Hexamethylphosphoric triamide , solvation

High-energy irradiation of the lower alcohols gives solvated electrons, solvated protons and radicals [64]. Solvated electrons are also obtained by irradiation of the aprotic amide solvents [65] frequently used in organic electrochemistry and by the irradiation of hexamethylphosphoric triamide [66], N-Methylpyrrolidone which has properties similar to dimethylformaraide, is a useful solvent for the generation of solvated electrons because the reaction between electrons and protons is relatively slower than with dimethylformamide [67],... 

A-Alkylation, -acylation and -sulfonation are also promoted by a polar solvent, such as HMPA (hexamethylphosphoric triamide).This acts to solvate the ions (promoting dissociation), but in a non-polar solvent like diethyl ether or tetrahydrofuran (THF), attack by most carbon electrophiles upon indolylmagnesium bromide proceeds at C-3 (Scheme 7.9). 

Hexamethylphosphor-triamide (HMPA) HMPA radical anion Solvated electron... 

Calcium bis[tetrahydroborate(l —)] is obtained as a fine powder which must be stored in well-sealed containers. It is stable in dry air at room temperature but traces of moisture decompose it with liberation of hydrogen. The gas evolution is quantitative when dilute hydrochloric acid is added. The compound decomposes at ca. 260°. The solvate Ca[BH4]2 -2THF loses THF on heating to 240° in vacuo. Calcium bis[tetra-hydroborate(l —)] dissolves exothermally in THF and crystalline Ca[BH4]2 -2THF may be isolated from the solution. The Ca[BH4]2 2THF is very soluble in dimethyl sulfoxide, in hexamethylphosphoric triamide, and in bis(2-methoxyethyl) ether (diglyme). 

The tendeney towards reaetion at the center with the maximum eleetron density inereases when dipolar non-HBD solvents are employed owing to the laek of speeifie solvation cf. solvents HCON(CH3)2 and CH3SOCH3 in Table 5-22). Thus, in the alkylation of the enolate ions of 1,3-dicarbonyl compounds, the greatest yields of the 0-alkylated isomers are obtained in hexamethylphosphoric triamide, followed by dipolar non-HBD solvents of the amide type [372-375]. 

A nonpolar solvent favors conformation A, whereas conformation B is favored by more polar solvents (e.g. dimethylformamide, hexamethylphosphoric triamide) because the cation is more solvated (cf. Table 9, entries 1 and 2). However, this solvent effect is absent when BujP Cu" is used as counterion. Conformation A is more favored by relatively small counterions, such as the lithium and sodium ion, as compared to the larger potassium ion, due to the higher degree of association of the former. Steric strain between ASG and ASG is minimized in conformation B. Conformations A and B lead to trans- and c -substituted cyclopropanes, respectively. A study of cyclopropane esters, -in which the stereoselectivity of the reaction of polymer-supported reagents was compared with molecules of low-molecular weight, made clear that the steric and polar microenvironment of the polymer-supported reaction is not different enough in bulk to influence the selectivity substantially. Nevertheless, a specific influence of the solid phase can be observed at low temperatures. 

While solvated electrons and their ion-pairs seem to be the most abundant species in liquid ammonia and hexamethylphosphoric-triamide, the negative alkali ions and their ion-pairs predominate in ethereal solvents. In fact, pulse radiolysis of tetrahydrofuran solutions of alkali tetraphenylborides revealed that the e, Met+ pairs, observed immediately after a pulse, rapidly decay, being converted into the corresponding Met" ions645. 

Solvents also play an important role in regulating the reactivity of anions. Hexamethylphosphoric triamide (HMPT) solvates alkali metal ions so strongly that the counteranions are practically free. Under such conditions the inherent bond strengths determine the ease of reaction. Since the O-C bond is stronger than the C-X bonds (X = Cl, Br, I), facile displacement of halide ions by carboxylates in HMPT is a logical consequence (35-38). 

Hexamethylphosphoric triamide (Figure 3.7). The data at 298 K show some water-water preferential association but only to a minor extent. The value of 8xww(max) = 0.050 atXs = 0.55 indicates essentially zero preferential mutual solvation. 

Phenyllithium is known to form, in equilibrium with the monomer, a dimer 25 (Scheme 1-21) held together by electron-deficient partial bonds. However, phenyllithium can also adopt the structure of a lithium lithiate ion pair 26 (Scheme 1-21). What makes the difference The solvent plays a capital role. As long as the solvation forces remain moderate as in diethyl ether, the doubly carbon-lithium-carbon bridged dimer 25 is energetically most favorable. In a 2 1 mixture of cylohexane and diethyl ether, phenyllithium even assembles a tetrameric cluster. " " in neat THF, the four-centered dimer 25 is found in equilibrium with the monomer. " However, in the presence of the powerful electron-donor hexamethylphosphoric triamide (HMPA), it is the ion pair 26 that coexists with the monomer. Analogous lithiate complexes have been identified with... 

In considering the solvation of charged species by a polar solvent, the polar solvents we considered were hydrogen bond donors (protic polar solvents) such as water and alcohols. Some polar solvents—for example, Ai,77-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and hexamethylphosphoric acid triamide (HMPA)—are not hydrogen bond donors (they are aprotic polar solvents) (Table 10.7). 

Doping of PBT with lithium perchlorate, LiC104, (Li" as counter ion) is ineffective in acetonitrile and propylene carbonate, but it is successful when the effective radius of the lithium ion is increased by using hexamethylphosphoric acid triamide as a solvent with high solvating power [250]. 

Solvation of Pb(CH3)4 in solution increases along the series of solvents cyclohexane < 1,2-dimethoxyethane dioxane = hexamethylphosphoric acid triamide < pyridine < tetrahydrothiophene < triethylamine < tetrahydrofuran < triethylphosphane < N,N,N, N -tetramethylethylenediamine = acetone < dimethylformamide < dimethyl sulfoxide as derived from the increase of the NMR coupling constants J( H, Pb). Coordination of only one solvent molecule and trigonal bipyramidal geometry of the complexes was supposed [8]. For studies of the dispersion interaction of Pb(CH3)4 and various solvents, see [9, 28]. For a correlation of the ionization potential and the solvation energy of Pb(CH3)4 and other tetraorganometal compounds in acetonitrile, see [36]. 

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