Tetra-alkylammonium halides

Whilst reaction can take place in the absence of catalysts it is more common to use such materials as tetra-alkylammonium halides and tertiary amines such as triethylenediamine. A major side reaction leads to the production of isocyanurate rings, particularly in the presence of tertiary amines. 

One of the most important transformations catalysed by palladium is the Heck reaction. Oxidative addition of palladium(O) into an unsaturated halide (or tri-flate), followed by reaction with an alkene, leads to overall substitution of a vinylic (or allylic) hydrogen atom with the unsaturated group. For example, formation of cinnamic acid derivatives from aromatic halides and acrylic acid or acrylate esters is possible (1.209). Unsaturated iodides react faster than the corresponding bromides and do not require a phosphine ligand. With an aryl bromide, the ligand tri-o-tolylphosphine is effective (1.210). The addition of a metal halide or tetra-alkylammonium halide can promote the Heck reaction. Acceleration of the coupling can also be achieved in the presence of silver(I) or thallium(I) salts, or by using electron-rich phosphines such as tri-tert-butylphosphine. ... 

The effect of added electrolytes (sodium and potassium halides of progressively increasing molecular volume) in the concentration range 0.125—3 M on the viscosity behaviour of an aqueous sucrose solution (292 mM) between 25 and 40 °C has been investigated. Conductance data on the interaction of the sodium salts of several low-carbon aliphatic acids with sucrose in water and in formamide solutions have been reported and interpreted in terms of the effects that hydrocarbon chains have on hydrogen bonding in saturated solutions of sucrose. Conductance data have also been reported for the interaction of sucrose with symmetrical tetra-alkylammonium halides in formamide and in water in the temperature range 25—70°C. 

Finally, and most recently, phosphonate-cavitands have been used as catalysts [222]. Taking advantaged of the strong affinity tetra-alkylammoniums have for these cavitands, Dutasta and Dufaud showed that bound tetra-alkylammonium halides catalytically oxidize epoxides in the presence of CO2 to form cyclic carbonates in excellent yields. Catalytic activity was improved dramatically over a system lacking the host cavitand, implying a cooperative effect in the catalytic host-guest system [222]. 

Fig. 7. Excess chemical potential difference for exchanging chloride with iodide in solutions of tetra-alkylammonium halides of increasing chain lengths. Results from experimentally determined activity coefficients.

Alkylation of P-dicarbonyl compounds and p-keto esters occurs preferentially on the carbon atom, whereas acylation produces the 0-acyl derivatives (see Chapter 3). There are indications that C- and 0-alkylated products are produced with simple haloalkanes and benzyl halides, but only C-alkylated derivatives are formed with propargyl and allyl halides [e.g. 90]. Di-C-alkylation frequently occurs and it has been reported that the use of tetra-alkylammonium 2-oxopyrrolidinyl salts are more effective catalysts (in place of aqueous sodium hydroxide and quaternary ammonium salt) for selective (-90%) mono-C-alkylation of p-dicarbonyl compounds [91]. 

Tetrabutylammonium(triphenylsilyl)difluorosilicate is a new fluoride source soluble in anhydrous, nonhygroscopic and neutral organic solvents. This reagent converts organic halides and sulfonates into fluorides, however, 4-6 equivalents are required. 1-Iodooctane is converted to 1-fluorooctane (33) in 74% yield, the rest (26%) being the alkene.221 Apparently with tetra-alkylammonium fluorides, the formation of alkenes is generally not completely suppressable. 

Even this is not all If the alkylation were to continue, the secondary and the tertiary amines would be produced all together in the reaction mixture. The reaction comes to an end only when the tetra-alkylammonium salt R4N+ is formed. This salt could be the product if a large excess of alkyl halide R1 is used, but other more controlled methods are needed for the synthesis of primary, secondary, and tertiary amines. 

In many electrolytes, one or more of the constituent ions are specifically adsorbed at the interface. Specific adsorption implies that the local ionic concentration is determined not just by electrostatic forces but also by specific chemical forces. For example, the larger halide ions are chemisorbed on mercury due to the covalent nature of the interaction between a mercury atom and the anion. Specific adsorption can also result from the hydrophobic nature of an ion. Thus, tetra-alkylammonium ions, which are soluble in water, are specifically adsorbed at the mercury water interface because of the hydrophobic nature of the alkyl groups. Specific adsorption of molecular solutes, such as the alcohols, occurs for the same reason. 

Diborane finds such wide use in steroid chemistry that a new and very simple procedure for its preparation is welcomed. Tetra-alkylammonium borohydrides, which are readily extracted from an aqueous solution of Na BH and R4N -HSOT into CH2CI2, are used in the dried CH2CI2 solution addition of an alkyl halide (e.g. Mel) generates diborane in situ, and allows all the usual reactions of reduction or hydroboronation to be carried out conveniently and in high yields. 

The two general modes of colloidal stabilization are electrostatic (left) and steric (right). In the electrostatic mode there is a bilayer of anions (often halides) and a second layer of cations (for example tetra alkylammonium). In the steric stabilization there is a single bulky molecule attached to the surface (usually a P, N, or S donor, alkyl thiols are common). 

Infrared spectra in the O—H and N—H stretching regions could be used also to measure the interaction between halide ions (Cl", Br , 1 as tetra-alkylammonium salts) and water or N-methyl acetamide (NMA) in tetrachloromethane. Formation constants for the monosolvates (at 22.5°C) have been estimated by Symons et al [Sy 80]. The frequency shifts were approximately proportional to the free energy of hydrogen-bond formation. Formation constants for the monoaquo complexes and the NMA complexes were similar to those of the corresponding methanol solvates however, the sensitivity of the N—H frequency was less than that of the O—H frequency. 

A range of carboxylic esters undergo mild and neutral transesterification when treated with iodotrimethylsilane followed by an alcohol. The reaction is limited to primary and secondary aliphatic alcohols, and yields between 40 and 98% are obtained. Another non protic trans-esterification method involves treatment of the methyl esters of various carboxylic acids with tetra-n-alkylammonium halide at 140 °C, leading to the production of the corresponding n-alkyl esters. The generality of this method clearly depends on the availability of the required tetra-n-alkylammonium halide. Lactones react with iodotrimethylsilane in the presence of an alcohol to provide a convenient route to iodoalkyl esters in good yield. ... 

Two papers this year have described the use of chromate ions for the conversion of alkyl halides or alcohols into aldehydes or ketones (Equation 4). The reaction can be conducted in HMPA in the presence of a crown ether, and although yields are good for allylic and benzylic halides (ca. 80%) they are not so high for saturated halides. " The chromate ion, however, can be supported on an insoluble polymer matrix as the tetra-alkylammonium salt and this both enhances the nucleophilicity of the ion and simplifies the work up procedure. Using the polymer-supported reagent a variety of primary and secondary alcohols were oxidized to the corresponding aldehyde or ketone in excellent yield (ca. 90%). ... 

C. M. French and K. H. Glover, Trans. Faraday Soc., 61,1427 (1955). Electrical conductance of solutions in AT-methylacetamide. Systems studied seven alkali metal halides and two tetra-alkylammonium salts. 

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