THEORETICAL AND EXPERIMENTAL STUDIES OF OPTICALLY ACTIVE BRIDGEHEAD-SUBSTITUTED ADAMANTANES AND RELATED COMPOUNDS

An experimental and theoretical study of the optical activity of α-bromopropionitrile (1) and an analogous bridgehead-substituted adamantane derivative, 3-methyl-5-bromo-1-cyanoadamantane (2c), has been carried out. The synthesis and resolution of 2c via the corresponding carboxylic acid (2a) and amide (2b) are described. Specific rotations of less than 1° are found for each of 2a-c, confirming expectations of a large reduction in rotation when molecular asymmetry is due to substituents at the adamantane bridgeheads. A theoretical interest in 1 and 2c exists because the symmetry of the substituents is such that first-order (pairwise) contributions to the rotation are expected to vanish. An “atom polarizability” model, treated earlier by Boys, is considered as a basis for calculating higher order contributions in these molecules. The basic assumption is that the atoms are isotropically polarizable point particles located at their nuclei. A simple generalization of polarizability theory is presented, in which perturbation contributions of any order may be easily calculated. The calculations are carried out for the third- and fourth-order contributions to rotation in 1 and 2c for the atom polarizability model, noting that the first and second orders vanish. The main conclusions are: (i) the calculated and observed rotatory dispersion constants are roughly comparable in magnitude, suggesting that the assumed mechanism of optical activity is significant; (ii) the fourth-order contributions are larger than those of third order, implying that a number of still higher terms are also important, and raising serious doubts about the usefulness of the perturbation expansion; (iii) it is not possible to correlate sign of rotation with absolute configuration on the basis of the third- and fourth-order calculations. © 1969, American Chemical Society. All rights reserved.