Alkoxide species

Surface oxidation pathways the activation at the C-H bonds and the nature of surface alkoxide species . 

Zinc hydroxide and alkoxide species are particularly relevant to catalytic processes, often forming the active species. The cooperative effects of more than one zinc ion and bridged hydroxides are exploited in some enzymatic systems. Zinc alkyl phosphate and carboxylate materials have been important in the formation of framework compounds, often containing large amounts of free space for the inclusion of guest molecules. Aldehyde and ketone compounds are of low stability due to the poor donor capabilities of the ligands however, a number of examples have recently been characterized. 

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... 

Analogous alkoxides, phenoxides, and carboxylates will also initiate the ROP of epoxides, all forming propagating alkoxide species.779 Block copolymers of epoxides with /3-butyrolactone have been prepared via the addition of EO or PO to living poly(ester) chains.782 The oxygen-bound enolate of living PMMA will also react with epoxides to yield diblocks such as PEO-b-PMMA and PPO-b-PMMA (Mn= 12,800, Mw/Mn = 1.16) 787... 

Reaction of tris(neopentyl) complexes of titanium, zirconium and hafnium with molecular oxygen furnishes the corresponding tris(neopentoxy) complexes [42, 43, 51]. A peroxo complex is an intermediate in this reaction, being relatively stable in the case of titanium [42]. The alkoxide species can also be formed upon reaction with alcohols under mild conditions [42, 52]. The alcoholysis reaction is fast, with a low dependence on the steric hindrance of the alkyl chain [42]. Hydrolysis leads to ](=SiO)M(OH)3] or ](=SiO)2M(OH)2], depending on the precursor species. Deu-... 

Kowalski A, Duda A, Penczek S (2000) Mechanism of cyclic ester polymerization initiated with tin(II) octoate. 2. Macromolecules fitted with tin(II) alkoxide species observed directly in MALDI-TOF spectra. Macromolecules 33 689-695... 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

Other attempts have used the cations dissolved in a chelating solvent such as methoxyethanol that forms alkoxide species (22)(23). When water is added to a solution of these elements in a ratio between 1 and 4 of H20/alkoxide, gels are obtained that are suitable for thin film deposition. 

The use of supported metal complexes in transesterification reactions of TGs is not new. An earlier patent claimed that supported metals in a hydroxylated solid could effectively catalyze transesterification. The catalyst preparation used an inert hydrocarbon solvent to attach transition metal alkoxide species to the support surface. The reaction, however, was carried out in the presence of water. The author claimed that water was essential in preparing materials with good catalytic activity. Among the metals employed, titanium catalysts showed the best activity. However, it was not clear from the preparation method if reproducibility could be easily achieved, an important requirement if such catalysts were to be commercially exploited. 

So far, fewer than 10 types of carbenium ions have been reported to be persistent species formed upon adsorption of olefins or alcohols on acidic zeolites. Instead, surface alkoxy (alkoxide) species with carbenium-ion-like properties are suggested to act, most likely, as catalytic intermediates in reactions catalyzed by acidic zeolites. Various groups have observed that, upon adsorption of olefins or alcohols on acidic zeolites, alkoxy species are formed the observations are based on both in situ and ex situ A MAS NMR spectroscopy (49,50,71-80). 

The Cu(l) bicarbonate complex previously mentioned (77) was synthesized by the reactions summarized in Scheme 7, which includes C02 insertion into copper hydroxide and alkoxide species. The insertion reaction of C02 with metal hydroxides to form bicarbonates is believed to occur... 

Specific poisoning studies with pyridine and TCNE have led to this picture that excludes the participation of Lewis acid sites in the reaction. It should, however, be mentioned that this interpretation of the reaction mechanism is not completely undisputed. Thus, Soma and co-workers (351) conclude from their dynamic treatment that a surface alkoxide species is the intermediate in olefin formation. Since the alkoxide is formed by dissociative adsorption on A1—O pair sites, one would expect a strong poisoning effect by pyridine. Bremer and co-workers (352, 353) propose a mechanism in which a coordinated held alcohol molecule... 

Zeolites Y, MOR and all silica-BEA showed no activity, although activity is shown by US-Y (a dealuminated, ultra stable form of Y), which contains some extra framework A1 and the less crystalline MCM-41, which react with the alcohol to form an activated alkoxide species. 

The structure of the mixed hydroxo-alkoxide species consists of two face-sharing bioctahedral Zr2X9 units each bound to Ba2+ by four X groups. Recent studies28 have shown that for the system Al(OPr )3—M(OPr )4, M = Zr and Hf, in isopropanol there is only one type of complex formed, namely, [(OPr )2Al(p,-OPr )2]2M(OPr )2. A rare heterotermetallic alkoxide which contains Cd, Ba, and Zr has been prepared29 according to... 

The stmcture of the derivative [EtZn(OEt2)3]+ [B(C6F5)4] could be determined by X-ray crystallography (Figure 56). The zinc atom is tetracoordinated with a slightly distorted tetrahedral geometry, and the zinc carbon bond length of 1.964 A is comparable with that observed in the methylzinc alkoxide species [MeZn(0-t-Bu)]4 (1.955 A). 

Most of the mechanistic work on this reaction has been devoted to determining the role of the base. Its most obvious function would be to complex the Lewis-acidic boron reagent, rendering it nucleophihc and thus activating it toward transmetallation. However, Miyaura, Suzuki, and coworkers noted that an electron-rich tetracoordinate boronate complex was less reactive than a bivalent boronic ester. From this, they surmised that the role of the base was not to activate the boron toward transmetallation, but rather to transform the palladium halide intermediate to the hydroxide or alkoxide species, which would then be more reactive toward boron. However, in a mass spectrometry study of a reaction between a pyridyl halide substrate and an aryl boroiuc acid, Aliprantis and Canary saw no evidence of palladium hydroxide or alkoxide intermediates, despite observing signals in the mass spectra assignable to every other palladium intermediate of the proposed catalytic cycle. ... 

The polymerization undergoes a coordination-insertion mechanism. The initiation step involves nucleophilic attack of the active group, such as a hydride, alkyl, amide or alkox-ide group, on the carbonyl carbon atom of a lactide or lactone to form a new lanthanide alkoxide species via acyl-oxygen cleavage. The continued monomer coordination and insertion into the active metal-alkoxo bond formed completes the propagation step as shown in Figure 8.50. 

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