch ester for hydrogen transfer and compound (21) as catalyst. This mild and operationally simple fragment coupling has been accomplished with a wide range of ketones in combination with aryl and heterocyclic amines.359... 

The direct enantioselective organocatalytic a-fluorination can also be performed with cinchona alkaloid derivatives as catalyst under phase-transfer reaction conditions [25]. The fluorination reaction by NFSI of / -ketoesters 21, readily enolizable substrates, generated a stereogenic quaternary C-F bond in high yields and with enantioselectivities up to 69% ee for the optically active products 26 (Eq. 6). 

The nucleophilic activation of hydrosilanes as HSi(OR)3 offers an opportunity to transfer one hydride on the carbon of ketones or imines [22]. The enantioselective organocatalytic hydrosilylation of ketones was first reported in 1999 by Matsu-mura et al. [23], the catalyst employed being a proline derivative 19 (Scheme 11.7). Amide 20 was also able to catalyze the hydrosilylation of ketimines, as indicated in Scheme 11.7 [24]. Improved results were recently reported by Kocovsky and Maikov [25], who prepared from valine some acyclic analogues of prolina-... 

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... 

Enantioselective organocatalytic synthesis of 3,4-dihydro-2H-pyran-2-ones can be accomplished through a nucleophile-catalyzed Michael addition-proton transfer—enol lactonization of a,P-unsaturated acyl chlorides and 1,3-dicarbonyl compounds (Scheme 47) (13AGE13688). 

Recently, asymmetric reactions catalyzed by metal-free compounds (organoca-talysis) have received increased attention due to the potential advantages of these types of systems [4]. This is primarily due to the operational simpUcity and the obvious industrial applications. This chapter covers recent developments, from 2007 to date, in the area of the enantioselective organocatalytic Henry reaction with an emphasis on catalytic methods based on Bronsted bases, Bronsted acids, and phase-transfer conditions (PTC) approaches. 

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. 

The first metal-free organocatalytic enantioselective hydrogen transfer involving unprotected ort/zo-hydroxybenzophenone N-H ketimines (317) using the chiral phosphoric acid catalyst (300) and a the Hantzsch ester... 

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