Zinc complexes selectivity

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

A detailed report on the structure of sulphamethoxypyridazine has been given recently [310]. Various complexes of sulphamethoxypyridazine have been prepared, including bismuth, cadmium, calcium, chromium, cobalt, copper, iron, nickel, silver and zinc complexes. Thus, for instance, Cu(II) and Ag(I) complexes have been found to be more potent against selected bacterial pathogens than (95, R1 = MeO R2 = H) [311]. 

The tetrasulfo-Pc complexes of a number of metals are made by the urea melt process by heating the powdered metal, or its acetate, with triammonium-4-sulfophthalate, urea, boric acid, and ammonium molybdate. The metals or metal compounds used are those of chromium (III), manganese(II), iron(II), iron(III), cobalt(II), and zinc(II). Selected synthetic examples of sulfo- and other derivatives of metal phthalocyanines are presented below. 

The high stereoselectivity can be explained again by means of the zinc complex A (see Scheme 3). The nucleophilic isonitrile attacks the glycosyl imine from the steri-cally less shielded Re-side. The bulky pivaloyl group at the 2-position and the formed zinc complex block the Si-side efficiently. The exchange of the pivaloyl for the acetyl group decreases the selectivity of the reaction (10 1 instead of 30 1) [13]. 

Substituted isoxazoline A -oxides 52 were synthesised by condensation of a-nitro-esters 51 and aldehydes catalysed by zinc complexes of amino acids. In DMSO the selectivity of the reaction was high and products 52 were obtained as single trans diastereomers in 77-88% yields <02TL5287>. 

In an extension of this work, either zinc(II), palladium(II), rhodium(I) or copper(I) salts were immobilised in an ionic liquid film (SILP, vide supra) onto diatomic earth and the catalysts tested for activity in the reaction between phenylacetylene and 4-isopropyl-phenylaminc.1 391401 The supported rhodium, ruthenium and zinc complexes afford higher rates and selectivities relative to their use under homogenous reaction conditions. Lower rates are, however, observed with the copper salt, which is rationalised by strong complexation of the ionic liquid to the Cu(I) centre. 

The synthesis of the zinc complex 115 according to Scheme 3 presents an efficient alternative to the previous method based on reduction of carbon diselenide by sodium metal <1990JCD115>. Compounds 22, 98-101, and 116 containing either (2-methoxycarbonyl)ethyl or 2-cyanoethyl as thiolate protecting groups are versatile compounds for the synthesis of a large selection of functionalized tetraselenafulvalenes via the (1) base-promoted deprotection and (2) thiolate alkylation protocol. 

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