7r-Orbital overlap

The asymmetric induction that has been observed in this reaction can be explained in terms of the model shown in Scheme 9. In the most stable conformation the appropriately positioned phenyl group shields selectively the Re,Re face of the chromadiene by 7r,7r-orbital overlap forcing the nucleophile to attack preferentially on the opposite side. 

The a-methylbenzyl cation (1) can be approached from the alcohol dehydration direction or the alkene protonation direction, as shown, and both of these processes have been the subject of ab initio molecular orbital calculations. It was found that the alcohol dehydration has a transition state about half way between the two stmctures shown, with the transition state and the carbocation having about the same amount of 7T-orbital overlap. However, the alkene protonation has an earlier transition state with less effective 7r-orbital overlap than that in the cation. This is held to explain the different Yukawa-Tsuno r+ values found for the two processes, 0.7-1.1 for alkene... 

It has been suggested that 7r-orbital overlap could explain the bonding in 86,94 but the strength of S-S-S bonds would then be rather small. The sulfoniumylid structure 87 complies with common valence theory and the octet rule for sulfur atoms. 

A polymer of structure 44 was obtained by chemical coupling reactions [247]. It has been observed that 44 shows a large shift in the absorption maximum with the appropriate ion and, as the shift occurs in the visible region, a dramatic color change is observed. This effect has been ascribed to conformational changes induced by the ions. In fact, metal complexation forces a rotation of the thiophene rings in order to accommodate maximum chelation, which reduces 7r-orbital overlap. 

Resonance effect (Section 16.4) The donation or withdrawal of electrons through orbital overlap with neighboring 7r bonds. For example, an oxygen or nitrogen substituent donates electrons to an aromatic ring by overlap of the O or N orbital with the aromatic ringp orbitals. 

Morris and Waring<49) suggested that to effect the necessary overlap to form (64), the 7r-orbital lobes at C-4, C-6 and C-l and C-5 must be twisted from planarity. This could explain the finding that only heavily substituted dienones lead to the bicydo[3.1.0]hex-3-en-2-one photolysis. 

Substituent Effects. When the carbyne substituent is changed to a good dr-donor such as NMe2, the nonequivalence of the 7r-orbital becomes more marked. This is indicated by a greater difference between the overlap populations for the two ir-orbitals (29). In terms of resonance formalism, the effect can be considered to be an increased contribution of hybrid B to... 

After the cr bond has formed, further interaction of the p orbitals on the two atoms is restricted to the px and py orbitals, which are perpendicular to the pz orbital. When these orbitals interact, the region of orbital overlap is not symmetrical around the internuclear axis but rather on either side of the internu-clear axis, and a 7r bond results. Orbital overlap of this type is also shown in Figures 3.5 and 3.6. The combinations of wave functions for the bonding -n orbitals can be written as... 

FIGURE 21.6 The overlap of a (/orbital on a metal with the antibonding 7r orbital on CO. 

Rotation of the aromatic ring out of the enediyne plane forces the aromatic 7r-orbitals to overlap with the in-plane 7r-orbitals of the enediyne system as illustrated... 

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