PTC conditions

NaOCL Sodium hypochlorite proves to be a useful reagent under PTC conditions for the following transformations (30) ... 

Cyclization of 2-bromo-2-perfluoroalkylethyl acetate under PTC conditions provides perfluoroalkyl oxiranes [fO] (equation 9)... 

In general, the stability of a PT catalyst is a function of cation structure, presence of anions, type of solvent, concentration, and temperature. Degradation of catalysts under PTC conditions may occur. For instance, ammonium and pho.sphonium salts may be subject to decomposition by internal displacement (usually at temperatures of 100 - 200 °C) ... 

Compounds 242 when treated with trimethylsulfonium iodide under phase-transfer catalysis (PTC) conditions ( -Bu4XI, dichloromethane, 50% NaOH) provide mixtures of the corresponding 2-oxiranylvinyl benzotriazoles 243 and 18-83% yields of heteropentalenes 244. The ratio of these two main products is highly dependent on the R substituent (Scheme 29) <2004ARK(ii)109>. 

Allenes as starting materials are still being explored. One example shown below is that the reaction of bromoallene with a 1,3-diketone under PTC condition provided a trisubstituted furan in high yield <060L5061>. 

Alkylation of dianhydrohexitols under phase-transfer catalysis (PTC) conditions... 

Dianhydrohexitols, important by-products of biomass (Scheme 3.10), were dialky-lated under PTC conditions in the presence of a small amount of xylene. 

Tab. 3.14 Yield (%) from dialkylation of dianhydrohexitols under PTC conditions.

Dealkoxycarbonylation of activated esters occurs classically under drastic thermal conditions [90]. It constitutes a typical example of a very slow-reacting system (with a late TS along the reaction coordinates) and is therefore prone to a microwave effect. The rate determining step involves a nucleophilic attack by halide anion and requires anionic activation, which can be provided by solvent-free PTC conditions under the action of microwave irradiation [91]. The above results illustrate the difficult example of cyclic /1-ketoesters with a quaternary carbon atom in the a position relative to each carbonyl group (Eq. 36). 

Tab. 3.16 The Krapcho reaction under solvent-free PTC conditions.

In general, under PTC conditions, three types of catalytic procedure can be considered (Scheme 5.1). 

Liquid-liquid PTC conditions in which weak organic acids (e. g. carboanions) react in the presence of concentrated aqueous sodium or potassium hydroxide which is in contact with the organic phase containing an anion precursor and organic reactants the anions are created on the phase boundary and continuously introduced, with the cations of the catalyst, into the organic phase, in which further reactions occur (Scheme 5.1 path b). 

Solid-liquid PTC conditions in which the nucleophilic salts (organic or mineral) are transferred from the solid state (as they are insoluble) to the organic phase by means of a phase-transfer agent. Most often the organic nucleophilic species can be formed by reaction of their conjugated acids with solid bases (sodium or potassium hydroxides, or potassium carbonate) (Scheme 5.1 path b). Another proposed mechanism suggests that interfacial reactions occur as a result of absorption of the liquid phase on the surface of the solid. 

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

In conventional methods, PTC has provided interesting procedures for O-alkylation, and coupling PTC conditions with microwave activation has proved to be quite fruitful for such reactions. 

Tab. 5.1 Alkylation of CH3COO K under MW + PTC conditions (domestic oven, 600 W).

Tab. 5.2 Synthesis ofn-octyl acetate under MW + PTC conditions (Synthewave, 5 min, 160°C).

Tab. 5.3 Reaction of benzyl halides with hexanoic acid under MW + PTC conditions (10 min, 560 W, 10% PhCH2N+Me3Cr).

A series of new ethers has been obtained by alkylation of dianhydrohexitols (i. e. iso-sorbide, isomannide, isoidide) under the action of microwave irradiation and PTC conditions. Yields exceeded 90%, a dramatically improvement compared with those from conventional heating, despite similar temperature profiles. The best yields, for example from isosorbide, were obtained in the presence of a small amount of xylene and TBAB as catalyst at 140 °C (Eq. 9 and Tab. 5.6) [20],... 

Catechol was reacted with /fmethallyl chloride under the action of MW and PTC conditions yields of 2-methallyloxyphenols varied from 59 to 68% under liquid-liquid conditions (Tab. 5.8), whereas no reaction was observed in a solid-liquid PTC procedure (Eq. 14) [25]. 

With the purpose of improvement and simplification of the synthesis of potential cosmetic compounds, alkylation of mono- and dihydroxybenzaldehydes with long-chain halides was efficiently realized under solvent-free MW + PTC conditions (Eqs. 16 and 17 and Tab. 5.9) [27, 28],... 

Tab. 5.9 Alkylation of 3,4-dihydroxybenzaldehyde with long chain alkyl bromides under the action of MW + PTC conditions.

The synthesis of 8-quinolinyl ethers from 8-hydroxyquinolines and organic halides has also recently been reported under PTC conditions with microwave irradiation (Eq. 18) [29]. 

Alkylations of phenols with epichlorohydrin under PTC conditions and microwave irradiation were described twice in 1998. Subsequently, ring-opening reactions of the epoxide group were also performed using microwaves (Eqs. 20 and 21) [31, 32]. In the first catalytic synthesis of chiral glycerol sulfide ethers was described [31] in the second biologically active amino ethers were prepared [32],... 

Tab. 5.11 N-Alkylation of saccharin with bromides under MW + PTC conditions (750 W).

Under solid-liquid PTC conditions 5,5-diethylbarbituric acid was N,N-dialkylated in a good yield in the presence of a lipophilic ammonium salts and potassium carbonate when reaction mixtures were irradiated in a household microwave oven (Eq. 26). 

N-Substituted amides and lactams can be rapidly N-alkylated under solid-liquid PTC conditions by use of microwave irradiation. The reactions were performed simply by mixing an amide with 50% excess of an alkyl halide and a catalytic amount of TBAB. These mixtures were absorbed on a mixture of potassium carbonate and potassium hydroxide [41] and then irradiated in an open vessel in a domestic micro-wave oven for 55-150 s (Eq. 28). 

N-Ethylaniline was alkylated by reaction with benzyl chloride under liquid-liquid PTC conditions in the presence of 30% sodium hydroxide solution and CTAB as a catalyst. Microwave irradiation (25 min) of the reaction mixture in a sealed vessel afforded N-benzyl-N-ethylaniline in 90% yield, compared with 16 h of conventional heating (oil bath) (Eq. 30) [24]. 

Tab. 5.14 Alkylation of phenylacetonitrile under MW + PTC conditions (NaOH 6 equiv.,TEBA 15%).

Tab. 5.15 Epoxyisophorone ring opening by reaction with several active methylene compounds under MW + PTC conditions.

Mercaptobenzimidazo[l,2-c]quinazoline reacts with dibromo derivatives under PTC conditions (K2C03/TBAB). Microwave irradiation enabled a striking reduction in reaction times (12 h compared with 15 min) with similar yields (86 and 81%) when compared with conventional heating (Eq. 40) [61]. 

Solvent-free SNAr reactions under solid-liquid PTC conditions were realized by using methoxide or phenoxide as nucleophiles. The main results, and comparison with those from classical heating, are indicated in Tab. 5.24 for activated (e.g. 4-nitro-halobenzenes) or nonactivated (e.g. a-naphthyl halides) substrates [74]. 

Tab. 5.24 Solvent-free PTC S tgAr reactions under MW + PTC conditions (potassium salts). ...

Tab. 5.28 Deethylation of 2-ethoxyanisole within 20 min under MW + PTC conditions (KOtBu 2 equiv. TDA-1 10%).

Tab. 5.31 Synthesis of diaryl-a-tetralones from isophorone and chalcone under MW + PTC conditions.

Hydrogen peroxide has also been used with microwave irradiation for the epoxida-tion of simple or cyclic alkenes. The reactions were accomplished under liquid-liquid PTC conditions in ethylene chloride solution in the presence of Na2W04 and Aliquat 336 as catalysts. The best results were obtained at 70 °C when the concentration of hydrogen peroxide was set to 8% and the pH of aqueous phase was kept below 2 (Eq. 63) [90],... 

Alkylation of 31 with dibromomethane and 1,2-dibromoethane was performed under solvent-PTC conditions with good yields and short irradiation times (15 min) [16]. The synthesis of original benzimidazo-[l,2-c]-quinazoline dimers 32(a,b) was successfully achieved by use of potassium carbonate in the microwave active DMF solvent (Scheme 8.13). 

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