Zusammenfassung
1H n.m.r. spectra and semi-empirical calculations of strain energies have shown that the ground state of the toluene derivative 3 is represented by conformation a (R CH3), the α-methine proton of which eclipses one of the o-methyl groups. Accordingly, the existence of two rotamers, e.g.15E and 15Z, was demonstrated in the case of the toluenes 15, 16, 17, 18, and 19 synthesized with two unequal o- ...
Zusammenfassung
1H n.m.r. spectra and semi-empirical calculations of strain energies have shown that the ground state of the toluene derivative 3 is represented by conformation a (R CH3), the α-methine proton of which eclipses one of the o-methyl groups. Accordingly, the existence of two rotamers, e.g.15E and 15Z, was demonstrated in the case of the toluenes 15, 16, 17, 18, and 19 synthesized with two unequal o- or m-substituents. The measured equilibrium ratios of rotamers (Table 2) are in good agreement with the results of the calculations for 15 and 16. - The free enthalpies of activation ΔGmath image for rotation about the Cαaryl bond vary from < 7.5 to 15 kcal/mole (Tables 1 and 2). Barriers of this kind were reproduced with sufficient accuracy by calculations for 3, 15, and 16 if the Cαaryl bond length and the bond angles were assumed to be flexible, i.e., if they were optimized for each rotational arrangement. The transition state turned out to be a conformation of type d, the CαH bond of which is arranged perpendicular to the benzene ring; potential energy curves for Cαaryl rotation are given in Fig. 3 and 4. Calculations presupposing a rigid molecular geometry erroneously result in conformation c as the transition state and yield barriers which are much higher than the measured ones.
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