Solvent-promoted

Shibasald et al. reported that lithium-containing, multifunctional, heterobimetallic catalysts such as LaLi3tris((l )-6,6 -dibromobinaphthoxide) 35, with moderate Lewis acidity in non-polar solvents, promote the asymmetric Diels-Alder reaction to give cycloadducts in high optical purity (86% ee) [53] (Scheme 1.67). The lithium... 

It was mentioned above that weakly ionizing solvents promote syn elimination when the leaving group is uncharged. This is probably caused by ion pairing, which is greatest in nonpolar solvents. Ion pairing can... 

The tissue sections must be washed with organic solvents when the detection targets include peptides and proteins. Washing with organic solvents promotes the ionization of peptides and proteins mainly by removing phospholipids from the sections.14 Washing also flushes out salts that could interfere with the crystallization of the matrix. 

The results of a thorough study of the kinetics, products and stereochemical course for the nucleophilic substitution and elimination reactions of ring-substituted 9-(l-Y-ethyl)fluorenes ([31]-Y, Y = Br, I, brosylate) have been reported (Scheme 19).121,122. The reactions of the halides [31]-Br and [31]-I were proposed to proceed exclusively by a solvent-promoted ElcB reaction or an E2 reaction with a large component of hydron transfer in the transition state .122... 

Kinetic effects of solvent polarity are also observed with the alkylacetoxycarbenes. Polar solvents promote 1,2-H shifts,22 83 and the 1,2-H/1,2-Ac migration ratio of 76 increases from 50/50 to 95/5 as the solvent is changed from isooctane to TCE a parallel alteration (26/74 to 62/38) occurs with carbene 75.92 Thus, bystander substituents or a polar solvent exert positive effects on the... 

Interestingly, Ng and Tsakiri127 also observed an effect of the solvent, whereby the more polar solvents promoted the water-gas shift rate, as shown in Table 46. 

The chemistry of the reactions involved in coupling peptides is the same as that for coupling TV-alkoxycarbonylamino acids. However, the oxazolone that is formed by the activated peptide is chirally unstable, it is formed more readily, and there is an added impetus for it to form because the rate of bond formation between segments is lower. In addition, segments usually have to be coupled in polar solvents because they are insoluble in nonpolar solvents, and polar solvents promote the undesirable side reaction. The result is that the number of procedures actually used for coupling peptides is rather small. The methods in question are addressed below. 

Alkylation of 2-ethoxycarbonylcyclohexanone, followed by hydrolysis and decarboxylation, has been used as a convenient route to 2-alkylcyclohexanones [74, 92]. Polymer-supported catalysts have also been used [e.g. 93] 2-acylcyclohexanones are C-alkylated at the 2-position almost exclusively, whereas cyclohexane-1,3-dione produces a mixture of mono- and di-C-alkylated products, together with the O-alkylated derivative. In all cases, less polar solvents promote the O-alkylation. 

Fig. 7.11 Reaction profiles for L-catalyzed isomerization of cis-to-trans ML2X2. In (A) an ionic intermediate is favored by a polar solvent. In (B) ion-pair formation arises with a less polar solvent. In (C) a non-polar solvent promotes a 5-coordinated intermediate. In (C), pseudo-rotation occurs. Based on D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973) see also Ref. 90. Reproduced with permission from D. G. Cooper and J. Powell, J. Amer. Chem. Soc. 95, 1102 (1973). (1973) American Chemical Society.

When particles or large molecules make contact with water or an aqueous solution, the polarity of the solvent promotes the formation of an electrically charged interface. The accumulation of charge can result from at least three mechanisms (a) ionization of acid and/or base groups on the particle s surface (b) the adsorption of anions, cations, ampholytes, and/or protons and (c) dissolution of ion-pairs that are discrete subunits of the crystalline particle, such as calcium-oxalate and calcium-phosphate complexes that are building blocks of kidney stone and bone crystal, respectively. The electric charging of the surface also influences how other solutes, ions, and water molecules are attracted to that surface. These interactions and the random thermal motion of ionic and polar solvent molecules establishes a diffuse part of what is termed the electric double layer, with the surface being the other part of this double layer. 

Polar aprotic solvents promote this type of reaction. 

The idea of solvent polarity refers not to bonds, nor to molecules, but to the solvent as an assembly of molecules. Qualitatively, polar solvents promote the separation of solute moieties with unlike charges and they make it possible for solute moieties with like charges to approach each other more closely. Polarity affects the solvent s overall solvation capability (solvation power) for solutes. The polarity depends on the action of all possible, nonspecific and specific, intermolecular interactions between solute ions or molecules and solvent molecules. It covers electrostatic, directional, inductive, dispersion, and charge-transfer forces, as well as hydrogen-bonding forces, but excludes interactions leading to definite chemical alterations of the ions or molecules of the solute. 

The type of solvent can affect the kinetics, and, as a consequence, the equilibrium is sometimes not reached within a reasonable time. For example, Kim and Shin studied the kinetically controlled synthesis of alitame precursor (Z-Asp(OEt)-D-Ala-NH2), and better results were obtained in the presence of dimethyl sulfoxide and 2-methoxyethyl acetate as adjuvants. These solvents promoted product precipitation while maintaining the reaction mixture in a homogeneous state, thus improving the conversion [64]. 

The compound of interest is dissolved in a high-boiling viscous solvent such as glycerol a drop is placed on a thin metal sheet, and the compound is ionized by the high-energy beam of xenon atoms (Xe). Ionization by translational energy minimizes the amount of vibrational excitation, and this results in less destruction of the ionized molecules. The polar solvent promotes ionization and allows diffusion of fresh sample to the surface. Thus ions are produced over a period of 20-30 min, in contrast to a few seconds for ions produced from solid samples. 

Effect of solvent on El vs. E2 vs. ElcB. With any reaction a more polar environment enhances the rate of mechanisms that involve ionic intermediates. For neutral leaving groups, it is expected that El and ElcB mechanisms will be aided by increasing polarity of solvent and by increasing ionic strength. With certain substrates, polar aprotic solvents promote elimination with weak bases (the E2C reaction). 

The effectiveness with which the solvent promotes migration decreases in the order methanol > ethanol water. The extremely low stability of complexes in water can be explained by the relatively great tendency of metal ions to associate with water molecules.70 The difference between the rate in ethanol and that in methanol can be attributed, at least partly, to the fact that salts are more highly dissociated into free ions in methanol71 a higher concentration of free cations would permit a higher concentration of positively charged carbohydrate species. 

The equilibrium proportions of the constituents vary with temperature, concentration, and solvent, and are characteristic of each individual aldonic acid. Dehydration in vacuo, or by evaporation from suitable solvents, promotes lactone formation. By suitable choice of conditions, many aldonic acids and both types of lactones have been obtained crystalline 1 considerations have been detailed by Isbell and Frush.63... 

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