Photolysis results

The photosensitized dimerization of isoprene in the presence of henzil has been investigated. Mixtures of substituted cyclobutanes, cyclohexenes, and cyclooctadienes were formed and identified (53). The reaction is beheved to proceed by formation of a reactive triplet intermediate. The energy for this triplet state presumably is obtained by interaction with the photoexcited henzil species. Under other conditions, photolysis results in the formation of a methylcydobutene (54,55). 

The reaction of vinylogous amides, or ketoaldehydes, with hydroxylamine produced 4,5,6,7-tetrahydro-l,2-benzisoxazole. A side product is the 2,1-benzisoxazole (Scheme 173) (67AHC(8)277). The ring system can also be prepared by the reaction of cyclohexanone enamines with nitrile oxides (Scheme 173) (78S43, 74KGS901). Base treatment produced ring fission products and photolysis resulted in isomerization to benzoxazoles (76JOC13). 

The catalysts were tested for their ability to c ytically photolyze water prior to their use in the methane conversion experiments. We were able to reproduce photolysis results reported in the literature [4] using these catalysts under similar conditions. 

Consistent with the involvement of the boron 2pz-orbital in this transition is the almost total reduction of the long-wavelength absorption of (PhCH2)3B above 250 nm by its complexation with ammonia. Strong chemical support for a multiply bonded boron in the excited state (48b) can be deduced from photolysis results with tribenzylborane (49).48 While photolysis of... 

We have also attempted to study the reactions of the tetrafluoro-o-phenylene di-radical (32) with benzene, by carrying out the photolysis of 1,2-di-iodotetrafluorobenzene in the presence of benzene. It is known that 1,2-di-iodoarenes give arynes on photolysis 70-72>. The only product derived immediately from o-dehydrotetrafluorobenzene was the 1,4-cyclo-adduct (24) which was, as expected, partially photoisomerised to (27) and a trace of (26) 73>. The photolysis resulted in the appearance of (24) and (27) before (26) was detected and hence we conclude that (26) was not a primary product. 

Photolysis of [Co(cyclam)(H20)(CH3)]2+ is known to produce methyl radicals via Co-C bond homolysis (235). The complex is stable in water and in the presence of oxygen, and it is capable of converting su-percoiled plasmid DNA into a nicked circular form after 2 h in ordinary room light (236). Longer photolysis results in increased yields of nicked DNA. The complex is not selective in its attack on nucleotides. 

The actual limit value of rr, below which the time constraint is met for a given transducer, is somewhat ambiguous. For a 0.5 MHz transducer (response time 2 xs), Mulder et al. [297] set this limit at 60 ns, based on the observation of a maximum of amplitude of the photoacoustic wave with the concentration of phenol and calculating rr from the rate constant of reaction 13.24, k = 3.3 x 108 mol-1 dm3 s-1 [298]. Later, Wayner et al. [293] empirically choose 100 ns as that limit and used laser flash photolysis results to adjust the phenol concentration until the lifetime of reaction 13.24 was less than that limit. In any case, the safest way of ensuring that the time constraint is being met is to verify it experimentally by varying the concentration of substrate until the observed waveform reaches a maximum (or, equivalently, until the final A0bs77 value reaches a maximum). 

When electron-deficient dienes were used, however, it proved that vigorously flushing the solution with an inert gas during photolysis resulted in higher reaction yields. This may indicate that the reaction can also proceed according to mechanism 2. A pathway according to 494 — 499 - 500 - 501 — 497 —498 could then explain the positive effect of a CO blanket in the case of electron-rich dienes305. 

The sensitized reaction yields the cyclopropanes 67 as the only product, whereas direct photolysis results in a Wolff-rearrangement yielding the cyclo-pentaneester 77 AWolff-rearrangement is only given by the singlet carbene... 

The conversion of 3-amino-4-oxo-l,jc-naphthyridines (117) into the corresponding 3-diazo-4-oxo compounds (118) followed by photolysis results in the formation of azaindoles (119) via nitrogen evolution and ring contraction (56LA(599)233, 60JCS1794). 

The transition state of singlet carbene cycloaddition to alkenes involves an electrophilic approach of the vacant p orbital to the n bond of alkenes. By contrast, the first step of the triplet addition process may involve the in-plane a orbital of the carbene. As in the case of C—H insertion (see Section 5.1), the difference in the transition structure between the singlet and triplet cycloaddition becomes important in the intramolecular process, especially when approach to a double bond is restricted by ring strain. Direct photolysis of ( )-2-(2-butenyl)phenyldiazomethane (99) in the presence of methanol gives l-ethenyl-l,la,6,6fl-tetrahydrocycloprop [fljindene [100, 29%, (E/Z)= 10 1] and l-(2-butenyl)-2-(methoxymethyl)benzene (101, 67%). Triplet-sensitized photolysis results in a marked increase in the indene (52%, EjZ) = 1.3.T) at the expense of the ether formation (4%) (Scheme 9.30). On the other hand, direct photolysis of phenyldiazomethane in an equimolar mixture of... 

The thermal decomposition of 8 in tetrachloroethene at 134 C gave a chromatographically separable mixture of cyclobutane 9 and 1,8-divinylnaphthalene 10 in 7 1 ratio. Although several experiments have been carried out to identify the spin multiplicity of the intermediate diradical, the results were inconclusive.17 A recent report stated that while triplet-sensitized photolysis resulted in predominant denitrogenation, laser/liquid jet photochemical reaction also gave cyclopentenes by 1,2-hydrogen shift.18 Indications are that the amounts of cyclopentenes increase with increasing lifetime of the intermediary 1,3-cyclopentadiyl triplet diradical.18... 

Similarly, 2,3-diazabicyclo[2.2.2]oct-2-enes underwent photoelimination of nitrogen via singlet 1,4-diradicals15 to give bicyclic cyclobutanes (Houben-Weyl, Vol. 4/4, pp 40-44). However, the parent compound on photolysis at 185 nm gave bicyclo[2,2.0]hexane (11 a) in only 9% yield,10 while benzophenone sensitized or direct photolysis resulted in mixtures of bicy-clo[2.2.0]hexane and hexa-1,5-diene (27 73 or 42 58, respectively).16 In a separate study, a triplet 1,4-cyclohexadiyl was trapped with molecular oxygen.17... 

From our analysis of the reported data for the photopolymerization and the flash photolysis results, we conclude that the explanation offered by... 

TABLE 17 Laser Flash Photolysis Results with Novel Xanthene Dyes Derivatives 1... 

A diazosilene is probably also involved in the photochemical or copper-catalyzed decomposition of bis(diazoacetate) 156 in benzene (equation 36). In both cases, dia-zoketene 157 was the only identified product72. Its formation was explained by the silylcarbene-to-acylsilene-to-silylketene sequence outlined in Scheme 5. Efforts to achieve the N2 extrusion from the remaining diazo function by thermolysis in boiling toluene or by prolonged photolysis resulted only in unspecific decomposition. 

When a methylene or dimethylgermyl group intervenes between the transition metal and the silyl group, as in 164, photolysis results in rearrangement to 165, which appears, for the methylene case, to involve the intermediacy of a metal-silene complex (Scheme 25). 

The photochemistry of the above oxatrisilacyclobutane 200 (R = CH2CMe3) was also investigated. Photolysis resulted in the extrusion of a silylene and the formation of a diradical 202, which in the presence of cyclohexane afforded the dihydride 203 or in its absence cyclized to the disilaoxirane 204, as shown in Scheme 35. If the photolysis was done in ethanol, the silylene was trapped, and ring opening of the oxirane occurred at... 

From NMR spectral patterns of the aromatic protons they deduce that L, 0 and P are incorporated into SDS aggregates, whereas M and N are attached to the double layer. Their flash photolysis results as well... 

Br)]2, exclusively. Low concentrations of 1,2-bromochloroethane, however, yield the mixed halide metal dimers Pt2(pop)4(Br)(Cl) " and Ir2(p-pz)2(C0D)2(Br)(Cl). This result is predicted by the proposed mechanism (Figure 5). Photolysis results in formation of Pt2(pop)4 (Br)4 or Xr2(p-pz)2(C0D)2(Br) as intermeditaes. The intermediate can react with another bromochloroethane molecule, as it does when the latter species is in high concentration, to yield the dibromide dimer or it can react with the chloroethane radical to yield the mixed halide metal species. The latter pathway becomes competitive at low halocarbon concentrations. In general, the oxidative addition of halocarbons is typical of the photochemistry arising from electron transfer from d -d metal dimers with the final product being the stable d -d metal-metal bonded dimers (24-25). 

Methyl and 10-phenyl thioxanthenium salts can be deprotonated at the benzylic group either by base or photolytically to generate the stable deep orange ylide or thiaanthracene. Secondary photolysis results in the conversion to the 9-substituted thioxanthene (Scheme 132) <1997CC709>. 

Selective sensitization gave ODPM rearrangement products, whereas direct photolysis resulted in efficient E Z isomerization and inefficient 1,3-acyl shift processes. The results indicated that y,5-double bonds or benzoyl groups present in these systems did not act as intramolecular triplet quencher. The new argument placed by the authors was the charge transfer interaction within the molecule enhances the internal conversion process there by a reduction in the ODPM rearrangement reactivity. 

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