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United States Patent Office Patented Oct 3,534,078 United States Patent Office Patented Oct. 13, 1970 1. 2 The nature of the anion associated with the "onium' 3,534,078 ONUM AMALGAMS cation is not of special importance and salts will generally Brian John Woodhall and John David Littlehailes, Run be chosen on such considerations as availability, ease of corn, England, assignors to Imperial Chemical indus manufacture, solubility in the chosen solvents, and cor tries Limited, London, England, a corporation of rosion effects on the apparatus. However, in the case of Great Britain the reaction involving alkali metals, since the electro No Drawing. Filed Jan. 26, 1968, Ser. No. 700,720 chemical potential promoting the reaction is essentially Claims priority, application Great Britain, Feb. 7, 1967, fixed, the reaction is an equilibrium and is therefore revers 5,842/67; Mar. 28, 1967, 14,098/67; Apr. 17, 1967, ible. The forward reaction to "onium” amalgam is not 17,529/67 inherently favoured, and so the anion should be such that Int. C. C07f 3/10, 3/12 O the alkali metal salt formed is substantially insoluble in U.S. C. 260-431 18 Claims the reaction medium, thereby influencing the course of the reaction in the desired direction. Anions which may be incorporated in the "onium' ABSTRACT OF THE DISCLOSURE 5 salts include paratoluene sulphonate, tetrafluoroborate, Amalgams of onium radicals represented by the gen chloride, bromide, iodide and bicarbonate. eral formula RM/Hg. Preferably, the onium radicals It is preferred that the organic radicals of the "onium' RM are quaternary ammonium (RN), quaternary phos salt and hence also of the "onium' amalgam are aliphatic. phonium (RP) or ternary sulphonium (RS). These Aromatic "onium' amalgams are rather unstable. Alkyl amalgams are prepared by reducing a solution of an 20 radicals, especially unbranched alkyl radicals, are most onium Salt at a mercury surface in a substantially inert, readily incorporated in the product amalgams and two or polar, aprotic medium. These amalgams are useful as more such radicals may be linked to form a heterocyclic reducing agents. ring structure. For example two of the four alkyl radicals of a quaternary ammonium amalgam may be linked to 25 form a nitrogen-containing heterocyclic system. "Onium' amalgams may be prepared from salts con This invention relates to amalgams of certain organic taining any one of the following cations: tetramethyl am cations known as "onium” ions, and to their preparation. monium, trimethyl ethyl ammonium, dimethyldiethyl am "Onium' cations form ionic, salt-like compounds with monium, tetraethyl ammonium, tetra-n-butyl ammonium, anions, the cation being a co-ordination complex of a non 30 tetra-n-propyl ammonium, triethylmethyl ammonium, metallic element. "Onium’ salts may be represented by dodecyltrimethyl ammonium, cetyl trimethyl ammonium, the general formula (RM) X- where R is an organic tetramethyl-phosphonium, tetra-n-butyl phosphonium, tri group, X is an anion of any acid capable of salt forma methyl sulphonium, triethyl sulphonium, tri-n-propyl sul tion and M represents the non-metallic element on which phonium, tri-n-butyl sulphonium and triphenyl sulpho the "onium” ion is based, and is, especially, nitrogen 35 (n=4) phosphorus (n=4) or sulphur (n=3). Furthermore "onium” amalgams may be prepared from The "onium” radical may therefore be represented by "onium' salts having two or more atoms of the non the general formula R.M. metallic element linked by a difunctional hydrocarbon The present invention provides amalgams of “onium group or groups, especially a poly(methylene) chain. An radicals, which may be represented by the general formula 40 example of such an "onium’ salt is tetramethylene bis RM/Hg. Preferably; the radicals are quaternary am (tri-n-butyl-phosphonium iodide). monium (RAN) quaternary phosphonium (RP) or ter Those amalgams formed from "onium' cations in nary sulphonium (RS). which all the substituents are methyl groups are generally "Onium” amalgams behave and analyse as though they the most stable. consist of an "alloy' of mercury and the “onium' radical The reactions are preferably carried out in the absence (RM) and are powerful reducing agents. of air, and at low temperatures, preferably between -30° A process for preparing "onium” amalgams comprises C. and --10 C. Phosphonium and sulphonium amalgams reducing a solution of an "onium” salt at a mercury sur should be prepared below 0° C. face in a Substantially inert, polar, aprotic medium. Pref When the process is carried out by the electrolytic erably, the process is carried out eithe (a) by electrolysis 50 method, it is convenient to use a cell having a porous par at a mercury cathode or (b) by reaction with an alkali tition separating the anode compartment from the cathode metal amalgam. compartment. The partition may be constructed of sintered The preferred alkali metal amalgams are sodium and po glass. The nature of anode material is unimportant, pro tassium amalgams, though such a preference is based only vided that it is inert; platinum is therefore very suitable. on economic considerations. 55 Current densities in the range 0.5-100 milliamps per It is desirable that the reaction medium is substan square centimetre are convenient to use. tially free of reducible components which could be dis When the process is carried out by the amalgam meth charged at the cathode or react with either the alkali metal od, it is not necessary to maintain the relative proportions or "onium' amalgam. The contaminant most likely to be of alkali metal amalgam and "onium’ salt within closely present is water, but other proton sources and metal ions 60 defined limits. It is convenient to use moderate excess of may also be present. When it is desired to isolate the "onium” amalgam the concentration of other proton alkali metal amalgams, although even where the "onium' sources and metal ions should be maintained as low as amalgam forms a distinct phase, some contamination possible. If the "onium” amalgam is to be used in situ as a with alkali metal is likely. In most uses, such contamina reagent, for example, to reduce an organic substrate, tion is not particularly disadvantageous, though it does water may be present. It is advantageous to dry com 65 indicate that to prepare a pure "onium' amalgam, the mercial grades of polar aprotic Solvents bypassing through electrolytic route is to be preferred. a molecular sieve or by distilling from a drying agent, Tetramethyl ammonium amalgam is a grey crystalline such as calcium hydride. Suitable polar aprotic solvents solid giving an X-ray powder photograph similar to solid are acetonitrile, dimethylformamide, hexamethylphos crystalline mercury, but with a distorted lattice. Tetra phoramide, N-methylpyrollidone and dimethyl sulphox 70 methyl, dimethyldiethyl, methyltriethyl, tetra - n-propyl ide. and tetra-n-butyl quaternary ammonium amalgams are () 5 34,078 3. 4. also all grey solids. They float on the Surface of mercury EXAMPLE 4. and the tetramethyl ammonium amalgam, at least, is A solution of tetramethyl ammonium borofluoride in moderately stable in the absence of air and Water. anhydrous acetonitrile (15 mls., 0.01 M), dried by treat The potentials of some of these amalgams were meas ment with an aluminium calcium silicate molecular sieve ured with reference to a Saturated calomel electrode by a 5 (BDH type 5A) was electrolysed at a temperature of 0 digital voltmeter. The following results were obtained. C. for a period of 2 hours at a current density of 20 ma./ Amalgam: Potential (volts) sq. cm., using a cell divided by a glass frit and fitted (CH)4N/Hg - - - - - - - - - - - - - - - - - - - - - - - - -2.6 With a mercury cathode and a platinum anode. Oxygen (CH-CH2)4N/Hg - - - - - - - - - - - - - - - VM- M -2.8 free nitrogen was used to de-aerate the solution. Tetra (CH3(CH2)2)N/Hg - - - - - - - - - - - - - - - - - a -2.9 O methyl-ammonium amalgam was produced in the cath (CH3(CH2)3)4N/Hg w - - - - - - - - - - - - - - - - - -3.0 ode compartment at 95% current efficiency as grey crys Amalgams prepared from cetyltrimethyl-, ethyltri tals on the mercury surface. On stirring the surface, the methyl-, dodecyltrimethyl-, phenyltrimethyl- and benzyl crystals dissolved in the mercury. trimethyl- quaternary ammonium compounds are soluble EXAMPLE 5 in mercury, but they may be identified by potential meas The procedure of Example 4 was repeated using a urements. Phosphonium amalgams are generally black amor Solution of tetraethyl ammonium iodide in anhydrous phous powders which float on the surface of mercury. acetonitrile (0.1 M). Tetraethyl ammonium amalgam was They are less stable than equivalent nitrogen contain formed as a grey, fluffy material on top of the mercury ing compounds, and should be stored at temperatures cathode, and was insoluble in mercury. The mercury was below -30° C. decanted off, and the amalgam was washed with cold Sulphonium amalgams are generally soluble in mer acetonitrile or ether under anhydrous and air-free con Cllry. ditions to remove unreacted tetraethyl ammonium iodide. "Onium' amalgams may be analyzed by acid hydroly EXAMPLE 6 sis, followed by conventional analytical methods to iden tify the “onium' salt which is formed. A solution of tetra-n-butylammonium iodide in an Oxonium amalgams have not been successfully pre hydrous dimethylformamide (0.1 M), dried by distilla pared by methods according to the present invention. tion from calcium hydride under reduced pressure, was Presumably they are too unstable to allow identification. 30 electrolysed at a mercury cathode in a divided cell at a AS mentioned above, "onium' amalgams are powerful temperature of -20° C. in the absence of air, using a reducing agents, especially for organic substrates and current of 25 ma. Tetra-n-butylammonium amalgam was may be used in situ or may be removed from the ap formed with a current efficiency of 70%. The amalgam paratus in which they are prepared.
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