Other Photofragmentations

6 Other Photofragmentations - [4+4] Cycloadducts of anthracenes and furan are both formed and cleaved photochemically. (o-Anthrylpolystyrenes have been shown to undergo reversible photodimerization, allowing photochemical control of molar rnass. The cycles can be conducted more than ten times without detectable degradation of the polymers. Polyurethanes and poly-ethers containing coumarin components exhibit similar reversible photopolymerization and cleavage. 

Studies of the generation of hydroxyl radicals by photocleavage of the N-O bonds in A -hydroxypyridine-2(1 0-thione, and N-hydroxy-2(l//)-pyridone have been reported. Alternative photochemical sources of hydroxyl radicals are 4-mercaptopyridine-A -oxides.  

Baer and W. H. Ease, Unimolecular Reaction Dynamics Theory and Experiment , Oxford University Press, New York, 1996. 

Horspool, in Chemistry of Triple-Bonded Functional Groups , S. Patai (ed.), Wiley, Chichester, 1994, Vol. 2, p. 375. 

Antonietti, R. Kublickas, O. Nuyken and B. Voit, Macromol. Rapid Commun., 1997,18,287. 

6 Other Photofragmentations - Photodissociation of tert-huty hydroperoxide at 266 nm gives OH radicals with dynamics which are similar to those found for OH from H2O2, and which are consistent with dissociation via a repulsive excited state. Rates of p-scission of the ter/-butoxy radical to acetone and methyl radicals have been determined in flash-photolysis experiments by monitoring its transient UV absorption and its laser-induced fluorescence.  

Glycosyl radicals are formed photochemically and thermally from telluro-glycosides, and addition of this type of radical to alkynes provides a 

Whitehead, Anna. Rep. Prog. Chem., Sect. C Phys. Chem., 1998, 94,293. 

Gudmundsdottir, S. Morgan and M. S. Platz, J. Photochem. Photobiol. A Chem, 1998,116, 1. 

Yamaguchi, N. Hirota and M. Terazima, Chem. Phys. Lett., 1998,286, 284. 

Hochstrate, M. Keller, F.-G. Klamer and H. Prinzbach, Angew. 

Photoformation of OH from carboxylic acids has also been studied, and a review of this topic has appeared. Recent research reports have been published 

Photo-induced demethylation of 4-nitro-iV,A-dimethylaniline,to give 4-nitro-N-methylaniline, has been observed upon excitation in the presence of an external acceptor in benzene or the methoxide ion in methanol. A quantum chemical study of the photodissociation of the alkyl C-N bond in AT-substituted anilines, PhNH-R (R = Me, PhCH2, Ph2CH, Ph3C), has also been carried out.  

Photocleavage of metal-carbon bonds has been investigated in a number of systems. Argon-matrix photolysis of trimethyldioxorhenium(VII) (Me3Re02) with UV-visible light (200 A. 800 nm) was monitored by IR spectroscopy, and resulted primarily in the elimination of methane. This process afforded a novel methylidene-rhenium(VII) compound, H2C=Re(CH3)02. A similar photoelimination of CH4 was observed in the matrix photolysis of tetramethyloxor-henium(VI) (Me4ReO), which afforded H2C=Re(0)Me2. Dissociative photosubstitution of Mo(CNPh)6 has been examined as a function of the nucleophile and 

Beier (FeBiT Ferrarius Biotechnology GmbH, Germany), German Patent 

Beier (FeBiT Ferrarius Biotechnology GmbH, Germany), German Patent 10,105,079, 8 Aug 2002 Chem. Abstr., 2002,137,140733p. 

Two methods for the photolytic generation of / -methoxybenzyl radicals, by C-H cleavage in /7-methoxytoluene and C-O cleavage in p-methoxybenzyl alcohol, have been studio in n-heptane solution by nanosecond fluorescence and absorp- 

Evidence has been provided for the intermediacy of A-methyl-A-phenylnitre-nium ion in the photolysis of iV-(methylphenylamino)-2,4,6-trimethylpyridinium tetrafluoroborate. Substituted aryltropylium ions have been generated by photolysis of a series of 7-methoxycycloheptatrienes, and found to have lifetimes strongly dependent on the donor capacity of the aryl substituent. Photolysis of a mixture of the two pinacols (96 and 97) in acetonitrile resulted in efficient fragmentation of both compounds, by cleavage of the central C-C bonds. This reaction has a quantum yield of 9 1, which suggest a chain process initiated by single electron-transfer quenching of excited (96) by (97). 

Erentova, V. Adamcik, A. Stasko, O. Nuyken, A. Lang and M. B. Leitner, Collect. Czech. Chem. Commun., 1997,62,855. 

Kritzenberger, D. Franzke, T. Kunz, A. Lang, O. Nuyken and A. Wokaxm, Angew. Makromol. Chem., 1998,254, 17. 

Dell Erba, M. Novi, G. Petrillo and C. Tavani, Gazz. Chim. Ital., 1997,127,361. 

Horspool and F. Lend (ed.), CRC Handbook of Photochemistry and Photobiology, 2nd edn., CRC Press, Boca Raton, Florida, 2004. 

Komazawa, M. Kurihara, H. Inoue, N. Miyata, H. Okuda, T. Tsuchiya and Y. Yamakoshi, Tetrahedron Lett., 2004, 45, 527-529. 

Saldyka and Z. Mielke, Phys. Chem. Chem. Phys., 2003, 5, 4790-4797. 

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Other siinjlai structures undergo the following photofragmentation reartangement decomposition processes ... 

In the case of 22b-e, the butadienes 24b-d and the hexatriene 24 e, respectively, are also obtained on trapping with cinnamaldehyde, the 1,3-diene 24a is even the sole reaction product. It is quite obvious that the olefins 24 are secondary products of the trapping reaction of 9 arising by photofragmentation of 22. The other product is phenyldioxophosphorane (23) which also numbers among the short-lived compounds of quinquevalent phosphorus with coordination number 3 (see Sect. 3.1). 

The photochemistry of Ru3(CO)y2 has been investigated in our laboratory (3-5) and others (6-11) and has been shown to involve both photofragmentation of the cluster (Equations 1 and 2) and photolabi-lization of carbonyls to give substituted trinuclear clusters Ru3(C0)] ] L (Equation 3). 

Alternative Probes. Even though C02 possesses a broad range of special virtues, other small molecules could also be used as reporter probes. Once the technique has been refined using C02, photofragmentation reactions that yield N2, CO, HCN, S02, 02, NO, N02 or any of a host of other small molecules should be amenable to similar mechanistic studies using Raman or FTIR spectroscopy. 

Photolyses of spiroanthronetriazolines (Scheme 35) are anomalous they fail to yield aziridines.193 Complex reaction mixtures are obtained when the starting quinone methide has R = Ph, the aryliminoanthrone and the 10-arylamino (or imino) dibenzotropolones are obtained, but not when R = H (Scheme 155). The quinone methide may arise from a photochemical deamination of the diradical, whereas photofragmentation and rearrangement may account for the other products.193... 

In indirect photofragmentation, on the other hand, a potential barrier or some other dynamical force hinders direct fragmentation of the excited complex and the lifetime amounts to at least several internal vibrational periods. The photodissociation of CH3ONO via the 51 state is a representative example. The middle part of Figure 1.11 shows the corresponding PES. Before CH30N0(5i) breaks apart it first performs several vibrations within the shallow well before a sufficient amount of energy is transferred from the N-0 vibrational bond to the O-N dissociation mode, which is necessary to surpass the small barrier. 

Scrutiny of a number of other five-membered C-, N- and S-containing heterocycles (78JCS(P1)1445) with the framework depicted in Scheme 2 has revealed that facile photofragmentation takes place when extrusion of a small inorganic fragment such as N2 (thiatriazole), N20 (thiatriazole 3-oxide), CO, C02, etc. is possible. In these cases formation of nitrile... 

The first quantitative measurement of the distribution of excited atomic states produced in the multiphoton dissociation of a metal carbonyl has been made for Cr(CO)6. Photodissociation does not yield spin- or parity-differentiated states, rather the state distribution appears to be statistical. Photofragmentation dynamics of Cr(CO)6 in the gas phase have been measured and two channels of dissociation revealed. One of these is a rapid predissociation (efficiency 36%) and the other a slow process (efficiency... 

Other examples of wavepacket calculations applied to photofragmentation may be found in Refs. [34], [57] and [58]. Most recently, time-dependent methods have been used to compute vector correlations and alignment parameters [59, 60]. 

Photolysis of o-Nitrobenzyl Derivatives - The cleavage of o-nitrobenzyl derivatives is one of the relatively few classes of photofragmentations in which the two fragments can both bear complex functionality. The protection of alcohols as o-nitrobenzyl ethers is well known, and provides a photochemical method of deprotection, but other applications include the design of photoclea-vable polymers and photochemically active links for molecules synthesized on polymer beads. 

In the case of unsymmetrically substituted oxiranes, the photofragmentation into a carbonyl compound and a cyanocarbene dominates by far the other possible fragmentation pattern. ... 

Like alkenes (Section 6.1.1), chromophores containing the N=N (azo compounds) or C=N (imines, oximes, etc.) bonds can undergo E Z (or trans cis) photoisomerization (Scheme 6.156) and the resulting isomer concentration ratio in the photostationary state (PSS) reflects the absorption properties of the isomers and isomerization quantum yields (see Scheme 6.1 in Section 6.1.1). Since conventional (dark) synthesis generally provides access to more stable E-isomers, photochemistry is an exceptional tool for preparing sterically hindered Z-isomers.1061,1062 The photoisomerization reaction can be induced by a direct irradiation or by sensitization and it often competes with other phototransformations, such as photofragmentation or photorearrangement (Section 6.4.2). 

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