Selective Coordination with Carbonyl Oxygen

The role of the catecholate group and fluoride is to delocalize negative charge and increase the Lewis acidity of the silicon center, which coordinates a carbonyl oxygen to form a hexaeoordinate silicate. The six-membered cyclic transition state in the chair conformation is consistent with high threo and erythro selectivity similar to that of allyl boronates [91]. It is interesting to see the structure-reactivity and structure-selectivity correlation shown in Sch. 53 [92]. 

With a-alkyl-substituted chiral carbonyl compounds bearing an alkoxy group in the -position, the diastereoselectivity of nucleophilic addition reactions is influenced not only by steric factors, which can be described by the models of Cram and Felkin (see Section 1.3.1.1.), but also by a possible coordination of the nucleophile counterion with the /J-oxygen atom. Thus, coordination of the metal cation with the carbonyl oxygen and the /J-alkoxy substituent leads to a chelated transition state 1 which implies attack of the nucleophile from the least hindered side, opposite to the pseudoequatorial substituent R1. Therefore, the anb-diastereomer 2 should be formed in excess. With respect to the stereogenic center in the a-position, the predominant formation of the anft-diastereomer means that anti-Cram selectivity has occurred. 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

A study518 of the mechanism of oxidation of alcohols by the reagent suggested that a reversible, oriented adsorption of the alcohol onto the surface of the oxidant occurs, with the oxygen atom of the alcohol forming a coordinate bond to a silver ion, followed by a concerted, irreversible, homolytic shift of electrons to generate silver atoms, carbon dioxide, water, and the carbonyl compound. The reactivity of a polyhydroxy compound may not, it appears, be deduced from the relative reactivity of its component functions, as the geometry of the adsorbed state, itself affected by solvent polarity, exerts an important influence on the selectivity observed.519... 

Uemura demonstrated that enantiomerically pure (arylaldehyde)tricarbonyl-chromium complexes afford exclusively the //iraj-pinacols, providing an asymmetric synthesis of hydrobenzoins (Scheme 5.4).13 An intermediate involving coordination of the Sm(III) metal centre with the carbonyl oxygen was proposed to account for the high selectivity observed. This was supported by coupling experiments in the presence of HMPA, an additive that is known to prevent complexation, which led to the preferential formation of the erythro product. 

Figure 19. Ca(polyP) solvated by PHB. A. Drawing depicting the solvation of Ca(polyP) by PHB in the bilayer forming a Ca2+-selective channel.13 B. Drawing of the cross section of PHB/polyP channel. C. Putative coordination geometry of Ca2+ in PHB/polyP. Ca2+ forms ionic bonds with four phosphoryl oxygens of polyP and ion-dipole bonds with four ester carbonyl oxygens of PHB to form a neutral complex with distorted cubic geometry.25...

The acetal is probably formed by hydride transfer to an intermediate ester. The terf-butyl group apparently stabilizes the second intermediate and consequently changes the course of the reaction. It should be noted that the first cyclic intermediate is stabilized by coordination of the borane with the oxygen of the carbonyl. The results are outstanding when = Me (92-98 %), although selectivity and yield (45-62 %) decrease when R = H (Eq. 48). 

It has been reported that several transition metal complexes catalyze the hetero-Diels-Alder reaction between a variety of aldehydes, in particular benzaldehyde, and Danishefsky s diene (Sch. 52). With the [CpRu(CHIRAPHOS)] complex the ee is modest (25 %) (entry 1) [192]. The chiral complex VO(HFBC)2 performs better in this reaction (entry 2) [193]. In experiments directed towards the synthesis of anthra-cyclones, this complex was used in cycloadditions between anthraquinone aldehydes with silyloxy dienes. One example is shown in Sch. 53 [194]. Compared with the chiral aluminum catalyst developed earlier by Yamamoto and co-workers [195], the vanadium catalyst results in lower enantioselectivity but has advantages such as ease of preparation, high solubility, stability towards air and moisture, and selective binding to an aldehyde carbonyl oxygen in the presence of others Lewis-basic coordination sites on the substrate. 

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