PTC—See Phase transfer catalysts

The title compounds known for their anti-inflammatory activity were prepared earlier in low yields/ Polymer supported phase transfer catalysts have also been used (for details see sec 7.4.5) for various reactions. These have now been prepared" by the condensation of 2-aminophenols with phenacyl bromide in presence of a PTC in aq. K2CO3 (Scheme 26). 

Diazo compounds can be dediazonized by transition metal complexes to generate metallocarbenes, which are important intermediates in various transformations [45-48]. Since tosylhydrazones have been found to be readily available precursors of diazo compounds through the Bamford-Stevens reaction, a series of transition metal-catalyzed reactions of aldehyde tosylhydrazone salts in the presence of base and phase transfer catalyst (PTC) have been reported since 2000 [49-53]. It has been considered that metal carbenes generated from the in situ generated diazo compounds are involved in the catalytic cycle of these reactions -see (7). 

About ten years ago a knowledgeable organic chemist offered the opinion that "almost all the things that can be done via phase transfer catalysis has already been done." He was wrong, of course, as one can now look back and see that the great bulk of PTC chemistry now known came after his comment was made. While it may be true that many of the obvious and direct applications of PTC, especially for anion transfer, have been identified, it seems most likely to this author that a vast amount of new applications and more complex catalyst systems based on PTC await discovery and exploitation. 

Phase-transfer catalytic (PTC) conditions, which are specific for anionic reactions (and anionic activation) are perfectly well tailored for microwave activation, because after ion exchange between a substrate and catalyst, the resulting nucleophilic ion pair is a highly polar species especially prone to interaction with microwaves [32]. Eventually, the mixture of neat reagents in an open vessel can lead to a reaction under microwave conditions provided that one of the reagents is liquid or a low melting solid that couples well with microwaves. On the other hand, even a small amount of a good microwave absorber (e.g,. H20, DMF for example, see Table 1.3) added to reaction mixtures that consist of substrates that do not absorb microwaves in the solid state can initiate an increase of reaction mixture temperature and then chemical reaction. 

One of the oldest techniques for overcoming these problems is the use of biphasic water/organic solvent systems using phase-transfer methods. In 1951, Jarrouse found that the reaction of water-soluble sodium cyanide with water-insoluble, but organic solvent-soluble 1-chlorooctane is dramatically enhanced by adding a catalytic amount of tetra-n-butylammonium chloride [878], This technique was further developed by Makosza et al. [879], Starks et al. [880], and others, and has become known as liquid-liquid phase-transfer catalysis (PTC) for reviews, see references [656-658, 879-882], The mechanism of this method is shown in Fig. 5-18 for the nucleophilic displacement reaction of a haloalkane with sodium cyanide in the presence of a quaternary ammonium chloride as FT catalyst. 

Using bromodichloromethyl(phenyl)mercury, vinyl acetate afforded 2-acetoxy-l,l-dichlo-rocyclopropane (1, 85%), dichlorocyclopropanation of other aldehyde enol esters would also be expected. The cyclopropane 1 ( 10%) together with 2-acetoxy-l,l,l-trichloropropane (2,10%) were formed when the dichlorocarbene was generated from sodium trichloroacetate, the chain product 2 results from the reaction of the trichloromethyl anion (for the mechanism, see ref 197). These reactions are described in Houben-Weyl, Vol. 4/3, pp 177-178. Under phase-transfer catalytic conditions (CHClj/base/PTC), with a typical catalyst such as benzyl-triethylammonium chloride, vinyl acetate gave 2 (65%) only (Houben-Weyl, Vol.E19b, ppl550-1551). 

The main use of crown ethers in PTC is in solid-liquid phase-transfer. In particular, it has been emphasized that they should be the catalysts of choice under such conditions. Due to its particular structure, the crown ether can approach the crystalline lattice so that the extraction and subsequent complexation of the cation require very little cation displacement, while the anion is contemporaneously associated to the complex. In the case of quaternary salts, on the other hand, the steric hindrance around the cationic center makes its interaction with the surface of the crystal difficult and the solution mechanism more complicated in this case only the anion must be extracted to displace the one originally associated with the lipophilic cation. These conclusions met with some scepticism Indeed, onium salts, cryptands, polypodes, polyamines, etc. have been successfully employed as solid-liquid PTC catalysts (see Sects. 4, 5.2, 5.3, 5.4). Moreover, when the catalytic activity of quaternary salts, crown ethers and polyamines was compared with respect to the extraction of anions from the crystalline state into an organic solvent, crown ethers were found to be the best system for the transport of CN . The catalytic effectiveness is completely reversed in the case of other anions, such as F and CHjCOO , the quaternary salt being the most efficient in these cases... 

The most attractive oxidant is hydrogen peroxide being cheap and environment friendly thus it has found wide application in many processes. There are two major variants of its use under PTC conditions direct transfer of OOH anions as TAA salts into the organic phase or extraction of molecular H2O2 by hydrogen bonding to the catalyst anion. Both of these approaches assure efficient conversion of electron-deficient alkenes such as chalcones, unsaturated nitriles, or carbonyl compounds into oxiranes (eq. 173 for review see Ref. 79). 

The quaternary onium salt transfers the anion from the aqueous phase into the organic one, where the reaction takes place. It then transfers the leaving group into the aqueous phase. This mechanism assumes a partition of the catalyst between the two phases. On the other hand, other conditions being the same, the efficiency of a PTC catalyst is directly related to its solubility in the organic phase (see Sect. 2.3.3). The modified scheme 13 may thus be proposed alternatively in this case, the electroneutrality of the phases is simply maintained by the transport of the anions. 

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