Miscellaneous Rearrangements and Bond Fission Processes

Miranda and his co-workers have studied the cycloreversion of the cyclobutanes (127) and (128) and of the oxetane (129). This work made use of the pyrylium salt sensitizers (130). The reactions arise from the triplet state of the sensitizers since there is clear evidence that the reactions are quenched by molecular oxygen. The ring opening involves an electron transfer, and the best sensitizer is the thiapyrylium salt (130b). The quantum yields for the three products using the three sensitizers are shown in Table 2.  

The formation of the isomers of the quinonemethide (131) by irradiation of the corresponding o-hydroxybenzyl alcohol has been described. A further study on the formation and reactivity of quinone methides has reported the flash photolysis of the phenol derivative (132) in perchloric acid solution. This affords the p-quinone methide (133). Irradiation of the benzene derivative (134) results in the formation of the quinodimethane (135). A two-colour laser method 

A patent has been lodged covering the sulfochlorination of gaseous alkanes. Photochemical bromination using N-bromosuccinimide of the arenes (136) has been examined under a nitrogen atmosphere. The remote functionalization of the alcohol (137) has been carried out using DIB/Ii/cyclohexane. The product obtained from this, after oxidation, is the iodoketone (138, 84%).  

Dibenzosuberene derivatives have been shown to undergo proton-transfer reactions to water from their excited singlet states. Biphenyl derivatives are also reactive under these conditions. A full report of the excited state proton transfer in phenols has been published. ° 

Newcomb and co-workers have described the synthesis of the radicals (139) by laser flash photolysis of suitable precursors. The radical (139) undergoes hetero-lytic loss of mesylate to afford the radical cation (140). The loss of mesylate is faster than loss of phosphate or bromide.  

Evidence for both C-O and N-O bond fission has been found following the irradiation of the pyridone derivatives (211). Irradiation in methanol purged with nitrogen and using wavelengths 340 nm results in the formation of five products identified as the pyridone (212), the corresponding alcohol (213), the aldehyde (214), the ether (215) and the hydroxypyridone (216). The first three products arise by N-O bond homolysis while the remaining two are the results 

Gravel and Bordeleau have demonstrated that the cyclohexanediol (219) can be converted into the deoxysugar (220) by irradiation in the presence of benzophenone, acetonitrile and thiophenol. The conversion of (219) into (220) involves the formation of the aldehyde (221) that is converted into the acetal, i.e. the deoxysugar. An extension of this work has demonstrated that deoxyazasugars can also be formed using the same conditions. Thus irradiation of (222) gave the aldehyde (223) which can then be cyclized by the same path used for the formation of (220). The conditions used were irradiation at 350 nm in acetonitrile solution with xanthene and thiophenol. 

Leigh and his co-workers have studied the photochemical decomposition of the silacyclobutanes (224). In hydrocarbon solution with added methanol, (224) undergoes decomposition and the formation of the alkoxysilanes (225). These are formed via the intermediacy of the silenes (226) formed by photochemical elimination of ethene. 

Kuriyama, H. Takeya, S. Oishi and M. Kojima, Chem. Lett., 1998, 843. 

Sket and B. Kosmrlj, Zb. Ref. Posvetovanja Slov. Chem., 1997, 99 Chem. Abstr., 1998,750100). 

1 Miscellaneous Rearrangements and Bond Fission Processes - Chan and Arnold have carried out calculations to confirm that the two electrons in the biscation (145) are delocalized three-dimensionally. A review has given details of specific photoreactions of acyclic and cyclic saturated hydrocarbons that can provide paths for their functionalization. A study of the SET-induced photochemical bond fission processes in the series of 2-alkoxyphenylethers (146), (147) and (148) has been studied. The reaction involves bond cleavage in the inter- 

Photochemical carbon acid dissociation has been studied in considerable detail by Wan and his coworkers. The current work has examined the photochemical behaviour of the dibenzocycloheptene derivatives (149). The influence of substituents on the process was studied. A review has highlighted the photochemical methods for the formation of carbanions. Within the general framework of photodeprotonation of benzylic systems Wan and his coworkers have also reported the results of a study into the photochemical deprotonation of the thioxanthenium salts (150)-(153). The reactions are carried out by irradiation at 254 nm in dry acetonitrile under an argon purge. Prolonged irradiation of compound (150), for example, results in the production of (154) in 60% yield. The authors reason that this product is formed via the intermediacy of the 

Interest has also been shown in the formation and photochemistry of cations such as those produced from (156). These species are generated by the photolysis of 9-fluorene in 1,1,1,3,3,3-hexafluoro-i-propylalcohol. The reaction of the cations with aromatic compounds has been studied. 

The paracyclophane (157) undergoes fission of the ethano bridge C-C bond when irradiated in an argon matrix at 10 K. Similar behaviour is reported for the analogue (158). Carbon-carbon bond fission has also been studied within the epoxide (159). Irradiation of this at 266 nm brings about conversion into the trans-y ide (160). The quantum yield for the process was measured as 0.099. A patent has described a method for the photochemical isomerization of a cis, trans-mixture of l-(4 -propylphenyl)-4-pentylcyclohexane to give the rru/w-isomer in 98% purity.  

Shi and Wan report that the three biarylmethyl alcohols (161), (162) and (163) undergo conversion into the corresponding pyrans (164), (165) and (166) on irradiation at 254 nm in acetonitrile or acetonitrile/water. The reaction involves the conversion of the diols into the corresponding quinone methide for example, (167) is formed from (161) by formal loss of water. Cyclization of this quino-methide intermediate affords the final products. Previously Wan and his coworkers have described this reactivity for biphenyl derivatives. The quantum 

2 Miscellaneous Rearrangements and Bond Fission Processes. - Methacrylo-nitrile undergoes C-CN bond fission on irradiation at 193 Substitution 

Durr and H. Bouas-Laurent (ed.), Photochromism, revised edn., Elsevier, Amsterdam, 2003. 

Amatatsu, Chem. Phys. Lett., 2003,369,673 (b) Y. Amatatsu, Chem. Abstr., 

Tanaka, T. Hiratsuka, S. Ohba, M.R. Naimi-jamal and G. Kaupp, J. Phys. 

Kawamura, T. Ishiduka and M. Tsukayama, hit. J. Modem Phys. B Condensed Matter Phys., Statistical Phys. Appl. Phys., 2003, 17, 1492 b) Y. Kawamura, T. Ishiduka and M. Tsukayama, Chem. Abstr., 2003,139,213988. 

The product (183) is formed on irradiation of acridine (184) phenothiazine (185) crystals in which the ratio of the reactants is 3 4, This reactivity is different from the solution phase (in acetonitrile) process which affords both (183) and the dihydro dimer (186). The photoinduced electron transfer reactions of some a-silyl ethers has been investigated. The sensitizing system uses DCA/ biphenyl and irradiation at A, 345 nm in acetonitrile/methanol. The irradiation brings about the formation of the radical cation (187) of the ether which undergoes cleavage to yield the radical (188), a hydroxymethyl equivalent. When these are generated in the presence of a,P-unsaturated esters such as (189) addition takes place affording the adducts (190). Additions to dimethyl maleate were also carried out successfully.  

High yields of alcohols can be obtained on irradiation of the ether derivatives (191). The ethers in acetonitrile/water solution are photo-cleavable on irradiation at 254 nm or 300 nm and the yields obtained are good to excellent. The multi-photon chemistry of the naphthalene derivative (192) has been studied 

A study of the reactivity induced by irradiation at 254 nm of the epoxides (202) has been carried out. The aim was to study the formation of the ylides (203). The reactions were carried out in acetonitrile solution with ethyl vinyl ether as the addend and are reported to be dependent to some extent on the substitution pattern. Thus, irradiation of (202a) fails to yield an adduct and only the enone (204, 33%) is formed. Epoxides (202b) and (202c) do produce ylides that add to the alkene to yield mixtures of the adducts (205) regiospecifically with a preponderance of the xo-adduct. The epoxide (202d) is also reactive and gives a low yield of the adduct (206, 2%). In general the overall yields are moderate. The 

Marquis, B. Henze, H. Bouas-Laurent and J. P. Desvergne, Tetrahedron Lett., 1998,39, 35. 

2265 (b) M. Irie, T. Lifka and K. Uchida, Mol. Cryst. Liq. Cryst. Sci. Technol, Sect. 

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