Photochemical transfer

Several iodonium ylides, thermally or photochemically, transferred their carbene moiety to alkenes which were converted into cyclopropane derivatives. The thermal decomposition of ylides was usually catalysed by copper or rhodium salts and was most efficient in intramolecular cyclopropanation. Reactions of PhI=C(C02Me)2 with styrenes, allylbenzene and phenylacetylene have established the intermediacy of carbenes in the presence of a chiral catalyst, intramolecular cyclopropanation resulted in the preparation of a product in 67% enantiomeric excess [12]. 

The reverse process, photochemical transfer of an electron from the solvent molecule to the central metal ion, has been postulated in the case of several of the more strongly oxidizing metal ions, and, more recently, on the basis of an extensive study of the spectra of free radicals such as halogen atoms and OH. By analogy with Eq. 

DUiydride photochemistry is not limited to reductive elimination. CpRe(PPh3)2H2 is known to photochemically catalyze H/D exchange between CeDe and other arenes or alkanes. Photochemical studies of the mechanism of this process have ruled out loss of phosphines and instead postulate photochemical transfer of one or both hydrides to the cyclopentadienyl ligand yielding a 14-e intermediate, ( -C5H7)Re(PPh3)2, as a hkely intermediate. ... 

Figure 4. Examples of low-temperature limit of rate constant of solid-state chamical reactions obtained in different laboratories of the USSR, United States, Canada, and Japan (1) formaldehyde polymerization chain growth (USSR, 1973 [56]) (2) reduction of coordination Fe-CO bond in heme group of mioglobin broken by laser pulse (United States, 1975 [65]) (3) H-atom transfer between neighboring radical pairs in y-irradiated dimethylglyoxime crystal (Japan, 1977, [72], (4, 5) H-atom abstraction by methyl radicals from neighboring molecules of glassy methanol matrix (4) and ethanol matrix (5) (Canada, United States, 1977 [11, 78]) (6) H-atom transfer under sterically hampered isomerization of aryl radicals (United States, 1978 [73]) (7) C-C bond formation in cyclopentadienyl biradicals (United States, 1979 [111]) (8) chain hydrobromination of ethylene (USSR, 1978 [119]) (9) chain chlorination of ethylene (USSR, 1986 [122]) (10) organic radical chlorination by molecular chlorine (USSR, 1980 [124,125]) (11) photochemical transfer of H atoms in doped monocrystals of fluorene (B. Prass, Y. P. Colpa, and D. Stehlik, J. Chem. Phys., in press.).

The reactivity of the aminoborylene complexes 1 and 2 under thermal conditions leads to the formation of semi-bridged borylene or bis-borylene complexes (vide infra). The photochemical transfer of the borylene ligand is useful for the preparation of borirenes [23], turned out to be an alternative for the synthesis of 1, and was also used for the synthesis of the first half-sandwich borylene complex [(775-C5H5)(OC)3V=B=N(SiMe3)2] (5) [24] (Scheme 4). In the crystal, the borylene ligand adopts the expected linear geometry (V-B-N 177.9(4)°) and the B-N bond (137.8(7) pm) is... 

The photochemical transfer of a single electron should result in the formation of species with unpaired electrons. These species can be expected to give EPR signals, and it has been of interest to see if the EPR signals produced by the absorption of light in photosynthetic material can be correlated with the P700 reaction. 

Photochemical Transfer of the Nicotinamide Moiety of NAD to a Specific Residue in the Catalytic Center of Diphtheria Toxin Fragment A... 

Samec, Z., A. R. Brown, L. J. YeUowlees, and H. H. Girault, Photochemical transfer of tetra-aryl ions across the interface between two immiscible electrolyte solutions, J Electroanal Chem, Vol. 288, (1990) p. 245. 

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

Knox R S and Gulen D 1993 Theory of polarized fluorescence from molecular pairs—Forster transfer at large electronic coupling Photochem. Photobiol. 57 40-3... 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

Fig. 20. Proposed photochemical mechanisms for the generation of acid from sulfonium salt photolysis. Shown ate examples illustrating photon absorption by the onium salt (direct irradiation) as well as electron transfer sensitization, initiated by irradiation of an aromatic hydrocarbon.

Fig. 21. Representative nonionic photoacid generators. A variety of photochemical mechanisms for acid production ate represented. In each case a sulfonic acid derivative is produced (25,56,58—60). (a) PAG that generates acid via 0-nitrobenzyl rearrangement (b) PAG that generates acid via electron transfer with phenohc matrix (c) PAG that is active at long wavelengths via electron-transfer sensitization (d) PAG that generates both carboxylic acid and...

Photochemical technology has been developed so as to increasingly exploit inorganic and organometaUic photochemistries (2,7), recognizing the importance of photoinduced electron transfer as the phenomenological basis of a majority of commercially successful photochemical technologies (5,8). 

M. GiAtzel, Heterogenous Photochemical Electron Transfer, CRC Press, Boca Raton, Ela., 1988. 

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