Steric effects on solvation

Hall also studied the base strengths of substituted piperidines in water and acetonitrile [189,339]. He found that field effects and steric effects on solvation were important in these compounds. 

The absence of steric effects in gas-phase proton-transfer equilibria has been shown to aid greatly the evaluation of steric effects on solvation and in the formation of Lewis acid-base complexes. 

Secondary steric effects on chemical reactivity can result from the shielding of an active site from the attack of a reagent, from solvation, or both. They may also be due to a steric effect on the reacting conformation of a chemical species that determines its concentration. 

Steric effects on both the amide and the acyloxyl side chain are similar. Tert-butyl and adamantyl groups on the amide side chain in 29v, 29x, 29c, and 29e (Table 2 entries 53 and 54, 63 and 65) result in lower stretch frequencies that, on average, are only 40 cm-1 higher than their precurser hydroxamic esters. Streck and coworkers have suggested that such changes in dialkyl ketones can be ascribed to destabilisation of resonance form II through steric hindrance to solvation which, in the case of tert-butyl counteracts the inductive stabilisation.127... 

Although the intermolecular selectivity of the nitration of alkylbenzenes by nitric acid in trifluoroacetic acid is controlled by both electronic and steric factors, it is argued that intramolecular selectivity is controlled by steric effects on transition state solvation. 

Solvation of the nucleophile has but little influence on the stereochemistry (entries 3 and 4), Notice however that in our model calculations, only one molecule of solvent is introduced. Therefore, the steric effects of solvation are not adequately accounted for. 

The stetic parameter accounts for steric effects on the solvation of a-amino and carboxyl groups caused by the side chain. We have not attempted to account for any charge transfer interactions as none of the compounds studied is a very effective charge transfer acceptor, and the water and n-octanol phases can only function as change transfer donors. Further support is provided by the correlation of log P values for Ph(CH2)mX (m = 1, 2, 3) with the equation... 

An interesting contribution of steric effects to solvation is the addition of benzenethiolate ions to N-ethylmaleimide in 95% EtOH (52). The reactivity ratio of the 4-H 4-Me 2-Me 2,6-Me2 derivatives is 1 2.16 1.6 1.6 thus, a small steric retardation is superimposed on a small electronic acceleration. However, the 2-t-Bu derivative is 8.7 times faster than its 4-t-Bu isomer. This finding was ascribed to a rate-enhancing steric inhibition to solvation of the anion, which raises its reactivity by more than the reactivity decrease due to crowding in the transition state. 

Andersson and coworkers investigated the role of solvation, dispersion, and steric effects on the enantioselectivity. The authors results agree with the fact that gas-phase B3LYP calculations describe the drop in enantioselectivity of 2,3,4,5,6-pentafluoroacetophenone compared to acetophenone where steric and electrostatic effects are the only major effects, but the approximation is too crude to reproduce quantitatively the extent of enantioselection observed experimentally. In order to extract the isolated contributions of steric effects, the authors correlated the same energetic parameter with respect to an empirical steric parameter called STERIMOL B1 [132]. Their results showed that bulkier alkyl groups tend to decrease the enantioselectivity, which correlates well with the B1 parameter. Based on this observation, it seems that an intrinsic steric factor enhances or depresses the enantioselectivity in the reduction of acetophenone and other n-alkyl aryl ketones. However, a generalized rationalization of this effect is not trivial, and investigation of similar catalysts reveals only a small role for steric effects [105, 109]. 

This is deduced from Hall s assumption of the steric effect on the solvation of tertiary ammonium ions °. The ionization constants of -(substituted-phenyl)gIycines reported recently provide interesting fects on the nature of the substituent, which in conjunction with spectral data, have been used to calculate the zwitterion equilibrium constant and the constants pertaining to the individual ionization process. 

The effect of a para substituent reflects both inductive and resonance contributions to the carbon atom bearing the carboxyl group. The effect of a meta substituent results only from inductive effects. The contribution of groups ortho to the carboxyl group is not easily interpreted because of steric effects on the conformation of the carboxyl group as well as solvation effects. 

Bagno, A., Boso, R.L., Ferrari, N. and Scorrano, G. (1995) Steric effects on the solvation of protonated di-ferf-butyl ketone and phenyl ferf-butyl ketone. J. Chem. Soc., Chem. Commun., 2053-2054. 

Contrary to the results obtained with carbon tetrachloride solvent and entirely in accord with the postulate that the effect arises from the steric hindrance to solvation, the rates of cleavage of ArSnR3 compounds in methanol decrease on increasing the size of the group R. This is shown by the rate coefficients in Table 265, though it is difficult to draw any conclusion from the Arrhenius parameters... 

The chemoselectivity of the other alkenes of Table 1 is more variable. It appears that bulky substituents favour bromide over methanol attack of the bromonium ion, since dibromlde increases from 39 to 70 % on going from methyl to tert-butyl in the monosubstituted series. The same trend is observed in the disubstituted series with a contraction of the chemoselectivity span (37 to 43 % on going from methyl to teH-butyl) for the trans isomers. Since the solvated bromide ion can be viewed as a nucleophile larger than methanol, the influence of steric effects, important in determining the regioselectivity, does not seem very significant as regards the chemoselectivity. This result has been interpreted in terms of a different balance between polar and steric effects of the substituents on these two selectivities. 

Contrary to earlier expectations (see Dorfman, 1965), Hentz and Kenney-Wallace (1972, 1974) failed to find any correlation between s and Emax. Actually, there is a better correlation of matrix polarity with the spectral shift from e(to e upon solvation and the time required to reach the equilibrium spectrum (Kevan, 1974). Furthermore, Hentz and Kenney-Wallace point out that emax is smaller f°r alcohols with branched alkyl groups, the spectrum being sensitive to the number, structure, and position of these groups relative to OH. Clearly, a steric effect is called for, and the authors claim that a successful theory must not rely too heavily on continuum interaction as appeared in the earlier theories ofjortner (1959,1964). Instead, the dominant interaction must be of short range, and probably the spectrum is determined by optimum configuration of dipoles within the first solvation shell. 

Scheme 5 shows a group of alkynylgold(i) complexes for which the studies focused on the UV-VIS electronic absorption and emission properties. Most of these compounds are of the type [(L)AuC=CR], for which the methods of synthesis have been summarized above. The products were found to show phosphorescence in various polymorphs and crystal forms of solvates. Although there are no metallophilic interactions discernible in the crystal between most of the monomers due to the steric effect of the large tertiary phosphines, there is nevertheless strong excitonic coupling based on other weak interactions, which depend on the organization of the molecules in the crystal.105,106... 

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