HMPA, solvent properties

Nelsen et al. (2007) have revealed one more aspect of solvent control over charge localization. Solvents with marked electron-donor properties contribute to charge localization in cation-radicals, whereas anion-radicals experience the same changes in better electron-accepting solvents. Thus, naked (non-ion-paired) anion-radicals of 4,4 -dinitrostilbene and 4,4 -dinitrotolane show the spectra of delocalized species in HMPA and THF, but essentially spectra of localized species in DMF, DMSO, and MeCN. 

The reaction involves the transfer of an electron from the alkali metal to naphthalene. The radical nature of the anion-radical has been established from electron spin resonance spectroscopy and the carbanion nature by their reaction with carbon dioxide to form the carboxylic acid derivative. The equilibrium in Eq. 5-65 depends on the electron affinity of the hydrocarbon and the donor properties of the solvent. Biphenyl is less useful than naphthalene since its equilibrium is far less toward the anion-radical than for naphthalene. Anthracene is also less useful even though it easily forms the anion-radical. The anthracene anion-radical is too stable to initiate polymerization. Polar solvents are needed to stabilize the anion-radical, primarily via solvation of the cation. Sodium naphthalene is formed quantitatively in tetrahy-drofuran (THF), but dilution with hydrocarbons results in precipitation of sodium and regeneration of naphthalene. For the less electropositive alkaline-earth metals, an even more polar solent than THF [e.g., hexamethylphosphoramide (HMPA)] is needed. 

Electrochemical properties of samarium(ii) iodide are very sensitive to the nature of solvents. Reduction potential increases by replacing THE with a more polar solvent, such as DME or CH3CN. Addition of HMPA to a THE solution of samarium(ii) iodide leads to a substantial increase in the electron-donating nature of samarium(ii). The principal samarium(ii) species in a mixed solvent of THE and HMPA is an ionic cluster of [Sm(HMPA)4(THE)2] 2I in HMPA-THF (4 1) or [Sm(HMPA)6] 2I in HMPA-THE (>10 1). The reactivity order of the samarium(ii) complexes is [Sm(HMPA)6] 2I > [Sm(HMPA)4(THF)2] 2P > Sml2 in the reaction with 1-iodobutane. 

The acid-base properties of a mixed solvent is also an important factor influencing the behavior of solutes. Thus, the parameters of the acidity and basicity of mixed solvents have been studied to some extent [35], Figure 2.10 shows the donor numbers of mixtures of nitromethane and other organic solvents. Because ni-tromethane has very weak basicity (DN= 2.7), the addition of small amounts of basic solvents (HMPA, DMSO, pyridine) increase the donor number remarkably. 

A co-solvent with properties and reaction enhancements similar to HMPA. It is a dipolar aprotic solvent, miscible in water and most organic solvents. Can be cooled to dry ice temperature. 

EPD solvents do not correctly reflect the ionizing properties of these solvents due to the differences in dielectric constants. Although Sn(CH3)3l is considerably ionized in pure tributylphosphate, the solutions are essentially nonconducting because of the very low dielectric constant e = 6.8 of this medium (see Section IV). Puoss-Krauss analysis of conductance data for Sn(CH3)3l in strong EPD solvents, such as dimethylformamide (DME), dimethyl sulfoxide (DMSO), pyridine, and hexamethylphosphoric amide (HMPA), reveal that the substrate is completely ionized and consequently behaves as a 1 1 electrolyte (33). 

For example, complexes with very strong EPD ligands, such as Ng ", NCS ", CN, or F may exist even in solvents of high DN such as HMPA or DMSO. In solvents of weak or medium EPD properties, complex formation is essentially quantitative. On the other hand, bromo and iodo complexes usually exist only in weak EPD solvents, such as NM, PDC, or AN, and are completely ionized in solvents such as DMF, DMSO, or HMPA. The stabilities of chloro complexes are somewhat higher in the respective solvents. According to Table VII the chloride ion has an EPD strength similar to that of DMF or DMSO. Consequently chloro complexes in these solvents (compare Table IV) are ionized to some extent, sometimes with autocomplex formation. 

Polymerization of I. I was polymerized in flame-dried equipment under N 2 at -40 °C as follows. A 25-mL round-bottom flask equipped with a poly(tetra-fluoroethylene) (Teflon)-covered magnetic stirring bar and rubber septum was charged with I (1.2 g, 10.9 mmol) (5, 6), THF (10 mL), and either HMPA (5 drops) or TMEDA (5 drops). n-Butyllithium (0.8 mL, 1.2 M, 0.96 mmol) was added slowly to this mixture. The mixture quickly became thick. The mixture was stirred for 1 h at -40 °C and then warmed to -20 °C, and saturated aqueous ammonium chloride was added. The organic layer was separated, washed with brine and water, and dried over molecular sieves (4 A). After filtration, the solvent was removed by evaporation under vacuum 1.10 g (92% yield) of polymer was isolated. The yields of polymer ( 2%) and their spectral properties were identical regardless of whether HMPA or TMEDA was used as cocatalyst. With n-butyllithium-TMEDA, a polymer with Mw and Mn of 158,000 and 69,000, respectively, was obtained, whereas with n-butyl-lithium-HMPA, a polymer with My, and M of 120,000 and 30,400, respectively, was isolated. 

Both monomeric and aggregated species (e.g., open dimers in Bu OMe and triple ions in HMPA/ THF) are reactive. In related work in which aggregate formation was maximized, it was shown that the rates of enolization in the presence of the mixed aggregates are much lower and solvent dependent. The autoinhibition correlates with the relative stabilities of the mixed aggregates the stabilities do not, however, correlate in a straightforward marmer with the ligating properties of the solvent. 

The chemistry of sulfones is dominated by the reactions of sulfonyl carbanions. The sulfone group has a unique ability to facilitate deprotonation of attached alkyl, alkenyl and aryl groups and will permit multiple deprotonation to yield polyanions. These properties, combined with the relative intertness of the sulfone (S02) group to nucleophilic attack, have made the S02 group the first choice for stabilisation of carbanions and account for the extensive application of sulfones in synthesis. Sulfonyl carbanions can be generated and reacted under a wide variety of conditions extending from aqueous phase transfer reactions using sodium hydroxide as base to the use of alkyllithiums in polar aprotic solvents. The reactivity of sulfonyl carbanions depends on the nature of the metal counterion (Li+, Na+, K+ and Mg2+ are the most important ones) and the presence of additives, e.g. TMEDA, HMPA and Lewis acids. 

If we compare Reaction (A) (Scheme 2) with the reaction of Scheme 3 we can say that r-BuONa communicates to NaNH2 in THF similar properties to those that this amide has in the presence of HMPA. We thought that if this supposition was true, it should be possible to perform in THF reactions which usually need HMPA as a solvent. 

It may be desirable to avoid ethers despite their valuable properties. Ethers are sometimes toxic (e.g., THF , but also HMPA ) and less attractive industrially because of their high inflammability and the hazards of peroxide formation. Ethers are also more expensive than, e.g., hydrocarbons. Consequently, considerable effort is invested in the preparation of the reagents in hydrocarbons and other nonethereal solvents. 

DN = 38.8) is a stronger EPD than DMSO (DN=29.8) but its ability to solvate anions is much smaller 43) compared to other aprotic solvents, and coordination of HMPA to Co2+ (as wdl as to several other metal cations) is definitely sterically hindered (see preceding section). The role of solvent donicity in complex formation is supported by polarographic studies of the reduction of Eu + to Eu2+ in different solvents using supporting electrolytes with anions of different EPD properties (Fig. 8)... 

The present study, concerned with dipolar aprotic media, had its origin in the 1960 s, with the discovery that certain solvents such as dimethyl sulfoxide (DMSO) dimethylformamide (DMF) and hexa-methylphosphortriamide (HMPA), had the property of accelerating enormously certain reactions relative to protic solvents such as alcohols(12-14). These reactions included nucleophilic substitutions, both aliphatic and aromatic, as well as proton abstraction processes proceeding by carbanion intermediates. 

The z v(OH - OMe) scale is devoid of this stoichiometric problem and is the solva-tochromic scale of choice for the correlation of solvent basicity-dependent properties. It can be scaled in a range from 0 (solvents obeying the comparison Equation 4.37) to 1 by dividing by the 2030 cm shift of HMPA ... 

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