Asymmetric organocatalysts pyridines

A review describes the utility of a new class of helically chiral pyridines as asymmetric organocatalysts for propargylation of aldehydes, as well as for several other unrelated transformations. ... 

Examples of nonasymmetric organocatalysts that were introduced in the 1950s include analogs of thiamine reported by Breslow in 1957 as an alternative to cyanide as a catalyst for the benzoin condensation [8]. Asymmetric versions of these thiazolium catalysts were used in organocatalytic benzoin condensations by Sheehan and Hunneman in 1966 [9]. In another important development, in 1969 the nucleophilic catalyst 4-(dimethylamino)pyridine (DMAP), which is now widely used for difficult esterifications, was reported by Steglich [10]. 

On the basis of their observation that achiral 2,2 -bipyridyl promotes the reaction between crotyltrichlorosilane and benzaldehyde, the Barrett group screened chiral pyridine molecules as Lewis-base catalysts for this reaction [175]. The pyridinylox-azoline 164a was identified as the most efficient organocatalyst. In the presence of this catalyst, which was, however, used in stoichiometric amounts, asymmetric addition of (E)-crotyltrichlorosilane 158b to aldehydes gave the anti products (S,S)-159 in yields of 61-91% and with enantioselectivity from 36 to 74% ee (Scheme 6.76) [175], Diastereoselectivity is high, because only the anti diastereomers were obtained. Aromatic aldehydes and cinnamylaldehyde were used as substrates. 

In addition to organometaUic dendrimers for asymmetric catalysis, a number of dendritic organocatalysts have been developed and successfully applied in different asymmetric reactions in the absence of any metal [63]. In 2009, Parquette and coworkers [64] reported a new kind of folded pyridine-2,6-dicarboxamide dendrons (36) for direct aldol reaction. These dendritic prolinamide catalysts were tested for the aldol reactions of various acycUc and cydic ketones with 4-nitrobenzaldehyde (Figure 4.33). Remarkable dendritic effects on both the enantioselectivity and... 

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