Catalytic groups, introduction

The next step in the elaboration of a synthetic catalyst based on the PEI matrix was the introduction of catalytic groups along with the binding groups. Reaction of PEI with chloromethyl imidazole and dodecyl iodide produced (26), a potent catalyst for the hydrolysis ofp-nitrophenylcaproate. 

Introduction of a single catalytic group in cyclodextrin generally affords enzyme models as shown in many examples listed in Table XVI. Thus, reasonable acceleration and substrate specificity were observed in these models. However, monosubstituted cyclodextrins seem to have limitations and introduction of two or more functional groups is usually necessary for multiple recognition and for a sophisticated enzyme model. 

The range of catalytic proficiencies for enzymes suggests that there are features of catalysis in enzymes that involve factors other than stabilization of transition states. One important distinction is that the enzyme active site contains catalytic groups that are able to access reactive intermediates, while intermediates formed in solution have lifetimes that are less than the time needed for a reagent to diffuse to the site of the reaction.33 In the enzyme, groups are initially associated with the bound substrate in a specific array and continue to be available through the course of the reaction. Diffusional introduction of catalytic groups is overcome by pre-association of the catalysts and reactant prior to the formation of any reactive intermediate. This accesses modes of catalysis that are not possible if the catalyst and intermediate must become associated after the intermediate has formed. 

Hydrocarbon polymers are nearly inert supports introduction of specific catalytic groups can lead to the formation of a selective catalyst. 

Introduction of catalytic groups or cofactors at the binding sites... 

Introduction of a 16P-methyl group into the corticosteroid molecule was effected by a reaction (83) whereby a 16-dehydropregnenolone (62) was treated with dia2omethane to form the pyra2oline (75) which was decomposed with perchloric acid in acetone to give the 16-methylpregn-16-en-20-one derivative (76). Catalytic hydrogenation yielded the 16P-methyl intermediate (77), which was converted into... 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

The relationship between 9 and its predecessor 10 is close. Oxidation of the allylic C-3 methylene group in 10 and elimination of the methoxy group could furnish enone 9. Retrosynthetic disassembly of ring E in 10 furnishes tertiary alcohol 11 as a viable precursor. That treatment of 11 with a catalytic amount of acid will induce the formation of a transient oxonium ion at C-12 which is then intercepted by the appropriately placed C-4 tertiary hydroxyl group is a very reasonable proposition. As we will see, the introduction of the requisite C-4 hydroxyl group is straightforward from intermediate 12. 

The chemical modification of unsaturated polymers via homogeneous catalytic means offers a potentially useful method for introduction of desirable functional groups on the polymer chains. 

---