Homogeneous immobilization

In cyclic voltammetry, an ideal (homogeneous) immobilized layer of molecules undergoing a reversible electron transfer that is uncomplicated by coupled chemical reactions gives a pair of reduction and oxidation peaks that are compact at all scan rates (the half-height width for a simple n-electron transfer is approximately... 

Two types of immobilization are used for immobilizing glucose isomerase. The intracellular enzyme is either immobilized within the bacterial cells to produce a whole-ceU product, or the enzyme is released from the cells, recovered, and immobilized onto an inert carrier. An example of the whole-ceU process is one in which cells are dismpted by homogenization, cross-linked with glutaraldehyde, flocculated using a cationic flocculent, and extmded (42). 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Because enzymes can be intraceUularly associated with cell membranes, whole microbial cells, viable or nonviable, can be used to exploit the activity of one or more types of enzyme and cofactor regeneration, eg, alcohol production from sugar with yeast cells. Viable cells may be further stabilized by entrapment in aqueous gel beads or attached to the surface of spherical particles. Otherwise cells are usually homogenized and cross-linked with glutaraldehyde [111-30-8] to form an insoluble yet penetrable matrix. This is the method upon which the principal industrial appHcations of immobilized enzymes is based. 

Since no special ligand design is usually required to dissolve transition metal complexes in ionic liquids, the application of ionic ligands can be an extremely useful tool with which to immobilize the catalyst in the ionic medium. In applications in which the ionic catalyst layer is intensively extracted with a non-miscible solvent (i.e., under the conditions of biphasic catalysis or during product recovery by extraction) it is important to ensure that the amount of catalyst washed from the ionic liquid is extremely low. Full immobilization of the (often quite expensive) transition metal catalyst, combined with the possibility of recycling it, is usually a crucial criterion for the large-scale use of homogeneous catalysis (for more details see Section 5.3.5). 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

The combination of ionic liquids and compressed CO2 - at opposite extremes of the volatility and polarity scales - offers a new and intriguing immobilization technique for homogeneous catalysis. 

The immobilization procedure may alter the behavior of the enzyme (compared to its behavior in homogeneous solution). For example, the apparent parameters of an enzyme-catalyzed reaction (optimum temperature or pH, maximum velocity, etc.) may all be changed when an enzyme is immobilized. Improved stability may also accrue from the minimization of enzyme unfolding associated with the immobilization step. Overall, careful engineering of the enzyme microenvironment (on the surface) can be used to greatly enhance the sensor performance. More information on enzyme immobilization schemes can be found in several reviews (7,8). 

Clarke and Shannon also supported copper bis(oxazoline) complexes onto the surfaces of inorganic mesoporous materials, such as MCM-41 and MCM-48, through the covalent binding of the ligand, modified by alkoxysilane functionalities [59]. The immobilized catalysts allowed the cyclopropanation of styrene with ethyldiazoacetate to be performed as for the corresponding homogeneous case, and were reused once with almost no loss of activity or selectivity. 

The enantioselectivity obtained in the hetero-Diels-Alder reaction (Scheme 12) was low (18% ee). This is, in part, due to the important temperature effect. For example, 50% ee was obtained in reactions carried out in homogeneous phase at - 60 °C and 95% ee in reactions at - 78 °C. However, at 0 °C the enantioselectivity dropped to 28% ee, a value closer to that obtained with the immobilized catalyst at the same temperature. Recycling was investigated and the solid was used four times with the same activity maintained. The 6b-Cu(OTf)2 catalyst proved to be less effective for this reaction and less stable in terms of recycling, a situation in agreement with the results obtained with exchanged catalysts [53]. 

In the case of the reaction between N-acryloyloxazolidin-2-one and cy-clopentadiene, both catalysts showed activities and enantioselectivities similar to those observed in homogeneous phase. However, a reversal of the major endo enantiomer obtained with the immobilized 6a-Cu(OTf)2 catalyst, with regard to the homogeneous phase reaction, was noted. Although this support effect on the enantioselectivity remains unexplained, it resembles the surface effect on enantioselectivity of cyclopropanation reaction with clay supports [58]. 

---