Catalyst biphasic conditions

Although not as popular as with other carbonylation catalysts, biphasic conditions (particularly with phase transfer catalysts) may be employed to accomplish carbonylation under very mild conditions for benzylic and vinyl bromides389. 

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. 

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. 

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... 

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... 

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. 

In a similar fashion, 2 a, 2 c, 4 a and 4 c were also tested in the hydrogenation of 1-hexene under fluorous biphasic conditions (l-hexene/PFMCH = l 2 (v v)) [12, 14]. Relatively low activities were found for all catalysts, with activities increasing in the order 4a<4c<2a<2c. 

The conversion of tetralin to a-tetralone may be achieved under aqueous biphasic conditions in the presence of 02, using NiCl2 as a catalyst, as shown in Scheme 9.3 [7], It is necessary to use tetraethylene pentamine (TEPA) as a surface-active ligand, as well as an emulsifier, dodecyl sodium sulfate. Some alcohol and naphthol by-products were also observed, and a radical chain mechanism has been proposed for this reaction. 

A similar reaction has been conducted under fluorous biphasic conditions, using a perfluoroalkylated bipyridine as ligand to ensure that the copper species resides in the fluorous phase [22], The oxidation of a range of primary alcohols to the corresponding aldehydes was found to be possible, an example of which is shown in Scheme 9.11. The catalyst could be successfully recycled by phase separation, with analytically pure products being isolated even after... 

The effectiveness of 5a as a catalyst for the addition of alcohols 2 to propio-late 3 was first established under conventional fluorous Uquid/organic biphase conditions. This set the stage for the sequence in Fig. 3. Compounds 2a, 3, and 5a were combined in n-octane at room temperature in a 2.0 1.0 0.1 ratio (10 mol% 5a). As would be expected from Fig. 2, there was no visually... 

The ketone hydrosilylation shown in Fig. 7 was used as a test reaction. This can be catalyzed by the fluorous rhodium complexes 16-Rf6 and 16-Rfs under fluorous/organic liquid/liquid biphase conditions [55,56]. These red-orange compounds have very httle or no solubihty in organic solvents at room temperature [57]. However, their solubilities increase markedly with temperature. Several features render this catalyst system a particularly challenging test for recovery via precipitation. First, a variety of rest states are possible (e.g., various Rh(H)(SiR3) or Rh(OR )(SiR3) species), each with unique solubility properties. Second, the first cycle exhibits an induction period, indicating some fundamental alteration of the catalyst precursor. 

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