Chelation products

Addition of one mole of P,P -dipheny1methy1enediphosphinic acid to tetraisopropyl titanate gives a chelated product, the solutions of which can be used as a primer coat for metals to enhance the adhesion of topcoats, eg, alkyds, polyalkyl acylates, and other polymeric surface coating products, and improve the corrosion resistance of the metal to salt water (102). 

The combination of metal tuning and double stereodifferentiation helps to prepare chelation and nonchelation products in the imine series7. In the case of an alkoxy substituent adjacent to the aldimino, the chelation product 10 is predominantly obtained with allylmagnesium chloride, chloromagnesium allyltriethylaluminate or allylzinc bromide, while the use of allyl-boronates or allyltitanium triisopropoxide, which lack the requisite Lewis acidity for chelation, gives 11 with good Cram selectivity. 

E- and Z-silyl thioketene acetals give the 2,3-anti product. The 3,4-syn ratio is 50 1, and is consistent with the Felkin model. When this nucleophile reacts with 2-benzyloxypropanal (Entry 8), a chelation product results. The facial selectivity with respect to the methyl group is now reversed. Both isomers of the silyl thioketene acetal give mainly the 2,3-syn-3A-syn product. The ratio is higher than 30 1 for the Z-enolate but only 3 1 for the F-enolate. 

Extensive investigations in our laboratories on the deactivation of rhodium and iridium catalysts has shown there to be a number of different mechanisms involved. Both, rhodium and iridium catalysts are generally less stable at higher temperatures, and have more labile ligands than their ruthenium counterparts. All of the catalysts are affected by pH, but the ruthenium catalysts seem to be more readily deactivated by acid. Indeed, these reactions are often quenched with acetic acid, whilst stronger acids are used to quench the rhodium reactions. Each of the catalysts can be deactivated by product inhibition, the ruthenium catalyst with aromatic substrates such as phenylethanol, and the rhodium and iridium ones by bidentate chelating products. 

Stiles and coworkers (70) have taken advantage of the effect of chelation to carboxylate compounds having active methylene groups. The reactions of these substances with C02 in the presence of aluminum or magnesium alkoxide readily produce chelate products (Equations 23 and 24). 

In the case of inert cobalt(m) complexes it is possible to isolate the chelated products of the reaction. Let us return to the hydrolysis of the complex cations [Co(en)2(H2NCH2C02R)Cl]2+ (3.1), which contain a monodentate iV-bonded amino acid ester, that we encountered in Fig. 3-8. The chelate effect would be expected to favour the conversion of this to the chelated didentate AO-bonded ligand. However, the cobalt(iu) centre is kinetically inert and the chloride ligand is non-labile. When silver(i)... 

Cram product Felkin-Anh product Cram chelate product... 

After Cram had discovered the selectivities now named after him, he proposed the transition state model for the formation of Cram chelate products that is still valid today. However, his explanation for the preferred formation of Cram products was different from current views. Cram assumed that the transition state for the addition of nucleophiles to a-alkylated carbonyl compounds was so early that he could model it with the carbonyl compound alone. His reasoning was that the preferred conformation of the free a-chiral carbonyl compound defines two sterically differently encumbered half-spaces on both sides of the plane of the C=0 double bond. The nucleophile was believed to approach from the less hindered half-space. 

In order for the Cram chelate product to predominate after the addition of a hydride donor to a chiral carbonyl compound, which contains a heteroatom in the a-position, this heteroatom and part of the reagent must he able to form a five-membered ring chelate. If this is not possible, one observes Felkin-Anh selectivity (provided one observes selectivity at all). This has the following interesting consequences for synthesis. 

The a-chiral ketone from Figure 10.18—the a-substituent is a benzyloxy group—is reduced to the Cram chelate product by Zn(BH4)2, a Lewis acidic reducing agent. The Zn2 ion first bonds the benzyl and the carbonyl oxygen to a chelate. Only this species is subsequently reduced by the BH 4 ion because a Zn2 -complexed C=0 group is a better elec-... 

In contrast, the diastereoselectivities of Figure 8.10 can be observed for many additions of hydride donors to carbonyl compounds which contain a stereocenter in the a position with an O or N atom bound to it. One of the product diastereomers and the relative configuration of its stereocenters is called the Felkin-Anh product. The other diastereomer and its stereochemistry are referred to as the so-called Cram chelate product. If the latter is produced preferentially, one also talks about the occurrence of chelation control or—only in laboratory jargon—of the predominance of the chelation-controlled product. ... 

The transsilylation reaction described above was studied with 8 and the silylated hydrazides l.33,34 The E,Z stereoisomers of la35 reacted in different ways, shown in Eqs. (6) and (7). Initially both isomers form unstable tetracoordinate silane intermediates (9a-E, 9a-Z), which can be observed at low temperatures in CD2C12 solutions by 29Si NMR spectroscopy. The intermediates 9a-E and 9a-Z could be observed for about 20 min at — 70 °C, before reacting further to form five-membered and six-membered chelate products 10a and 11a, respectively. At ambient temperature the reaction proceeds immediately without observation of intermediates. 

Beryllium chemistry includes its S-diketonate complexes formed from dimedone (9), acetylacetone and some other S-diketones such as a,a,a-trifluoroacetylacetone. However, unlike the monomeric chelate products from acetylacetone and its fluorinated derivative, the enolate species of dimedone (9) cannot form chelates and as the complex is polymeric, it cannot be distilled and is more labile to hydrolysis, as might be expected for an unstabilized alkoxide. However, dimedone has a gas phase deprotonation enthalpy of 1418 9 kJmoD while acetylacetone enol (the more stable tautomer) is somewhat less acidic with a deprotonation enthalpy of 1438 10 klmoD Accordingly, had beryllium acetylacetonate not been a chelate, this species would have been more, not less, susceptible to hydrolysis. There is a formal similarity (roughly 7r-isoelectronic structures) between cyclic S-diketonates and complexes of dimedone with benzene and poly acetylene (10). The difference between the enthalpies of formation of these hydrocarbons is ca... 

The importance of cobalt in the physiology and ecology of cyanobacteria is underscored by evidence showing that they produce strong, specific cobalt chelators. Production of such cobalt... 

The reaction between p-tolylarsenic dichloride and BAL, (HSCH2CH(SH)CH,OH), gives the chelate product 58, with normal length As—S single bonds (2.225,2.276 A) and... 

NH-inserted seven-membered chelate product, but a rearrangement product, 120, also forms in which carbene insertion into a C—S bond effected... 

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