Meso catalysts asymmetric amplification

Blackmond pointed out that asymmetric amplification always has, as a consequence, a decrease in reactivity when compared to the enantiopure catalyst. This can be calculated on the various models proposed for the interpretation of nonlinear effects. It is qualitatively visible in the reservoir model above as well as in the ML2 model, where the asymmetric amplification given by g < 1 (low reactivity of the meso catalyst) has as consequence the overall slowdown in reaction rate. The generalized model ML has been discussed (for n = 2,3,4) when the various species are in equilibrium. The complexity of the curve can increase sharply as soon as n > 2. 

In order to achieve an amplification of chirality, it requires that/> 1. If P = 0 (no meso catalyst) or g = 1 (same reactivity of meso and homochiral catalysts), then/= 1. The condition/> 1 is achieved for 1 + p > 1 + g ), or g < 1. Thus the necessary condition for asymmetric amplification in the above model is for the heterochiral or meso catalyst to be less reactive than the homochiral catalyst. If the meso catalyst is more reactive, then/< 1, and hence a negative nonlinear effect is observed. The size of the asymmetric amplification is regulated by the value off, which increases as K does. The more meso catalyst (of the lowest possible reactivity) there is, the higher will be eeproduct. This is well illustrated by computed curves in Scheme 11. The variation of eeproduct with eeaux is represented for various values of g (the relative reactivity of the meso complex) with K = 4 (corresponding to a statistical distribution of ligands Scheme 11, top). The variation in the relative amounts of the three complexes with eeaux is also represented for a statistical distribution of ligands (Scheme 11, bottom). 

The asymmetric amplification is a consequence of an in situ increase in the ee of the active catalyst, since racemic ligand is trapped in the unreactive or weakly reactive meso catalyst. In the reservoir effect a similar phenomenon occurs outside the catalytic cycle. Let us assume that part of the initial chiral ligand, characterized by eeaux, is diverted into a set of catalytically inactive complexes (Scheme 12). 

The presence of a nonlinear effect, either negative or positive, is a useful piece of information for the mechanistic study of a reaction. It implies that diastereomeric species are formed from the chiral auxiliary. If an asymmetric amplification is observed, it can be indicative of the formation of meso dimers (or tetramers etc.) of low reactivity. When the kinetic study of an asymmetric catalysis shows a rate second order with respect to catalyst concentration, it may be useful to investigate the possibility of nonlinear effects in the system. Jacobsen et al., for example, studied the... 

In the enantioselective addition of diethylzinc to aldehydes catalyzed by nonracemic amino alcohols, the phenomenon named asymmetric amplification has been well recognized as a consequence of an in situ increase in the enantiopurity of the active catalyst, as the racemic ligand is trapped in the more stable, unreactive meso species [3, 11]. Although the reaction will definitely give racemic product if only racemic Hgands are used alone, the addition of an easily accessible alternative... 

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