Triphasic conditions

Bentley et al.m recently improved upon Julia s epoxidation reaction. By using urea-hydrogen peroxide complex as the oxidant, l,8-diazabicyclo[5,4,0]undec-7-ene (DBU) as the base and the Itsuno s immobilized poly-D-leucine (Figure 4.2) as the catalyst, the epoxidation of a, (3-unsaturated ketones was carried out in tetrahydrofuran solution. This process greatly reduces the time required when compared to the original reaction using the triphasic conditions. 

Phase transfer catalysis can be conducted under liquid-liquid conditions, liquid-solid conditions, or liquid-liquid-solid triphasic conditions. 

Half-life times of the reaction of n-C,H17Br (in toluene) with aqueous potassium salts under triphase conditions ... 

For a triphasic reaction to work, reactants from a solid phase and two immiscible liquid phases must come together. The rates of reactions conducted under triphasic conditions are therefore very sensitive to mass transport effects. Fast mixing reduces the thickness of the thin, slow moving liquid layer at the surface of the solid (known as the quiet film or Nemst layer), so there is little difference in the concentration between the bulk liquid and the catalyst surface. When the intrinsic reaction rate is so high (or diffusion so slow) that the reaction is mass transport limited, the reaction will occur only at the catalyst surface, and the rate of diffusion into the polymeric matrix becomes irrelevant. Figure 5.17 shows schematic representations of the effect of mixing on the substrate concentration. 

Alkyl bromides have been converted into the corresponding iodides by treatment under solid solid liquid triphase conditions with silica-impregnated tetramethyl-ammonium salts (cf. ester formation, Section 3.4) [40]. 

Kumar, R., Mukherjee, P., and Bhaumik, A. (1999) Enhancement in the reaction rates in the hydroxylation of aromatics over TS-I/H2O2 under solvent free triphase conditions. Catal Today, 49,185-191. 

Table 1. Epoxidation of Enones using Triphasic Conditions ...

It is immediately apparent that the main attraction of the biphasic method is the much shorter reaction times it affords. This is illustrated well by the chalcone system. Using triphasic conditions, the epoxidation of chalcone can take up to 8 hours, while the biphasic reaction (Table 2, Entry 1) is complete in 30 minutes... 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Under biphase conditons (in dry dioxane) the activity of 46 for the reaction of 1-bromo-heptane with sodium phenoxide decreased with an increase in the length of aliphatic chains. Under triphase conditions, however, only catalyst 46 with n-octyl groups was active for chloride displacement reactions, and catalysts with groups smaller than n-butyl showed no activity 165). Polyacrylamides 46 with short aliphatic chains are too hydrophilic to act as catalysts under triphase conditions. Similar behavior has been observed for ammonium salts supported on dextran 95). 

When the reactions of alkyl bromides (n-Q-Cg) with phenoxide were carried out in the presence of cosolvent catalyst 51 (n = 1 or 2,17 % RS) under triphase conditions without stirring, rates increased with decreased chain length of the alkyl halide 82). The substrate selectivity between 1-bromobutane and 1-bromooctane approached 60-fold. Lesser selectivity was observed for polymer-supported HMPA analogue 44 (5-fold), whereas the selectivity was only 1,4-fold for polymer-supported phosphonium ion catalyst 1. This large substrate selectivity was suggested to arise from differences in the effective concentration of the substrates at the active sites. In practice, absorption data showed that polymer-supported polyethylene glycol) 51 and HMPA analogues 44 absorbed 1-bromobutane in preference to 1-bromooctane (6-7 % excess), while polymer-supported phosphonium ion catalyst 1 absorbed both bromides to nearly the same extent. 

The most straightforward way to obtain polymeric phosphonium salts involves introducing the phosphonio groups on to a suitable polymeric structure, for example by reacting tertiary phosphines with a poly(chloromethylstyrene) (reaction 99). The polymeric phosphonium salts obtained in this way are mostly used as polymer-supported phase-transfer catalysts for nucleophilic substitutions reactions under triphase conditions. 

The hydrophobicity of TS-1 could also explain why the oxidation of hydrocarbons in aqueous H2C>2 is faster without added organic solvent (triphase catalysis) than in organic solution (biphase catalysis) e.g. benzene hydroxylation under triphase conditions was up to 20 times faster than in acetonitrile or acetone (biphase conditions).1741 Indeed, benzene competes more favourably with water than with organic solvents for adsorption within the micropores of hydrophobic TS-1, as furthermore confirmed through adsorption experiments.1471... 

Ear greater yields were reported for the hydroxylation of benzene under the so-called triphasic conditions, that is with the solid catalyst, aqueous hydrogen peroxide and an immiscible aromatic phase [56]. Others, however, could not reproduce these results [54]. 

Benzyl chloride was significantly p-hydroxylated under triphase conditions, and underwent only side-chain attack in acetonitrile solution (Table 4) [21]. Little or no propensity for ring hydroxylation was shown by acetophenone and benzyl alcohol, respectively. In agreement with the empirical rule that the double bond is the preferred site of attack by TS-I/H2O2, epoxidation was the only reac-... 

More informative studies were published by Wu et al. [28,29]. They compared, operating under triphase conditions, the catalytic and transport properties of [Ti,Al]-MOR and TS-1 of similar Ti content and crystal dimensions (1 and 0.2 pm, respectively). The oxidations were performed employing unusually low... 

In 1980, Julia and coworkers reported the discovery of poly(amino acid)-catalyzed epoxidation of chalcone under triphasic conditions. In the presence of solid poly-L-alanine 156 (E)-chalcone 157 undergoes epoxidation yielding product 158 with a high enantioselectivity up to 96% ee within 24-28 h (Figure 54a). 

For the reduction of PhN02, a more complex system involving the reaction of Fe3(CO)i2 under triphase conditions have also been reported [18], The reaction mixture (NaOH IM in water, Fe3(CO)i2 and ArN02 in 1 1 ratio in benzene, and chloromethylated polystyrene as its ammonium salt, obtained by reaction with NEts, NBu3 or N-N -dimethylaniline) was stirred under nitrogen at 40-45 °C. A conversion of 50 % was achieved in 7 h (for ethylnitrobenzene). The system is similar to the previously discussed one operating under biphasic conditions [10-12], but is less efficient. The phase transfer catalyst can be recovered by filtration and be repeatedly used. The active species was proposed to be [HFe3(CO)ii], but this is questionable (yide supra). 

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