Reaction center, synthetic

Studies on the formation and reactivity of phosphorus-centered radicals continue to be a versatile source of mechanistic information and reactions in synthetic chemistry. The recent literature devoted to the chemical reactivity of these derivatives is surveyed by S. Marque and P. Tordo in Chap. 2. Various new persistent or stable phosphorus-centered radicals have been described and find applications, for example, as paramagnetic probes. 

Owing to the low barriers to bond formation, reactant conformation often plays a decisive role in the outcome of these reactions. Carbocations, carbene, and radicals frequently undergo very efficient intramolecular reactions that depend on the proximity of the reaction centers. Conversely, because of the short lifetimes of the intermediates, reactions through unfavorable conformations are unusual. Mechanistic analyses and synthetic designs that involve carbocations, carbenes, and radicals must pay particularly close attention to conformational factors. 

Madjet, M. E., E. Abdurahman, and T. Renger. 2006. Intermolecular Coulomb couplings from ab initio electrostatic potentials Application to optical transitions of strongly coupled pigments in photo synthetic antennae and reaction centers. J. Phys. Chem. B 110 17268-17281. 

The last two decades have witnessed rapid development of organic synthetic methods based on benzotriazole derivatives. Thus, introduction of benzotriazole moiety to organic molecules provides several practical advantages. Among other benefits, a benzotriazolyl substituent activates the reaction center, stabilizes intermediates, increases regio- and stereoselectivity, and simplifies separation and purification of the products. After the desired molecular assembly is constructed, the bond with benzotriazole is cleaved off to provide the final product. A vast variety of... 

The molecular weight (M 10,200,000) represents the highest molecular weight known to date for a linear, synthetic copolymer. DFT calculations suggest that steric congestion, derived from the triethylsilyl group and the amine moiety, near the polymerization reaction center diminishes the rates of chain termination or transfer processes yet permits the monomer access to the active site and the monomer s insertion into the metal-carbon bond (Fig. 21). 

In contrast, the need to evaluate the relative rates of competing radical reactions pervades synthetic planning of radical additions and cyclizations. Further, absolute rate constants are now accurately known for many prototypical radical reactions over wide temperature ranges.19,33 3S These absolute rate constants serve to calibrate a much larger body of known relative rates of radical reactions.33 Because rates of radical reactions show small solvent dependence, rate constants that are measured in one solvent can often be applied to reactions in another, especially if the two solvents are similar in polarity. Finally, because the effects of substituents near a radical center are often predictable, and because the effects of substituents at remote centers are often negligible, rate constants measured on simple compounds can often provide useful models for the reactions of complex substrates with similar substitution patterns. 

Michel, H. and J. Deisenhofer (1988). Relevance of the photo synthetic reaction center from purple bacteria to the structure of photosystem II. Biochemistry, 27 1-7. 

Gust and co-workers designed a synthetic antenna reaction center capable of undergoing energy transfer followed by electron transfer.11461 Four ZnP chromo-phores are covalently linked to a FBP electron donor, forming a cross, and the FBP is in turn attached to a C60 electron acceptor (compound 27). Excitation of a ZnP... 

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules. 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

H. Suginome, Reaction and synthetic application of oxygen-centered radicals photo-chemically generated from alkyl hypoio-dites in CRC Handbook of Organic Photochemistry and Photobiology, 2nd Edition, W. Horspool and F. Lenci (Eds.), pages 109-1-109-44 CRC Press 2004. 

In the natural photosynthetic reaction center, ubiquinones (QA and QB), which are organized in the protein matrix, are used as electron acceptors. Thus, covalently and non-covalently linked porphyrin-quinone dyads constitute one of the most extensively investigated photosynthetic models, in which the fast photoinduced electron transfer from the porphyrin singlet excited state to the quinone occurs to produce the CS state, mimicking well the photo synthetic electron transfer [45-47]. However, the CR rates of the CS state of porphyrin-quinone dyads are also fast and the CS lifetimes are mostly of the order of picoseconds or subnanoseconds in solution [45-47]. A three-dimensional it-compound, C60, is super-... 

The best molecule mimicking multi-step electron-transfer processes in the photo synthetic reaction center so far reported is a ferrocene-meso, meso-linked porphyrin trimer-fullerene pentad [Fc-(ZnP)3-C60] in Fig. 13.16b, where the C60 and the ferrocene (Fc) are tethered at both the ends of (ZnP)3 (R = 46.9 A)... 

B. Rotaxanes as Models of the Photo synthetic Reaction Centers... 

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