Norcaradienes, structure

In one famous case, the release of ring strain is almost exactly counterbalanced by the formation of a a bond at the expense of a it bond. Cycloheptatriene exists in equilibrium with a blcyclic isomer known as norcaradiene. Usually cycloheptatriene is the major component of the equilibrium, but the norcaradiene structure is favoured if R is an electron-withdrawing group. 

Attempts were made to influence the position of the equilibrium by incorporation of C(7) in a notional norcaradiene into a spiro system. The presence inter alia of a multiplet at 3.60 ppm requires the norcaradiene 268 a symmetrical multiplet at 3.53 ppm supports the norcaradiene structure, 269 . By way of contrast the hexafluoro compound 270... 

However, it has been shown that substituents can change the position of the cycloheptatriene/ norcaradiene equilibrium. Thus n-electron-withdrawing substituents in 1 (X = Y = CN X = COjH, Y = H ) are able to stabilize the norcaradiene structure by shortening the distal (C2-C3) bond and lengthening the vicinal (C1-C2, C1-C3) bonds. With 7r-electron-do-nating substituents the situation is more complex. Only strong n-donors (e.g. 0 , CHj) effect stabilization by lengthening the cyclopropane bonds."... 

According to theoretical studies, the HOMO of benzocyclopropene is mainly located at the bridging rr-bond and it therefore would be predicted that it would participate in cycloadditions with electron-deficient dienes. Stable adducts 1 with the norcaradiene structure were obtained upon heating benzocyclopropene with 4,5-dibromo-l,2-benzoquinone, 4,5-dichloro-l,2-ben-zoquinone and tetrachlorobenzoquinone. ... 

Benzocyclopropene reacted with benzonitrile oxide to give a Diels-Alder adduct 2a with norcaradiene structure in 10% yield together with a mixture of stereoiso- and regioisomeric bis-adducts using 2,4,6-trimethylbenzonitrile oxide the yield of monoadduct 2b increased to 27%. The bis-adducts are derived from reaction of the nitrile oxide with the diene moiety of the monoadducts. 

When the parent compound of the bridged [lOjannulene series, l,6-methano-[10]annulene 11>120) 28 (see Table 6), was first prepared, it was necessary to distinguish between three possible structures a) the "double norcaradiene structure 20, b) a rapidly fluctuating system (c.f. cyclooctatetraene), possibly proceeding through low equilibrium concentrations of the double norcaradiene, 21, or c) a delocalized 10 -electron system 22. 

The molecules taking part in a valence tautomerization need not be equivalent. Thus, NMR spectra indicate that a true valence tautomerization exists at room temperature between the cycloheptatriene 110 and the norcaradiene (111). In this case one isomer (111) has the cw-l,2-divinylcyclopropane structure, while the other does not. In an analogous interconversion, benzene oxide and oxepin exist in a tautomeric equilibrium at room temperature. 

As to the cation-radical version of this isomerization, there are testimonies on the transition of the norcaradiene carcass into the cycloheptatriene skeleton. Calculations at the B3LYP level shows that cycloheptatriene cation-radical is more stable than norcaradiene cation-radical by ca. 29 kJ mol (Norberg et al. 2006). Hydrocarbon ion-radicals with strained ring structures have a tendency to undergo facile rearrangement to enforce the unpaired electron delocalization and release their strain energy. 

The structure of the corresponding radical cations should be determined by the symmetry of the two fragment FMOs at the points of union. The butadiene HOMO is antisymmetric at the positions of attachment in the norcaradiene framework, it may interact with the antisymmetric cyclopropane HOMO (as shown above). Indeed, norcaradiene derivatives provide the most promising examples of radical... 

While the nuclei of the aromatic segments show the identical signal directions, the cyclopropane protons show characteristic differences. This suggests significantly different spin-density distributions for the cyclopropane moieties of the two species and, thus, different structures. Like the norcaradiene HOMO, the styrene HOMO is antisymmetric at the positions of attachment, suggesting preferred interaction with the antisymmetric cyclopropane HOMO. In the norcaradiene system, the natural structure ( Aj) of the cyclopropane radical cation is altered by the interaction with the diolefin entity. 

For radical cations of norcaradiene and derivatives, the interaction of the cyclopropane in-plane e orbitals with the butadiene frontier MO favors the type B structure. The assignments are based on ab-initio calculations, CIDNP results, and the ET photochemistry. The norcaradiene radical cation (lla ) has a electronic ground state (Cj symmetry). The Cl—C6 bond is shortened on ionization (—3.4 pm) while the lateral bonds are lengthened (+2.8 pm). The delocalization of spin density to C7 (py = 0.246 py 5 = 0.359) and the hyperfine coupling constants of the cyclopropane moiety a e = 1.36 mT oysyn = —0.057 mT flvanti = —0.063 mT) support a type B structure. 

Although the structural elements supporting cyclopropyl homoaromaticity and nobond homoaromaticity are now generally understood, it is not clear under what conditions a homoconjugated molecule will prefer to occupy a single minimum or to adopt classical forms connected by a valence tautomeric equilibrium. Of course, one can explain that the norcaradiene/cycloheptatriene system is characterized by a valence tautomeric equilibrium while the homotropenylium cation possesses a single minimum PES. This has simply... 

A related >/4-norcaradiene tricarbonyliron complex is obtained upon reaction of tricy-clo[4.3.1.0l6]deca-2,4-diene with Fe3(CO)12 in boiling benzene (equation 143). However, the [4.3.1]propellane ring system is not retained in the analogous tricarbonylchromium complex. Instead, as suggested from solution NMR and solid state X-ray analyses, the complex assumes a homoaromatic structure, which is intermediate between a norcaradi-ene and a cycloheptatriene system (equation 144)193,194. It is noteworthy that the Cr(CO)3 group prefers the same conformation as the Fe(CO)3 group in the analogous norcaradiene iron complex. 

---