Biphasic conditions

Usually, iodides and bromides are used for the carbonylation, and chlorides are inert. I lowever, oxidative addition of aryl chlorides can be facilitated by use of bidcntatc phosphine, which forms a six-membered chelate structure and increa.scs (he electron density of Pd. For example, benzoate is prepared by the carbonylation of chlorobenzene using bis(diisopropylphosphino)propane (dippp) (456) as a ligand at 150 [308]. The use of tricyclohexylphosphine for the carbonylation of neat aryl chlorides in aqueous KOH under biphasic conditions is also recommended[309,310]. 

A large number of Brpnsted and Lewis acid catalysts have been employed in the Fischer indole synthesis. Only a few have been found to be sufficiently useful for general use. It is worth noting that some Fischer indolizations are unsuccessful simply due to the sensitivity of the reaction intermediates or products under acidic conditions. In many such cases the thermal indolization process may be of use if the reaction intermediates or products are thermally stable (vide infra). If the products (intermediates) are labile to either thermal or acidic conditions, the use of pyridine chloride in pyridine or biphasic conditions are employed. The general mechanism for the acid catalyzed reaction is believed to be facilitated by the equilibrium between the aryl-hydrazone 13 (R = FF or Lewis acid) and the ene-hydrazine tautomer 14, presumably stabilizing the latter intermediate 14 by either protonation or complex formation (i.e. Lewis acid) at the more basic nitrogen atom (i.e. the 2-nitrogen atom in the arylhydrazone) is important. 

Singer and Scammells have investigated the y-Mn02 oxidation of codeine methyl ether (CME) to thebaine in the ionic liquid [BMIM][BF4] [63]. The ionic liquid was used in different ways and with mixed results (Scheme 5.1-35). For example, the oxidation of CME in the ionic liquid gave 38 % yield after 120 hours. A similar reaction under biphasic conditions (with diethyl ether) gave a 36 % yield of thebaine. This reaction gave a 25 % yield of thebaine when carried out in tetrahydrofuran... 

Table 7.2-4 Polymerization of isobutene to high molecular weight poly(isobutene)s in the ionic liquid [EMIM]CI/AICl3 under biphasic conditions [35].

A related study used the air- and moisture-stable ionic liquids [RMIM][PFg] (R = butyl-decyl) as solvents for the oligomerization of ethylene to higher a-olefins [49]. The reaction used the cationic nickel complex 2 (Figure 7.4-1) under biphasic conditions to give oligomers of up to nine repeat units, with better selectivity and reactivity than obtained in conventional solvents. Recycling of the catalyst/ionic liquid solution was possible with little change in selectivity, and only a small drop in activity was observed. 

Room temperature ionic liquids are air stable, non-flammable, nonexplosive, immiscible with many Diels-Alder components and adducts, do not evaporate easily and act as support for the catalyst. They are useful solvents, especially for moisture and oxygen-sensitive reactants and products. In addition they are easy to handle, can be used in a large thermal range (typically —40 °C to 200 °C) and can be recovered and reused. This last point is particularly important when ionic liquids are used for catalytic reactions. The reactions are carried out under biphasic conditions and the products can be isolated by decanting the organic layer. 

Table 1 Results of the alkene epoxidation reactions with fluorinated (salen)Mn complexes under biphasic conditions ...

The solid is used as a heterogeneous catalyst or as a water-soluble system in biphasic conditions in the hydrogenation of benzene and pheny-lacetylene [65]. The heterogeneous system Rh-PVP is investigated in the solid/liquid catalytic hydrogenation of benzene with a ratio of 1/34000 at 80 °C and 20 bar H2. The conversion into cyclohexane is about 60% after 200 h of reaction time. In a water/benzene biphasic condition at 30 °C and under 7 bar H2, complete hydrogenation (Scheme 2) for a molar ratio of 2000 is observed after 8 h giving a TOF = 675 h (related to H2 consumed), never-... 

Figure 28.5 shows another experiment that we did and this case we used 1-dodecene as the olefin. Once again we started under a strictly biphasic condition in which the catalyst was dissolved in water and the olefin dissolved in heptane. Under... 

Structurally, a carbene is the smallest member of the alkene family. As carbenes have no charge, they are expected to have a certain stability toward water. In fact, in some of the earliest work, carbenes were generated in aqueous medium under biphasic conditions via the reaction of chloroform with a strong base such as NaOH. 

Water-soluble dicationic palladium(II) complexes [(R.2P(CH2)3PR.2)Pd-(NCMe)2][BF4]2 proved to be highly active in the carbon monox-ide/ethene copolymerization under biphasic conditions (water-toluene). In the presence of an emulsifier and methanol as activator, the catalytic activity increased by a factor of about three. Also higher olefins could be successfully incorporated into the copolymerization with CO and the terpolymerization with ethene and CO.184... 

Ru3(CO)12(117)3] and [H4Ru4(CO)11(117)] as catalyst precursors in the hydrogenation of non-activated alkenes under biphasic conditions. Each cluster displays activity under moderate conditions, ca. 60 atm. H2 at 60 °C with catalytic turnovers up to ca. 500. The trinuclear clusters undergo transformations during reaction but can be used repeatedly without loss of activity.325... 

Various other biphasic solutions to the separation problem are considered in other chapters of this book, but an especially attractive alternative was introduced by Horvath and co-workers in 1994.[1] He coined the term catalysis in the fluorous biphase and the process uses the temperature dependent miscibility of fluorinated solvents (organic solvents in which most or all of the hydrogen atoms have been replaced by fluorine atoms) with normal organic solvents, to provide a possible answer to the biphasic hydroformylation of long-chain alkenes. At temperatures close to the operating temperature of many catalytic reactions (60-120°C), the fluorous and organic solvents mix, but at temperatures near ambient they phase separate cleanly. Since that time, many other reactions have been demonstrated under fluorous biphasic conditions and these form the basis of this chapter. The subject has been comprehensively reviewed, [2-6] so this chapter gives an overview and finishes with some process considerations. 

Figure 6.6. Hydrosilation of alkenes under fluorous biphasic conditions.[34]...

Figure 6.28. Acetylation of cyclohexanol carried out continuously under fluorous biphasic conditions for 500...

As part of a search for catalysts that can be used under biphasic conditions, zwitterionic Rh(sulphos)(cod) derivatives were studied. The isomerization of allyl alcohol proceeded within 1 h at 100 °C using only 1 mol% catalyst to give propanal in quantitative yield (Equation (12)).46 After separation of the product, the catalyst could be recycled three times with a slight deactivation after each run. 

---