Bimetallic hetero

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. 

The reduction steps on active Co sites are strongly affected by activated hydrogen transferred from promoter metal particles (Pt and Ru). Several indications for the existence and importance of hetero-bimetallic centers have been obtained.63 [Cp Co(CO)2] in the presence of PEt3 and Mel catalyzes the carbonylation of methanol with initial rates up to 44 mol L 1 h 1 before decaying to a second catalytic phase with rates of 3 mol L 1 h-1.64 HOAc-AcOMe mixtures were prepared by reaction of MeOH with CO in the presence of Co(II) acetate, iodine, and additional Pt or Pd salts, e.g., [(Ph3P)2PdCl2] at 120-80 °C and 160-250 atm.65... 

A) in Scheme l).9 Furthermore, asymmetric Diels-Alder reactions have been developed in LPDE solutions or using hetero-bimetallic catalysts containing lithium ions (compound... 

Reactions involving bimetallic catalysts, either homo-dinuclear or hetero-bimetallic complexes, and chemzymes were highlighted by Steinhagen and Helmchen96c in 1996. Some examples are discussed in Chapter 2. Among these examples, Shibasaki s reports have been of particular significance.97 Shibasaki s catalyst is illustrated as 130, which consists of one central metal M1 (La+3, Ba+2, or A1+3), three other metal ions (M2)+ [(M2)+ can be Li+, Na+, or K+], and three bidentated ligands, such as (R)- or (iS )-BINOL. The catalyst exhibits both Lewis acidic properties because of the existence of central metal and the Lewis basic properties because of the presence of the outer metal ions. 

LA represents Lewis acid in the catalyst, and M represents Bren sled base. In Scheme 8-49, Bronsted base functionality in the hetero-bimetalic chiral catalyst I can deprotonate a ketone to produce the corresponding enolate II, while at the same time the Lewis acid functionality activates an aldehyde to give intermediate III. Intramolecular aldol reaction then proceeds in a chelation-controlled manner to give //-keto metal alkoxide IV. Proton exchange between the metal alkoxide moiety and an aromatic hydroxy proton or an a-proton of a ketone leads to the production of an optically active aldol product and the regeneration of the catalyst I, thus finishing the catalytic cycle. 

Table 10. Catalytic enantioselective epoxide ring opening with 4-methoxyphenol 105 promoted by gallium hetero-bimetallic complexes in the presence of MS 4A.

T. Iida, N. Yamamoto, N. Matsunaga, H.-G. Woo, M. Shibasaki, Enantioselective Ring Opening of Epoxides with 4-Methoxyphenol Catalyzed by Gallium Hetero-bimetallic Complexes An Efficient Method for the Synthesis of Optically Active 1,2-Diol Monoethers, Angew. Chem Int. Ed. EngL 1998,32 2223-2226. 

The use of hetero-metallic (MM )carbonyl complexes as precursors can lead to the preparation of supported catalysts having weU-defined bimetallic entities in which the intimate contact between M and M remains in the final catalyst and the atomic ratio M/M of the aggregates is that of the bimetallic carbonyl precursor used. This is illustrated in Figure 8.1, in which the definite interaction of the MjM (CO) complex with the functional group (F) of a surface (S) produces a new anchored surface species. This new surface species could evolve with an appropriate treatment producing tailored bimetallic particles. 

While today the construction of such bidentate phosphine ligands with the use of an assembly metal is referred to as supramolecular chemistry, this is not to say that there are no examples in older literature utilizing this principle. Actually, there are many and using as a search term hetero bimetallic complexes (a named coined in the early 1980s) leads us to a plethora of complexes. For instance in complex 29, reported by Rauchfuss in 1982, one would consider copper as the assembly metal and platinum as the potential catalytic metal [70]. 

The catalytic asymmetric nitroaldol reaction was extended to a direct catalytic asymmetric nitro-Mannich-type reaction promoted by hetero-bimetallic catalysts (Scheme 2) [53-55] or by EtjNBOX-Cu complexes [56]. These topics are reviewed in Chap. 28.2. 

Partial quantum yields3 of Nd111 and quantum yields of Yb111 luminescence upon ligand excitation (300 nm) in homo-and hetero-bimetallic complexes with p>-fert-butylcalix[8]arene in dmf, at room temperature (Korovin et al., 2000)... 

Shibasaki and coworkers have conducted extensive research on the use of hetero-bimetallic complexes as catalysts for asymmetric synthesis [11]. The reactions are catalyzed by heterobimetallic complexes that function as both a Lewis acid and a Bronsted base. Among these, LaLi3tris(binaphthoxide) catalyst 1 (LLB) was proven to be an effective catalyst in direct asymmetric aldol reactions (Fig. 1) [12]. On the basis of this research, Shibasaki et al. reported the first report of a direct catalytic asymmetric Mannich reaction [13],... 

Attempts to structurally characterize the generated Ln(III)/Al hetero-bimetallic complexes were not successful. Addition of coordinating solvents, such as THF or pyridine, afforded complex mixtures from which only lanthanide(III) chloride donor-adducts crystallized. The presence of reactive ethyl groups was confirmed by the reaction with D20 (generating CH3CH2D) [134], The catalytic relevance of such heterobimetallic complexes was confirmed by the quantitative conversion of a 7500-fold excess of iso-prene into polyisoprene within 5-10 minutes at ambient temperature, after the addition of one equivalent of i-Bu3Al cocatalyst. 

Scheme 17 Reactivity of [(CsMes Sm -ChCPhlh toward triethylaluminum. Two hetero-bimetallic Sm(III)/Al complexes were isolated via fractionated crystallization [116]...

It should be mentioned that S. Cesca and coworkers reported a hetero-bimetallic Ln/Al complex from a slow reaction of Ce(Oz -Pr)4(HOz-Pr) with triethylaluminum as early as 1977 [176]. The crystalline cerium(III) complex (/78-C8H8)Ce(/r-Oz-Pr)2AlEt2 was generated in the presence of cyclooctate-traene in low yields (30%). It was characterized by means of mass spectrometry, alcoholysis, deuterolysis, IR and NMR spectroscopy. 

Compared to the extensive use of phenolate ligands and the isolation and structural characterization of a broad variety of differently alkylated hetero-bimetallic Ln(III)/Al complexes, only two examples of anilides AIR3 adduct formation and [anilide] [alkyl] exchange are reported in the literature. There exist no reports on the use of simple anilide ligands in diene polymerization. 

Fig. 11. Molecular-orbital diagram for the [(NH3)4PtJI-PdII(NH3)H2]2+ model (4 3 hetero-bimetallic species). <7-Bonding MO is part of 7 (omitted for clarity) +1 filled d orbitals.

Bimetallic clusters of gold were reviewed by Braunstein and Rose (6) in 1985, and Hall and Mingos (7) surveyed both homonuclear and hetero-nuclear gold clusters in the previous year. In addition, Steggerda et al. (8) reviewed the preparation and properties of gold clusters in 1982, Jones (9) has listed gold clusters studied by single-crystal X-ray diffraction up to mid... 

The following sub-sections describe the preparation of hetero-bimetallic complexes, and are grouped in terms of the earliest metal present in the complex. 

Another early-late bimetallic complex related to the (71) group can be prepared by connecting the two Cp rings by a carbon bridge. Its reaction with an excess of AlMes affords a doubly bridged hetero bimetallic complex (equation 34). ... 

Chiral C2-symmetric early-late complexes can be obtained via similar complexation starting from bis phosphinoenolato zirconocene. Assembly of phenyl-substituted phospha-metallocene and [(binap)Rh(cod)] + leads also to a chiral hetero dinuclear complex. The unprecedented thermal room temperature isomerization ofZr derivative was exploited to achieve its dynamic resolution producing enantiopure bimetallic ansa metallocene. ... 

Bi, W.-Y.,Lii, X.-Q., Chai, W.-L., etal. (2008) Construction and NIR luminescent property of hetero-bimetallic Zn—Nd complexes from two chiral salen-type Schiff-base Ugands. Journal of Molecular Structure, 891, 450. 

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