Benzene to benzvalene

Figure 39. Benzene to benzvalene reaction, (a) Assuming that the prebenzvalene structure is a transition state. The two benzvalene isomers are anchors, (b) Assuming that prebenzvalene is an intermediate, A two-anchor loop results, compare Figure 12.

Ratio of cyclopentadiene The formation of bicyclopentene is formally an electro-cyclic reaction like that which converts the benzene system to a Dewar benzene [47], and the formation of 3 is crossed cycloaddition like that which converts benzene to benzvalene [48]. Stmctural derivatives of 3 were known prior to the isolation of the parent Masamune prepared 1,5-diphenyl tricyclo[2.1.0.0 Jpentan-3-one by photolysis of a diazocyclopropenyl ketone [49] (X-ray analysis of a derivative confirmed the structure in the face of skepticism [50,51] and Closs and Larrabee prepared 2,4-dimethyltricyclo[2.1.0.0 ]pentane [52] by heating the sodium salt of a cyclobutenyl tosylhydrazone both kinds of reactions are known to generate carbenes or carbe-noids, and presumably involve insertion of a carbene-type carbon into a carbon-carbon double bond ... 

The situations of this type are not, fortunately, very frequent for thermal reactions (e.g. the benzene to benzvalene or cyclooktatetraene to cubane transformations), but in photochemistry are quite common. For the analysis of these reactions the concept of the so-called natural correlation diagrams was introduced [31]. The construction of these diagrams is not, however, so straightforward as the construction of the... 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

Although valence isomerization reactions of aromatic compounds have found little by the way of practical application, they are a fascinating area for mechanistic and theoretical study. The details are not completely dear, but it seems that, for benzene itself, benzvalene arises from the lowest excited singlet state, perhaps by way of a biradical intermediate (3.32) that could also be a precursor to fulvene bicyclohexadiene is probably produced from the second excited singlet state. For some other aromatic compounds the electronic nature of 5, and S2 may be reversed, or at least the states are much closer in energy, so that the preference for benzvalene or bicyclohexadiene formation under conditions of long-wavelength irradiation can be rationalized. 

As already mentioned, the triplet state of benzene was identified by the cis-trans isomerization of the 2-butenes. Nevertheless, Wilzbach and Kaplan in the liquid phase, following work of Srinivasan,42 find a photochemical adduct of benzene to olefins. This adduct they find to be an adduct of benzvalene and not of benzene itself.414 There is also evidence of formation of a small amount of this adduct even in the vapor phase.35 Benzene exposed to ultraviolet radiation in a flowing system diluted with nitrogen and condensed shows compound formation with trifluoroethanol, CF3CH2OH. Again the adduct is one of benzvalene.410... 

Shifts of atoms or groups in fluorine-containing aromatic or heteroaromatic compounds are covered in this section. Shifts due to valence isomerization (benzene-Dewar benzene-prismane-benzvalene), however, are covered in Section 5.3.4.3. A major review1 on polyfluoroaromatic and heteroaromatic compounds has been written by G.M. Brooke, one of the principal contributors in this area, covering the literature until the end of 1995 (see pp 56-59 of ref 1 for the section on rearrangement reactions). 

Figure 10 (a) Photochemical transformation of benzene to fulvene and benzvalene. (b) The change in spin coupling for a circuit around the apex of the Cl in the plane xt, x2. Bold lines between atoms represent bonding interaction, whereas up and down arrows are used to designate +1/2 and - 1/2 electron spin, respectively. 

Ultraviolet irradiation causes changes in carbon skeleton of many compounds. A classical example is conversion of hexafluorobenzene to hexafluoro Dewar benzene [759], and isomerization of benzene to three isomers. The same is true of hexakis(trifluoromethyl)benzene, which rearranges to the perfluorinated isomers of hexamethyl Dewar benzene (F), prismane (G), and benzvalene (H) [140]. 

On the other hand, shock waves generate high pressures as well as high temperatures, and, consequently, some fector in addition to heat must be involved in the shock reactioa Drickamer [145], for example, has suggested a close relationship between photochemistry and liigh-pressure chemistry. He experimentally showed that high-pressure conditions promoted the formation of pentacene dimers with cross-linked structure, the formation of which usually occurred in the photochemical reaction. If the shock reaction is a type of some reactions in excited states such as a photochemical reaction, many valence isomers such as Dewar benzene and benzvalene would be generated from benzene by shock waves, and the interaction between these isomers would produce various com-poimds such as derivatives of fiilvene. Such valence isomers are imstable and would not have been detected in our study. 

Among perfluorinated aromatic compounds, only hexakis(trifluoromethyl)benzene was converted to benzvalene. Using an ultraviolet lamp of longer wavelength (307 nm), benzvalene was selectively obtained in high yield (12)14,1S). This benzvalene is very stable and relatively easy to handle. Its reactions will be discussed in Section 2.5. 

Non-sensitized isomerization of unsubstituted Dewar benzene and benzvalene was found to proceed in high quantum yield (45, 46) 48). 

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). 

The photochemistry of unsubstituted benzene has recently been summarized by Bryce-Smith S9> and will not be repeated here. Several very recent reports should be mentioned, however. Ward and Wishnok 6°) studied the liquid phase vacuum ultraviolet photolysis of benzene and were ible to identify Dewar benzene 78, benzvalene 79 and fulvene 80 in the relative amounts of 1 5 2, respectively. The quantum yields for the formation of 78—80 were estimated to be 0.006, 0.03 and 0.012, respectively. The low quantum yields suggest that radiationless transitions to... 

Reactions involving excited states of benzene include the photoaddition of benzene to furan,41 to olefins,42 and to amines,43 the photochemical chlorination of benzenes,44 the photo-oxidation of benzene,48 and the photo-reactions of tetrafluoroiodobenzene.48 These papers will be discussed in detail in Part 1 of this volume. The gas-phase structures of benzvalene and hexamethylprismane and hexamethylbenzene have been evaluated.47... 

The kinetics and thermochemistry of the quantitative rearrangement of benzvalene (228) and bisdeuteriobenzvalene (229) into benzene and 1,2-dideuteriobenzene have been investigated. " Reactions are first order in the temperature range 313—330 K, with AH = 25.9 0.2kcalmol and AS = 1.6 + 0.7calK mol the heat of reaction for the Ag -ion catalysed process in chlorobenzene solution was estimated at 67.54 0.66 kcalmol" (i.e. more exothermic than the Dewar-benzene to benzene valence isomerization). In the presence of 9,10-diphenylanthracene or 9,10-dibromoanthracene (traps for benzene triplet states) the yield of excited benzene... 

The 7T MOs are drawn for the symmmetrical benzene molecule, numbered as in Fig 9.9 but labeled on their left by their irreps in the subgroup of T>2h appropriate to benzvalene. It is noteworthy that ips and 1 4 have the irrep 62 in while 2 and 5 are a2. Both components of Sj are therefore totally symmetric, whereas those of S2 have B symmetry like Si of BV. Only one component of each transition appears in the correspondence diagram, because - at the orbital level of approximation - excitation from 2( 2) would leave it singly-occupied, and destroy its direct correspondence with (74(02). However, one orbital correspondence is sufficient to allow the reaction. 

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]. 

---