Zinc alcoholates

CH2CI2. A colourless liquid with a chloroform-like odour b.p. 4I°C. Prepared by heating chloroform with zinc, alcohol and hydrochloric acid manufactured by the direct chlorination of methane. Decomposed by water at 200°C to give methanoic and hydrochloric acids. Largely used as a solvent for polar and non-polar substances, particularly for paint removal (30%), dissolving cellulose acetate and degreasing (10%). It is more stable than carbon tetrachloride or chloroform especially towards moisture or alkali. It is somewhat toxic. U.S. production 1981 280000 tonnes. 

The polymerisation of the bispropylene spiroorthocarbonate with zinc-based coordination catalysts probably involves zinc alcoholate propagating species, which is shown schematically as follows ... 

The tendency towards alternation in the oxirane/cyclic acid anhydride system with the zinc-based coordination catalyst is connected, according to literature data [193], with the stronger nucleophilic properties of the oxygen atom of the zinc alcoholate species and the weaker electrophilic properties of the zinc atom in such species compared with the respective properties of zinc carboxylate species. Studies of the copolymerisation of propylene oxide and maleic anhydride in the presence of catalysts such as diethylzinc-monoprotic compound (1 1) showed an increasing tendency towards alternation in systems with catalysts of decreasing electrophilicity of the zinc atom [186], which may corroborate the coordination mechanism proposed scheme (24). 

It seems that the initiation step of the copolymerisation most likely involves the oxirane reaction [according to scheme (3)]. Zinc alcoholate species formed in this reaction can easily propagate the copolymer chain, coordinating and enchaining both the oxirane [scheme (3)] and the cyclic carbonate [scheme (15)] comonomers. However, in the case of the cyclic carbonate, its enchainment may also proceed according to scheme (14), leading to decarboxylation. Thus, the obtained poly(ether-carbonate)s are characterised by a lower content of carbonate units with respect to the ether units [82,146]. 

The formation of a cyclic carbonate, e.g. propylene carbonate, accompanying the copolymerisation has been explained in terms of the backbiting reaction involving zinc alcoholate species [206,207], This has been confirmed recently [147] by degrading polypropylene carbonate) by using catalysts with zinc phenolate species. The degraded copolymer thus obtained was terminated in its chains with zinc alcoholate species and phenylcarbonate groups. The course... 

On the other hand, polypropylene carbonate) degradation rim in the presence of catalysts containing zinc alcoholate species resulted in the formation of degraded copolymer chains terminated with zinc carbonate species [scheme (31)] [147] ... 

Zinc carbonate species appeared to undergo slow decarboxylation [scheme (14)] and then, as zinc alcoholate species, were involved in chain depolymerisation via backbiting reaction according to the following scheme [147] ... 

Limonene (88) is known to react with one or two equivalents of hydrogen chloride the monohydrochloride (105) is conveniently prepared in carbon disulphide. The dihydrochloride (106) reacts with chlorine in a complex way Carman and Venzke have shown how the trichloride (107) and tetrachloride (108) are formed, the latter being identical with the product obtained from terpinolene (89) and iodobenzene dichloride. Zinc-alcohol reduction of the tetrachloride leads to the 1,2-cts-dichloride (109), which will, in turn, add hydrogen chloride to give a second trichloride (110). " Carman and Venzke have shown that the... 

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. 

P-Aminoalcohols are excellent catalysts for the enantioselective addition of dioorganozincs to aromatic aldehydes (enantioselectivities of up to 90-98% ee). This reaction was discovered by Oguni et al. in 1983 [85]. In a sense it may be considered as a case of organometallic catalysis since zinc alcoholates are involved in the catalytic cycle [86]. 

Curve A in Scheme 6, represents an example of the asymmetric ampHfication obtained by Noyori et al. [12], which still remains the highest (-i-)-NLE known. The authors found that the reaction is first-order with respect to the catalyst and that the asymmetric ampHfication originates from the accumulation of unreactive meso-dimers of the zinc alcoholate of (-)-DAIB. The (-l-)-NLE maybe seen as the consequence of a reservoir, weakly coupled to the catalytic cycle and storing some racemic Hgand. 

Initially, mainly 3-amino alcohols were used as chiral ligands to catalyze the addition of organozincs to aldehydes. In most of the cases investigated a (-l-)-NLE was observed. Presumably this is a consequence of the greater stability of the dimeric meso-zinc alcoholate derived from the 3-amino alcohol. In addition to... 

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