Bimetallic enolate complex

C02-Bridged bimetallic zirconocene complexes have been formed from 1 and metallocarboxylic acids [229]. Reachon of 1 with metal enolates Cp(CO)3WCHR COX (X = OEt, Me, Ph) gives Cp(CO)3WCH(R )CH(R)OZrCp2(Cl). The structure for R = H and R = Me was solved by an X-ray analysis and the chemical reactivity of these organometallic products have been studied [230]. 

Initiation of MMA polymerization by complexes such as (192) was shown to proceed via a bimetallic bis(enolate) intermediate, arising from the dimerization of a radical anion.478" 80 Such a mechanism481,482 explains why efficiencies with such initiators (calculated from polymer molecular weights) are always <50%. Using a similar methodology, the bimetallic bisallyl complex (198) was shown to polymerize MMA in a living fashion (Mw/Mn 1.1) and triblock copolymers with methacrylate and acrylate segments have been prepared. 

Figure 4.2 Bimetallic chelate-bridged ester enolate complex.

SCHEME 23.8 Generation of an active bimetallic initiator system for the polymerization of MMA from the (poorly active) cationic enolate complex 26. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

This finding is also in agreement with another three-component Michael/aldol addition reaction reported by Shibasaki and coworkers [14]. Here, as a catalyst the chiral AlLibis[(S)-binaphthoxide] complex (ALB) (2-37) was used. Such hetero-bimetallic compounds show both Bronsted basicity and Lewis acidity, and can catalyze aldol [15] and Michael/aldol [14, 16] processes. Reaction of cyclopentenone 2-29b, aldehyde 2-35, and dibenzyl methylmalonate (2-36) at r.t. in the presence of 5 mol% of 2-37 led to 3-hydroxy ketones 2-38 as a mixture of diastereomers in 84% yield. Transformation of 2-38 by a mesylation/elimination sequence afforded 2-39 with 92 % ee recrystallization gave enantiopure 2-39, which was used in the synthesis of ll-deoxy-PGFla (2-40) (Scheme 2.8). The transition states 2-41 and 2-42 illustrate the stereochemical result (Scheme 2.9). The coordination of the enone to the aluminum not only results in its activation, but also fixes its position for the Michael addition, as demonstrated in TS-2-41. It is of importance that the following aldol reaction of 2-42 is faster than a protonation of the enolate moiety. 

Divalent organolanthanide complexes can also initiate MMA polymerization. A divalent lanthanide complex, as a single-electron transfer reagent, can readily react with the monomer to generate a radical anion species, which subsequently couples into a bimetallic trivalent lanthanide enolate intermediate, which is the active center. Therefore, divalent organolanthanide complexes serve as bisinitiators for MMA polymerization [160]. 

Asymmetric addition of an oxindole enolate to nitroalkenes has been observed by Matsunaga, Shibasaki, and coworkers [25]. As illustrated in Scheme 9, chiral oxindole 35 was prepared with good diastereoselective and enantioselective control (30 1 dr, 97% ee) upon treatment of oxindole 32 with nitroalkene 33 in the presence of the bimetallic (Mn2) Schiff Base complex 34. Control experiments with various heterobimetallic complexes (Cu/Mn or Pd/Mn with organic catalyst 34) and a mononuclear complex of Mn with organic catalyst 34 led to decreased selectivity, highlighting the importance of the homodinuclear Mn2-34 complex for promoting the stereoselective transformation. 

Novak and Boffa, in studying lanthanide complexes, observed an unusual facile organometaUic electron transfer process takes place that generates in situ bimetallic lanthanide(lll) initiators for polymerizations of methacrylates [239]. They concluded that methyl methacrylate polymerizations initiated by the Cp 2Sm complexes occur through reductive dimerizations of methyl methacrylate molecules to form bisinitiators that consists of two samarium(lll) enolates joined through their double bond terminally [239]. Their conclusion is based on the tendency of Cp 2Sm complexes to reductively couple unsaturated molecules ... 

Trost has described very efficient and versatile bimetallic zinc catalyst 10 generated in situ from diethyl zinc and a chiral Ugand derived from proline and p-cresol (Scheme 10) [46], For example, this complex can promote catalytic aldol reactions with high enantiomeric excess. The role of two proximal zinc species is for one of them to form the enolate and for the second one to function as a Lewis acid to activate the aldehyde. 

Probably a bimetallic complex 181 is formed with the bidentate ligand quinine (or quinidine, respectively) coordinating to the lithium enolate (Figure 5.2.8). The incorporation of a second metal ion M (LT, Al , Mg into the complex should stabilize this complex by forming a very rigid structure, in which one face of the enolate is shielded by the bicyclic substructure of the quinine. This would explain the high ee s obtained with this system. Obviously, the alcoholates are not involved in enolate formation direcdy, but they have an influence on the reaction via the bimetallic complex. The similar ee values obtained might be explained by similar ion radii of the metal ions (li 0.60 A, Mg 0.65 A, Al 0.51 A). 

Scheme 2.40 Enantioselective conjugate addition catalyzed by bimetallic complex 135 and consecutive aldol addition of aluminum enolate 136.

Various examples of both C- and O-bound tautomers were described for enolates of palladium, and the corresponding complexes were characterized. In addition, both bonding modes were postulated as intermediate palladium enolates in catalyses. For a palladium(II) enolate of acetophenone, the monomeric C-bound form 40, and the dimer 41 were characterized by crystal structure analyses. In both complexes, palladium is tetra-coordinated in a distorted square planar arrangement [75aj. Similarly, palladium enolates derived from acetic esters were isolated and characterized under the form of monomeric enolate 42 as well as bimetallic complex 43 [75b]. The dimeric complexes 41 and 43 feature the structural motif of an C,0-bridging enolate moiety (Scheme 3.14). 

---