Catalysis In Water

Another environmental issue is the use of organic solvents. The use of chlorinated hydrocarbons, for example, has been severely curtailed. In fact, so many of the solvents favored by organic chemists are now on the black list that the whole question of solvents requires rethinking. The best solvent is no solvent, and if a solvent (diluent) is needed, then water has a lot to recommend it. This provides a golden opportunity for biocatalysis, since the replacement of classic chemical methods in organic solvents by enzymatic procedures in water at ambient temperature and pressure can provide substantial environmental and economic benefits. Similarly, there is a marked trend toward the application of organometal-lic catalysis in aqueous biphasic systems and other nonconventional media, such as fluorous biphasic, supercritical carbon dioxide and ionic liquids.10

A prime advantage of such biphasic systems is that the catalyst resides in one phase and the starting materials and products are in the second phase, thus providing for easy recovery and recycling of the catalyst by simple phase separation. A pertinent example is the aerobic oxidation of alcohols catalyzed by a water-soluble Pd-bathophenanthroline complex (Figure 9.5).11 The only solvent used is water, the oxidant is air, and the catalyst is recycled by phase separation.

The Boots Hoechst Celanese (BHC) ibuprofen process12 involves palladium-catalyzed carbonylation of a benzylic alcohol (IBPE). More recently, we performed this reaction in an aqueous biphasic system using Pd/tppts as the catalyst (Figure 9.6; tppts = triphenylphosphinetrisulfonate). This process has the advantage of easy removal of the catalyst, resulting in less contamination of the product.

R2 OH

Ibuprofen Preparation Bhc Company
;Pd(OAc)2

Pd2+-Bathophenanthroline (L)

Pd2 Bathophenanthroline

Figure 9.5 Aerobic oxidation of alcohols catalyzed by Pd(II)/bathophenanthroline in water.

JYJJ OH pd/tppts/H+ I /XCOOH

COOH

IBPE Ibuprofen 3-IPPA

Conversion: 83% tppts = P Selectivity to ibuprofen: 82%

S03Na'3

Figure 9.6 Ibuprofen synthesis by Pd/tppts-catalyzed biphasic carbonylation in water.

Figure 9.6 Ibuprofen synthesis by Pd/tppts-catalyzed biphasic carbonylation in water.

60 bar

60 bar

Figure 9.7 Carbonylation of benzyl alcohol-catalyzed by tppts/Pd.

In the same way, the biphasic carbonylation of benzyl alcohol (Figure 9.7) was achieved.13 Phenylacetic acid was obtained in 77% yield, 100% selectivity, and 100% atom utilization.

Similarly, acylamino acids can be prepared with 100% atom utilization via palladium-catalyzed amidocarbonylation.14 The method was used for the synthesis of a surfactant from sarcosine (Figure 9.8).

100% atom efficient

Example: CnH23\ ^NK

CO/CH2O C11H23-V.N^.COOH

o 86% yield

Sarcoside surfactant Figure 9.8 Acylamino acids via palladium-catalyzed amidocarbonylation.

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