Rotatory dispersion in the amine series. IV. The optical activity of diamines.

Abstract

In Parts I and II of this series (Lowry and Baldwin 1937; Baldwin 1937), attempts were made to account for the effects on rotatory power which are observed when a simple optically active amine is ionized through neutra-lization by a mineral acid. It was found that the simple amino bases could be classified roughly into two groups, viz. ( a ) those whose specific rotation undergoes a large change of magnitude or even of sign on neutralization, and ( b ) a few which do not show this “acid effect ”. It was also shown that, when the “acid effect” occurs, (i) the amino group is usually in close proxi-mity to an asymmetric carbon grouping; (ii) this configuration leads to circular dichroism of the absorption bands of the lone pair of electrons on the nitrogen atom, and thus to a large induced rotation due to the amino group; (iii) this induced rotation is eliminated on neutralization of the base, the rotatory power is reduced, and may even change in sign; whilst at the same time the dispersion constant in a Drude equation for the rotatory dispersion is displaced towards the Schumann region, owing to the elimina-tion of absorption bands at longer wave-lengths. Confirmation of these results is found in the investigations of Raman spectra by Freymann and Freymann (1936), who showed that the frequencies characteristic of the basic NH radical disappear when the nitrogen atom becomes quadrivalent. In the work now described, it is shown that ionization by mere neutralization is not the only mechanism by which the “acid effect” can be produced. If it be correct to attribute the phenomenon in, say, d -secondary butylamine to the lone pair of electrons, the same type of effect should be observed when these electrons become bound in any way; and Table I shows that, in fact, conversion to a quaternary bromide leads to a result similar to that produced by simple ionization. The “acid effect” may there-fore be associated directly with the existence of induced dissymmetry in the amino group. The reversal of sign in the case of d-triethylbutylammonium bromide is of particular interest, since it invalidates the general rule for the determination of configuration put forward by Boys in 1934. Nevertheless, his model finds great success when its application is restricted to the —CH—NH 2 - CH 3 residue, and only one anomaly can be found when it is applied to the compounds of the amine series as described in Part II. These effects are, how-ever, by no means general, as was shown by a full investigation of 1:1'- diamino- cyclo butane- spirocyclo butane (Lowry and Baldwin 1937). In this simple case ionization actually led to an increase in molecular rotatory power, and the assumption was made that a configuration such as is offered by a classical asymmetrical carbon grouping is usually necessary for development of induced dissymmetry in the amino group, and that molecular dissymmetry alone does not necessarily lead to the same activation of the lone pair of electrons on the amino nitrogen.