Excited state hydrogen atom transfer

Barbara PF, Walsh PK, Brus LE (1989) Picosecond kinetic and vibrationally resolved spectroscopic studies of intramolecular excited-state hydrogen atom transfer. J Phys Chem 93 29-34... 

C. Manca, C. Tanner and S. Leutwyler, Excited-state hydrogen atom transfer in ammonia-wire and water-wire clusters, Ini. Rev. Phys. Chem., 24 (2005) 457-488. 

C. Tanner, C. Manca and S. Leutwyler, Exploring excited-state hydrogen atom transfer along an ammonia wire cluster Competitive reaction paths and vibrational mode selectivity, J. Chem. Phys., 122 (2005) 204236. 

Although the primary photoprocess for alkyl halide photoreduction may not be atom transfer in all cases, 3(da pa) excited-state hydrogen atom transfer has been established as the mechanism of the reactions between several binuclear d3 complexes and a number of organic and organometallic substrates (Roundhill, D. M. Che, C.-M. Gray, H. B. Accts. Chem. Res., in press, 6-8). Initial work in this area focused on Pt2(P204H2)4 ", for which the catalytic conversion of isopropanol to acetone (Equation 1) had been first observed (18). 

Garcia Garibay, M. A., Gamamik, A., Bise, R., Pang, L., Jenks, W. S., Primary Isotope Effects on Excited state Hydrogen atom Transfer Reactions Activated and Tunneling Mechanisms in an ortho Methylanthrone, J. Am. Chem. Soc. 1995, 117, 10264 10275. 

However, the mechanism of the transfer is not the excited state hydrogen atom transfer (ESHAT) but should be more appropriately characterized as a coupled proton-electron transfer in excited state. (See Ref. [159, 192] for reviews of proton-coupled electron transfer.)... 

Our previous results on hypericin indicate that excited-state H-atom transfer occurs even when one of the carbonyls is prohibited from accepting a hydrogen. The presence of such a transfer is apparent under very acidic conditions in AOT reverse micelles and cannot be excluded upon chelation of Tb3+ [76]. There is thus no evidence for a concerted H-atom-transfer mechanism in hypericin. In the present study, contrary to our initial expectations, we are not even able to demonstrate that hypomycin B executes an excited-state H-atom transfer hence our investigation sheds no light on the general question of how many H atoms are transferred in the perylene quinones and whether the transfer is concerted or stepwise. On the other hand, if further investigation reveals that H-atom transfer does not occur in hypomycin B, the result would have considerable implications for an understanding of the reaction coordinate for the H-atom transfer. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

Ikegami M, Arai T (2000) Laser flash photolysis study on hydrogen atom transfer of 2-(2-hydn>xyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole in the triplet excited state. Chem Lett 9 996-997... 

Electron transfer does not follow the simple rule of order of reactivity observed for n-7T, tt-tt and CT states for hydrogen atom transfer. The 77—77 singlet excited state (S of 9-cyanoanthracene for instance also undergoes electron transfer with amines at a diffusion-controlled rate. 

In some cases when detailed ab initio calculations of potential surfaces have become available, they have confirmed the major qualitative features of the surfaces deduced from experimental data and from preliminary data on correlation diagrams and the asymptotic properties of reactant, product, and intermediate states. One such case is the C+-H2 system,451 which has aroused considerable interst. Angular and energy distributions were determined experimentally for the CH+ + H products from the reactions of the C+(2P) ground state and the C+(4P) excited state with H2 (Table I, Jones et al.9b and references cited therein, and Jones et al.326). Chemiluminescence was also observed from this hydrogen-atom transfer reaction (Table IV).442,443 In addition, the reaction... 

Hydrogen atom transfer from anthracene, excited into its lowest excited singlet state, to anthraquinone impurity molecules creates a radical pair that strongly quenches the fluorescence from anthracene crystals. The reverse transfer rate constant, found from measurements of fluorescence intensity and its characteristic lifetime at different moments after the creation of the radical pair, varies from 106 to 10s s 1 in the range 110-65 K, kc = 4 x 104 s 1, TC = 60K. The kc values drops to 102 s 1 in the deuteroanthracene crystal [Lavrushko and Benderskii, 1978]. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type II reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to oxygen through a six-membered transition state (Scheme 5). [19a] An adamantyl ketone derivative 27 was found to crystallize from ethanol in very large prisms in the chiral space group P2 2 2. Upon irradiation of these crystals to approximately 10% conversion, the chiral cyclobutanol derivatives 28 were afforded as the major products in 80% ee. 

As in isolated phenol and in phenol-ammonia/water clusters, the OH bond is broken homolytically in 7HQ-A3, resulting in the transfer of a hydrogen atom rather than proton transfer. As found for phenol-A /W and naphthol-A /W clusters, ammonia is a better hydrogen acceptor than water. Excited-state hydrogen transfer processes are thus strongly favoured in an ammonia environment. 

Scheffer et al. provided another unimolecular asymmetric transformation involving the Norrish type 2 reaction, a well-known excited state process of ketones that is initiated by an intramolecular hydrogen atom transfer from carbon to... 

The hydrogen atom-transfer reactivity of the 3(da pa) excited state has been generalized to other complexes. Among these metal com-... 

The 3(do po) excited state of the dS-d complexes has been shown to be involved in the photochemical hydrogen atom-transfer reaction. The atom-transfer reactivity of this state is attributed to the presence of a hole in the do orbital, analogous to the 3nir state of organic ketones. Interaction of the oxidizing hole wiUi the electron pair of the C-H bond is the presumed pathway. 

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