Meso catalysts

Figure 7.2. The ML2 model with competition between homochiral and meso catalysts.

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. 

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The comonomer response (compare rac-catalysts 2 and 3 with the corresponding meso-catalysts 4 and 5 in Fig. 2) can be improved by using meso-isomers of C2 symmetric metallocenes [60]. In copolymerisation of ethene with 1-hexene the reactivity ratios of siloxy-substituted meso-isomers, catalysts 4 and 5, are comparable with those of Me2Si(2-Me-Benz(e)Ind)2ZrCl2 [52], which is considered to be a highly efficient copolymerisation catalyst. 

A simple calculation has been performed2 to express eeproducl as a function of the relative amount of meso catalyst present p and of its relative reactivity g ... 

An additional parameter K is needed to calculate the distribution of the three complexes. It is defined as the equilibrium constant of interconversion between the three complexes and is related to the relative amount ( J) of meso catalyst. The kinetic calculations give equation (18), where ees are defined as being < 1. 

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). 

As stated earlier, chemical reactors are characterized by multiple length (or time) scales and these disparate scales are typically characterized by three representative ones, namely, micro (molecular), meso (catalyst particle or tube diameter) and macro (reactor or process) scales. Figures 1-4 show examples of simple to complex chemical reactors, in each of which scale separation exists. 

The two enantiopure catalysts ML/ L/ and ML5L5 give identical reaction rates (rRR = rss) and reaction products of opposite enantioselectivities (ee0 and -ee0, respectively). A racemic product is formed from the meso catalyst ML Ls, which exhibits a reactivity of g relative to the enantiopure catalysts (g = kns/kRR). The enantioselectivity of the reaction product eepro<], will vary with ee in the following way... 

The meso catalyst precursor, on the other hand, generates a considerably poorer hydroformylation catalyst. The racemic catalyst is 22 times faster than the meso... 

Mesoporous (fine 3-5 nm in diameter) Pt DP n-Si, micro and meso, catalyst for fuel cells Brito-Neto et al. (2006), Hayase et al. (2004)... 

Fig. 13 Fe-Nx-C catalysts EESEM images of Fe-Nx-C SBA15 (a) and Fe-Nx-C meso (b) BET adsorbed volumes, included also the non-templated Fe—Nx-C catalyst (c), and XPS analysis of the Fe-Nx-C meso catalyst (d). Data from [71]...

The Fe-Nx C meso catalyst resulted more stable compared to the other Fe-Nx/ C catalysts home-made previously mentioned (Fig. 11). 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

One of the earliest useful methods for asymmetric opening of meso-epoxides with sulfur-centered nucleophiles was reported by Yamashita and Mukaiyama, who employed a heterogeneous zinc tartrate catalyst (Scheme 7.10) [20]. Epoxides other than cydohexene oxide were not investigated, and the enantioselectivity depended strongly on the identity of the thiol. 

Hou reported the use of a chiral (salen)titanium catalyst for the desymmetriza-tion of meso-epoxides with thiols (Scheme 7.14). The complex, fonned in situ... 

Although the enantioselective intermolecular addition of aliphatic alcohols to meso-epoxides with (salen)metal systems has not been reported, intramolecular asymmetric ring-opening of meso-epoxy alcohols has been demonstrated. By use of monomeric cobalt acetate catalyst 8, several complex cyclic and bicydic products can be accessed in highly enantioenriched form from the readily available meso-epoxy alcohols (Scheme 7.17) [32]. 

An impressive application of the (salen) Co-catalyzed intramolecular ARO of meso-epoxy alcohols in the context of total synthesis was reported recently by Danishefsky [33], Enantioselective desymmetrization of intermediate 9 by use of the cobalt acetate catalyst 8 at low temperatures afforded compound 10, which was obtained in 86% ee and >86% yield (Scheme 7.18). Straightforward manipulation of 10 eventually produced an intermediate that intersected Danishefsky s previ-... 

Jacobsen developed a method employing (pybox)YbCl3 for TMSCN addition to meso-epoxides (Scheme 7.22) [46] with enantioselectivities as high as 92%. Unfortunately, the practical utility of this method is limited because low temperatures must be maintained for very long reaction times (up to seven days). This reaction displayed a second-order dependence on catalyst concentration and a positive nonlinear effect, suggesting a cooperative bimetallic mechanism analogous to that proposed for (salen)Cr-catalyzed ARO reactions (Scheme 7.5). 

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