Solvents dissociating

Equilibrium between the various enolates of a ketone can be established by the presence of an excess of the ketone, which permits proton transfer. Equilibration is also favored by the presence of dissociating solvents such as HMPA. The composition of the equilibrium enolate mixture is usually more closely balanced than for kinetically... 

In dissociating solvents, perfluoroalkyl iodides react with a zinc-copper... 

Under other reaction conditions, the product can result from thermodynamic control. Aldol reactions can be effected for many compounds using less than a stoichiometric amount of base. In these circumstances, the aldol reaction is reversible and the product ratio is determined by the relative stability of the various possible products. Thermodynamic conditions also permit equilibration among the enolates of the nucleophile. The conditions that lead to equilibration include higher reaction temperatures, protic or polar dissociating solvents, and the use of weakly coordinating cations. Thermodynamic conditions can be used to enrich the composition in the most stable of the isomeric products. 

The Triphenylmethyl Ion.—A solution of triphenylchloromethane in a dissociating solvent conducts the electric current (Walden). Since, on electrolysis, triphenylmethyl is liberated at the cathode, it follows... 

Depending on the linear, branched or cyclic structure of the unsaturated compound, a variety of dialkyl sulfides has been obtained in the reaction with H2S (equations 16-18). The regiochemistry depended markedly on the structure of the diene. For a mechanistic purpose, some experiments have been carried out using deuterium sulfide, D2S. The results have been interpreted in terms similar to those of Nordlander and coworkers9 (vide infra). The thiylation of 1,3-dienes was assumed to start with a regiospecific addition of a proton or a deuteron to one of the two double bonds to form two isomeric ion pairs as in equation 14 which, in the poorly dissociating solvent, collapse into products with equal probability. 

The regio-, stereo- and chemoselectivities have been mainly interpreted in terms of bridging of the ionic intermediate and/or ion pair dissociation. Solvent-separated ion pairs and free ions have often been considered to explain the product selectivities of these reactions. Nevertheless, the stereochemical outcomes can also be determined by the relative rates of the ion pair dissociation and of the nucleophilic trapping of the intermediate, i.e. by the lifetime of the intermediate . 

The photochemical addition of tetranitromethane to aromatic compounds under conditions of excitation of the [ArH C(N02)4l charge-transfer complex by light matching the wavelength of the charge-transfer band results in a recombination within [ArH+, NOj, C(N02)3 ] triad. The destiny of triad depends on the nature of the solvent (Sankararaman et al. 1987, Sankararaman and Kochi 1991). In dissociating solvents, radical substitution is predominant, leading to nitro products and trinitromethane ... 

Although the dielectric constants of molten salts are generally quite small (8r = 2-3), they behave as strongly dissociating solvents. This is due to the ability of the solvent ions to exchange places with solute ions of the same charge. 

In dissociating solvents, perfluoroalkyl iodides react with a zinc-copper couple to give perfluoroalkylzmc compounds, R Znl, which react with alkyl carbonates or pyrocarbonates to give perfluorocarboxylic acids or esters, respectively [55] (equations 47 and 48)... 

The destiny of the triad depends on the nature of the solvent (Sankararaman et al. 1987 Sankararaman Kochi 1991). In dissociating solvents, radical substitution is predominant, leading to nitro products and trinitromethane ... 

A similar mechanism was recently suggested for the hydrochlorination of styrene and t-butylethylene by Fahey and McPherson (1969). The proposed mechanistic scheme appears reasonable in a weakly dissociating solvent, such as acetic acid, although it seems somewhat surprising that the product distribution and the stereochemistry are insensitive to addition of diluents such as formic acid up to 1 1m. 

Physical measurements in dissociating solvents confirm this value. 

These labile residues then form carbanion intermediates rapidly in the strongly dissociative solvent environment. 

An example of the Sn2 rate dependence on the nature of the counterion is given by the reaction of -butyl 4-bromobenzene sulfonate with lithium- and tetra-n-butylammonium halides in the weakly dissociating solvent acetone (cr = 20.6) [279]. 

The ion-pair dissociation of ambident alkali enolates, which results in increasing 0/C alkylation ratios, can be promoted not only by dissociating solvents but also by specific cation solvation. In the latter case, EPD solvents cf. DMF and DMSO in Table 5-22b) or macro(poly)cyclic ligands such as coronands ( crown ethers ) or cryptands are used [376, 377, 660]. For example, the alkylation of sodium y9-naphtholate with (bromomethyl)benzene or iodomethane in the presence of benzo[18]crown-6 gives high O/C alkylation ratios when tetrahydrofuran or benzene are the solvents [660]. In dissociating solvents such as A,A-dimethylformamide or acetonitrile, however, so far no... 

To summarize, it can be stated that the freer the ambident anion in every respect, the larger the 0/C-alkylation ratio in the case of 1,3-dicarbonyl compounds [365]. Thus, if 0-alkylation products are desired in the alkylation of enolates, dipolar non-HBD and dissociating solvents such as A, A -dimethylformamide, dimethyl sulfoxide, or, especially, hexamethylphosphoric triamide should be used. If C-alkylation is desired, protic solvents like water, fluorinated alcohols, or, in the case of phenols, the parent phenol will be the best choice [365]. 

In non-HBD, non-dissociating solvents, a corresponding proposal can be made hard counterions (alkali metal cations) should associate preferably with the hard site, and the substrate RX with the soft site in the activated complex composed of RX and the ambident ion pair [366]. With increasing hardness of the counterion (increasing charge density), the fraction of C-alkylation should increase in non-HBD solvents and decrease on solvent insertion into the ion pair. Indeed, the C-ethylation of M (ethyl acetoacetate) in dimethyl sulfoxide or hexamethylphosphoric triamide increases in the order M = R4N < Cs < K < Na < Li [373]. 

The solvent-influenced synjanti dichotomy for bimolecular eliminations of acyclic and medium-ring bromides, tosylates, and onium salts has been reviewed [395, 693] and will be mentioned only briefly. As a rule, the s jn-elimination pathway gains importance in non-dissociating solvents, while dissociating solvents facilitate the more common anti-elimination reaction. The more unusual s jn-elimination is favoured in non-dissociating solvents because of ion-pair association, which favours a cycHc six-membered activated eomplex as shown in Eq. (5-151a) see reference [395]. 

The advantage of s jn-elimination in solvents of low relative permittivity lies also in the formation of a eontact ion pair in the product, whereas anti-elimination produces a produet-separated ion pair according to Eq. (5-15 lb). Thus, reaction of free ions pro-eeeds in standard anti fashion, while ion pairs (or higher aggregates) tend to react by a syn pathway. Solvent separation of the RO M ion pair in the reactant state removes the driving force for s-jn-elimination i.e. dissociating solvents, increase in cation size, addition of crown ethers, etc.). 

---