Tetra-n-butylammonium halides

E-(P-Alkylvinyl)phenyliodonium salts react with tetra-n-butylammonium halides to yield the correspondingly substituted Z-haloethenes (80-100% for chloro-, bromo- and iodo-derivatives) [41], In contrast, in the corresponding reaction with Z-(2-benzenesulphonyl-ethenyl)phenyliodonium salts, nucleophilic substitution occurs with retention of configuration to yield the Z-2-benzenesulphonyl-l-haloethenes [42], The ammonium fluorides fail to yield the fluoroethenes, but produce the ethynes by simple elimination [41]. Where carboxylic acids have low solubility in organic solvents, their conversion into the acid chlorides is frequently difficult. Phase-transfer catalysis not only allows the conversion to be effected rapidly, it also results in high yields of a wide range of acid chlorides [43]. 

Di-n-propylzinc reacts with benzaldehyde in ether at 25 °C to give both addition products and reduction products. The second-order rate coefficient for total reaction is 2.1 x 10-3 l.mole-1.min-1, but is increased in value on addition of tetra-n-butylammonium halides. Addition of these halides also considerably increases the ratio of addition to reduction30. 

These salt effects are schematically depicted in Scheme 8. As we will discuss later more in detail (Sections Vl.B.3 and VII.E.3), mechanistically, salts may act in two different ways. In polar solvents they will suppress the free ions and considerably reduce their lifetime. This often converts bimodal MWD to monomodal MWD and provides controlled polymers. However, in polymerization of vinyl ethers initiated by strong Lewis acids such as SnCl4, where only ion pairs are present after addition of a few percent of salts or in nonpolar toluene, control is still very poor (Fig. 17B). Controlled polymers can be obtained only after addition of a more than equimolar amount of tetra-n-butylammonium halides. This implies that the salts change the weakly nucleophilic counterion SnCIs-to the more nucleophilic SnCl62 , which faster converts growing carbo-cations to covalent species. Another effect of added salts is related to... 

An example of the Sn2 rate dependence on the nature of the counterion is given by the reaction of -butyl 4-bromobenzene sulfonate with lithium- and tetra-n-butylammonium halides in the weakly dissociating solvent acetone (cr = 20.6) [279]. 

ALKYL HALIDE EXCHANGE Tetra-n-butylammonium halides. 

Tetra- -butylammonium bromide, 601 Tetra-n-butylammonium fluoride, 78, 81 Tetra-n-butylammonium halides, 565 Tetra-n-butylammonium hydrogen sulfate,... 

An attempt to study the feasibility of clatharate formation under conditions that might be considered as physiological has been evaluated by X-ray diffraction. One of the tetra-n -butylammonium halides, which seem to readily form clatharates In water at high hydrostatic pressure, when examined by X-ray diffraction in concentrated aqueous solution at ordinary pressuresshows a radial distribution function that is In good agreement... 

In what appears, initially, to be a closely similar reaction, acid chlorides react with alkyl halides under solidtliquid two-phase conditions using sodium hydrogen carbonate in the presence of sodium iodide and tetra-n-butylammonium bromide [45]. Although the mechanism is not clear, it has been proposed that the acid chloride is initially converted into the carboxylate anion. It is also probable that the halogen interchange between the sodium iodide and the alkyl halides enhances their reactivity. Although the yields are high, the availability of the alkyl halides and alcohols are usually similar and there appears to be little to commend this process over the catalysed reaction of the acid chlorides with the alcohols. 

It is noteworthy that benzyltriethylammonium chloride is a slightly better catalyst than the more lipophilic Aliquat or tetra-n-butylammonium salts (Table 5.2). These observations obviously point to a mechanism in which deprotonation of the amine is not a key catalysed step. As an extension of the known ability of quaternary ammonium halides to form complex ion-pairs with halogen acids in dichloromethane [8], it has been proposed that a hydrogen-bonded ion-pair is formed between the catalyst and the amine of the type [Q+X—H-NRAr] [5]. Subsequent alkylation of this ion-pair, followed by release of the cationic alkylated species, ArRR NH4, from the ion-pair and its deprotonation at the phase boundary is compatible with all of the observed facts. 

It can be assumed that the azoles are deprotonated by the interfacial exchange mechanism, but it is noteworthy that it has been suggested that the rate of alkylation of indole under liquiddiquid two-phase conditions decreases with an increase in the concentration of the sodium hydroxide [8]. The choice of catalyst appears to have little effect on the reaction rate or on the overall yields of alkylated azole. Benzyltriethylammonium chloride, Aliquat, and tetra-n-butylammonium hydrogen sulphate or bromide have all been used at ca. 1-10% molar equivalents (relative to the concentration of the azole) for alkylation reactions, but N-arylation of indole with an activated aryl halide requires a stoichiometric amount of the catalyst [8]. 

Halogens are frequently used as oxidation agents and, under two-phase conditions, they can either be employed as ammonium complex halide salts [3], or in the molecular state with or without an added quaternary ammonium catalyst [4]. Stoichiometric amounts of tetra-n-butylammonium tribromide under pH controlled conditions oxidize primary alcohols and low-molecular-weight alkyl ethers to esters, a,cyclic ethers produce lactones [3], and secondary alcohols yield ketones. Benzoins are oxidized to the corresponding benzils (80-90%) by the tribromide salts in acetonitrile in the presence of benzoyl peroxide [5]. 

Amino acid synthesis (8, 389). Alkylation of the aldimine (1) from glycine ethyl ester and /j-chlorobenzaldehyde under phase-transfer conditions offers a general route to amino acids. Either liquid-liquid phase-transfer or solid-liquid phase-transfer catalytic conditions are satisfactory with active halides, but alkylation with allylic halides and less active alkyl halides is best effected under ion-pair extraction conditions (6,41), with 1 equiv. of tetra-n-butylammonium hydrogen sulfate (76-95% yields).1... 

It was found that sodium salt (II) can be easily and cleanly monoalkylated with a variety of polyfunctional organic halides under strictly anhydrous conditions. The reactions are generally carried out in refluxing benzene in the presence of 10 mol-% of tetra-n-butylammonium bromide (TBAB) as phase-transfer catalyst ... 

Table 2. Thermodynamic parameters of complexation of calixpyrrole receptors with halides and dihydrogen phosphate anions (tetra-n-butylammonium as counterion) in acetonitrile, dichloromethane, A/,A/-dimethylformamide, dimethyl sulphoxide and propylene carbonate... 

Examination of the Sn2 reaction between ethyl tosylate and halide ions in hexamethylphosphoric triamide (fir = 29.3) with a variety of counter ions [Li , [n-C4H9)4N ] has shown that the rates obtained with Hthium salts are always higher than those with the corresponding tetra-n-butylammonium salts [341]. This is in contrast to the situation observed in acetone [279]. This means that, in this particular solvent, lithium salts are more dissociated than tetraalkylammonium salts. This has indeed been confirmed by conductivity measurements [341, 342], The lithium cation apparently has specific interactions with strong EPD solvents such as [(CH3)2N]3PO cf. Section 3.3.2). 

It is a palladium-catalyzed cross-coupling reaction between organosilanes (vinyl, ethynyl and allylsilanes) and organic halides (aryl, vinyl and allyl halides). Allylpal-ladium chloride dimmer [( ri -C3H5PdCl)2] and either tris(diethylamino)sulfonium difluorotrimethylsilicate (TASF) or tetra-n-butylammonium fluoride (TBAF) are used as catalysts. Fluoride ion acts as an activator for the coupling, forming an intermediate hypervalent anionic silicon species, which can then transmetallate with palladium as a preliminary reaction to coupling. 

Hydroxydithiocinnamic acids (1) can be converted into the monosalt by treatment with tetra-n-butylammonium hydrogen sulfate and NaOCHj, which on alkylation gives the dithioesters (2) in high yield. Mercaptals (3) can be prepared in 70-88% yield by treatment of (2) with thallous ethoxide (2, 407 411) and an alkyl halide. 

One of the two hydrogen atoms of phosphoramidates can be temporarily blocked with a trimethylsilyl group. The sodium salt of diethyl V-(trimethylsilyl)phosphoramidate (102) reacted with alkyl bromides in benzene in the presence of 10 mol % of tetra-n-butylammonium bromide to afford, after desilylation, the corresponding V-alkyl derivatives (103) in 79-95% yields for primary alkyl halides. Secondary alkyl halides gave poor results (Scheme 43). The addition of the quaternary ammonium salt is essential to promote the alkylation reaction. Hexamethyldisilazane and its cyclic analogs can also be utilized in the preparation of amines under moderate conditions (Scheme 43). ... 

Thallium(I) cyanide was introduced by Taylor and McKillop as a reagent. Aromatic and heteroaromatic acyl cyanides are produced in go yield, whereas aliphatic acid halides lead under these conditions mainly to dimerization products. 18-Crown-6 is a good catalyst for the preparation of cyanoformate in methylene chloride with potassium cyanide and chloroformates. Similarly, tetraethylammonium cyanide gives cyanoformates in high yield under very mild conditions. Aroyl cyanides are generated easily by phase transfer catalysis with tetra-n-butylammonium bromide. Tri- -butyltin cyanide proved successful only with aromatic acid halides, leading to dimerization products with aliphatic compounds. ... 

To overcome the problems derived from formation of anomeric mixtures when O-benzylated glycosyl halides are used as precursors, Ledford and Carreira52 have disclosed an elegant synthesis of 2,3,4-tri-O-benzyl-a-D-glucopyranosyl isothiocyanate (10) based on the use of l,6-anhydro-2,3,4-tri-0-benzyl-/t-i)-glucopyranose (9) as the glycosyl donor. Treatment of 9 with tetra-n-butylammonium thiocyanate and boron trifluoride-etherate complex provided 10 in 50% yield with total control of the anomeric configuration (Scheme 5). 

Somewhat less clear is the nature of catalysis by halogens and positive halogen compounds, such as I2 [176, 177], NBS [178], tetra-N-butylammonium tribromide [179], and hexamethylenetetramine-bromine complex [180]. These compounds would be expected to react with indoles by halogenations, generating the hydrogen halide as a by-product. Another halogenated catalyst, 2,4,6-trichlorotriazine, is believed to function by generation of HCl in the presence of moisture [181]. 

Potassium carbonate I tetra-n-butylammonium bromide Thiolphosphinic acid esters from thionophosphinic acids and halides Solid-liq. phase transfer catalysis... 

Li et al. developed the solvent-free C-N coupling between A-heterocycles and aryl/ heteroaryl halides promoted by the nano-Cu2O/l,10-phen/tetra-n-butylammonium fluoride (TBAF) catalytic system. Different types of CU2O were evaluated, and the studies showed that CU2O nanoparticles (especially the cubic form) were the most efficient for the C-N coupling reactions (Scheme 4.28) (Tang et al., 2008). 

Ketones and benzaldehydes are obtained in high yield from secondary and benzylic halides respectively, by oxidation with tetra-n-butylammonium dichromate (prepared and isolated prior to use). ... 

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