Norcaradiene hepta-2,4-diene

Cyclopropanation of monocyclic aromatic rings (benzene and its derivatives) yields 7-acyl-bicyclo[4.1.0]hepta-2,5-dienes that are subject to the norcaradiene/cycloheptatriene valence equilibrium (see Houben-Weyl, Vol.4/3, p509 and Section 1.2.I.2.4.2.6.2.). Whereas 7-mono-... 

Photolysis of a-diazo esters in the presence of benzene or benzene derivatives often results in [2-1-1] cycloaddition of the intermediate acylcarbene to the aromatic ring, thus providing access to the norcaradiene (bicyclo[4.1.0]hepta-2,5-diene)/cyclohepta-l,3,5-triene valence equilibrium. The diverse effects that influence this equilibrium have been discussed (see Houben-Weyl, Vol. 4/3, p509). To summarize, the 7-monosubstituted systems obtained from a-diazoacetic esters exist completely in the cycloheptatriene form, whereas a number of 7,7-disubstituted compounds maintain a rapid valence equilibrium in solution. On the other hand, several stable 7-cyanonor-caradienes are known which have a second 7t-acceptor substituent at C7 (see Section 1.2.1.2.4.3). Subsequent photochemical isomerization reactions of the cycloheptatriene form may destroy the norcaradiene/cycloheptatriene valence equilibrium. Cyclopropanation of the aromatic ring often must compete with other reactions of the acylcarbene, such as insertion into an aromatic C H bond or in the benzylic C H bond of alkylbenzenes (Table 7). 

Although nonaromatic cyclic alkaoligoenes always react with transient phosphorus-substituted carbenes to give 1 1 adducts, aromatic derivatives sometimes give rise to a mixture of 1 1 and 1 2 adducts. Numerous bicyclo[4.1.0]hepta-2,4-dienes (norcaradienes) have been prepared by reacting transient phosphorus-substituted carbenes with aromatic compounds in order to study the influence of the substituents on the norcaradiene-cycloheptatriene equilibrium. Benzene itself and its functionalized derivatives, in which the three bonds are not equivalent, will be reviewed in separate sections. 

Various bi- and oligocyclic ring systems, such as norcaradiene (1 bicyclo[4.1.0]hepta-2,4-diene), bicyclo[6.1.0]nona-2,4,6-triene (2), bullvalene (3 tricyclo[3.3.2.0 ]deca-3,6,9-triene), semibul-Ivalene (4 tricyclo[3.3.0" ]octa-2,7-diene), barbaralone (5 tricyclo[3.3.1.0 ]nona-3,6-dien-10-one) and many other systems containing divinylcyclopropane subunits, exhibit interesting transition metal mediated reaction pathways leading to new carbon skeletons. ... 

The quantum yield for toluene formation is very low in solution but approaches unity in the gas phase at low pressures . The toluene was suggested to be formed from vibrationally excited ground state molecules, following rapid internal conversion from the excited singlet state manifold, perhaps involving the intermediacy of norcaradiene (bicyclo[4.1.0]hepta-2,4-diene) . The hot ground state mechanism for toluene formation has received considerable support from time-resolved and steady-state experiments on cycloheptatriene and several of its derivatives - ". ... 

Activation energies for conversion of cyclic trienes to bicyclic dienes are smaller than those for ring closures of acyclic trienes and activation energies for formation of bicyclo[4 3.0] systems are smaller than those for formation of bicyclo[4.2.0] systems. The only data for formation of a bicyclo[4.1.0] system are for cyclization of l-oxa-2,4,6-cycloheptatriene, which has a very low energy of activation. The equilibrium for cyclization of cycloheptatriene to bicyclo[4.1.0]hepta-2,3-diene (norcaradiene) is too far toward cycloheptatriene to permit study of this interesting reaction. 

The experimental enthalpy of activation for disrotatory thermal isomerization of cis-l,3,5-hexatriene to 1,3-cyclohexadiene in the gas phase at 100 C is 29.2 kcal/mol [13]. The reaction is exothermic by 14.5 kcal/mol [14, p. 127], so of the reverse reaction is 43.7 kcal/mol, but - in spite of its high activation energy - it is characterized as allowed by all of the common orbital symmetry criteria. In norcaradiene ([4.1.0]hepta-2,4-diene), the cyclopropane ring bridging Cl and Ce of cyclohexadiene has built the disrotation into the molecule, desymmetrizing it - and its monocyclic isomer, cycloheptatriene - to C, in which the 61 and ai orbitals correlate directly (Fig. 5.3). The rate of isomerization is so much faster that it had to be measured at low temperature (ca. 100 K) in a hydrocarbon glass [15] is only 6.3 kcal/mol ... 

Geometrical restraints can also impose fomiidable energy barriers to reaction. In the case of bicyclo[4,l,0]hepta-2,4-diene (i.e. norcaradiene) ring opening to cyclohepta-I,3,5-triene can only occur readily by the single dis-rotatory mode indicated in Equation (3.18). The alternative disrotatory process and the two conrotatory modes can be ruled out because of their... 

---