Active site 440 Benzylic alcohols

Experiments (Table XIV) showed that the oxidation of activated alcohols is faster than of aliphatic ones. It is likely that hydrogen abstraction from the a-carbon atom by TEMPO (Fig. 33, species III to IV) is involved in the ratedetermining step. No reaction, however, was observed for 1-phenylethanol, an activated secondary benzylic alcohol, or for octan-2-ol. This selectivity for the oxidation of primary alcohols, resembles the activity and selectivity exhibited by GOase (see above). Indeed, the proposed active species for this catalytic oxidation can be considered as a model of the active site of GOase (Fig. 7). The bipyridine N atoms mimic the two histidine ligands, while TEMPO plays the role of both the phenolate and the phenoxyl groups. 

Improved control was observed, however, upon addition of benzyl alcohol to the dinuclear complexes.887 X-ray crystallography revealed that whereas (296) simply binds the alcohol, (297) reacts to form a trinuclear species bearing four terminal alkoxides. The resultant cluster, (298), polymerizes rac-LA in a relatively controlled manner (Mw/Mn=1.15) up to 70% conversion thereafter GPC traces become bimodal as transesterification becomes increasingly prevalent. NMR spectroscopy demonstrates that the PLA bears BnO end-groups and the number of active sites was determined to be 2.5 0.2. When CL is initiated by (298) only 1.5 alkoxides are active and kinetic analysis suggests that the propagation mechanisms for the two monomers are different, the rate law being first order in LA, but zero order in CL. 

For example, whereas the solid oxidation catalyst MCM-41-entrapped perruthenate can be used for the conversion of benzyl alcohols only, a similarly perruthenated-doped amorphous ORMOSIL is equally well suited for a variety of different alcohol substrates.35 On the other hand, a uniform pore structure ensures access to the active centres, while in an amorphous material made of non-regular porosity hindered or even blocked sites can well exist (Figure 1.16), rendering the choice of the polycondensation conditions of paramount importance. 

In both the biphenyl case and the benzyl case the aromatic ring is free to rotate and if sufficient energy is available at the active site the molecule could simply twist to an active configuration. However, this would not be true if the 2- and 2 -positions of the biphenyl alcohol were bridged with an alkyl chain where the conformation is fixed within narrow limits that depend on the length of the chain (17). ... 

These systems are also described as normal copper proteins due to their conventional ESR features. In the oxidized state, their color is light blue (almost undetectable) due to weak d-d transitions of the single Cu ion. The coordination sphere around Cu, which has either square planar or distorted tetrahedral geometry, contains four ligands with N and/or 0 donor atoms [ 12, 22]. Representative examples of proteins with this active site structure (see Fig. 1) and their respective catalytic function include galactose oxidase (1) (oxidation of primary alcohols) [23,24], phenylalanine hydroxylase (hydroxy-lation of aromatic substrates) [25,26], dopamine- 6-hydroxylase (C-Hbond activation of benzylic substrates) [27] and CuZn superoxide dismutase (disproportionation of 02 superoxide anion) [28,29]. 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

These benzaldehydes could then be directly used as a feedstock for various polymeric products or reduced to form phenolic benzylic alcohol derivatives (i.e., p-methylol groups). The p-methylol groups would thus be active sites, whereas in unmodified lignins, the C-l site is blocked and unreactive. In addition, the oxidative-cleavage step will hydrolyze a portion of the lignin interunit ether bonds, and thus increase the total fraction of free phenolic units to further enhance the reactivity. Other possible benefits are that the lignin would be extensively depolymerized and would form a more uniform feedstock material both conditions would give a product that is easier to handle. 

A Co(salophen)/zeolite catalyst was prepared by the template synthesis method. This catalyst proved to be active in the ruthenium catalyzed oxidation of benzyl alcohol. The heteroge-nized Co(salophen), having the same amount of complex produced a higher rate in the oxidation reactions than the free complex. It can be explained by the sites isolation theory. In the case of the heterogenized catalyst it was not necessary to use an extra axial ligand such as triphenylphosphine. It was also found that in the case of Co(salophen)/zeolite catalyst the choice of the solvent was not so critical, as in the case of the free complex. 

Supported silver catalysts are relatively commonly used in gas phase oxidations of alcohols.74,75 Benzyl alcohol can be selectively oxidised to benzaldehyde using a 0.6% Ag/pumice catalyst76 with 100% selectivity, although its activity is less than a similar Pd material. However, a mixed Pd-Ag/pumice bimetallic increases the activity whilst retaining the 100% selectivity to benzaldehyde. The authors of this study concluded that the role of the Pd was to activate the substrate whereas the highly dispersed silver particles served to activate the oxygen. Hence, the mechanism was one of cooperation between the Ag° and Pd° sites, the alloy phase, detected by EXAFS, was considered not to play an important role. 

Figure 9.3. A schematic of the active site of horse liver alcohol dehydrogenase. The bound substrate (In this case benzyl alcohol) and the cofactor NAD are sho A/n together A/ith several residues in the active site.

Mechanistic Implications. The native Zn(II)-substituted enzyme has been shown to undergo a conformational change from an open structure, where the active site clefts are solvent accessible, to a closed structure on formation of ternary complexes where the substrate binding cleft becomes a narrow hydrophobic tunnel from which solvent is excluded.The d d transitions of the Co(II)-substituted enzyme are sensitive both to the change from an open to a closed conformation and to the electronic properties of inner sphere coordinated ligands. It is likely that in the enzyme-NAD" complexes, the ionization of the coordinated ligand (water, benzyl alcohol, or trifluoroethanol) is accompanied by a change in protein conformation from the open conformation to the closed conformation. ... 

---