Rotational barriers peri substituent, effect

Establishing that a smaller substituent in the peri position can raise the barrier, Oki and co-workers (149) were interested in finding the peri substituent effect on the barrier to rotation, and prepared a series of 9-(l,l-dimethyl-2-phenyl-ethyl)triptycenes (104). Data in Table 22 indicate that the barrier to rotation... 

In order to see the effect of the peri substituent on the barrier to rotation, 2,3-dichloro-9-(l-cyano-l-methylethyl)triptycene and 2,3-dichloro-9-(l-meth-oxycarbonyl-l-methylethyl)triptycene (103, X = H, Y = Cl) were prepared (147). The data are included in Table 21. The barrier to rotation becomes definitely higher when the peri substituent is removed. Thus the peri substituent in... 

In order to see the effect of buttressing and the effect of size of the peri substituent on the barrier to rotation, 1-chloro- (112) and 1,3-di-tm-butyl-9-isopropyltriptycene (113) were prepared (161). The barrier for 112 was 22.9 kcal/mol. Comparing this to the tetrachloro compound 111 (all substituents Cl), AG = 25.5 kcal/mol, a normal buttressing effect seems to operate, in contrast to what is observed for the 9-rm-butyltriptycenes as discussed in the previous section. The barrier to rotation in 113 was 20.8 kcal/mol, which is very low... 

The barrier to rotation was 24.8 kcal/mol at 48°C, and the population ratio was 2.0 in chloroform-, which is the statistical value. The size of the peri substituent has an important effect on the barrier to rotation about the CH2—C(9) bond, because if a 1,4-dimethoxybenzeno bridge is introduced in place of the 1,4-dimethylbenzeno, the coalescence of the AB quartet due to the benzylic CH2 protons is observed at 167°C corresponding to a free energy of activation of 22 kcal/mol, which is too low for isolation of the atropisomers at room temperature. 

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See also in sourсe #XX -- [ , , ]

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