Benzyltriethylammonium cations

As indicated in Chapter 1, the hydroxide ion is not readily transported into the organic phase, particularly when the benzyltriethylammonium ion is employed as the catalytic cation. Hence, the reaction of chloroform with the hydroxide ion must occur by an interfacial mechanism. The interfacial reaction initially produces the trichloromethyl anion, which immediately forms an effective ion-pair with the benzyltriethylammonium cation. Diffusion of the ion-pair into the bulk of the organic phase occurs, followed by a slow decomposition of the trichloromethyl anion... 

Kinetics show that the reaction is pseudo-first order in the RX concentration and that there is a linear correlation in the rate of consumption of RX with the concentration of the catalyst. The need for a high rate of stirring indicates that, as discussed in Chapter 1, the base-initiated formation of the cobalt tetracarbonyl anion results from an interfacial exchange process. It is significant that, when preformed NaCo(CO)4 is used, the extractability of the anion by benzyltriethylammonium cation into diisopropyl ether is three times less efficient than it is into benzene or dichloromethane, but kinetic studies show that, in spite of the lower concentration of the anion in the ether, the rate of reaction with RX in that solvent is generally higher [3]. 

Cationic accelerants vary in their efficacy [161]. Other types of accelerant have also been evaluated. In one study [162], comparisons were made between tetra-ethylammonium bromide, benzyltriethylammonium chloride, polyfdiallyldimethylammonium chloride) and the diethyldimethylammonium derivative of a benzenesulphonate polyglycol ester. It was found that the cationic polymers had a greater effect than the simple quaternary ammonium compounds of lower molecular mass. This effect was attributed to the capability of the polymers to enter into hydrophobic interaction with the fibre surface. Ethylenediamine has also been found to accelerate the alkaline hydrolysis of polyester [163]. 

It is worth mentioning at this point that according to Normant et al. (1975) simple polyamines such as tetramethylethylenediamine (TMEDA) are even more active than [2.2.2]-cryptand in the benzylation of acetates in acetonitrile under liquid-solid conditions. These authors suggested that the activity was due to salt solubilization by cation complexation and not to formation of a quaternary ammonium ion since the latter showed no activity. This statement, however, is not in line with the results of Cote and Bauer (1977), who were unable to detect any interaction between K+ and TMEDA in acetonitrile. Furthermore, Vander Zwan and Hartner (1978) found Aliquat 336 (tricaprylylmethylammonium chloride) to be almost as effective as TMEDA in this reaction (Table 30). It might well be, however, that in amine-catalysed benzylation reactions the quaternary salt formed in situ acts both as a reactant and as a phase-transfer catalyst, since Dou et al. (1977) have shown that the benzyltriethylammonium ion is a powerful benzylation agent. 

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. 

Solid lithium aluminium hydride can be solublized in non-polar organic solvents with benzyltriethylammonium chloride. Initially, the catalytic effect of the lithium cation in the reduction of carbonyl compounds was emphasized [l-3], but this has since been refuted. A more recent evaluation of the use of quaternary ammonium aluminium hydrides shows that the purity of the lithium aluminium hydride and the dryness of the solvent are critical, but it has also been noted that trace amounts of water in the solid liquid system are beneficial to the reaction [4]. The quaternary ammonium aluminium hydrides have greater hydrolytic stability than the lithium salt the tetramethylammonium aluminium hydride is hydrolysed slowly in dilute aqueous acid and more lipophilic ammonium salts are more stable [4, 5]. 

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. 

Of a series of quaternary permanganate salts examined as organic oxidants, those of the unsaturated cations benzyltrimethylammonium, benzyltriethylammonium, methyltriphenylphosphonium, ethylenebis(triphenylphosphonium) and hexadecyl-pyridinium all decomposed explosively at —80—90°C, and of tetraphenylarsonium at 120—130°C. The permanganate salts of the saturated cations... 

Another version of the Schmidt-type rearrangement starts from silyl enol ethers (181) prepared from acylbenzenes and does not need any acid catalysts." The silyl enol ethers (181) react with N-chlorosuc-cinimide (NCS) in the presence of sodium azide and benzyltriethylammonium chloride to give chloro azido derivatives (182) via siloxycarbinyl cations. Subsequent thermal rearrangement of the azido products (182) leads to anilide derivatives (183 equation 51). 

The TEACC oxidation of organic sulfides replicated the results of similar oxidation by MCC (ORM 2009, p. 121 Ref. 43) except that a sulfenium cation intermediate is formed in the TEACC reaction, whereas an intermediate sulfonium cation had been proposed in the MCC oxidation. The oxidation of primary aliphatic alcohols by IFC replicated the results of benzyltriethylammonium chlorochromate (BTEACC) oxidation of secondary alcohols with the exception that primary alcohols exhibited Michaelis-Menten kinetics while secondary alcohols had first-order dependence (ORM 2007, p. 73 Ref. 35). 

---