Aldol transition states

Enzyme Catalyzed. The enzyme aldolases are the most important catalysts for catalyzing carbon-carbon bond formations in nature.248 A multienzyme system has also been developed for forming C-C bonds.249 Recently, an antibody was developed by Schultz and co-workers that can catalyze the retro-aldol reaction and Henry-type reactions.250 These results demonstrate that antibodies can stabilize the aldol transition state but point to the need for improved strategies for enolate formation under aqueous conditions. 

Scheme 7.15 Diketone aldol transition states to hypocrellin A...

One possible explanation for the diastereoface selection (AAGt -78°C 3 kcal/mol) observed for these chiral enolates is illustrated in Scheme 23. In the respective aldol transition states derived from conformers A and B leading to erythro diastereomers A and B, it may be assumed that developing imide resonance (118) will lock the chiral auxiliary in one of the in-plane conformations illustrated in products A or B. Based on an examination of models, it is projected that developing CHj R, allyhc strain steric interaction (37) disfavors that transition state leading to A. These steric considerations are largely attenuated in the transition state leading to the observed erythro adduct B. ... 

Allyl- and crotylborane aldol transition state Vulpetti 1993b... 

Allyl- and crotylboronate aldol transition state Gennari 1994 Allyl palladium complexes... 

The geometry of the aldol transition state was interpreted in terms of the variation in mode of the aldehyde/catalyst complexation [Felkin-Ahn (nonchelating) model versus chelating model] and size of KSA. The same complex efficiently catalyzes the Michael reaction of a,/9-unsaturated ketones with KSA [124],... 

Stereoselectivities of aldol additions catalysed by histidine have been shown to contrast with those for proline. ° Quanmm mechanical calculations suggest that the imidazolium and CO2H functionalities of histidine stabilize the cyclic aldolization transition state through hydrogen bonding and that stereoselectivity is a consequence of minimization of gauche interactions around the forming C-C bond. Extensive computations have been used to support rules that enable prediction of the outcome for asymmetric cross-aldol additions between enolizable aldehydes catalysed by histidine. ... 

Mukaiyama aldol reactions, whereby trimethylsilyl enol ethers react with aldehydes in aqueous solution to form -ketoalcohols, have been promoted by new chiral lanthanide-containing complexes and a chiral Fe(II)-bipyridine complex with 0 outstanding diastereo- and enantio-selectivities. Factors controlling the diastereoselec-tivity of Lewis-acid-catalysed Mukaiyama reactions have been studied using DFT to reveal the transition-state influences of substituents on the enol carbon, the a-carbon of the silyl ether, and the aldehyde. The relative steric effects of the Lewis acid and 0 trimethyl silyl groups and the influence of E/Z isomerism on the aldol transition state were explored. Catalytic asymmetric Mukaiyama aldol reaction of difluoroenoxysilanes with /-unsaturated a-ketoesters has been reported for the first time and studied extensively. ... 

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