Zinc hydroxyl species

Tungsten oxide is not the only nanomaterial that forms unusual morphologies in benzyl alcohol. Another example is ZnO that grows into fan-like nanorod bundles [168]. The chemical formation mechanism, elaborated from the analysis of the organic compounds detected by GC coupled with mass spectrometry, involved a nucleophilic attack of the hydroxyl function of benzyl alcohol on one of the carbonyl groups of the acetylacetonate ligand of the precursor molecule (Scheme 2.4). Release of acetone (in its enol form) and benzyl acetate resulted in the formation of a zinc hydroxyl species, which then underwent condensation to a Zn—O—Zn bridge. 

Scheme 2.4 Chemical reaction mechanism for the formation of ZnO from zinc acetylaceto-nate hydrate and benzyl alcohol (a) formation of the zinc hydroxyl species and (b) condensation reaction.

Figure 20.4). Ribulose 1,5-bisphosphate binds to Mg2+ through its keto group and an adjacent hydroxyl group. This complex is readily deprotonated to form an enediolate intermediate. This reactive species, analogous to the zinc-hydroxide species in carbonic anhydrase (Section 9.2.2). couples with CO2, forming the new carbon-carbon bond. The... 

The three major types of oxysalt bonded AB cement are the zinc oxychloride, the magnesium chloride and the magnesium oxysulphate cements. The bases employed, therefore, are either zinc oxide or magnesium oxide, both of which readily undergo hydration in aqueous solution, behaving as M(OH)2 species and acting as a source of hydroxyl ions. They are thus both clearly bases in the Bronsted-Lowry sense. 

Ibe principle found for zinc(II) was applied to ) complex models by Young et al. (25). The hydroxyl function of copper complex 27a deprotonates with a p value of 8.8 to yield 27b, which cleaves phosphodiester bis(2,4-dinitrophenyl) phosphate (BDP ) by transesterification to produce 28 ( (BDP ) = 7.2 x -1 M-1sec-1 at 25°C see Scheme 5). The analogous complex with a hydroxyethyl pendent cleaves the diester predominantly by hydrolysis, which suggests that the reactive species is not Cun-alkoxide, but —OH-. The rate A(BDP") of... 

This is of relevance to the mechanism of carbonic anhydrase. This enzyme, which catalyzes the hydration of C02, has at its active site a Zn2+ ion ligated to the imidazole rings of three of its histidines. The classic mechanism for the reaction is that the fourth ligand is a water molecule which ionizes with a pKa of 7.37 The reactive species is considered to be the zinc-bound hydroxyl. Chemical studies show that zinc-bound hydroxyls are no exception to the rule of high reactivity. The H20 in structure 2.31 ionizes with a pKa of 8.7 and catalyzes the hydration of carbon dioxide and acetaldehyde.38... 

To demonstrate the versatility of his S3mthesis strategy Yamada used ketoester 151 as relais substance to S3mthesize two further picrotoxane alkaloids isolated from Dendrobium species, nobilonine (90) and 2-hydroxydendrobine (87) (Scheme 14) (84). Monobromination of 151 with bromine in dioxane and subsequent treatment with water resulted in hydroxy-y-lactam 152, whereas attempts to hydroxylate 151 by Barton oxidation led to rearrangements. Chemo- and stereoselective reduction with zinc borohydride converted 152 into the en fo-alcohol. To counterbalance the unfavorable conformational equilibrium this alcohol had to be converted into the alcoholate to achieve lactonization. Chemoselective reduction of the hydroxylac-tam moiety of lactone 153 again followed Borch s protocol, which led in this case to boron complexed amino compounds necessitating successive acid treatment to obtain racemic 2-hydroxydendrobine (87) in low yield accompanied by dendrobine (82). 2-Hydroxydendrobine (87) was converted into nobilonine (90) by Eschweiler-Clark methylation. 

The best catalysts for CPL polymerisation initiated by hydroxyl groups are the alcoholates of aluminium, titanium, zinc and lanthanides (the yttrium alcoholates are very efficient) [46-49] or tin salts (e.g., stannous octoate). The polymerisation with these catalysts is in fact an aniono-coordinative polymerisation, the monomer being activated by the coordination at the catalyst. The attack of the anionic species takes place at the carbonyl group (carbonyl - oxygen scission), as shown in the reactions in 8.30. 

Another way to stabilize an eclipsed or gauche conformation is to coordinate heteroatom substituents with a metal ion via chelation. Oishi and co-workers reduction of 153 with zinc borohydride proceeds via a chelated species, 154. 2 Chelation of zinc to the hydroxyl and carbonyl groups effectively locks the conformation into that shown in the transition state required for reaction. The methyl and hydrogen are held in place, and the hydride is delivered from the less hindered face (over the hydrogen in 154) to complete the reaction (see secs. 4.4.B and 4.7.B). Since transition metal salts usually behave as Lewis acids, the presence of a heteroatom with... 

Yamanaka et al. have demonstrated that aerobic hydroxylation of methane in trifluoroacetic acid at 40 °C is catalyzed by NH4VO3 or VO(acac)2 if zinc powder is present [56m]. Methyl trifluoroacetate is formed with a turnover number ca. 10 after 1 h. The authors suggested that the active oxygen species in... 

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