Photoproduction successive

Even entrapment of entire cells within reversed micelles without loss of their functionality has been achieved. For example, mitochondria and bacteria (Actinobacter cal-coaceticus, Escherichia coli, Corynebacterium equi) have been successfully solubilized in a microemulsion consisting of isopropyl pahnitate, polyoxyethylene sorbitan trioleate [162], Enhanced hydrogen photoproduction by the bacterium Rhodopseudomonas sphaeroides or by the coupled system Halobacterium halobium and chloroplasts organelles entrapped inside the aqueous core of reversed micelles with respect to the same cells suspended in normal aqueous medium has been reported [183,184],... 

The first successful transformation of protoberberines to benzo[c]-phenanthridines was reported by Onda et al. (122,123). Irradiation of the enamines 200 and 195, the Hofmann degradation products of the corresponding protoberberines, in benzene afforded the initial photoproducts 201, which immediately rearranged to the tetrahydrobenzo[c]phenanthridines 202 in 70% yield (Scheme 37). Dehydrogenation of 202 afforded dihydro-chelerythrine (203) and dihydrosanguinarine (204), which were further oxidized with dichlorodicyanobenzoquinone (DDQ) to yield chelerythrine (205) and sanguinarine (206), respectively. 

The HPLC-MS/MS assay was also successfully applied to the measurement of UV-induced dimeric pyrimidine photoproducts [123, 124]. The latter lesions were released from DNA as modified dinucleoside monophosphates due to resistance of the intra-dimer phosphodiester group to the exonuclease activity during the hydrolysis step [125, 126]. The hydrolyzed photoproducts exhibit mass spectrometry and chromatographic features that allow simultaneous quantification of the three main classes of photolesions, namely cyclobutane dimers, (6-4) photoproducts, and Dewar valence isomers, for each of the four possible bipyrimidine sequences. It may be added that these analyses are coupled to UV detection of normal nucleosides in order to correct for the amount of DNA in the sample and obtain a precise ratio of oxidized bases or dimeric photoproducts to normal nucleosides. 

Intramolecular photocyclodimerizations of styrene systems of type 66 have been applied50 for the synthesis of cyclophanes (Scheme 14). The irradiation of compound 66a afforded straight photoproducts 67, subsequently transformed, upon reductive cleavage of the produced four-membered ring, to cyclophane 68. The approach seems successful for longer chains between the styrene units and allows introduction of heteroatoms such as oxygen51, 52 (69) and silicon53 (66b). 

The photosensitized electron transfer process involves two successive steps (eq. 5) In the primary event an encounter cage complex of the photoproducts is formed. This can either recombine to yield the original reactants or dissociate into separated photoproducts. The separated photoproducts can then recombine by a diffusional back electron transfer reaction to form the original reactants. We have introduced two conceptional approaches as a means for assisting the separation of the encounter cage complex and for the stabilization... 

Consistent with most tethered photocycloadditions, the photoproducts obtained from silicon tethered irradiations are formed with complete regiocontrol [19]. Several examples of silyl tethered groups have been reported. Scheme 16 lists the variants that have been successful. As mentioned previously, the mechanistic pathway that is most consistent with the results involves interaction between the alkenes prior to cycloaddition. The experimental observation is that cycloadducts are formed only when the alkenes have extended n systems attached. 

A final comment on Table 4 concerns the reaction shown in entry 8. Because the di-TT-methane photorearrangement of benzonorbomadiene derivatives requires triplet energy sensitization, we could not use typical, passive amines such as (/ )-( + )-l-phenylethylamine as chiral auxiliaries. We therefore prepared an optically pure amine to which a sensitizing benzophenone moiety was tethered, namely, the 4-benzoylphenyl ester of l-valine [25]. Photolysis of the salt of this amine at wavelengths where only the benzophenone chromophore absorbs led to the photoproduct in 91% ee at 100% conversion, a gratifying vindication of the concept. Optically active photosensitizers have been used in solution with limited success [33], but this represents the first example of simultaneous triplet-triplet energy transfer and asymmetric induction in the crystalline state. 

In principle, any of the photoproducts shown in Table 4 could have been prepared in enantiomerically pure form by irradiating their achiral precursors in solution to form a racemate and then separating the enantiomers by means of the classical Pasteur resolution procedure [36]. This sequence is shown in the lower half of Fig. 3. The top half of Fig. 3 depicts the steps involved in the solid-state ionic chiral auxiliary method of asymmetric synthesis. The difference between this approach and the Pasteur method is one of timing. In the ionic chiral auxiliary method, salt formation between the achiral reactant and an optically pure amine precedes the photochemical step, whereas in the Pasteur procedure, the photochemical step comes first and is followed by treatment of the racemate with an optically pure amine to form a pair of diastereomeric salts. The two methods are similar in that the crystalline state is crucial to their success. The Pasteur resolution procedure relies on fractional crystallization for the separation of the diastereomeric salts, and the ionic chiral auxiliary approach only gives good ees when the photochemistry is carried out in the crystalline state. 

From detailed studies of this system, it was concluded that the primary photoprocess is abstraction of the a-hydrogen by the 3Pt2 to form a monohydride species (directly observed by transient absorption spectroscopy for a number of substrates) and the organic radical (Equation 2), with the final photoproduct being Pt2H2 and acetone (Equation 3). The Pt2H2 complex has been characterized by NMR, UV-Vis, and IR (but has not been successfully isolated) (19). 

The absorption lines of the low temperature photoreaction products in TS-6 monomer crystals are summarized in the diagram of Fig. 7. The correlation of the A, B, C,. .. photoproduct series to diradical DR intermediates and of the b, c, d,... photoproducts to asymmetric carbene AC intermediates is based on the ESR experiments discussed below. The correlation of the y, 8,6,... series to stable oligomers SO is based on their thermal and optical stability. The correlation of dimer, trimer, tetramer,... molecules follows from the chemical reaction sequences observed in the time resolved optical and ESR measurements as well as from the widths of the one-dimensional potential wells used in the simple electron gas theory , which already has proved successful in its application to dye molecules. Following Exarhos et al. the explicit dependence is given by... 

From an evaluation of the highest specific rates of H2 photoproduction by different purple bacteria using various organic substrates published by different authors, one notes little success in acceleration of this important parameter during the last two decades (Table 1). 

The weak point is that in order that a photoehemical process takes place, a photon must be absorbed, and this generally implies using a low concentration of the substrate, otherwise all of the light is absorbed in the first thin layer of the solution close to the surface of the vessel (causing an inefficient conversion, particularly if the photoproduct absorb light of the same wavelength and sometimes causing successive photoreactions and polymerizations). 

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