Cyclization pulsed irradiation

The photoenolization of the quinone (286) can be carried by irradiation at 313 or 365 nm in acid solution. The steady state irradiation has identified the product as the unstable hydroxylated compound (287) which is formed via the enol (288). The presence of this intermediate was detected in a laser flash study of the reaction. The quinones (289) undergo cyclization when irradiated with visible light.The mechanism by which the compounds (289) are transformed into the derivatives (290) involves the production of an excited state that is either a zwitterion or a biradical. After the transfer of a hydrogen the intermediate (291) is formed. It is within this species that cyclization occurs to give the final products. (2+2)-Cyclo-adducts such as (292) and oxetanes can be obtained by the photochemical addition of quinones to homobenzvalene. Interest in the photo-SET in quinone systems has led to the synthesis of the pyropheophytin substituted naphthoquinone dyads (293). A pulse radiolysis study of vitamin K in solution has been reported. 

The behaviour of irradiated butyrophenone in the presence of triphenyl-phosphine has been studied.Direct irradiation of the acetophenone derivatives (44) using a pulsed nitrogen laser (337 nm output) as the energy source yields the cyclobutanols (45) as the principal products. The minor product in each case was the corresponding acetophenone, and the formation of the cyclobutanols as the main product was solvent-independent. When crystalline samples were irradiated, less cyclobutanol was formed in all cases. An X-ray study was also carried out in conjunction with the above this showed that the P-hydrogen (relative to the carbonyl) was accessible for H-abstraction in the crystal, but no confirmatory evidence for this was obtained from the photochemistry. The change of product ratio from solution to solid phase is attributed to restricted cyclization within the crystal rather than an enhancement of the fragmentation path. 

Many studies used radiation chemistry to produce the radical and radical cations and anions of various dienes in order to measure their properties. Extensive work was devoted to the radical cation of norbomadiene in order to solve the question whether it is identical with the cation radical of quadricyclane . Desrosiers and Trifunac produced radical cations of 1,4-cyclohexadiene by pulse radiolysis in several solvents and measured by time-resolved fluorescence-detected magnetic resonance the ESR spectra of the cation radical. The cation radical of 1,4-cyclohexadiene was produced by charge transfer from saturated hydrocarbon cations formed by radiolysis of the solvent. In a similar system, the radical cations of 1,3- and 1,4-cyclohexadiene were studied in a zeolite matrix and their isomerization reactions were studied. Dienyl radicals similar to many other kinds of radicals were formed by radiolysis inside an admantane matrix. Korth and coworkers used this method to create cyclooctatrienyl radicals by radiolysis of bicyclo[5.1.0]octa-2,5-diene in admantane-Di6 matrix, or of bromocyclooctatriene in the same matrix. Williams and coworkers irradiated 1,5-hexadiene in CFCI3 matrix to obtain the radical cation which was found to undergo cyclization to the cyclohexene radical cation through the intermediate cyclohexane-1,4-diyl radical cation. 

Interesting behaviour, dependent on the wavelength used, has been demonstrated for the bis diazo compound 492. Conventional irradiation at 254 nm leads to consecutive loss of nitrogen and cyclization to yield the mixture of isomers 493300,301 Using pulsed-laser irradiation at 248 nm affords the bis carbene 494, which dimerizes to yield the cyclo-octatriene 495. 

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