Chemoselectivity, solvent effects

More recent work has focused on understanding the mechanism or mechanisms of selectivity. Some of these studies have been performed on well-characterized catalysts about which particle size information is available. Still others have been performed on single crystals. So conclusions may be reached about the effects on chemoselectivity of planes, edges, and corners that are related to particle size (structure sensitivity). A number of these studies, mostly on Pt, are summarized in Table 2.6. Since these studies have usually been performed in the vapor phase, information about solvent effects and their possible influence on chemoselectivity is unavailable. 

The solvent has no influence on the stereoselectivity of bromine addition to alkenes (Rolston and Yates, 1969b), but it could have some effect on the regioselectivity, since this latter depends not only on polar but also on steric effects. Obviously, it modified the chemoselectivity. For example, in acetic acid Rolston and Yates find that 2-butenes give 98% dibromides and 2% solvent-incorporated products whereas, in methanol with 0.2 m NaBr, dibromide is only about 40% and methoxybromide 60%. There are no extensive data, however, on the solvent effects on the regio- and chemoselectivity which would allow reliable predictions. 

A remarkable solvent effect on the chemoselectivity was discovered by Agarwala and Bandyopadhyay (Scheme 3.24, B) [114]. When cyclohexene la was oxidized with tBuOOH in the presence of an electronegative substituted iron(III) porphyrin complex in CH2Cl2-MeOH, epoxide 4a was the predominant product (69% yield) in addition to alcohol 2a and ketone 3a as byproducts in 20% and 11% yields,... 

Solvent effects on chemoselectivity. Use of donor solvents (THF or HMPT) enhances the rate of displacement reactions of lithium organocuprates with alkyl halides, but retards conjugate addition to enones. House and Lee have examined the effect of solvents on the reaction of (CH3)2CuLi with the bromo enone 1. Reaction of 1 with the cuprate in ether-(CH3)2S gives the product of... 

Ligand and solvent effect on the chemoselectivity of Pd-catalyzed cross cou-... 

Maugard, T., Remaud-Simeon, M. and Monsan, P. (1998) Kinetic study of chemoselective acylation of amino-alditol by immobilized lipase in organic solvent effect of substrate ionization. 

For the methyl-substituted ethylenes, i.e. in the absence of any steric effects, there is a roughly linear relationship between the chemoselectivity and the 13C nmr chemical shift of the most substituted carbon atom of the bromonium ions (Dubois and Chretien, 1978). This selectivity is therefore discussed in terms of the magnitude of the charge on the carbon atom and the relative hardness of the competing nucleophiles, according to Pearson s theory (Ho, 1977). However, this interpretation does not take into account the substituent dependence of the nucleophilic solvent assistance, which must play a role in determining this chemoselectivity. 

The current research areas with ruthenium chemistry include the effective asymmetric hydrogenation of other substrates such as imines and epoxides, the synthesis of more chemoselective and enantioselective catalysts, COz hydrogenation and utilization, new methods for recovering and recycling homogeneous catalysts, new solvent systems, catalysis in two or three phases, and the replace-... 

With this mechanistic scheme, the chemoselectivity of the addition and the formation of rearranged chlorides (but not acetates) have been chosen as criteria to differentiate the ion pair mechanism from the purely ionic one and, on the basis of both criteria, the authors suggest the involvement of a tight ion pair for the addition of ArSCl in AcOH to diene 62 and of solvent separated ion pairs to triene 108. The effects related to the presence of added electrolytes, which favor the formation of rearranged acetates, have been considered in this work127 as evidence that even a larger separation of ions, which should lead to more electrophilic species, is possible. 

Finally, the effect of reaction conditions and solvents on chemoselectivity should also be considered.18 Empirically, polar, more basic solvents, e.g. HMPA, DMF, serve to minimize counterion effects by formation of solvent-separated ion pairs and promote electron-transfer processes which are conducive for... 

Chemoselective reductions. The reactivity of NaBH4 can be decreased by use of a lower temperature or by a mixed solvent such as methanol or ethanolic methylene chloride. This simple strategy can be used to effect selective reduction of ketones in the presence of enones,1 and of aldehydes in the presence of ketones. ... 

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