Quaternary onium salts

A multiphase system consisting of a hydrocarbon solvent, a strong alkaline solution, and a quaternary onium salt, in the presence of a Pd/C catalyst with hydrogen that was bubbled at atmospheric pressure through the organic phase, allows the rapid displacement of chlorine from polyhalogenated benzenes. The onium salt, insoluble in both phases, is localized in the interfaces, coats the Pd/C catalyst, and constitutes the phase in which the reaction takes... 

These two phase reactions summarized in Scheme 1 are strongly promoted by catalytic amounts of the quaternary onium salts which will transfer between aqueous and organic phases, as shown in Scheme 2 for the substitution of halides with cyanides. The effect was termed by Starks... 

Oxidation of —CHtOH — —CHO (cf., 12,479-480). This oxidation can be effected in high yield with sodium hypochlorite (slight excess) in buffered H20/ CH2C12 with this nitroxyl radical and KBr as the catalysts.1 The oxidation is exothermic, and the temperature should be maintained at 0-15° with a salt-ice bath. Saturated primary alcohols are converted to aldehydes in 88-93% yield yields are lower in the case of unsaturated substrates. Addition of quaternary onium salts permits further oxidation to carboxylic acids. 

Probably the most important group of phase transfer reactions, and certainly the commonest, are those in which an anion is transferred from the aqueous phase into the organic solvent, where nucleophilic substitution occurs. These would once have been performed in a dipolar aprotic solvent such as DMF. A good example is the reaction between an alkyl halide (such as 1-chlorooctane), and aqueous sodium cyanide, shown in Scheme 5.5. Without PTC, the biphasic mixture can be stirred and heated together for 2 weeks and the only observable reaction will be hydrolysis of the cyanide group. Addition of a catalytic amount of a quaternary onium salt, or a crown ether, however, will lead to the quantitative conversion to the nitrile within 2 h. 

The commonly used quaternary onium salts work by associating with the inorganic anion and transferring it into the organic phase. Complexing agents such... 

The first catalysts utilized in phase transfer processes were quaternary onium salts. In particular, benzyltriethylammonium chloride was favored by Makosza (7 ) whereas Starks utilized the more thermally stable phosphonium salts (6,8). In either case, the catalytic process worked in the same way the ammonium or phosphonium cation exchanged for the cation associated with the nucleophilic reagent salt. The new reagent, Q+Nu , dissolved in the organic phase and effected substitution. 

The reaction was originally used by Starks ( ) in his early studies of ptc mechanism. The reaction was monitored by gas-liquid chromatography and decane served both as solvent and internal standard. The reaction was run using a quaternary onium salt, Aliquat 336, and 18-crown-6 for calibration. The rates are shown in Table II. Please note that the relative rate of 1.00 corresponds to an absolute rate at 105 of 1.34 X 10 M-lsec . ... 

Quaternary onium salts were the first phase-transfer catalysts used subsequently, a number of compounds (linear polyethers, polypodands, crown-ethers, cryptands, cage-compounds, etc.) were found effective for the anion activation in two-phase systems. These structurally different systems must satisfy at least two fundamental conditions in order to behave as phase-transfer catalysts i) solubility in the organic phase ii) steric hindrance around the cationic center leading to a good cation-anion separation within the ion-pair. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

The direct quaternization of chloromethylated polystyrenes by tertiary amines or phosphines represents the easiest way to obtain polymer-supported quaternary onium salt (12,13). A lipophilic character of quaternary cation and a topology allowing sufficient cation-anion separation also play an important role (35,36). A linear spacer chain (of about 10 carbon atoms) between the catalytic site and the polymer backbone substantially increases the reaction rates. The loading of quaternary onium groups also affects catalytic efficiency, the influence being different for directly bonded and spaced groups, e.g. 10 and 11, respectively (37). 

Conversion of saturated, primary alkyl and aryl alkyl alcohols into the corresponding aldehydes can be achieved by this method provided that the alcohols are entirely dissolved in the organic phase. Relatively unstable protective groups are not affected, as in the oxidation of the acetonide of 1,2,6-hexanetriol, whereas conjugated and isolated double bonds give rise to side reactions which considerably decrease selectivities and yields.4 Some examples of aldehydes synthesized with this method are reported in Table 1. Under the same conditions, secondary alcohols are oxidized to ketones. Addition of catalytic amounts of quaternary onium salts allows fast and total conversion of primary alcohols and aldehydes into carboxylic acids making this methodology very versatile 4... 

Under the same reaction conditions, catalytic activity is greater for cryptands than crown ethers or quaternary onium salts (111, 113). A series of polymer cryptands derived from the vinyl-containing monomer 16 have been discussed and could well find useful application (114). 

PEGs have been extensively investigated as PTCs in many commercial processes to replace expensive and environmentally harmful PTCs [46, 47], Compared with the commonly used PTCs, linear PEGs are much cheaper than analogous macrocyclic crown ethers and cryptands [48]. PEGs are also more stable at high temperatures, up to 150-200 °C, and show higher stability to acidic and basic conditions than quaternary onium salts [49]. 

The mechanism of living copolymerisation in the presence of aluminium porphyrin coupled with quaternary onium salt was proposed to involve activation of the anion of the onium salt as the nucleophile by the metallopor-phyrin the structure of the active species was found to be six-coordinate aluminium prophyrin carrying one reactive axial ligand on both sides of its square planar AIN4 skeleton [188] ... 

Considerations about the possible mechanism of copolymerisation with the (tpp) AICI EtPh PBr catalyst should include the fact that quaternary onium salts themselves promote the cyclisation of epoxide and carbon dioxide [243],... 

Thus, the possibility of the nucleophilic attack of the anion from the quaternary onium salt on the oxirane coordinated at the aluminium atom should also be taken into account ... 

Ue M, Ida K, Mori S. Electrochemical properties of organic liquid electrolytes based on quaternary onium salts for electrical double-layer capacitors. Journal of the Electrochemical Society 1994 141(ll) 2989-2995. 

Polymer supported quaternary onium salts [68] and polymer supported catalysts containing phosphorus palladium complexes and quaternary onium... 

The crystal standard of (7r-cp)2Ti(NCO)2 has been determined and confirms that, in the solid state, the cyanate group is here N-bonded it was only in the last stages of the analysis that the 0-bonded alternative could be finally eliminated (i). Hexa-iV-cyanato complexes of ytterbium, erbium, and neodymium have been reported as quaternary onium salts (16) and reference made to the series of tetraethylammonium salts of [Ln(NCO)0] (Ln = Eu-Yb) (22) these results extend Table XLII. The ESR spectra of series of complexes CuL2(NCO)2 (L = an, or substituted an) have been interpreted to show Cu— N(CO)—Cu bridges with no indication of any Cu-0 interactions (19). The ESCA spectra of [M(NCO)4] (M = Mn, Co, Zn) have been recorded (12). 

Of all aluminum electrolyte systems, to our knowledge type 3 is today the technically most accepted. This type of electrolyte was discovered and intensely studied by Ziegler and Lehmkuhl [118, 217, 218, 221]. The companies Siemens AG, HGA, MBB, SEDEC, Interatom, and ALU 2000 have developed the industrial scale process (Sigal Process = Siemens galvanoaluminum). The used electrolytes consist of aluminumalkyls as well as alkali metal halides or quaternary onium salts, which are dissolved in aromatics (i.e., toluene). Electrolysis is carried out at temperatures around 90-100°C. Electrolytes of this kind have demonstrated their high produc-... 

Landini, D., Albanese, D., Mottadelli, S., Penso, M. Finkelstein reaction with aqueous hydrogen halides efficiently catalyzed by lipophilic quaternary onium salts. J. Chem. Soc., Perkin Trans. 1 1992, 2309-2311. 

Landini, D. Maia, A. Rampoldi, A., Stability of Quaternary Onium Salts under Phase-Transfer Conditions in the Presence of Aqueous Alkaline Solutions /. Org. Chem. 1986, 51,3187. 

Strategy C. A catalytically active quaternary onium salt is used either under solvent-lfee conditions or in solution. The value of the catalyst... 

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