Photochemical functionalizations

Photoresponsive polyelectrolytes tethered with a photochemical functional group were first reported in 1964 by Lovrien and Waddington [24] who prepared copolymers of iV-azobenzeneacrylamide and acrylic or methacrylic acid (1). 

With inflexible substrates, this photochemical functionalization process showed significant selectivity. As our first example, photolysis of steroid ester 70 led to carbonyl insertion into the C-H bonds at C-7 and C-14 [149,150]. Other steroid esters afforded selective functionalizations in various positions [e.g. 151, 152], and compound 71 directly dehydrogenated the steroid to form the product olefin 72 [150]. 

Photochemical functionalizations with synthetic potential have been achieved using benzophenone esters of steroids (Scheme 10). In some cases attack occurs on several hydrogens for instance, a mixture of and A -alkenes is produced on irradiation of (2). However, with compound (3) photolysis produces only (4) as a new steroid. The yield of 55% involves some photoreduction of the benzophenone unit by solvent, so the other significant product is starting material with a reduced ketone group. Many other photolyses of benzophenone steroid esters have been studied they lead to useful infoimation about conformations, but not the directed single-site functionalizations that would make them synthetically useful. 

Remote Photochemical Functionalization 13 3.2 Template-directed Epoxidation... 

R Hill (1965) The biochemist s green mansions the photosynthetic electron-transport chain In plants. In PN Campbell and GD Greville (eds) Essays in Biochemistry 1 121-151. Acad Press, London Govindjee and E Rabinowitch (1960) Two forms of chlorophyll a In vivo with distinct photochemical function. Science 132 355-356... 

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Wiedmann TS, Pates RD. Beach JM, Salmon A, Brown MF. Lipid-protein interactions mediate the photochemical function of rhodopsin. Biochemistry 1988 27 6469-6474. 

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Photochemical Functionalizations by Complexed Benzophenones and Chlorinations Directed by Ion-paired Templates... 

Scheme 6-12 Photochemical functionalizations of alkyl and steroidal esters by complexed ben-zophenones.

Scheme 6-13 The doubly ion-paired template 26 gives high regioselectivity in the photochemical functionalization of 27, whereas the single ion-pairing substrates 28 and 29 are not functionalized with high specificity.

Two very curious but heretofore unexplained phenomena relate to the interaction of reduced polyoxometalates with O2 and are addressed by our experiments described right below. The first involves the lack of oxygenated products seen in the photochemical functionalization of heptacyclotetradecane (HCTD) in the presence of O2 by Christina Prosser-McCartha. The second involves two observations by Neumann and Levin, namely, the suppression of tetralin autoxidation and the clean conversion of a-terpinene to p-cymene in the presence of O2 without formation of oxygenated products. 

Heteroarenes have been photochemically functionalized by PET reactions forming new C—C bonds both in an inter- and intramolecular fashion via a similar mechanism [46]. The heteroarenes could serve both as electron donor (e.g. pyrroles or indoles) or electron acceptor (e.g. cyanopyridines or cyanopyrazines). Again, fragmentation of the radical cation, coupled with the radical anion and loss of the anion, led to overall ipso-substitution. In addition to the cyano group, halides could also function as leaving groups, such that in some cases an attack at an unsubstituted position took place [46],... 

Alvarez, N.T., KittreH, C., Schmidt, H.K., Hauge, R.H., Engel, P.S., and Tour, J.M. (2008) Selective photochemical functionalization of surfactant-dispersed single wall carbon nanotubes in water. 

Electrochemical functionalization methods 3 Electrochemical reaction 6 Photochemical functionalization methods 4 Photochemical reaction 6 Silicon-carbon bond formation 1 Thermal functionalization methods 3 Thermal-Grignard and organolithium reagents 5 Thermal-heat reaction 3 Thermal-hydride abstraction 5 Thermal-Lewis acid catalysis 4 Thermal-radical initiators 3 Thermal-transition metal catalysis 4... 

The first system we examined involved photochemical functionalization of flexible molecules [2], and later of steroids [3]. We found that flexible molecules could be attacked when they were converted to esters of benzophenonecarboxylic acids, and that the positions attacked were those expected from a detailed consideration of the geometries of the molecules. However, with flexible substrates a number of conformations were available, and the resulting selectivity for any particular position of attack was not high. We will return to this point later, when we discuss ways to immobilize such flexible chains. 

The preceding data establish that the bilayer(and monolayer) membrane is formed from a variety of single-chain and double-chain amphiphiles. It is clear that the biolipid bilayer is a special case of the general membrane formation. The peculiar characteristics of these synthetic membranes can be used for constructing new types of the reaction system. The ordered aqueous aggregates will find many applications in the basic and applied research. Some important aspects of the synthetic membrane e.g., chiral synthetic bilayers(26), the membrane-polymer interaction(27), and photochemical functions (28,29) were not mentioned in this paper. 

Renneke, R., Kadkhodayan, M., Pasquah, M., etal. (1991). Roles of Surface Protonation on the Photodynamic, Catalytic, and Other Properties of Polyoxometalates Probed by the Photochemical Functionalization of Alkanes. Implications for Irradiated Semiconductor Metal Oxides, J. Am. Chem. Soc., 113, pp. 8357-8367. 

Jin R-H, Aida T, Inoue S (1993) Caged porphyrin the first dendritic molecule having a core photochemical functionality. J Chem Soc Chem Commun (16) 1260-1262... 

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