Solvent participation

It has been shown that different types of glycosyl donors (e.g. trichi oroacetimidates, fluorides, phosphates and pentenyl-, vinyl- and thio-glycosides) feature the ability to form highly reactive nitrilium intermediates in the presence of acetonitrile. The best (3 selectivities are 

Finally, an important requirement for control of the anomeric centre by a solvent is the absence of a neighbouring participating functionality at C(2). 

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Supercritical fluid solvents have been tested for reactive extractions of liquid and gaseous fuels from heavy oils, coal, oil shale, and biomass. In some cases the solvent participates in the reactions, as in the hydrolysis of coal and heavy oils with water. Related applications include conversion of cellulose to glucose in water, dehgnincation of wood with ammonia, and liquefaction of lignin in water. 

The importance of solvent participation in the borderline mechanisms should be noted. Nucleophilic participation is minimized by high electronegativity, which reduces the Lewis basicity and polarizability of the solvent molecules. Trifluoroacetic acid and perfiuoro alcohols are among the least nucleophilic of the solvents used in solvolysis studies. These solvents are used to define the characteristics of reactions proceeding without nucleophilic solvent participation. Solvent nucleophilicity increases with the electron-donating capacity of the molecule. The order trifluoroacetic acid < trifluoroetha-nol <acetic acid < water < ethanol gives a qualitative indication of the trend in solvent nucleophilicity. More will be said about solvent nucleophilicity in Section 5.5. 

Reactant structure will also influence the degree of nucleophilic solvent participation. Solvation is minimized by steric hindrance. The 2-adamantyl system is regarded as being a... 

The reason the adamantyl system is much more sensitive to the substitutions of CH3 for H is that its cage structure prevents solvent participation whereas the i-propyl system has much stronger solvent participation. The internal stabilizing effect of the methyl substituent is therefore more important in the adamantyl system. 

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

If Y is to be a valid measure of solvent ionizing power, presumably the defining reaction should proceed via the Lim (pure SnI) process. This was the basis for the original choice of r-butyl chloride. It is now believed that /-butyl chloride sol-volyzes with some solvent participation, and modern versions of Y are based on other compounds, of which 2-adamantyl tosylate (p-toluenesulfonate, OTs), 6, is the most favored." ... 

Although this is a secondary substrate, complete shielding from backside attack by nucleophiles leads to S l solvolysis without solvent participation. The correspond-... 

It is clear from the work of Siekierska, Sokolowska and Campbell that the initial ligands are not essential if the solvent molecules suffice. On the other hand it is shown by Merz and Riedel that solvent participation is unnecessary, and that target s own ligands are also involved. 

Galvanic cells of the Nerst type are also termed cells with dissolution membranes or solvent type membranes [3]. Such systems are defined by the distribution equilibria in which all ions, present in aqueous and in organic solvents, participate (Section III.A). The general examples of the liquid concentration and chemical galvanic cells of this type are presented in the form of Schemes 8 and 9. 

Another general method for converting alcohols to halides involves reactions with halides of certain nonmetallic elements. Thionyl chloride, phosphorus trichloride, and phosphorus tribromide are the most common examples of this group of reagents. These reagents are suitable for alcohols that are neither acid sensitive nor prone to structural rearrangement. The reaction of alcohols with thionyl chloride initially results in the formation of a chlorosulfite ester. There are two mechanisms by which the chlorosulfite can be converted to a chloride. In aprotic nucleophilic solvents, such as dioxane, solvent participation can lead to overall retention of configuration.7... 

In the absence of solvent participation, chloride attack on the chlorosulfite ester leads to product with inversion of configuration. 

Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Formation of chlorohydrins in acidic aqueous solution. 

Protolytic reactions can also occur in fused salts. The solvent participates in these reactions provided that at least one of its ions has protogenic and/or protophilic character. An example of a solvent in which the cation is aprotic and the anion protophilic is ethylpyridinium bromide (m.p. 114°C). The acid HA is protolysed in this solvent (HA -I- Br HBr + A"). Hydrogen bromide acts as a solvated proton and the acidity is expressed as... 

I FIGURE 20.8 Substitution in a square planar complex in which the solvent participates in a second-order step. 

FIGURE 1. Solvent participation in the racemization of chiral chlorogermanes... 

Assessment of a typical analytical reaction in solution requires consideration of the solvent participation. It is likely that both reactants and products will undergo significant solvation, and the... 

A similar explanation of accelerative effects observed in grafting with other solvents such as the higher straight chain alcohols, DMF, DMSO, acetone, chloroform and cyclohexane (Table VI) has been advanced (6). It appears that there is a relationship between G(H) value of solvent and the extent to which the solvent participates in accelerated grafting. The radiolysis pathway thus contributes, but not exclusively, to the mechanism of the overall copolymerisation reaction. 

A transannular solvent participation has instead been observed in the IN3 addition in CH3CN to tricyclo[4.2.2.02-5]deca-3,7-diene derivatives 62 and 63, which give adducts 64 and 65 as well as tetrazoles 66 and 67 via Hassner-Ritter reaction (equation 69). 

The next topic of our consideration is the ion-radical incipiency. Generally, the mechanism of the ion-radical generation in frozen solution is as follows. Irradiation drives electrons out from a solvent. An organic precmsor (P) transforms into an ion-radical. At first glance, two reactions might be expected to take place electron capture (P -F e P ) and electron detachment (P + e P+ -F 2e). In fact, an indirect redox process takes place, with solvent participation. The example in Scheme 2.41 visualizes 2-methyltetrahydrofman (MeTHF) participation in the redox process, when P is a substance of electron affinity higher than that of the solvent. 

These aggravating reactions proceed at room temperature or under moderate heating up to 60°C without light excitation and in the absence of oxygen, that is, in conditions common to electron-transfer reactions. (Some of these reactions just take place in nonpolar solvents of the decane or xylene type.) Hence, application of nitrosobenzene as spin traps can be complicated by solvent participation. 

There is considerable uncertainty as to how to translate the results of deamination studies to solvolysis experiments. Deamination normally proceeds with a substantially lower activation energy and is expected to produce a hotter or freer ion because of the greater exothermicity of the reaction. Whether this should aid or hinder bridging, ion pairing or solvent participation is not any clearer than... 

ELECTROPHILIC ADDITIONS TO CARBON-CARBON MULTIPLE BONDS A. Chlorinating agents Sodium hypochlorite solution 7V-Chloro succi n i m i de Antimony pentachloride Formation of chlorohydrins from alkenes Chlorination with solvent participation and cyclization Controlled chlorination of acetylenes... 

The examples of this section summarise the recent application of the solvent-drop grinding approach to solid-state cocrystal preparation. The approach has been shown in certain instances to provide for either acceleration of CO crystallisation kinetics or selection of a particular polymorph via solid-state grinding. The approach is attractive, as it appears to incorporate some of the beneficial aspects of solvent participation while maintaining an essentially green, eco-friendly process. 

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