Enantioselective heterocyclic carbene catalysts

Recently, the Ye group found the [3 -I- 4] annulation of enals with aurones in the presence of a bifunctional N-heterocyclic carbene catalyst. The reaction gave the corresponding benzofuran-fused s-lactones 130 in good yields with good diastereoselectivities and excellent enantioselectivities (Scheme 20.55). 

Both y-mono and disubstituted allylphosphates react with silylboronates in the presence of a copperW-heterocyclic carbene catalyst and NaOH giving y-substituted allyl-silanes with high regioselectivities (5 275 2 ratios between 80/20 and >99/1) and high enantioselectivities (86 to 95%) ee Primary and secondary alkyl, aryl, halide, acetal, and ester groups are tolerated on C,. 

The same authors have also used this chiral IV-heterocyclic carbene catalyst in combination with Ti(Oz-Pr)4 to promote a highly diastereo- and enantioselective annulation reaction between a,p-unsaturated aldehydes and enones, which provided the corresponding chiral substituted ds-cyclo-pentenes in good yields and remarkable diastereo- and enantioselectivities of 90% de and 98 to 99% ee, respectively, as shown in Scheme 7.14. ... 

Reaction of a chiral A-heterocyclic carbene catalyst and a,/ -unsaturated aldehyde gives , -unsaturated acyl azolium that participates in enantioselective annulation via a Coates-Claisen rearrangement that invokes the formation of a hemiacetal before a sigmatropic rearrangement to give dihydropyranone products (Scheme 14)7 ... 

New catalysts and methods for enantioselective metal carbene reactions in syntheses of O- and N-heterocycles 98PAC1123. 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

V-Heterocyclic carbenes were demonstrated to be efficient catalysts for the Staudinger reaction of ketenes with /V-aryl-, /V-alkylcarbonyl imines [121]. Chiral /V-heterocyclic carbenes gave the corresponding < /.v-(3-lactams in good yields with good diastereoselectivities and excellent enantioselectivities (ee>99%). 

Grubbs and co-workers have recently reported a new class of Ru catalysts (83, Eq 2) [29] that bear a chiral monodentate N-heterocyclic carbene ligand [30]. The reactions illustrated in Eq 2 include the highest ee reported (13-90% ee) asymmetric induction is clearly dependent on the degree of olefin substitution (cf. Schemes 18 and 4 for comparison with the Mo-catalyzed reactions of the same substrates). As is the case with nearly catalytic enantioselective reactions [4], the identity of the optimal catalyst depends on the substrate a number of chiral... 

He M, Uc GJ, Bode JW (2006a) Chiral A-heterocyclic carbene catalyzed, enantioselective oxodiene Diels-Alder reactions with low catalyst loadings. J Am Chem Soc 128 15088-15089... 

In this context, the first attempts to employ chiral iV-heterocyclic carbenes as organocatalysts in an enantioselective reaction were carried out in the context of the benzoin condensation, which combines two molecules of an aldehyde (usually an aromatic one). The first pioneering examples were developed by Sheehan and coworkers, which were followed by several authors, highlighting the report by Enders in 2002, in which, for the first time, a highly enantioselective A -heterocyclic carbene-catalyzed benzoin condensation was reported (Scheme 6.2). As can be seen in this scheme, the general principle applied to the design of these catalysts relied on a thiazolium central unit as the precursor of... 

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