Dinuclear catalysts

Cai et al. (71) examined the use of dinuclear copper complexes as catalysts in the cyclopropanation reaction. Their ligand design, based on the success exhibited by the Aratani system, incorporates a diimine aryloxide. A comparison of the mononuclear catalyst 99 with the corresponding dinuclear catalyst 100 showed certain modest benefits conferred by the latter, Eq. 52. The authors note that these catalysts are effective at ambient temperature but isolated yields are higher at 50°C with no loss in enantioselectivity. 

A breakthrough was reported by Stack and co-workers in 1998 (212) who reported the first biomimetic catalytic system for the oxidation of primary alcohols by air. Independently, in the same year Chaudhuri, et al. (216) reported efficient aerobic oxidation of primary and secondary alcohols by the dinuclear catalyst [Cu2 2(Ls )2]C12 (216). Next, we will briefly review the salient features of these two systems. 

The low rate-enhancement brought about by the dinuclear complex in the reaction of 14 suggests a very modest formation, in the very dilute solution, of the productive intermediate 17 [SrOEt] [SrO2CR], a supramolecular complex composed of one molecule of ditopic ligand 17, two Sr ions, one EtO ion, and one substrate molecule. In contrast, the absence of any difference between mononuclear and dinuclear catalyst in the cleavage of 20 demonstrates that only one metal... 

The dinuclear catalyst 21-Ba2, in which the azacrown ether units are linked to vicinal positions of the calix 4 arcnc scaffold, is not only superior to its diagonal regioisomer 22-Ba2 in all cases, but is also superior to 17-Ba2 in the reactions of esters 14, 23, and 24. Modest levels of cooperation between metal ions are seen in the catalyzed reactions of the longest substrate 25, which indicates that dinuclear complexes cannot expand their intermetal distances to adapt to the long carboxylate-carbonyl distance in 25. 

Since the dinuclear catalysts transform the intermolecular reaction of ethoxide with substrate into an intramolecular reaction within a supramolecular complex (Scheme 5.3), the effective molarity (EM) parameter, defined as kintra/fcinten strictly applies to the catalytic process at hand and, more in general, to processes in which molecular receptors promote the reaction of two simultaneously complexed reactants [35]. 

A common theme in this catalysis has been the proposal of dinuclear catalyst active sites, often termed bimetallic catalysis. This section will review various dinuclear and dimeric complexes which have shown activity for copolymerization. 

Use. The oligomerization of olefins has generally been carried out with zero-valent transition metal complexes (mononuclear catalysts) and usually leads to an array of dienes see 1, 259). Schrauzer et at.1 of the Shell Development Co. reasoned that a dinuclear catalyst such as ZnfCo(CO).,J2 in which the two cobalt centers are connected close to each other will lead to new transition state formation from which different products can form. As a model, they examined the dimerization of nor-hornadiene and with the new catalyst obtained in almost quantitative yield a single dimer, m.p. 65-65.6°, shown unequivocably by elemental analysis (C14H16), infrared, nuclear magnetic resonance, and mass spectrometry to have the structure (2). 

The catalytic efficiencies of regioisomeric complexes 14-Ba2 and IS-Baa in the basic ethanolysis of esters 18-21 are compared in Table 8.1 with that of the dinuclear barium complex of the closely related ligand 16, as well as with that of the mononuclear control 17-Ba. Rate measurements refer to conditions where no less than 90% of the ester is bound to the catalyst in the productive (Michaelis) complex n (Scheme 8.4), while the concentration of the unproductive 2 1 complex HI is negligibly low. The superiority of dinuclear catalysts over the mononuclear counterpart demonstrates that the two metal ions work in a cooperative fashion, in accordance with the bifunctional catalytic mechanism in Scheme 8.4, but the extent of cooperation is strongly dependent on the catalyst-substrate combination. This is not really surprising, as the catalytic mechanism implies that the substrate and the catalyst must form a well matched pair in terms of size and geometrical features. A convenient comparison of catalytic performances of regioisomeric... 

Plots of EM vs. the carboxylate-carbonyl distances in the ester substrates, taken as a gross measure of their size, are shown in Figure 8.1. It is apparent that the dinuclear catalyst 14-Ba2, in which the azacrown units are linked to vicinal positions of the calix[4] arene scaffold, is not only far superior to its diagonal regioisomer 15-Ba2 in all cases, but it is also superior to 16-Ba2 in the reactions of esters 18-20. The EM profile shows that the catalytic efficiency of 14-Ba2 reaches its maximum value in the reaction of ester 19, and drops to a very low value in the reaction of the longest ester 21. This indicates that 14-Ba2 cannot expand its intermetal distance to fit the long carboxylate-carbonyl distance in 21. In conclusion, reactivity data obtained for the various catalyst-substrate combinations indeed show that a close fit of ester size to metal-to-metal distance is an important prerequisite for catalysis. However, the marked superiority of 14-Ba2 over... 

Examples of dinuclear catalysts for the re-Incorporatlon of olefinic polymers. 

Second, dinuclear complexes may exhibit enhanced reactivity. However, conclusions about the molecularity of the elementary steps based on the observed rate law are tenuous at best, and are likely wrong. Indeed, to date, we have not seen a CuAAC reaction that exhibits uniform second order in the catalyst as it progresses. It is possible that nuclearity of the catalytic species is maintained throughout the catalytic cycle and, as a consequence, all elementary steps are effectively bimolecular, exhibiting the commonly observed first order in the catalyst, even though the reaction is catalyzed by a dinuclear catalyst. 

On the other hand, a dinuclear chiral nickel catalyst, (R)-3, was developed by Mitsunuma and Matsunaga to promote the conjugate addition of a-sub-stituted p-keto esters to nitroethylene. " As shown in Scheme 2.6 the use of 1-10 mol% of this Schiff base dinuclear catalyst in a mixture of EtOAc and toluene as solvent allowed a range of cyclic as well as acyclic p-keto esters to be added to nitroethylene, providing the corresponding chiral Michael products in moderate to almost quantitative yields (73-92%), combined with good to high enantioselectivities of up to 98% ee in the case of cyclic substrates, and... 

The role of the ammonium cocatalyst in the aluminium-salen-catalysed carbonate synthesis was precisely investigated a couple years earlier by the group of North. This group achieved in 2007 a major development in aluminium-salen chemistry and its related carbon-dioxide chemistry with a new class of dinuclear aluminium salens produced via a controlled hydrolysis of the related reactive monomeric aluminium-salens, as shown in Scheme 18.39. This dinuclear catalyst used with tetrabutylammonium bromide as cocatalyst reached a very high efficiency in the formation of cyclic carbonates in some cases under very mild operating conditions (cat range 1 to 2.5 mol% recyclable more than 60 times) as exemplified with styrene oxide that afforded styrene carbonate in 86% yield after 24 h. 

The marked superiority shown by dinuclear catalysts over the mononuclear model 17—Ba confirms that the two metal ions act cooperatively. In the cleavage of 5, for example, the reaction rate in the presence of 0.1 mM 14 Ba2 is more than three orders of magnitude higher than the rate observed in the presence of 0.2 mM 17-Ba. 

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