Valence isomerization of benzene

The experimental results on valence isomerization of benzene can be rationalized by means of correlation diagrams that were first discussed for hexa-fluorobenzene by Haller (l%7). The orbital correlation diagram for the conversion of benzene to dewarbenzene (Figure 7.44a) has been constructed on the basis of a classification of the benzene Jt MOs according to C , symmetry. The full symmetry of benzene is D ,. The symmetry elements common to both point groups Cj, and are E, Cj, and o,(x,z) = o which is the plane through... 

Oxepin and its derivatives have attracted attention for several reasons. Oxepin is closely related to cycloheptatriene and its aza analog azepine and it is a potential antiaromatic system with 871-elcctrons. Oxepin can undergo valence isomerization to benzene oxide, and the isomeric benzene oxide is the first step in the metabolic oxidation of aromatic compounds by the enzyme monooxygenase. 

Three decades ago the preparation of oxepin represented a considerable synthetic challenge. The theoretical impetus for these efforts was the consideration that oxepin can be regarded as an analog of cyclooctatetraene in the same sense that furan is an analog of benzene. The possibility of such an electronic relationship was supported by molecular orbital calculations suggesting that oxepin might possess a certain amount of aromatic character, despite the fact that it appears to violate the [4n + 2] requirement for aromaticity. By analogy with the closely related cycloheptatriene/norcaradiene system, it was also postulated that oxepin represents a valence tautomer of benzene oxide. Other isomers of oxepin are 7-oxanorbornadiene and 3-oxaquadricyclane.1 Both have been shown to isomerize to oxepin and benzene oxide, respectively (see Section 1.1.2.1.). 

Depending upon the substitution pattern, a thermal valence isomerization of 1,4-dioxocins 4 to the tricyclic jyn-benzene dioxides (xyn-3,8-dioxatricyclo[5.1,0.02-4]oct-5-enes) 3 can be detected. On the other hand, the valence isomerization of sin-benzene dioxides (anti-benzene dioxides do not undergo such rearrangements) provides a general approach to 1,4-dioxocins 4. 

In an attempt to prepare the parent system via the valence isomerization of the. ryn-benzene-bisepisulfide 2, which in turn was generated from the syw-benzenebisepoxide 3 via diol 4, no valence isomerization to 1,4-dithiocin (1) was detected.3 The bisepisulfide 2 is a thermally rather unstable compound and decomposes in solution even at 20 C, leading to benzene and sulfur as the only isolated products. 

In connection with the chemistry of the reactive transient species, nitrene, the chemistry of azepines is well documented u. Also, the chemistry of oxepins has been widely developed due to the recent interest in the valence isomerization between benzene oxide and oxepin and in the metabolism of aromatic hydrocarbons 2). In sharp contrast to these two heteropins, the chemistry of thiepins still remains an unexplored field because of the pronounced thermal instability of the thiepin ring due to ready sulfur extrusion. Although several thiepin derivatives annelated with aromatic ring(s) have been synthesized, the parent thiepin has never been characterized even as a transient species3). 

Secondly, in view of the generally accepted mechanism for the sulfur extrusion reaction, which involves valence isomerization of the thiepin ring to its corresponding thianorcaradiene (benzene episulfide) isomer followed by irreversible loss of sulfur,... 

In a mechanistic investigation of the valence isomerization of hexamethyldewar-benzene to hexamethylbenzene, Eq. (39), a surface-bound cation radical chain mech-... 

Migration of substituents on a benzene ring caused by valence isomerization of the ring is discussed in Section 5.3.4.3. 

The special case of perfluorinated Dewar benzenes and heteroaromatic analogs is discussed in Section 5.3.4., together with other valence isomerizations of six-membered and larger rings. 

Theoretical calculations for the parent thiepine and its valence isomer benzene sulfide were conducted at a high level of theory (Scheme 1) <1997PCA3371>. The enthalpy change of valence isomerization from benzene sulfide 10 to... 

Paracyclophane, which exists as the minor component of an equilibrium with its valence-isomeric Dewar benzene, is the smallest member of this series to exhibit sufficient stability to be studied directly [3]. In order to further stabilize the [5]paracyclophane unit, Bickelhaupt [4] recently synthesized a benzannulated version, namely [5](l,4)naphthalenophane (2, Scheme 1). Even though this compound also exists in equilibrium with the Dewar naphthalene 1 from which it was produced, the proportion of the cy-clophane (35 % of the mixture) is higher than that in any previously reported case. A particularly interesting feature of this system is that there are two observable bridge conformers of 2 in solution in a ratio of 95 5. Unfortunately, despite considerable effort, assignment of the two conformers was not possible. 

While the isomerization of benzene oxides to oxepins occurs spontaneously at low temperature, the analogous mobile valence tautomerization of m-benzene dioxide 143 (Figure 10) to the 10-tt-heterocycle, 1,4-dioxocin, was only evident at temperatures > 50°The latter process is symmetry-allowed and is formally equivalent to a retro-Diels-Alder reaction. The mobile equilibrium at 60°C appeared... 

Hogeveen and Volger have also reported the effects of different types of catalysts on the valence isomerization of XII to hexamethyl-Dewar-benzene (XIII) and hexamethylbenzene (XIV) (10). 

Two reports of transition metal-catalyzed disrotatoiy cyclobutene ring opening have appeared. Volger and Hogeveen described the valence isomerization of hexamethyl-Dewar-benzene (IV) to hexamethyl-benzene (V) catalyzed by /t-dichlorodi(hexamethyl-Dewar-benzene)-dirhodium in the temperature range 60-70° (37). 

Prismane is the most highly strained valence-bond isomer of a benzene. Therefore, it isomerizes to the benzene isomer possibly via its Dewar isomer. However, since the activation energy of the isomerization of the Dewar isomer to the benzene is small, the Dewar isomer is rarely observed. Many photochemical isomerizations of benzene derivatives are assumed to proceed via Dewar benzenes and prismanes but in most of cases, such isomers have not been observed. 

For example, the quantum yield for a CdS-, TiOj-, or ZnO-mediated valence isomerization of hexamethyl-dewar benzene to hexamethylbenzene is greater than xmity [106]. A cation radical chain reaction mechanism accounts for this observation (Fig. 2). 

Soon after the first synthesis, a notable landmark, the second route to la, was achieved by Jones and Bickelhaupt they reported that the thermal valence isomerization of Dewar benzene type isomer of la, [6.2.2]propelladiene (8a), proceeded smoothly, giving la quantitatively (Scheme 2) [4]. This reaction provided an entirely new general synthetic method for [n]paracyclophanes. However, the overall sequence of reactions was not enough to give a large quantity of la at that time, because the precursor 8a was prepared as a minor product of silver-(I)-catalyzed isomerization of the bicyclopropenyl derivative 7. 

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