Stereoselective control solvent effects

Clifford, Rayner and co-workers at the University of Leeds [66,67] have used the density of SCCO2 to control the stereoselectivity of reactions in a novel way. Previous workers have observed an influence of density on selectivity in reactions very close to the critical point. The novelty of the Leeds work is that the effects are observed at densities considerably above pc and temperatures higher than Tc. This selectivity has no obvious counterpart in reaction chemistry in conventional solvents. Effects have been observed in a whole range of reactions, and this approach may well have widespread applicability. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

Solvent effects in heterogeneous catalysis are examined in terms of physical or chemical modifications to control the chemo-, regio- and stereoselectivity of a reaction. The main factors affecting selectivity are reactant solubility, polarity, reactivity or acido-basicity of solvents and competitive chemisorption of products and solvents In the special case of molecular sieves, selectivity control of a reaction by competitive adsorption, diffusion or shape selectivity and confinement catalysis are also examined. 

The stereoselective polymerization of various acrylates and methacrylates has been studied using initiators such as atkyllithium [Bywater, 1989 Pasquon et al., 1989 Quirk, 1995, 2002]. Table 8-12 illustrates the effects of counterion, solvent, and temperature on the stereochemistry of the anionic polymerization of methyl methacrylate (MMA). In polar solvents (pyridine and THF versus toluene), the counterion is removed from the vicinity of the propagating center and does not exert an influence on entry of the next monomer unit. The tendency is toward syndiotactic placement via chain end control. The extent of syndiotacticity... 

C6H5(CH3)2SiH, in the presence of Bu4NF or tris(diethylamino)sulfonium difluo-rotrimethylsilicate (TASF, 13, 336) in aprotic polar solvents, particularly HMPT. The actual reagent may well be a hexavalent fluorosilicate involving HMPT. CsF and KF are far less effective than Bu4NF. With proper control of all the parameters, near quantitative yields can be achieved, and high stereoselectivity is also possible, as in the case with methylcyclohexanone. 

A new probe of solvent accessibility of bound sensitizers has been described and tested for the particular case of a series of Ru" and Os photosensitizers bound to sodium lauryl sulphate micelles. The method depends upon the large solvent deuterium effect on excited-state lifetimes, and a correlation has been established between accessibility of bound complexes and hydrophobicity of the ligands. Luminescence properties of amphiphilic annelide-type complexes of ruthenium in micellar phases have been described. In the case of [4,4 -bis(nonadecyl)-2,2 -bipyridyl]bis-[4,4 -di-(10,13,16-trioxaundecyl)-2,2 -bipyridyl]ruthenium dichloride, intramicellar self-quenching effects have an influence on the excited-state lifetime, and the mechanism of self-quenching has been determined. Deactivation of [Ru(bipy)3] by [Co(EDTA)] has been studied in a micellar environment and found to occur by electron transfer at diffusion-controlled rates a stereoselective effect has been observed. ... 

Systematic studies on the chelation-controlled additions were carried out, varying the type of alkoxy group, the carbon nucleophile, the solvent and the temperature. It was found that a-alkoxy ketones react highly stereoselectively with Grignard reagents in THF (equation 24). Alkyllithiums were not effective. The generalization was made use of in the synthesis of the polyether antibiotic monensin. ... 

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