Hexamethylphosphoramide, solution alkali metals

Examples of alkali metal stannides are mainly limited to tris(trimethylsilyl)- and triphenylstannides. NMR studies of trimethyl- and tributylstannide in ether, THF, and hexamethylphosphoramide (HMPA, OP(NMe2)3) reveal the compounds to be monomeric in solution, as indicated by the large Li-Sn coupling. The addition of HMPA produces an... 

The most commonly used procedure is that established by Wooster and extensively developed by Birch, i.e. the reduction of a solution of the substrate in a mixture of liquid ammonia with an alcohol (usually ethanol or r-butyl alcohol) and an inert cosolvent (e.g. diethyl ether, tetrahydrofuran) with an alkali metal (lithium, sodium or potassium). Low molecular weight amines have been utilized in place of the ammonia, although the procedure then leads to more extensive reduction. Hexamethylphosphoramide may also serve in place of the ammonia, but there is no apparent advantage to offset its higher cost, toxicity and carcinogenicity. Of rather more interest is the potential of electrochemical and photochemical approaches, which may give complementary outcomes. 

Metals commonly utilized include the alkali metals, mainly lithium, sodium and potassium, and also calcium, zinc, magnesium, tin and iron. Alkali metals and calcium have been used in liquid ammonia," in low molecular weight aliphatic amines," in hexamethylphosphoramide, in ether or in THF containing crown ethers, or in very dilute solutions in polyethers such as 1,2-dimethoxyethane (DME)." Reactions with metal solutions in liquid ammonia often use a cosolvent, such as ether, THF or DME, to increase solubility of the organic substrate in the reaction mixture. These same metals, as well as zinc and magnesium, have been used as suspensions in various solvents including ether, toluene, xylene, etc. 

Solutions of alkali metals in ammonia have been the best studied, but other metals and other solvents give similar results. The alkaline earth metals except- beryllium form similar solutions readily, but upon evaporation a solid ammoniate," MINI-lj),. is formed. Lanthanide elements with stable + 2 oxidation states (europium, ytterbium) also form solutions. Cathodic reduction of solutions of aluminum iodide, beryllium chloride, and tetraalkylammonium halides yields blue solutions, presumably containing Al, 3e Be", 2e and R4N, e respectively. Other solvents such as various amines, ethers, and hexamethylphosphoramide have been investigated and show some propensity to form this type of solution. Although none does so as readily as ammonia, stabilization of the cation by complexation results in typical blue solutions... 

The isolation and study of triatomic radical anions is also a matter of considerable interest. The ozonide ion has been successfully studied as alkali metal salts in an argon matrix (74) and the S3 ion as a substitutional species in a sodium chloride crystal as host 69,75). The latter species has also been identified by resonance Raman studies as the one responsible for the blue colour of ultramarine blue and lapis lazuli (70). Sulphur also forms deep blue solutions under certain circumstances in many other media (e.g. hexamethylphosphoramide and dimethylformamide), and it is unquestionably the S3 ion which is the species responsible for the colour in these cases also. 

The spectral maxima at 320, 390, and 590 nm were found to correspond to absorptions by the ions S , S , and S7, respectively. Direct observations (resonance Raman, i.r., and e.s.r. spectra) and indirect measurements (visible— u.v. spectra, conductivity, and magnetic susceptibility) strongly suggest that the intensely blue species formed by alkali-metal polysulphides or elemental sulphur in hexamethylphosphoramide can be attributed to the Sj radical anion. Cryoscopic, conductance, and magnetic measurements euid i.r. and u.v. spectral data have been used to study the cations formed in solutions of sulphur in disulphuric acid. The blue coloration was shown to be due to the formation of the Si" " ion and the colourless solutions were found to contain the S ion. The solid compounds S4S30ioand 8383010, formed by the action of SO3 on elemental sulphur in 8O2 media at low temperatures, have been isolated and characterized. 

Alkali metals dissolve in liquid ammonia and other donor solvents, such as aliphatic amines (NR3, in which R = alkyl) and OP(NMe2)3, hexamethylphosphoramide, to give blue solutions believed to contain solvated electrons ... 

A powerful aprotic solvent capable of dissolving alkali and alkaline earth metals is hexamethylphosphoramide. The characteristic blue paramagnetic solution is observed [101]. It had been demonstrated that the ammoniated electron could be produced in the electrolysis of liquid ammonia [102] and recently it has been demonstrated that the solvated electron is produced in the electrolysis of hexeimethylphosphoramide [103, 104]. It is, however, doubtful whether eaq can be produced in this manner [102]. 

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