Planar diene rule

Walborsky and coworkers206 have reported the CD of series conjugated cyclohexylidene aldehydes and ketones. These compounds served as additional examples supporting the planar diene rule for CD jt tt transition208. 

Dienes in quasi-s-fraws conformation are found only in cyclic structures where perfect planarity is hindered. The DR also holds valid for this kind of conformation, as demonstrated by the considerations of Section II.D.l.a and also confirmed by all the reported calculations. Indeed, contrary to what is sometimes found for cisoid systems, the rotational strength evaluated by many types of calculation is invariably found to follow the diene rule for transoid systems. However, very small skew angles are usually found in real molecules and this implies that the main contribution to the observed optical activity cannot come from the weak intrinsic distortion, but is more likely to stem from the dissymmetric perturbations, notably of the allylic axial substituents. 

Walborsky and coworkers44 proposed a sector rule, the planar diene rale, to correlate the sign of the Ag —> transition Cotton effect of these derivatives with the absolute... 

The ultraviolet spectra of crowded olefins and substituted butadienes have been studied the sign of the Cotton effect has been related to the chirality of a series of w-molecular complexes of olefins with tetracyanoethylene the sign of the chiral-optical effects of non-planar heteroannular cisoid dienes is opposite to that predicted from the diene rule. Studies of linear and circular dichroism in mono-olefins and magnetic circular dichroism of conjugated olefins have been made, and the experimental ionization potentials of fourteen alkenes have been correlated with the inductive effects of substituents. ... 

The Hiickel rule predicts aromaticity for the six-7c-electron cation derived from cycloheptatriene by hydride abstraction and antiaromaticity for the planar eight-rc-electron anion that would be formed by deprotonation. The cation is indeed very stable, with a P Cr+ of -1-4.7. ° Salts containing the cation can be isolated as a product of a variety of preparative procedures. On the other hand, the pK of cycloheptatriene has been estimated at 36. ° This value is similar to those of normal 1,4-dienes and does not indicate strong destabilization. Thus, the seven-membered eight-rc-electron anion is probably nonplanar. This would be similar to the situation in the nonplanar eight-rc-electron hydrocarbon, cyclooctatetraene. 

Mason and coworkers10 studied the chiral bicyclic derivatives 2, 3, 33 and 34, having known absolute configuration. These molecules possess a planar s-cis diene chromophore and formally their chirality is due to the presence of the D or CH3 substituents, which rule out all the symmetry planes. However, it is interesting to point out a peculiar structural... 

Recent theoretical and spectroscopic studies indicate that in aliphatic dienes and trienes, excitation to the spectroscopic l1 state usually results in facile twisting about the termini in the stereochemical sense dictated by orbital symmetry selection rules for the appropriate electrocyclic ring closure, motions which are often accompanied by some degree of planarization of the carbon framework. In general, relatively minor distortions... 

The next molecules to be discussed are the seven-membered monocyclic dienes and polyenes. i,3-Cycloheptadiene has been studied by GED (Table 7). The molecule has also been studied by MW. This study did not include a complete structure determination, but it was concluded that the carbon skeleton is planar, except for the carbon, corresponding to Cs symmetry. This is in agreement with the GED results. Ab initio calculation at the Mp2 level, utilizing the 6-3lG(d,p) basis set, has recently been carried out. Both the Cs and C2 conformers of 1,3-cycloheptadiene were considered, and the Mp3/6-3lG(d,p)//Mp2/6-3iG(d,p) calculations predict the Cs conformer to be 3.3 kJmol lower in energy than the C2 conformer. It was concluded that the calculated Cs conformer at the Mp2 level is in excellent agreement with the available MW and GED data. The Mp2 and Mp3 energetics results allow, however, for the possibility of the presence of a C2 conformer, as evidenced by NMR data. The experimental results do not rule out the presence of small amounts of a C2 conformer that is twisted about the diene region. 

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