Zinc catalysis

The 100 MHz H-NMR spectra in benzene-d6 solution afforded additional information. Isotactic polymers prepared by zinc catalysis had a symmetrical quartet at 6.38, 6.48, 6.67, and 6.77 ppm, whereas amorphous polymers prepared by KOH catalysis had an additional quartet of equal intensity at 6.36,6.46,6.60, and 6.70 ppm. Therefore, the former absorption could be assigned to the isotactic dyad and the latter to the syndiotactic one. Amorphous polymers prepared by aluminum catalysis had an additional quartet assigned to a tail-to-tail dyad. 

Other chiral morpholine derivatives containing bromine, which is readily substituted under zinc catalysis by enolates, enol ethers, and allyl compounds to give amino acids (Section D.1.4.5.) have been prepared from (1 / ,2.S )-2-amino-l,2-diphenylcthanol or its enantiomer41, both of which are available by resolution of the racemate with L-glutamic acid42 or other resolving agents (Section 2.3.2.). 

Scheme 3.1 Three-component reaction catalysed by a combination of chiral copper catalysis and zinc catalysis.

Aloulou, A., Rodriguez, JA Frederic, C. (2006). Exploring the specific features of interfacial enzymology based on lipase studies. Biochimica et Biophysica Acta(Molecular cell biology og lipids), Vol. 1761,Vol.9, pp. 995-1013 Bockmann, A. C. R. (2006). Structural and dynamic studies of proteins by high-resolution solid-state NMR Chimie, Vol 9, No.3-4, pp 381-392 Auld, D. S. (1997). Zinc Catalysis in Metalloproteases. Structure and Bonding, Vol. 89, pp. 29-50... 

B.M. Trost, J. Hitce, Direct asymmetric Michael addition to nitroalkenes vinylogous nudeophiUcity under dinudear zinc catalysis, J. Am. Chem. Soc. 131 (2009) 4572-4573. 

Other compounds which may be found in crude oil are metals such as vanadium, nickel, copper, zinc and iron, but these are usually of little consequence. Vanadium, if present, is often distilled from the feed stock of catalytic cracking processes, since it may spoil catalysis. The treatment of emulsion sludges by bio-treatment may lead to the concentration of metals and radioactive material, causing subsequent disposal problems. 

In the previous section efficient catalysis of the Diels-Alder reaction by copper(II)nitrate was encountered. Likewise, other bivalent metal ions that share the same row in the periodic system show catalytic activity. The effects of cobalt(II)nitrate, nickel(II)nitrate, copper(II)nitrate and zinc(ll)nitrate... 

Terminal alkynes react with propargylic carbonates at room temperature to afford the alka-l, 2-dien-4-yne 14 (allenylalkyne) in good yield with catalysis by Pd(0) and Cul[5], The reaction can be explained by the transmetallation of the (7-allenylpailadium methoxide 4 with copper acetylides to form the allenyKalk-ynyl)palladium 13, which undergoes reductive elimination to form the allenyl alkyne 14. In addition to propargylic carbonates, propargylic chlorides and acetates (in the presence of ZnCb) also react with terminal alkynes to afford allenylalkynes[6], Allenylalkynes are prepared by the reaction of the alkynyl-oxiranes 15 with zinc acetylides[7]. 

Side chain reactivity is also enhanced and is typified by the difference in reactivity of 2-methylpyrazine and 2-methylpyrazine 1,4-dioxide towards anion formation and subsequent condensation reactions. 2-Methylpyrazine undergoes condensation with benzal-dehyde at 180 °C, with zinc chloride catalysis, to yield the styrylpyrazine (58), whereas the corresponding reaction of 2-methylpyrazine 1,4-dioxide proceeds at 25 °C under base catalysis (67KGS419). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Carboxypeptidases are zinc-containing enzymes that catalyze the hydrolysis of polypeptides at the C-terminal peptide bond. The bovine enzyme form A is a monomeric protein comprising 307 amino acid residues. The structure was determined in the laboratory of William Lipscomb, Harvard University, in 1970 and later refined to 1.5 A resolution. Biochemical and x-ray studies have shown that the zinc atom is essential for catalysis by binding to the carbonyl oxygen of the substrate. This binding weakens the C =0 bond by... 

Resoles are usually those phenolics made under alkaline conditions with an excess of aldehyde. The name denotes a phenol alcohol, which is the dominant species in most resoles. The most common catalyst is sodium hydroxide, though lithium, potassium, magnesium, calcium, strontium, and barium hydroxides or oxides are also frequently used. Amine catalysis is also common. Occasionally, a Lewis acid salt, such as zinc acetate or tin chloride will be used to achieve some special property. Due to inclusion of excess aldehyde, resoles are capable of curing without addition of methylene donors. Although cure accelerators are available, it is common to cure resoles by application of heat alone. 

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