Solvent assistance

This is no longer the case when (iii) motion along the reaction patir occurs on a time scale comparable to other relaxation times of the solute or the solvent, i.e. the system is partially non-relaxed. In this situation dynamic effects have to be taken into account explicitly, such as solvent-assisted intramolecular vibrational energy redistribution (IVR) in the solute, solvent-induced electronic surface hopping, dephasing, solute-solvent energy transfer, dynamic caging, rotational relaxation, or solvent dielectric and momentum relaxation. 

The relations 4- > 2-position in rate and 4- < 2-position in will apparently apply to reactions with anions, but the reverse relation is observed in piperidination, presumably due to 2-substitution being favored by hydrogen bonding in the zwitterionic transition state (cf. 47, 59, and 277) or by solvent-assisted proton removal from the intermediate complex (235). Substitutions of polychloroquino-lines (in which there is a combined effect of azine-nitrogen and unequal mutual activation of the chlorine substituents) also show 4- > 2-position in reactivity contrary statements are documented by these same references. Examples are cited below of the relation 2- > 4-position when a protonated substrate or a cyclic transition state is involved. 

This stereospecific reaction is, therefore, a rare example of stereochemical control by nucleophilic solvent assistance of an ionization process. 

An alternative view that also favors an intermediate mechanism is that of Schleyer and co-workers, who believe that the key to the problem is varying degrees of nucleophilic solvent assistance to ion-pair formation. They have proposed an Sn2 (intermediate) mechanism. ... 

Again, this aspect of carbon monoxide insertion has not received much attention. Hart-Davis and Mawby (108), in a comparative study, examined the reaction of 7r-C9H7Mo(CO)jMe (VII) with various P donor ligands. They find that, in THF, (VII) undergoes CO insertion, via a solvent-assisted... 

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

These reactions involve activation of the carbonyl group by the Lewis acid. A nucleophile, either a ligand from the Lewis acid or the solvent, assists in the desilylation step. 

Solvent-assisted decaffeination of coffee can result in residues of solvent reaching the consumer.208 The use of chlorinated hydrocarbon solvents such as chloroform,209 methylene chloride, trichloroethylene,208 and difluoromonochloromethane (Freon),210 will probably be replaced by compounds already found in roasted coffee. The use of an ethyl acetate and 2-butanone mixture leaves a 26-ppm residue in green coffee, but zero residue in roasted coffee.211 Other solvent compounds used or suggested for coffee improvement or decaffeination include propane, butane,212 carbon dioxide,213 214 acetone215 dimethyl succinate,2161,1-dimethoxymethane, and 1,1-dimethoxyethane.217 Of all these, supercritical carbon dioxide, ethyl acetate, and methylene chloride are the solvents most used currently in decaffeination processes. 

Assuming that the Final Protein -Formaldehyde Reactions Found After Paraff n-Embedding Are Different from Those Found After Aqueous Fixation, Will Careful Heating in Nonpolar Solvents Assist in the Reversal of These Reactions ... 

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. 

Bromination is less stereoselective, and the reactions of cis- and trans-olefins tend to be stereoconvergent. The stereospecific formation of the mixed bromoadducts in protic media, such as methanol or acetic acid, could be interpreted in the light of the recent finding (Ruasse et al., 1991) that these solvents assist the formation of the ionic intermediate nucleophilically. If a solvent molecule is close to the cationic part of the transition state in the rate-limiting step, the intermediate can be trapped by this solvent molecule in a necessarily trans mode with respect to the first bromine, before the two components of the ion-pair diffuse away from each other (15). This would... 

In addition to the fact that steric crowding can slow the reaction by hindering bromine approach to the double bond, it appears now that bulky substituents can modify the bromination mechanism by inhibiting nucleophilic solvent assistance to ionization of the CTC and/or nucleophilic trapping of the ionic intermediates. Assistance to the rate-limiting ionization step by... 

Kinetic solvent isotope effect as a measure of electrophilic assistance to bromide ion departure limiting values rate data in ethanol, methanol and their aqueous mixtures using Bentley s TBr scale its decrease corresponds to the involvement of nucleophilic assistance. R = (/caqhtOII//cAcoH)r as a measure of nucleophilic solvent assistance. Model for a limiting bromination mechanism. Ruasse et al. (1991). /Ruasse and Zhang (1984). 9Argile and Ruasse (to be published). Modro et al. (1979). 

Table 21 Solvent effects in bromination of conjugated olefins transition-state shifts and nucleophilic solvent assistance.

The only series of alkenes that show anomalously low KSIEs is that of highly congested adamantylidenealkanes (last rows of Table 20). The decrease in the isotope effect, associated with particularly small m-values and the absence of nucleophilic solvent assistance, can be attributed to reversible... 

The first step in the interpretation of mBr-values has consisted in searching for an alkene for which nucleophilic solvent assistance is impossible. Methylideneadamantane, in which steric constraints can restrict nucleophile... 

Finally, the last group of alkenes in Table 20 (congested alkenes) behaves very differently as regards solvent effects. The mBr, R and even KSIEs are systematically smaller than those observed for the two previous series. The attenuation of these coefficients can be reasonably attributed neither to earlier transition states nor to increases in nucleophilic solvent assistance. As described in the paragraph on return (p. 279), this trend is more consistent... 

It can be concluded that mBr depends on the magnitude of the charge at the transition state and also on its delocalization either by the substituents or by the solvent. It therefore seems difficult to separate these effects, since R, the measure of solvent assistance, depends also on the same factors. The idea that transition-state shifts contribute to m-variations is supported by substituent effects. Consequently, it would be useful to obtain p-m correlations to compare the influence of the solvent and the substituents in determining the position of the bromination transition state. 

The most significant difference between brominations in protic and non-protic solvents concerns the kinetic law. Whereas in protic media the reaction is first-order in bromine, in halogenated media it is second-order (Bellucci et ai, 1980). CTC ionization is electrophilically assisted by hydrogen bonding by a protic solvent to the leaving bromide and leads to a bromonium-bromide ion pair. In non-protic media, assistance to the bromination step is provided by a second bromine molecule, leading to a bromonium-tribromide ion pair. In other words, in protic media bromination is solvent-assisted (56) while in halogenated media it is bromine-catalysed (57). 

The reactant R2 can also be considered to be a solvent molecule. The global kinetics become pseudo first order in Rl. For a SNl mechanism, the bond breaking in R1 can be solvent assisted in the sense that the ionic fluctuation state is stabilized by solvent polarization effects and the probability of having an interconversion via heterolytic decomposition is facilitated by the solvent. This is actually found when external and/or reaction field effects are introduced in the quantum chemical calculation of the energy of such species [2]. The kinetics, however, may depend on the process moving the system from the contact ionic-pair to a solvent-separated ionic pair, but the interconversion step takes place inside the contact ion-pair following the quantum mechanical mechanism described in section 4.1. Solvation then should ensure quantum resonance conditions. 

Nonpolar solvents generally aid the formation of the (3-phase, while polar solvents assist in producing the y-modification. 

The Forster cycle method is quite simple, which explains why it has been extensively used. One of the important features of this cycle is that it can be used even in cases where the equilibrium is not established within the excited-state lifetime. However, use of the Forster cycle is difficult or questionable when (i) two absorption bands overlap (ii) the electronic levels invert during the excited-state lifetime (usually in a solvent-assisted relaxation process) (iii) the excited acidic and basic forms are of different orbital origins (electronic configuration or state symmetry) and (iv) the changes in dipole moment upon excitation are different for the acidic and basic forms. 

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