Very Large Rotation Barriers

Along with very strained molecules, a related goal has been the development of structures in which severe steric interactions are present in the transition state for a C-C rotation, but not (or much less so) in the ground state. This steric interference would produce very large rotation barriers. In some cases, Emi is so large that different conformers can be separated and remain stable at room temperature (such structures are termed atropisomers—see Chapter 6). 

Structures with very large rotation barriers. Values are in kcal/mol. 

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With BH2 kept in the ethylene plane, we get a 4-electron trans-butadiene (fig. 11 (II)), and a large rotational barrier (Ei 10 kcal.mol-1). This is not a surprise, because all the occupied MO s are bonding along CO. When the BH2 group is frozen perpendicular to the conjugation plane, the empty p orbital cannot conjugates, and we obtain a potential energy curve very similar to the one of the parent enol (E w 5.3 kcal.mol-1). 

As Ford and associates pointed out (95), introduction of a methyl group into the 1-position of the fluorene group in 9-(l-naphthyI)fluorene raises the barrier. This effect was examined in several compounds. The barrier to rotation in l-methyl-9-( 1 -naphthyl)fluorene was 21.4 kcal/mol at 433 K, which is ca. 4 kcal/ mol higher than that in the parent compound (101). Introduction of a 1-methyl group into 9-(8-methyl-l-naphthyl)fluorene raised the barrier for the sc (S ) —> ac (/J ) process to 25.2 kcal/mol at 307 K. Thus it was possible to isolate the sc (S ) isomer (59). Another pair of enantiomers, ac (/ ), was isolated. The equilibrium constant was again very large, 33 in favor of ac (/ ) (100). 

Barriers to rotation about the aryl-to-fluorene bond in 9-arylfluorene (63), where the aryl group is an o-alkylphenyl, were examined by two groups of investigators (93,105). Results given by Nakamura and Oki are shown in Table 13. Both the free energy of activation for rotation and the equilibrium constant are affected by the size of the substituent. Especially noteworthy is the case of 9-(2-rm-butylphenyl)fluorene, where the equilibrium is quite lopsided and the barrier for the process sp -> ap is very large. The ap form of this compound was prepared by protonation of the corresponding lithio compound, and equi-... 

In the case of dialkylated succinic acid, we have seen that, due to the occurrence of two barriers in the rotational potential of the racemic form (and not of the meso form) with the bulkier alkyl groups (and not the smaller ones), it is likely that the system will freeze-in into a mixture of two components. This is exactly where we observed very large negative cooperativity in the experimental data shown in Table 4.6. One cannot avoid the conclusion that at least a substantial part of the observed cooperativity is spurious. 

With a double bond, rotation would destroy the tt bond that arises from overlap of p orbitals consequently, there is a very large barrier to rotation. It is of the order of 263 kJmol , which is very much higher than any of the barriers to rotation about single bonds that we have seen for conformational isomerism. Accordingly, cis and trans isomers do not interconvert under normal conditions. Ring systems can also lead to geometric isomerism, and cis and trans isomers... 

A careful analysis of the NMR line shape provides the BH2 + /BH + ratio. Because of a large difference in activation energies between the rotation barriers in the mono-and diprotonated aldehyde, the observed rates are very sensitive to the concentration of BH+. Thus ionization ratios of the order of 10-4 could be measured and approximately 4 log units of Hq could be covered with the same indicator.45... 

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