With carbonyl group Coordination

Ir(OH)(cod)]2 catalyzed a formal [3+2] cycloaddition of 2-formylphenylboronic acid and 1,3-dienes (Scheme 11.41) [50]. The transmetaUation of boronic acid with iridium would yield aryliridium, where the carbonyl group coordinates to the metal. An electrophihc attack of the diene terminus to formyl carbon would then... 

The 2-acetamido-2-deoxy derivative 146 (Scheme 39) of glucono-1,5-lactam 126 (Scheme 31) was another ligand (Kx 2 pM) probed with the bacterial O-GlcNAcase from B. thetaiotaomicron (GH 84).308 The lactam (PDB 2XM1) was found in the 4E conformation with its carbonyl group coordinated by the catalytic acid-base Asp 243, Tyr 282 as well as a water molecule. 

Diazenes with carbonyl groups like bis(benzoyl)diazene prefer coordination at oxygen and nitrogen76 affording metallocycles 25. However, (CO)2Mo(S2CNEt2)2 probably also forms a a-complex.68 ... 

When the approaching monomer is in syndiotactic conformation and is added as such (c), the / carbon is fixed after addition, but the a carbon can rotate (d) and assume isotactic conformation after the addition of the next unit. The effect of the lithium cation is again assumed to be the driving force for the rotation of the potentially syndiotactic unit to the isotactic conformation. Repeating steps (a) and (b) leads to a threo-diisotactic chain, and repeated steps (c), (d) and (e) to an erythrodiisotac-tic chain. The addition of tetrahydrofuran reduces the number of threo-diisotactic units because it competes with carbonyl groups for coordination to Li+. This enhances the amount of monomer approaching the centre by the syndiotactic route. 

Antamanide is a cyclic decapeptide antibiotic represented as Cyclo Val -Pro -Pro .Ala -Phe -Phe T>ro -Pro -Phe -Phe ). Antamanide also forms very stable complexes with Li and K in nonpolar solvents such as acetonitrile. Hie crystalline structure of Li -antamanide complex has been investigated 137). It has been made clear that two peptide bonds Pro -Pro and Pro -Pro are cis, there are two intramolecular hydrogen bonds, and four carbonyl groups coordinate to li. This structure is depicted in Pig. 32, where the fifth ligand for Li is solvent acetonitrile. This... 

The proposed reaction mechanism (see Scheme 4) is similar to that which is well established for olefin reduction, with the insertion of a meial-hydride bond into the carbonyl group coordinated to (he metal center. A mechanism in which reduction occurs by a simple nucIcopliiUc attack of the hydrogen atom of the transition metal hydride (which behaves like Blit or AlHa ) into the carbonyl group of the aldehyde, cannot be ignored ... 

The necessity for ethylene to compete with CO for a coordination site (at high CO pressures the rate of copolymerization becomes negative-order in CO) could also explain the lower rate of copolymer formation compared with dimerization. It is believed that this explanation cannot be the whole picture, but that internal coordination of the growing polymer chain to palladium may be responsible for the observations. Each time, after an ethylene insertion, the polymer chain-end acts as a chelating ligand, in which the oxygen atom of the chain-end s carbonyl group coordinates to the electrophilic palladium ion. 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Olefin synthesis starts usually from carbonyl compounds and carbanions with relatively electropositive, redox-active substituents mostly containing phosphorus, sulfur, or silicon. The carbanions add to the carbonyl group and the oxy anion attacks the oxidizable atom Y in-tramolecularly. The oxide Y—O" is then eliminated and a new C—C bond is formed. Such reactions take place because the formation of a Y—0 bond is thermodynamically favored and because Y is able to expand its coordination sphere and to raise its oxidation number. 

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