Prefulvene formation

In the photochemistry of benzene, the so-called channel 3 represents a well-known decay route along which fluorescence is quenched above a vibrational excess of 3000 cm [57], The decay takes place through a prefulvenic conical intersection characterized by an out of plane bending [52,58] and results in the formation of benzvalene and fulvene. The purpose of this study is to find distinct radiationless decay pathways that could be selected by exciting specific combinations of photoactive modes in the initial wavepacket created by a laser pulse. For this, we carry out quantum dynamics simulations on potential energy surfaces of reduced dimension, using the analysis outlined above for the choice of the coordinates. 

The regioselectivity of the addition with respect to the arene has been interpreted in terms of both meta addition of the alkene to the Sj arene via an exciplex followed by formation of the vinylcyclopropane system,and initial rearrangement of the arene to a prefulvene species which then undergoes concerted alkene addition (see below). The orientation of the substituents on the wcfa-cycloadducts from some systems requires that both mechanisms operate in competition. The reaction shows no apparent regioselectivity with simple alkenes thus 2-methylpropene gives both wc/a-cycloadduct isomers in approximatively equal amounts. ... 

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with 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, Sg has a flat biradicaloid region of high energy 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 (Dreyerand Klessinger, 1995). 

Dreyer, J., Klessinger, M., The Photochemical Formation of Fulvene from Benzene via Prefulvene A Theoretical Study, Chem. Eur. J. 1996, 2, 335 341. 

Bryce-Smith and Gilbert [37] postulate the presence along the singlet path-way of a biradical intermediate, prefulvene, with fused three-membered and five-membered rings. A triplet biradical of that structure was found by Oikawa et al. [38] in their computational investigation of the lowest triplet pathway. The Ti states of benzene and benzvalene have the same space-symmetry (Bi) in C2y and Si(Ai) (but not S2(-Bi) ) can indeed cross to the T ,(5i) triplet component at the surface crossing. Spin-non-conservative formation of the triplet prefulvene biradical from Si can therefore not be excluded, particularly since fulvene is produced along with benzvalene [37]. 

Photoirradiation of benzene and its derivatives with alkenes give ortho-, meta-, para-cycloaddition products. In most cases, either meta- or orr/io-adducts are obtained as major products [5]. Bryce-Smith and Gilbert suggested a prefulvene type diradical intermediate mechanism (Path A) for the weta-adduct [6], whereas Morrison and Srinivasan groups [7, 8] proposed the exciplex mechanism (Path B) for the formation of these adducts (Scheme 9.1). The exciplex intermediates undergo photo-induced electron transfer processes between donor (D) and acceptor (A) to produce radical ion pairs as intermediates, stabilized by coulombic interactions to give adducts [9]. 

Fig. 5.10 (a). Correlation diagram for benzene-ethene meta-cycloaddition through prefulvene intermediate (initial formation of (i) bond)... 

In meta-addition either direct concerted cycloaddition or initial meta-bonding in excited singlet state to give prefulvene type intermediate takes place. Formation of bond (i) can be before (ii) and (iii) or vice versa or subsequent to (ii) and (iii). This reaction is stereospecific and insensitive to proton donor and solvent which suggests that (ii) and (iii) may be formed is concerted step. 

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