Hydrogen transfer, photosensitization

The hydrogen transfer photosensitization has been applied to the diastereo-selective alkylation of chiral fumaric acid derivatives, where again the mild conditions of the photochemical method are advantageous (Figure 3.8). ... 

In the presence of benzophenone, (8) was again the major product (>95°/0) and only trace amounts of the cyclohexane products were produced. These results suggest the intermediacy of a singlet 1,6-hexylene biradical in the direct photolysis and a longer lived triplet 1,6-diradical in the sensitized photolysis. In the triplet biradical more time is available for 1,6-hydrogen transfer to occur prior to spin inversion and hence more olefin (8) is produced. Similar results were reported for the direct and photosensitized photolysis of the 3,8-dimethyl derivative of (7). 

DHS can photosensitize reactions involving hydrogen atom transfer, which likely involve triplet state intermediates. For example, hydrogen transfer from the nitrogen of aniline to the sensitizer occurs at much higher rates than observed in the aniline photoreaction in distilled water. 

Many compounds sensitize biomolecules to damage by UVA (320-380 nm) and visible light. Two general mechanisms of sensitization are encountered. The Type I mechanism involves electron or hydrogen transfer from the target molecule to the photosensitizer in its triplet state. If 02 is present, this can be reduced to 02 by the reduced sensitizer. In the Type II mechanism, the excited sensitizer is quenched by 02, which is excited to the singlet state (typically A"g) and attacks the target molecule. Photosensitization is exploited in photodynamic therapy (PDT) for the destruction of cancerous or other unwanted cells. 

The direct irradiation of 1,2-diazocines, 40 [R = H, Me (isom. mix., Ph-(frans)] produced disproportionation (102) and coupled (103) products. The ratio of 102/103 increased with increasing amounts of nonketonic photosensitizer, indicating the intermediacy of a relatively long-lived 1,6-diradical in which more time is allowed for intramolecular hydrogen transfer (70JA4922). 

In a few less common procedures, the photocatalyst serves as a carrier of the hydrogen atom, subsequently forming a radical intermediate that must be regenerated via another hydrogen transfer step. For instance, a lower than stoichiometric amount of benzophenone is sufficient to trigger a photocatalysed addition of alkynes substituted with electron-withdrawing groups to cycloalkanes (575 is produced in less than 70% chemical yield Scheme 6.274).1491 In addition, photosensitized isomerization via the Schenck mechanism (Scheme 6.15 in Section 6.1.1) may also represent a photocatalytic radical process. 

Reduced 1-benzylnicotinamide (BNAH) is capable for reduction of carbonyl compounds in the presence of divalent ions such as Mg2+ and Zn2+. It can be (regenerated from BNA+ by a photochemically assisted hydrogen transfer from ascorbate, using [Ru(bipy)3]3+ as photosensitizer and [RhCl(TPPMS)3] as catalyst. The BNAH, generated continously in this reaction, was used for the reduction of benzil to benzoin [339],... 

Other studies have dealt with hydrogen abstraction from the solvent by the photosensitizing drug nalidixic acid [98a], hydrogen transfer in an-thraquinone/xanthene systems [98b], photoreductions of quinones by alcohols [98c] and of acetylenic ketones by various hydrogen donors [98d], the oxidation of NADH analogues [98e-98g], and the reaction of 4-methyl-2-quinolinecarbonitrile with optically active phenylpropionic acid [98h],... 

Formal intramolecular hydrogen abstraction (proceeding through coupled electron and proton transfer) occurs in the titi triplet excited state of furanone derivatives, upon acetone photosensitization. After hydrogen transfer from the tetrahydropyran to the (I position of the furanone moiety, radical recombination leads to the final products (18). The results of computational studies on model structures are in accordance with experimental observations and reveal that the reactivity and selectivity are mainly determined by the hydrogen-abstraction step. ... 

In contrast to the Ru + Rh bimetallic system described above, [Pt2(H2P205)4l potassium tetrakis (p-pyrophosphito)-diplatinate(II) is itself capable for playing the role of both the photosensitizer and the hydrogen transferring species. However, in this case only propan-2 ol could be used as H-donor. Oleate in DOPC, dispersed in phosphate buffer of pH = 5, containing 1 h propan-2-ol, was hydrogenated to stearate with 13 h conversion upon 30 min irradiation. 

Amines form relatively stable radical cations, and back electron transfer often overrides chemical reactions, except in the presence of good electrofugal groups in the a—position. Nevertheless, a-deprotonation (or alternatively a proton or hydrogen transfer process within an exciplex) may be fast enough to cause addition to unsaturated esters and ketones, " aromatics, and arylalkenes." " Deprotonation and further oxidation of some tertiary amines appear to produce aminium salts that are then aUylated by aUyl radicals simultaneously produced by fragmentation of allylsilane. Both processes are brought about by photosensitized electron transfer." ... 

In the Hg( Pi)-photosensitized decomposition of N2H4, the intermediate N2H3 reacts with CH3 radicals (from added dimethyl mercury) to give CH3NHNH2. A second reaction channel discussed is a fast reaction yielding CH4 and N2H2 [25]. The enthalpy of this hydrogen transfer reaction is A H —218 25 [25] and —88 kJ/mol (ab initio calculation) [26]. 

Photo-induced Diels Alder reaction occurs either by direct photo activation of a diene or dienophile or by irradiation of a photosensitizer (Rose Bengal, Methylene Blue, hematoporphyrin, tetraphenylporphyrin) that interacts with diene or dienophile. These processes produce an electronically excited reagent (energy transfer) or a radical cation (electron transfer) or a radical (hydrogen abstraction) that is subsequently trapped by the other reagent. 

When a photosynthetic organism is omitted, the addition of a photosensitizer is necessary. The methods use light energy to promote the transfer of an electron from a photosensitizer to NAD(P) via an electron transport reagent [6g]. Recently, carbon dioxide cvas reduced to formic acid with FDH from Saccharomyces cerevisiae in the presence of methylviologen (MV ) as a mediator, zinc tetrakis(4-methylpyridyl) porphyrin (ZnTMPyP) as a photosensitizer, and triethanolamine (TEOA) as a hydrogen source (Figure 8.8) [6h]. 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

Deeble DJ, Schuchmann MN, Steenken S, von Sonntag C (1990) Direct evidence for the formation of thymine radical cations from the reaction of SO/" with thymine derivatives a pulse radiolysis study with optical and conductance detection. J Phys Chem 94 8186-8192 DeFelippis MR, Murthy CP, Faraggi M, Klapper MH (1989) Pulse radiolytic measurement of redox potentials the tyrosine and tryptophan radicals. Biochemistry 28 4847-4853 Delatour T, Douki T, D Ham C, Cadet J (1998) Photosensitization of thymine nucleobase by benzo-phenone through energy transfer, hydrogen abstraction and one-electron oxidation. J Photo-chem Photobiol 44 191-198... 

Adam W, Arnold MA, Nau WM, Pischel U, Saha-Moller CR (2001b) Structure-dependent reactivity of oxyfunctionalized acetophenones in the photooxidation of DNA base oxidation and strand breaks through photolytical radical formation (spin trapping, EPR spectroscopy, transient kinetics) versus photosensitization (electron transfer, hydrogen-atom-abstraction). Nucleic Acids... 

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