Biphenyl , interacting fragments

The radical cation of spirofluorene, 19, is mentioned only in passing. The orientation of the fragments allows only the second highest MO of the biphenyl moiety to interact with a cyclopropane FMO (the A HOMO). Thus, the type B structure should be stabilized to a lesser extent. The extent of homoconjugation also is expected to be weak because of a more serious mismatch in orbital energies [110, 127]. 

Figure 4.16. Derivation of the correlation diagram for the concerted fragmentation of cyclobutenophenanthrene from the orbital energy-level scheme of biphenyl and the orbital correlation diagram for the fragmentation of cyclobutane into two ethylenes. (The additional double bond has been neglected in the simplified treatment.) The arrows indicate the magnitude of orbital interactions between the two superimposed systems (by permission from Michl, 1974b).

Sometimes the interactions typical for 1.2-disubstituted arenes have a formal correspondence in the decompositions of 2.2 substituted biphenyl derivatives. Thus the highly specific piperidine eliminations from 14330 as well as the formation of 146 correspond to the fragmentations of structurally analogous benzene derivative. 

Table 14.5 Transition energy A (in eV), osdUator strength (in parentheses), collectivity number K, excitation distribution La, CT numbers (14.37) for intra-fragment (/i = h->-]) and interfragment (/i- ii) interactions, and CT,ot (in %) in the lowest p-transitions of biphenyl...

---