Zinc alkoxide complex

A series of zinc alkoxide complexes were characterized of the form [RZnOR ]ra where n 2 or 4 (Figure 4). Complexes of the form [Zn3 0(2,6-i-Pr2C6H3) 4R2] were produced by stoichiometry... 

Recently, N,N,0-tridentate Schiff-based zinc alkoxide complexes 52a-53e have been developed by our group [76]. All complexes efficiently initiate the polymerization of L-lactide at 25 °C with >90% conversion within 30-240 min, with only one exception, 52c which is inactive. The polymerization was well-controlled (PDI = 1.04—1.09) and showed that the reactivity decreases with an electron-withdrawing... 

Deprotonation of the zinc alcohol complexes shown in Fig. 12 to produce zinc alkoxide species has not been reported. Instead, mononuclear, tetrahedral zinc alkoxide complexes, supported by hydrotris(pyrazolyl)borate ligands, ([TpBut,Me or Tpph,Me, Scheme 8), have been generated via treatment of zinc hydride precursor complexes with aliphatic alcohols.68-70 A zinc ethoxide complex, [TpBut,Me]Zn-OEt, was also prepared via decarboxylation of the ethyl carbonate complex, [TpBut,Me]Zn-0C(0)0Et.49 X-ray crystallographic studies of [Tpph Me] Zn-OCH3 and [TpBut,Me]Zn-OEt revealed Zn-O bond lengths of 1.874(2) and 1.826(2) A, respectively.68,71 These bond distances are 0.1 A shorter than found for the alcohol complexes shown in Fig. 12. 

Reaction of the hydrotris(pyrazolyl)borate zinc alkoxide complex [TpCum,Me]Zn-OiPr with the NAD + mimic A-benzylnicotinamide chloride in isopropanol yielded a... 

Scheme 10 Hydride transfer reactivity of zinc alkoxide complexes.

But Cheng et al. produced some bulky p-diiminate zinc alkoxide complexes, 2, for the use in this reaction [5]. Among different complexes they checked in this process only methoxy and isopropoxy of some P-diimine counterparts, showed acceptable catalytic activity (Table 7.1, entry 1-7). These active catalysts produced atactic polymer with over 94% carbonate linkage and narrow molecular weight distribution. The lower activity of others was attributed to the lower steric hindrance and, therefore, higher stability of dimer complexes that prevented equilibration to the monomeric structures which was necessary to initiation of the polymerization. 

Chamberlain et al. [13] prepared a series of zinc alkoxide complexes that act as single-site living initiators for the polymerization of DLA or LLA to isotactic PLA, rac-LA to atactic PLA, and mcra-LA to syndiotactic PLA. The synthesis of chiral aluminum and yttrium alkoxides as stereospecific catalysts used for LA polymerization has also been reported by Ovitt and Coates [14]. 

Understanding the importance of the zinc alkoxide, the iodomethylzinc iodide and the zinc sulfonamide allowed Denmark to propose a revised transition state structure xv (Fig. 3.22) [82]. In this picture, the complex, polymetallic aggregate invoked by Rickborn and later by Kobayashi is featured. 

The next step in the calculations involves consideration of the allylic alcohol-carbe-noid complexes (Fig. 3.28). The simple alkoxide is represented by RT3. Coordination of this zinc alkoxide with any number of other molecules can be envisioned. The complexation of ZnCl2 to the oxygen of the alkoxide yields RT4. Due to the Lewis acidic nature of the zinc atom, dimerization of the zinc alkoxide cannot be ruled out. Hence, a simplified dimeric structure is represented in RTS. The remaining structures, RT6 and RT7 (Fig. 3.29), represent alternative zinc chloride complexes of RT3 differing from RT4. Analysis of the energetics of the cyclopropanation from each of these encounter complexes should yield information regarding the structure of the methylene transfer transition state. 

The activation energy for the favored transition state TS4 (22.8 kcal mol ) is still somewhat high. Still, the qualitative predictions of enhanced reactivity of the zinc alkoxide-zinc chloride complexes are in full agreement with contemporary ideas about this reaction and represent a major advance in the theoretical understanding of the cyclopropanation process. 

We came up with the idea of using a dummy ligand, as shown in Scheme 1.23 [34]. Reaction of dimethylzinc with our chiral modifier (amino-alcohol) 46 provided the methylzinc complex 62, which was subsequently reacted with 1 equiv of MeOH, to form chiral zinc alkoxide 63, generating a total of 2 moles of methane. Addition of lithium acetylide to 63 would generate an ate complex 64. The ate complex 64 should exist in equilibrium with the monomeric zincate 65 and the dimer 66. However, we expected that the monomer ate complex 64 and the mono-... 

Zinc alkoxide and aryloxide complexes have been of particular interest as enzyme models and catalysts. Tetrameric alkyl zinc alkoxides are a common structurally characterized motif.81... 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

Kimura and co-workers have synthesized a series of alkoxide complexes with the alcohol functionality as a pendent arm.447 674 737 A zinc complex of l-(4-bromophenacyl)-l, 4,7,10-tetraaza-cyclododecane was also synthesized by the same workers to mimic the active site of class II aldolases. The X-ray structure shows a six-coordinate zinc center with five donors from the ligand and a water molecule bound. The ketone is bound with a Zn—O distance of 2.159(3) A (Figure 12). Potentiometric titration indicated formation of a mixture of the hydroxide and the enolate. Enolate formation was also independently carried out by reaction with sodium methoxide, allowing full characterization.738... 

Figure 5-14 illustrates the transition state in the reaction. The free hydroxyl group is necessary for producing an effective chiral environment, probably through complexation as a zinc alkoxide.118... 

While high polymers of /3-lactones can also be formed by cationic polymerization, most of the commercial production seems to be by the anionic route. Carboxylate salts such as sodium acetate or benzoate are commonly the initiators, but other nucleophiles, such as triethylamine, betaine, potassium f-butoxide, aluminum and zinc alkoxides, various metal oxides and tris(dimethylamino)benzylphosphonium chloride (the anion of which is the initiator), are of value. Addition of crown ethers to complex the counter cation increases the rate of reaction. When the reaction is carried out in inert but somewhat polar organic solvents, such as THF or ethyk acetate, or without solvent, chain propagation is very fast and proceeds without transfer reactions. 

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