Photoisomerization of benzene

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S (B2u) reaction path from benzene toward prefulvene starts at an excited-state minimum with D(,h symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sq has a flat biradicaloid region of high enei y with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyer and Klessinger, 1995). 

In low-temperature argon matrices, dewarbenzene has been shown to be a primary product of benzene photolysis at 253.7 nm (Johnstone and So-deau, 1991). Hexafluorobenzene yields hexafluorodewarbenzene upon irradiation in the liquid or vapor phase (Haller, l%7). 

Direct irradiation as well as sensitized triplet excitation of prismane results in rearomatization to produce benzene and in isomerization to form dewarbenzene, which predominates even in the triplet reaction. An adiabatic rearrangement to excited benzene is not observed (Turro et al., 1977b). 

While the electrocyclic ring opening to o-quinodimethanes is the major reaction pathway in the irradiation of substituted benzocyclobutenes (cf. Example 6.14), the irradiation of unsubstituted benzocyclobutene yields 1,2-dihydropentalene (119) and 1,5-dihydropentalene (120) as major products. The mechanism shown with prebenzvalene (118) as primary photochemical intermediate has been proposed to explain the formation of the isomeric dihydropentalenes (Turro et al., 1988). Supporting calculations that yield the same mechanism for the benzene-to-fulvene transformation have been published (Dreyer and Klessinger, 1995). 

Inversion of configuration at C-3, which is expected for an anti-disrota-tory reaction path, has been observed, for instance, for the acyclic chiral 1,4-diene 121. 

The stereochemistry of the di- r-methane rearrangement has thus been shown to be in complete agreement with a concerted reaction course (Zimmerman et al., 1974). 

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E. Photoisomerization of benzene. Photolysis of liquid benzene by excitation to its (7t, jr ) states brings about interesting photochemical transform ations from vibronically excited singlet as well as triplet states (Figure 7.8). 

In recent years there has been a sharp decline in the number of reports dealing with photoisomerizations of benzenes into fulvenes, benzvalenes, bicyclo[2.2.0]hexadienes (Dewar-benzenes), and prismanes. There is now... 

The photoisomerization of benzene into Dewar benzene, benzvalene and fulvene is a well-known process, and in 1995 the interconversion between the [4] paracyclophane (3) and the 1,4-bridged Dewar isomer (4) in a matrix at 77 K was described. The analogous Dewar isomers (5) and (6) have been synthesized by conventional means in order to investigate the properties of the derived substituted [4]paracyclophanes and the kinetic stabilization of this skeleton by substituents which sterically hinder reactions at the bridgehead sites. Irradiation under matrix isolation conditions of (5) and (6) yields the corresponding cyclophanes which have widely differing half-lives (1 min at — 90°C to 0.5h at — 20°C) for their cycloreversions the authors also discuss in detail the aromaticity of the extremely bent benzene ring in the [4]paracyclophanes. 

The photoisomerization of benzene rings has also been studied, using 1,3,5-tri-t-butyl benzene. The composition of the photostationary state is shown below ... 

Figure 10.7. Correspondence diagrams for photoisomerization of benzene (Deh) benzvalene (Cfv) and Dewar benzene (C )...

Before 1956, benzene 1 and other substituted aromatics were considered to be photochemically unreactive. The historically important discovery in that year of the photoisomerization of benzene to fulvene 2 changed this perception. Soon reports appeared on the photochemical preparation and characterization of other C5H5 isomers, t-butyl derivatives of prismane 3 (1965), > benzvalene 4 (1967), and Dewar benzene 5 (1968). Many examples of substituted cases are now also known. 

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