Cooperative bimetallic mechanism

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 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... 

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... 

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]. 

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). 

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. 

Synthetic binuclear complexes as catalysts, aiming at high activity, have been subjects of intensive research [3], However, the number of novel bimetallic catalysts that are active and actually operating via a bimetallic mechanism is extremely scarce. Stanley and co-workers reported a bimetallic rhodium complex that is an active and selective hydroformylation catalyst that was proposed to operate via a cooperative mechanism. 

Even more efficient bimetallic cooperativity was achieved by the dinuclear complex 36 [53]. It was demonstrated to cleave 2, 3 -cAMP (298 K) and ApA (323 K) with high efficiency at pH 6, which results in 300-500-fold rate increase compared to the mononuclear complex Cu(II)-[9]aneN at pH 7.3. The pH-metric study showed two overlapped deprotonations of the metal-bound water molecules near pH 6. The observed bell-shaped pH-rate profiles indicate that the monohydroxy form is the active species. The proposed mechanism for both 2, 3 -cAMP and ApA hydrolysis consists of a double Lewis-acid activation of the substrates, while the metal-bound hydroxide acts as general base for activating the nucleophilic 2 -OH group in the case of ApA (36a). Based on the 1000-fold higher activity of the dinuclear complex toward 2, 3 -cAMP, the authors suggest nucleophilic catalysis of the Cu(II)-OH unit in 36b. The latter mechanism is comparable to those of protein phosphatase 1 and fructose 1,6-diphosphatase. 

Even more interesting is the observed regioselectivity of 37 its reaction with 2, 3 -cCMP and 2, 3 -cUMP resulted in formation of more than 90% of 2 -phosphate (3 -OH) isomer. The postulated mechanisms for 37 consists of a double Lewis-acid activation, while the metal-bound hydroxide and water act as nucleophilic catalyst and general acid, respectively (see 39). The substrate-ligand interaction probably favors only one of the depicted substrate orientations, which may be responsible for the observed regioselectivity. Complex 38 may operate in a similar way but with single Lewis-acid activation, which would explain the lower bimetallic cooperativity and the lack of regioselectivity. Both proposed mechanisms show similarities to that of the native phospho-monoesterases (37 protein phosphatase 1 and fructose 1,6-diphosphatase, 38 purple acid phosphatase). 

The authors presume that transient vinylidene intermediates are involved in the observed reactions. This hypothesis is consistent with the observed (Z)-selectivity of dimerization and the unusual facility with which complexes of type 58 form isolable vinylidenes. Intermetallic cooperation in bimetallic complexes has been recognized to facilitate vinylidene-mediated processes [23] however, the operative mechanism in the present case remains unknown. 

This led Stanley to propose the bimetallic cooperativity mechanism shown in Scheme 16. The key bimetallic cooperativity step involves rotation about the central bridging methylene group to give a H /CO double-bridged intermediate (or transition state) species, which directly leads to an... 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols.74,75 Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst76 with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence, the mechanism was one of cooperation between the Ag° and Pd° sites, the alloy phase, detected by EXAFS, was considered not to play an important role. 

The modification of platinum catalysts by the presence of ad-layers of a less noble metal such as ruthenium has been studied before [15-28]. A cooperative mechanism of the platinurmruthenium bimetallic system that causes the surface catalytic process between the two types of active species has been demonstrated [18], This system has attracted interest because it is regarded as a model for the platinurmruthenium alloy catalysts in fuel cell technology. Numerous studies on the methanol oxidation of ruthenium-decorated single crystals have reported that the Pt(l 11)/Ru surface shows the highest activity among all platinurmruthenium surfaces [21-26]. The development of carbon-supported electrocatalysts for direct methanol fuel cells (DMFC) indicates that the reactivity for methanol oxidation depends on the amount of the noble metal in the carbon-supported catalyst. 

The overall efficiency of these nonmolecular bimetallic sysems to promote phosphate ester hydrolysis was illustrated with ApA as RNA model (0.1 mM) hydrolysis occurred with a mixture of LaCls and FeCls (10 mM each) to more than 70% at 50°C and neutral pH in 5 min (351). The products of the reaction included adenosine and 2 -and 3 -monophosphate adenonsine, indicating that the mechanism involved a first trans-esterification step by the vicinal 2 -OH of ribose. With the same mixture of metal ions, the DNA model TpT (0.1 mM) (Fig. 18), left, B = thymine) was converted to thymidine and 3 - and 5 -monophosphate th5miidine with 36% of conversion in 24 hr at 70°C at neutral pH. The important point is the notion of cooperativity between the two metals. 

In 1961 Heck proposed what is now generally considered to be the correct monometallic mechanism for [HCo(CO)4]-catalyzed hydroformylation [10]. He also proposed, but did not favor, a bimetallic pathway involving an intermolecular hydride transfer between [HCo(CO)4] and [Co(acyl)(CO)4] to eliminate aldehyde product (Scheme 2). Most proposals concerning polymetallic cooperativity in hydroformylation have, therefore, centered on the use of inter- or intramolecular hydride transfers to accelerate the elimination of aldehyde product. Bergman, Halpem, Norton, and Marko have all performed elegant stoichiometric mechanistic studies demonstrating that intermolecular hydride transfers can indeed take place between metal-hydride and metal-acyl species to eliminate aldehyde products [11-14]. The monometallic [HCo(CO)4] pathway involving reaction of the acyl intermediate with H2, however, has been repeatedly shown to be the dominant catalytic mechanism for 1-aUcenes and cyclohexane [15, 16]. 

The proposed bimetallic hydroformylation mechanism for 15r is shown in Fig. 13 and is based on our DFT calculations. In many ways the mechanistic steps parallel those for the dicationic catalyst, and bimetallic cooperativity once... 

Abstract Bimetallic catalysts are capable of activating alkynes to undergo a diverse array of reactions. The unique electronic structure of alkynes enables them to coordinate to two metals in a variety of different arrangements. A number of well-characterised bimetallic complexes have been discovered that utilise the versatile coordination modes of alkynes to enhance the rate of a bimetallic catalysed process. Yet, for many other bimetallic catalyst systems, which have achieved incredible improvements to a reactions rate and selectivity, the mechanism of alkyne activation remains unknown. This chapter summarises the many different approaches that bimetallic catalysts may be utilised to achieve cooperative activation of the alkyne triple bond. 

This review is not intended to be fully comprehensive but instead should serve to highlight current understanding of bimetallic cooperative catalysis as it applies to the activation of the alkyne triple bond. We have divided the review into four sections, separated by reaction type, which emphasise different aspects of the bimetallic alkyne activation mechanism. These four sections are as follows ... 

In the breakdown of the intermediate, the water bound to Ce(IV) functions as acid catalyst. With this catalysis, the alkoxide ion of 5 -OH can be promptly removed from the phosphorus atom. Otherwise, the leaving group is poor and hard to be removed. Consistently, the DNA hydrolysis by Ce(IV) is accompanied by a notable D2O solvent isotope effect ( 20/ 20 = 2.2-2.4). The proton transfer is, at least partially, rate limiting. The large coordination-number of the Ce(IV) ions in the bimetallic cluster is favorable for this acid catalysis, exactly as described above for the first step of DNA hydrolysis (formation of the pentacoordinated intermediate). This mechanism is proposed on the basis of the kinetic analysis in acidic solutions. When DNA hydrolysis is carried out at around pH 7, similar acid-base cooperation by Ce(IV)-bound water molecule and the corresponding hydroxide should occur in higher aggregates of [Ce 2(OH)4] +. 

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