The Multiphasic System

The third liquid catalyst-philic phase was constituted in the majority of cases by Aliquat 336® [tricapryl-methylammonium chloride, (C8H17)3N+CH3Cl": A336], a well-known phase-transfer agent that is liquid at room temperature, and that dissolves in toluene and in iso-octane (Figure 6.15a). The peculiarity here is that, when water (even a drop) is added to the A336/isooctane solution, three liquid phases separate out (Figure 6.15b).

Figure 6.15 shows the triphasic system at rest: in order to visualize the different phases, macroscopic amounts of the three components were combined. In

Figure 6.15 (a) A336/isooctane solution; (b) phase separation after addition of water.

Figure 6.15 (a) A336/isooctane solution; (b) phase separation after addition of water.

practice, the experimental setup appeared different, in that the A336 phase was used in catalytic amounts (0.20 molar) with respect to the substrates. Under the operative conditions the system appeared biphasic, with a thin layer of A336 at the iso-octane-water interface. When the heterogeneous catalyst was added to the triphasic system it resided in the A336 catalyst-philic phase, as is shown in macroscopic quantities in Figure 6.16.

What should be highlighted is that the figures show the triphasic system at rest. When stirred, which was necessary in order to achieve high interfacial area and reduce mass transport limitations, the system was better represented by catalyst particles coated by a layer of A336, immersed in the isooctane-water biphasic system (Figure 6.17). This system—where the catalyst-philic phase was A336—also could be considered the other way around as an A336-philic catalyst that is dispersed in the bulk biphasic system. However, inverting the factors does not change the result. The thin film of A336 acted as an interfacial boundary layer and was in close proximity to the catalytically active sites. This vicinity, and the ability of the A336 membrane to mediate the transfer of the reagents and products to and from the catalyst, was used to explain the selectivity and kinetics enhancements described in the following sections.

Figure 6.16 L-L-L-S system.
Phase Transfer Catalyst

Figure 6.17

Unlike some of the multiphasic systems described earlier, this system uses only a catalytic amount of the third phase, thereby eliminating the need for large quantities of expensive phase-transfer catalysts or ionic liquids. Along with

TABLE 6.1 Third Phase Constituents

Reference

TABLE 6.1 Third Phase Constituents

Reference

1

Aliquat 336 (A336)

41-63

2

C16 H33(C18 H3l)3N+Br

43,45

3

C16H33(„-Bu)3P+Br"

42-46

4

Cl6H33(py)+Br"

42

5

PhCH2(C2H5)3N+Br2

43

6

(„-Bu)4N+HSO4"

43

l

MeO(CH2CH2O)nH

43,44

S

PEG 6000

43

9

PPG 2000

43

10

Brij 35, 52, 56, 5S

49

11

Brij 52

49

12

Brij 5S

49

13

Brij 56

49

14

PhCH2(CH3CH2)3N+Cl2

49,50

15

PhCH2(„-Bu)3N+Cl2

49

16

Et2NH

50

1l

Et3N

50

1S

„-Bu3N

50

19

(PhCH2)3N

50

20

„-CsHliNH2

50

21

Cinchonidine

53

22

Cinchonine

Bulk isooctane

Pictorial view of the stirred L-L-L-S multiphasic system.

ammonium salts—in particular, Aliquat 336—other third phases were used as well: phosphonium salts, polyethylene glycols, amines, as shown in Table 6.1.

Not all of these salts formed a true separate phase, but all adhered to the heterogeneous catalysts that were used, and had an effect in modifying the catalytic activity and the reaction parameters.

The following discussion is organized by reaction type, while other parameters (metal catalyst, third phase, solvents, base concentration, etc.) are addressed as they arise.

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