Sensitizer-tethered substrates

Although ferryl intermediates of horseradish peroxidase and microperoxidase-8 have been produced in reactions with photogenerated [Ru(bpy)3]3+ [5], analogous experiments with P450s were unsuccessful, presumably due to the inefficiency of electron transfer from the buried heme active site through the protein backbone [6]. Photoactive molecular wires (sometimes referred to as metal-diimine wires, sensitizer-tethered substrates, or electron tunneling wires) were developed to circumvent this problem by providing a direct ET pathway between [Ru(bpy)3]3+ and the heme. These molecular wires, which combine the excellent photophysical properties of metal-diimine complexes... 

The synthesis of a variety of dihydrobenzofuranes can be accomplished via the Aryl SN1 mechanism. The acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents affords the aryl cation, which adds onto the tethered double bond following the 5-exo-trig mode leading to (dihydro)benzofurans. The reaction is solvent-dependent, and byproducts are also generally obtained. The best results are found when ethyl acetate is used. Depending on the substrate employed, the cation intermediate can yield products as a result of proton elimination or trapping by chloride ions (Scheme 10.54) [70],... 

Two measures to reproduce the key aspects of the enzyme have had some success to use an aromatic thiol and to block attack on the iron by building organic frameworks over the metal. Other approaches, illustrated in Fig. 4.10, include substituting iron with manganese, which is less sensitive to aerial oxidation, and to tether a cyclodextrin to the porphyrin so that the organic substrate could be positioned correctly for oxidation. 

There are several advantages of using CDx as a host (1) the cavity is inherently chiral, (2) the tethered sensitizer promotes tight packing and conformational fixing of the substrate/sensitizer pair in the cavity, and most importantly, (3) undesirable sensitization outside the chiral cavity is automatically prohibited... 

Studies of tethered electroactive species are less sensitive to pinholes than experiments with solution reactants and blocking layers, although heterogeneity and roughness of the substrate and film defects can still play a role. The rate constant, k, in this case has units of a first-order reaction (s ). Rate constants can be determined by a voltammetric method as described earlier for electroactive monolayers (Section 14.3.3). In addition potential-step chronoamperometry can be employed, in which case the current follows a simple exponential decay (88, 90, 91) ... 

The practical advantages of the radical relay process over the use of a tethered arylio-dine dichloride reagent are several. First of all, an excess of the solution chlorinating agent can be used, so complete conversion of the substrate to product is achieved. Secondly, there is no need to premake an attached aryl dichloride by using CI2, so sensitive substrates can be used. Finally, other templates can be used that can capture and relay a chlorine atom but cannot themselves be converted to dichloride reagents. These will be described later. 

More recently, the Brummond laboratory reported a series of Rh-catalyzed allenic Alder-ene reactions affording 5- and e-lactams 89 from amide-tethered alleneynes 88 (Scheme 46) (94). Higher temperature was required for the reaction to proceed efficiently. This observation could be attributed to the secondary amide, because the reaction of protected amides proceeded at room temperature to afford cyclized product in less than 1 h. The reaction is somewhat sensitive to terminal alkyne in that product yields for amide-tethered allenyne substrates with terminal alkyne were lower. 

The present study aims to understand the influence of solvent quality on the molecular-level friction mechanism of tethered, brushlike polymers. It involves complementary adsorption studies of PLL-,g-PEG by means of optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance with dissipation (QCM-D) as well as friction studies performed on the nanoscale using colloidal-probe lateral force microscopy (LFM). The adsorbed mass measured by QGM-D includes a contribution from solvent molecules absorbed within the surface-bound polymer fllm. This is in contrast to optical techniques, such as OWLS, which are sensitive only to the dry mass of a polymer adsorbed onto the surface of the waveguide.By subtracting the dry mass , derived from OWLS measurements, from the wet mass , derived from QCM-D measurements, it is therefore possible to determine the mass of the solvent per unit substrate area absorbed in the brushlike structure of PLL- -PEG, expressed as areal solvation, P. Areal solvation was varied by choosing solvents (aqueous buffer solution, methanol, ethanol, and 2-propanol) of different quality with respect to the PEG brush. The solvents were characterized in terms of the three-component Hansen solubility parameters, and these values were compared with measured areal solvation of the PEG brush. 

Recently, the investigation of polymer brushes has been focused on the synthesis of new tethered polymer systems primarily through surface-initiated polymerization (SIP). Previously, the term polymer brushes has been limited to the investigation of block copolymers (qv) or end-functional linear polymers that have been physically or chemically adsorbed to surfaces, respectively (3,4). Recent synthetic efforts using different polymerization mechanisms have resulted in the discovery of many novel properties of polymer brushes. This has been aided no less than the use of innovative and unique surface-sensitive analysis methods as applied to flat substrates and particles. The study of polymer brushes has benefited from improved dielectric, optical, spectroscopic, and microscopic characterization methods. Understanding the chemistry of these grafting reactions and how... 

In a recent improvement to this approach, poly(4-vinylpyridine) and poly(4-vinylpyridine) N-oxides were used as the catalyst carrier [91]. The MTO catalyst obtained from 25% cross-linked poly(4-vinylpyridine) proved to efficiently catalyze the formation of even hydrolytically sensitive epoxides in the presence of aqueous hydrogen peroxide (30%). This catalyst could be recycled up to 5 times without any significant loss of activity. Attempts have been made to immobilize MTO with the use ofeither microencapsulation techniques, including sol-gel techniques, to form silica-bound rhenium compounds, or by the attachment of MTO to silica tethered with polyethers. These approaches have provided catalysts with good activity using aqueous hydrogen peroxide as the terminal oxidant [91-93]. In the latter case, high selectivity for epoxide formation was also obtained for very sensitive substrates (e.g., indene). 

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