Chelated salt, zinc chloride

Zinc chloride has also been observed to have a significant influence on the enantiose-lectivity of these reactions. But several other important parameters have been pointed out, such as the temperature (best performances are obtained at very low temperatures), the solvent (HMPA is often required) or the concentration of the lithium amide96. As underlined above about the stereoselectivity of the deprotonation, the formation of lithium amides-lithium chloride mixed aggregates (1 1 or 2 1) has been proposed to rationalize the important salt effects (Scheme 20), the amide homogeneous dimer being supposed to provide only mediocre enantioselections. With diamines, the intramolecular chelation would provide a fairly rigid system. 

The effect of metal oxides and chelates has already been mentioned. The metals involved can react with HCl and their products of reaction are catalytic to PVC thermal degradation and thus HCl is produced more rapidly, which contributes to increase flammability. Zinc borate is a quite popular metal salt used in this application. In addition to zinc chloride, boron chloride reduces flammability. Some studies show that when boron chloride is produced, the B2O3 glass layer is destroyed and boron is volatilized, which reduces its flame stability action. 

Many variations were undertaken concerning the metal salts used for chelation. Zinc chloride, which normally gives the best results, provided the rearrangement product in excellent yields but with only moderate ee s (entry 4). Application of calcium chloride (entry 5) or magnesium chloride (entry 6) provided ee s between 65 and 70%, but the metal salts of choice were Mg(OEt)2 (entry 7) and Al(OiPr)3 (entry 8). With these salts excellent yields and diastereoselectivities were obtained (up to 98% each) as well as a very high induction (up to 86% ee), especially if 2 to 2.5 equiv of quinine were used. Unfortunately this excess of chiral ligand is necessary because the coordination of the ligand to the chelated metal decreases the... 

The selected organic salts of Ba, Cd, Zn, Sn, Pb, and Ca perform with varying degrees of acceptability. Barium-cadmium and barium-cadmium-zinc are the most widely used tin mercaptides and organotin complexes are utilized where ultimate clarity is required lead salts dominate in electrical applications, and calcium-zinc combinations have been traditionally used when FDA approval is necessary. Organic phosphites are also utilized as so-called chelators, complexing the half-chloride salt... 

For most commercial applications, it is necessary to add some form of accelerator to the formulation to speed the rate of cure. Both acidic and basic accelerators can be used. Consequently, several latent accelerators have appeared on the commercial scene in recent years. Included among these are quaternary ammonium halides such as benzyltrimethylammonium chloride (9 ), stannous octoate ( ), zinc stearate (1 ), extra-coordinate siliconate salts(11), triethanolamine borate (12), triethanolamine titanate (13), and various other metal chelates (14). However, all of these materials have been rejected for one reason or another and... 

Mixed metal stabilizers are commonly used where more specialized applications (e.g. food contact) or specific properties (e.g. transparency) are requirements. These combinations of metal salts are chosen to balance the ability of their carbot late ligands to replace labile ehlorines on the ehain backbone and the resultant properties of the metal chlorides eventually formed. For example, a combination of barium-zinc stabilizer would be selected so that the chlorides end up as banum chloride, a weak Lewis acid, and the zinc compound coordinates strongly with the labile chlorine on the polymer chain, enabling its displacement. This rather simple stabilization reaction is further enhanced by the inclusion in these mixed metal stabilizers of chelating agents ... 

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