Covalent tethering

A number of different strategies for the immobilization of chiral catalysts are introduced, which include covalent tethering, the use of electrostatic interactions, and encapsulation methodologies. 

The most favored approach for the immobilization of homogeneous or enzyme catalysts involves the covalent tethering of the catalyst to a support [7]. Suitable supports for immobilization of a metal complex include functionalized polymers, 

The confined space of the pores can be tailored to optimize catalytic performance. One example of this behavior is the study by Thomas and coworkers [8] who found that anchoring the chiral ligand (R,R)-diphenylethylenediamine (dpen) to the inner 

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In contrast, if photoactive chromophores are covalently tethered to the polyelectrolyte main chain, the chromophore reactivity in the photoinduced ET is much more greatly changed because the reaction sites are totally constrained to the polyelectrolyte molecular surface. 

Oxidative repair is not a unique feature of our Rh(III) complexes. We also demonstrated efficient long-range repair using a covalently tethered naphthalene diimide intercalator (li /0 1.9 V vs NHE) [151]. An intercalated ethidium derivative was ineffective at dimer repair, consistent with the fact that the reduction potential of Et is significantly below the potential of the dimer. Thymine dimer repair by a series of anthraquinone derivatives was also evaluated [151]. Despite the fact that the excited triplets are of sufficient potential to oxidize the thymine dimer ( 3 -/0 1.9 V vs NHE), the anthraquinone derivatives were unable to effect repair [152]. We attribute the lack of repair by these anthraquinone derivatives to their particularly short-lived singlet states anthraquinone derivatives that do not rapidly interconvert to the excited triplet state are indeed effective at thymine dimer repair [151]. These observations suggest that interaction of the dimer with the singlet state may be essential for repair. 

The ruthenium complex of NH-benzyH 17T2,S )-norephedrine covalently tethered to silica showed a high activity and enantioselectivity in the reduction of acetophenone.310... 

J. D. (2005) Properties ofhuman serum albumin entrapped in sol-gel-derived silica bearing covalently tethered sugars. Chemistry of Materials, 17, 1174-1182. 

Fig. 16.8 Schematic of a diphosphine rhodium complex covalently tethered to a cross-linked styrene/divinylbenzene matrix, used for the hydrogenation of quinoline.

Certain enzymes catalyze their reactions by way of a multisite mechanism in which the covalently linked intermediate is attached to a long arm that swings from one subsite to another subsite within the enzyme. In some cases, the covalently tethered intermediate can actually be transferred between subunits that form the active site. An example is Propionibacterium shermanii transcarboxylase an enzyme that catalyzes the biotin-dependent conversion of methylmalonyl-CoA and pyruvate to propionyl-CoA and oxaloacetate. Carboxylated biotin allows the two catalytic subsites to operate on the same reaction intermediate. 

SIP involves the growth of a polymer chain from an initiator moiety that has been covalently tethered to a surface. The advent of this reaction approach, along... 

Nitrenium ions have been applied to the synthesis of macrocycles and other medicinally interesting compounds. The most successful reactions have been in cases where the nitrenium ion is covalently tethered to its intended target. Further efforts aimed at modulating the selectivity of these intermediates would increase their synthetic utility. 

D. Displacement of a Covalently Tethered Reporter Site by Analyte... 

Guanidinium salt has special properties of high thermal and chemical stability, tunable groups attached on the three N atoms in the molecular structure, and excellent catalytic activity for cycloaddition of C02 and epoxides to produce cyclic carbonate [11, 12]. A functionalized-PEG, hexaalkylguanidinium bromide (Scheme 5.3) being covalently tethered to PEG (MW = 6000) is utilized as an active homogeneous catalyst, which includes the benefits of recyclability and high catalytic activity, for the synthesis of cyclic carbonates from C02 and epoxides with almost quantitative yield and excellent selectivity [13]. 

As expected, the amount of shape-persistent macrocycle in the crude reaction product depends strongly on the length of the covalent tether (Scheme 6.15)... 

In electric-field poling, a guest dipolar chromophore and a chemically and optically passive host polymer (the chromophore can also be covalently tethered to the polymer [74] or incorporated into the main chain of the polymer) is heated to above the glass transition temperature, Tg, of the mixture. A large electric... 

Whereas all experiments described so far used systems in which the diene was covalently tethered to the RNA, true catalysis with multiple turnovers was the final goal of this project. We therefore performed measurements in which the tether... 

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