Photochemistry dimerization

In contrast, the photochemistry of uracil, thymine and related bases has a large and detailed literature because most of the adverse effects produced by UV irradiation of tissues seem to result from dimer formation involving adjacent thymine residues in DNA. Three types of reaction are recognizable (i) photohydration of uracil but not thymine (see Section 2.13.2.1.2), (ii) the oxidation of both bases during irradiation and (iii) photodimer formation. 

Nonhomolytic cleavage pathways in the photochemistry of metal-metal bonded carbonyl dimers. A, E. Stiegman and D. R. Tyler, Acc. Chem. Res , 1984,17, 61-66 (36). 

Majeti11 has studied the photochemistry of simple /I-ketosulfoxides, PhCOCH2SOCH3, and found cleavage of the sulfur-carbon bond, especially in polar solvents, and the Norrish Type II process to be the predominant pathways, leading to both 1,2-dibenzoylethane and methyl methanethiolsulfonate by radical dimerization, as well as acetophenone (equation 3). Nozaki and coworkers12 independently revealed similar results and reported in addition a pH-dependent distribution of products. Miyamoto and Nozaki13 have shown the incorporation of protic solvents into methyl styryl sulfoxide, by a polar addition mechanism. 

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

In the past decade, Ishikawa et al. have investigated the photochemistry of aryldisilanes.66 73 76 84 Lately, dimers derived from these unusual silenes have been observed from the first time.78 Thus, photolysis of 1,4-bis(penta-methyldisilyl)benzene 76 in hexane presumably gave rise to the silene 77 but, on workup employing crystallization, two head-to-head stereoiso-meric dimers 78 and 79 were obtained in about 45% yield in a 1 1 ratio. These were said to be formed from the silenecyclohexadienes 77 by the two alternative pathways shown in Scheme 12. Related dimers were also... 

Finally a few sentences are deserved for the vast area of DNA photochemistry. Thymine dimerization is the most common photochemical reaction with the quantum yield of formation in isolated DNA of all-thymine oligodeoxynucleotides 2-3% [3], Furthermore, a recent study based on femtosecond time-resolved transient absorption spectroscopy showed that thymine dimers are formed in less than 1 ps when the strand has an appropriate conformation [258], The low quantum yield of the reaction in regular DNA is suggested to be due to the infrequency of these appropriate reactive conformations. 

Very important information concerning the photochemistry of the nucleic acids was furnished by the report of Beukers and Berends that irradiation of frozen solutions of thymine produces a stable photoproduct corresponding to dimerization of the thymine/58 This photoproduct has been subsequently identified as a cis-syn-cis dimer ... 

There is always interest in the photochemistry of the pyrimidine nucleic acid bases and related simple pyrimidinones, due to its importance in genetic mutation. In addition to damaging DNA, photo-induced reactions may also repair the damage, as in the reduction, by FADH, of the thymine glycol 64 back to thymine <06JACS10934>. Another report related to repair of DNA involved a model study, by means of the linked dimer 65, of the involvement of tryptophan in the electron-transfer leading to reversion of thymine oxetane adducts <06OBC291>. 

Pyran-4-ones bear an obvious structural similarity to the all-carbon cyclohexadienones discussed above. However, the original studies of their photochemical behavior revealed only dimerization processes to produce a cage product resulting from two successive head-to-tail [2 + 2]-photocycloadditions (Scheme 29)54. Much later, small amounts of substituted furfural 121 were observed during the irradiation of 11955a. It was speculated that 121 could arise from bicyclic epoxide 120, an intermediate analogous to those formed in cyclohexadienone photochemistry. Subsequent reports noted that further irradiation of... 

Two additional systems were exploited in order to confirm the involvement of free-radical processes during vindoline oxidations. These were the enzyme peroxidase and photochemistry. Horseradish peroxidase (HRP) oxidized both vindoline and 16-O-acetylvindoline in the presence of hydrogen peroxide. Vindoline was converted to the enamine dimer 59 (78). During the reaction, the following sequence of redox reactions occurs ... 

Photochemical reactions, like any chemical reaction, can be classified into various groups, depending on the reactants and products, for example, elimination, isomerization, dimerization, reduction, oxidation, or chain reaction. One important practical field of photochemistry is organic photochemistry. In solution photochemical reactions, the nature of the solvent can markedly influence the reaction. The absorbtion of the solvent and of the reaction products is an important parameter for the choice of the reaction conditions. It is useful to have a solvent with a relatively low absorption in the desired wavelength. Sometimes photosensitizers are used these are substances that absorb light to further activate another substance, which decomposes. 

Nitrosobenzene was studied by NMR and UV absorption spectra at low temperature146. Nitrosobenzene crystallizes as its dimer in the cis- and fraws-azodioxy forms, but in dilute solution at room temperature it exists only in the monomeric form. At low temperature (—60 °C), the dilute solutions of the dimers could be obtained because the thermal equilibrium favours the dimer. The only photochemistry observed at < — 60 °C is a very efficient photodissociation of dimer to monomer, that takes place with a quantum yield close to unity even at —170 °C. The rotational state distribution of NO produced by dissociation of nitrosobenzene at 225-nm excitation was studied by resonance-enhanced multiphoton ionization. The possible coupling between the parent bending vibration and the fragment rotation was explored. 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

The last example of a sequential approach is from Sanov (excerpt 130). A series of increasingly complex experiments is proposed to study the photochemistry of 02, and OCS . Sanov begins with the easier diatomic anions (02 and which will serve as prototypes for subsequent experiments. Next, he will study a larger, polyatomic anion (OCS ) and its cluster ions, 0CS (H20)]j. In the future, he will study even larger dimers and trimers (OCS)n (n > 2) and their hydrated counterparts. 

Traynor, N.J. and Gibbs, N.K. (1999) The phototumorigenic fiuoroqumolone lomefioxacin photosensitizes pyrimidine dimer formation in hrunan keratinocytes in vitro. Photochemistry and Photobiology, 70, 957-959. 

The cross sections for formation of the two dimers are similar, but not identical, with those for the formation of two of the dimers from UpU, and the wavelength dependencies are similar. Hydrate formation occurs more rapidly in d-UpU than in UpU. The marked quantitative and qualitative differences in the photochemistry of these two closely similar dinucleotides, along with the additional difference in results observed with poly U, to be discussed below, make it clear that the factors influencing the course of the photolysis of polynucleotides are not well known. It must be pointed out that the structures of the various... 

If the starting carbonyl compound or olefin has other chromo-phores which can undergo photochemistry, this may lead to complications. In the case of the photocycloaddition of 9-anthraldehyde to 2-methyl-2-butene (pp. 305, 324), prolonged irradiation of the oxetane 53 leads to anthracene-type dimers 87.25 When fluorenone is irradiated... 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

Several naphthynes have already been investigated in the early 1970s. Griitzma-cher and Lohmann ° generated some derivatives by pyrolysis of different precursors and measured the ionisation potentials of the pyrolysis products. At about the same time, Lohmann investigated the photochemistry of the isomeric naphthalenedicar-boxylic anhydrides 61 and 62 using LFP and found that dimerization of the 2,3-naphthyne (57) is significantly faster than that of 4. The 1,2-isomer 50, on the other hand, hardly dimerizes at all. 

The photochemistry of Zn+—(CH4) and Zn —(CH3OH) complexes has been studied in detail In that context, the possibility of the formation of a metal-hydroxo insertion complex [HO—Zn—CH3]+ (the isomer of [Zn—(CH30H)]+) has also been discussed . In other series of studies, the mechanism of dimethylzinc zinc monoethyl cation, diethylzinc diethylzinc dimer and dipropylzinc photolysis has been investigated by photoionization techniques. It was the study of Borsella and Larciprete that first observed different gas phase photodissociation mechanisms for Mc2Zn and Et2Zn by using one- and two-color multiphoton ionization combined with TOE MS. 

Nitro-compounds fRNOj) are isomeric with nitrites, but their electronic structure, excited states and photochemistry are very different. There is no very low-lying (n.jt ) state, and nitroalkanes show n — 3i absorption with a maximum around 275 nm ( —201 mol - cm In cyclohexane solution, nitromethane (CH1NOi) is photoreduced to nitrosomethane(CH,NO, but nitroethane under the same conditions gives rise to a nitroso-dimer derived from the solvent CS.47). The latter process is probably initiated by cleavage of the carbon-nitrogen bond in the nitroalkane. In basic solution (when the nitroalkane is converted to a nitronate anion) irradiation can lead to efficient formation of a hydroxamic acid (S.48), and this reaction most likely proceeds through formation of an intermediate three-mem bered cyclic species. 

The photochemistry of aryl azides is quite complex, suggesting that the nitrene 14 may not be the only reactive intermediate and that insertion reactions may not be the only route to form photoconjugates.Although aryl nitrenes are much less susceptible to rearrangements than acyl nitrenes, they may still occur and lead to the formation of reactive intermediates such as azepines, which may go on to react with nucleophiles.[911 141 Addition of nitrenes to double bonds will generate azirines, while dimerization will produce azobenzenesJ11 Aryl azides are stable to most of the procedures used in the course of peptide synthesis except for reduction reactions. Non-photochemical reduction of aryl azides to the primary amines by thiols has been reported by Staros et al.[15]... 

The photochemistry of thiocoumarins is virtually unknown, though 4-acetoxy compounds dimerize to cyclobutane species at the 3,4-double bond on irradiation. 

BM) forms with a quantum yield of 0.017 0.005 when Re2Cl " is irradiated at 313 nm. Cleavage of the quadruple bond to form ReCl4(CH3CN)2 is followed by rapid conversion to the neutral monomer by means of acetonitrile replacement of chloride. No reaction occurs in the absence of irradiation at reflux, and the 88 excited state is not responsible for the observed photochemistry, since 632.8 nm radiation does not lead to cleavage of the rhenium dimer (135). Flash-photolysis studies suggest that the dominant photochemical process involves internal conversion to a transient 65 excited... 

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