Norbornadiene interaction

The heuristic success of the concepts of through-space and through-bond interactions introduced by Hoffmann200 made it desirable to link it to many electron procedures, at least on the level of semiempirical or ab initio SCF calculations. Such a treatment, which had been proposed some time ago204, has been reviewed in detail elsewhere173. For this reason we shall only sketch the essential steps involved, using norbornadiene 75 as an example. 

FIGURE 18. The symmetry-adapted, orthogonal linear combinations of the localized a-orbitals of norbornadiene 75 belonging to the irreducible representations / and 62 of the point group C2v- The A] and B2 combinations are the relay orbitals for through-bond interaction between jra and 7Tb which define, according to equation 56, the orbitals jt and jt... 

As shown in Table 6 and Figure 1, the oxidation potentials of 2-substituted norbornadienes (1), 2-substituted bicyclo[2.2.2]octa-2,5-dienes (2) and 4-substituted [2.2]paracyclophanes (3) clearly indicate that the transannular interaction between two double bonds contributes already at the stage of the first electron transfer. Namely, in compounds 1-3, the electron is transferred from the unsaturated bond which is not substituted by the electron-withdrawing group, Figure 1 shows the... 

No discussion about strained-ring radical cations would be complete without the valence isomers quadricyclane (15 +) and norbornadiene, (16 +) 15 features two adjacent rigidly held cyclopropane rings, whereas 16 contains two ethene n systems well suited to probe through-space interactions.Molecular orbital considerations suggest the antisymmetric combination of the ethene n orbitals (16) or cyclopropane Walsh orbitals (15) as respective HOMOs of the two parent molecules. The radical ions have different state symmetries and their SOMOs have different orbital symmetries. 

Figure 3.7. Four-electron, two-orbital interaction diagram for norbornadiene and its ionization...

The through-space interaction of the two n bonds of norbornadiene was presented in Chapter 3 as exemplifying a four-electron, two-orbital interaction. The interaction of the nonconjugated n bonds of 1,4-cyclohexadiene cannot be treated in the same way ... 

Bertolini et al (377) determined the VEEL spectra and NEXAFS of but-l-ene and 1,3-butadiene on Pd(lll) and Pt(lll). They concluded that at 300 K but-l-ene was 77-bonded on Pd and di-cr-bonded on Pt, as is now becoming an expected pattern. 1,3-Butadiene was concluded to be di-77-bonded on Pd, but di-cr-bonded to Pt via one C-C double bond. Hostetler et al. (378, 378a) concluded from RAIRS and VEELS that norbornadiene at 130-220 K was di-cr-bonded to the Pt(lll) surface via one double bond, also forming an agostic soft-bond interaction of the bridgehead CH2 group... 

The photosensitized cyclization of norbornadiene to quadricyclane, 18, has been known for some time.270 When barralene, 19, is subjected to photosensitization, all three double bonds are involved and compound 20, which Zimmermann has named semibullvalene, results.271 Irradiation of dibenzobarralene, 21, in acetone produces the dibenzo analog of semibullvalene, 22, in high yield.272 Since there is appreciable interaction between the double bonds in both norbornadiene and barralene, these reactions may be special cases. 

A successive polarization of the exocyclic double bond, as monitored by the carbon shifts in the 7-methylene derivatives of norbornane, norbornene and norbornadiene (Table 4.12), is attributed to homoconjugative interaction of it bonds, pushing it electrons towards the exo methylene carbon [239]. 

Even stronger polarizations of double bonds in alkenes are induced by electron withdrawing substituents, as present in enol ethers, enones, and enamines (Sections 4.6.2, 4.7, and 4.9.2). Deshielding of C-7 in norbornadiene (75.5 ppm, Table 4.12) is understood as arising from interaction of antibonding n orbitals at the olefinic carbon atoms with o orbitals of the bridgehead bonds [214, 216]. Spiroconjugation in spiro[4.4]nonatetraene is interpreted similarly [242]. 

For example, norbornadiene does not possess a stability that is significantly different from that of a diene without any through-space interactions. Its heat of formation can be... 

The lowest energy configuration of the molecule is a twisted-boat geometry, illustrated in Fig. 8(a), in which the two C=C bonds, separated by 3.20 A, are twisted by 23° with respect to one another this arrangement is quite different from that in norbornadiene, for which the two C=C bonds are parallel. Figures 8(b) and (c) schematically show two possible ways of interaction with Si dimers, termed endo and exo , respectively. In order to bond to two adjacent dimers 1,5-cyclooctadiene makes the endo... 

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