Cooperative bimetallic

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). 

In contrast, Cozzi and Umani-Ronchi found the (salen)Cr-Cl complex 2 to be very effective for the desymmetrization of meso-slilbene oxide with use of substituted indoles as nucleophiles (Scheme 7.25) [49]. The reaction is high-yielding, highly enantioselective, and takes place exclusively at sp2-hybridized C3, independently of the indole substitution pattern at positions 1 and 2. The successful use of N-alkyl substrates (Scheme 7.25, entries 2 and 4) suggests that nucleophile activation does not occur in this reaction, in stark contrast with the highly enantioselective cooperative bimetallic mechanism of the (salen)Cr-Cl-catalyzed asymmetric azidolysis reaction (Scheme 7.5). However, no kinetic studies on this reaction were reported. 

The HKR reactions follow the cooperative bimetallic catalysis where epoxide and nucleophile activate simultaneously by two different (salen)Co-AlX3 catalyst molecules. The linking of two (salen)Co unit through the A1 induces the cooperative mechanism, albeit through a far less enantio-discriminating transition state than that attained with the catalyst la and la (Scheme2). 

Fig. 32 Proposed cooperative bimetallic intramolecular mechanism for the enantioselective Michael addition of a-cyanoesters 57 to vinylketones...

A very successful example for the use of dendritic polymeric supports in asymmetric synthesis was recently described by Breinbauer and Jacobsen [76]. PA-MAM-dendrimers with [Co(salen)]complexes were used for the hydrolytic kinetic resolution (HKR) of terminal epoxides. For such asymmetric ring opening reactions catalyzed by [Co(salen)]complexes, the proposed mechanism involves cooperative, bimetallic catalysis. For the study of this hypothesis, PAMAM dendrimers of different generation [G1-G3] were derivatized with a covalent salen Hgand through an amide bond (Fig. 7.22). The separation was achieved by precipitation and SEC. The catalytically active [Co "(salen)]dendrimer was subsequently obtained by quantitative oxidation with elemental iodine (Fig. 7.22). 

The mechanism of the Jacobsen HKR and ARO are analogous. There is a second order dependence on the catalyst and a cooperative bimetallic mechanism is most likely. Both epoxide enantiomers bind to the catalyst equally well so the enantioselectivity depends on the selective reaction of one of the epoxide complexes. The active species is the Co(lll)salen-OH complex, which is generated from a complex where L OH. The enantioselectivity is counterion dependent when L is only weakly nucleophilic, the resolution proceeds with very high levels of enantioselectivity. 

Some homogeneous solutions of mixed hydroxo clusters of appropriate combinations of two metal ions have been prepared (by avoiding the precipitation of polymeric aggregates of metal-hydroxo species) (349-351). The two combined metals, in a 1 1 mixed cluster, were located close to each other and were shown to participate in a cooperative bimetallic mechanism of phosphate hydrolysis. The metal... 

Scheme 2.26 Catalytic asymmetric CDC reaction by a cooperative bimetallic catalyst system reported by Feng.

To demonstrate that a cooperative bimetallic catalysis is operating, such fine kinetics and in situ analyses are essential to discriminate a bimetallic mechanism from a monometallic one where the second metallic species is poorly efficient or just a spectator. 

It has been shown that a number of active sites of enzymes contain two metal ions friat give high activity via a cooperative bimetallic mechanism... 

Replacement of 10 mol% cadmium chloride catalyst with complex 8 resulted in an increase in the reaction yield and rate, highlighting the crucial roles of ligand configuration and the cooperative bimetallic catalytic effect as operative for 8. With the optimal conditions already established, the reaction scope was also investigated, by the synthesis of a series of C-N coupling products in good to excellent yields (Table 17.1, entries 1-5). 

Highly active oligomeric (salen)Co complexes such as (198) were designed for asymmetric hydrolysis of meso-epoxides and kinetic resolution of terminal epoxides, based on cooperative bimetallic mechanism postulated for epoxide ring-opening reactions (Scheme 16.59) [83, 84]. 

Heman-Gomez A, Bradley TD, Kennedy AR, Livingstone Z, Robertson SD, Hevia E. Developing catalytic applications of cooperative bimetallics competitive hydroamination/trimerization reactions of isocyanates catalysed by sodium magnesiates. Chem Commun. 2013 49(77) 8659—8661. 

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