Isolation of rotational isomers and developments derived therefrom

Abstract

Isolation of rotational isomer models of ethane-type molecules is described. We could experimentally prove that, if rotational isomers whose molecular shape was chiral, the molecule could be optically active, even though it did not carry an asymmetric carbon atom. As an extension, other types of stereochemically fundamental and optically active molecules were isolated and their absolute stereochemistry was determined. One example is the model of meso-tartaric acid, for which optical inactivity had been attributed to internal compensation but is now explained as follows. On dissolution of meso-tartaric acid in a solvent, the molecule gives two kinds of conformers, one of which is a C(i) molecule and the other is a C(1) molecule. Although the latter is intrinsically optically active, the optical activity is cancelled by its enantiomer. The theory of internal compensation is recommended to be abandoned. As an extension to another area, some reactions of conformers are also discussed.

Scheme 1.

Optically active biphenyls.

Scheme 2.

Rotational isomers of N-methylformamide.

Scheme 3.

Compounds which showed hopeful rotational barriers.

Scheme 4.

First example of which rotational isomers were separately isolated.

Scheme 5.

Division of circles for the Klyne–Prelog convention.

Scheme 6.

ORTEP diagram of 5*.

Scheme 7.

Isolated rotational isomers of triptycene derivatives.

Scheme 8.

Possible skeletons which were expected to provide stable rotational isomers.

Scheme 9.

Bitriptycyls of which rotational isomers were separated by Schwartz et al.²⁵⁾

Scheme 10.

Stereostructures of C_2h molecules, butane (X = CH₃) and 1,2-dichloroethane (X = Cl), and their rotamers in perspectives and in Newman projections.

Scheme 11.

Three rotational isomers of 10, optically inactive and active, and a compound 10* used for determination of absolute conformation by Toyota et al.²⁶⁾

Scheme 12.

ORTEP diagram of 10*.

Scheme 13.

Synthesis of the model compound of tartaric acid.

Scheme 14.

Rotational circuits of R,S- and R*,R*-tartaric acids.

Scheme 15.

Simplified rotational circuits of R,S- and R*,R*-tartaric acids.

Scheme 16.

Newman projections for correlation of 13′ with 13″.

Scheme 17.

Optically active models of R,S-tartaric acid conformers.

Scheme 18.

Diazotization of primary amine 17 conformers.

Scheme 19.

Diazotization of rotamers of tetrachloro compounds 23.

Scheme 20.

Expected and unexpected products from diazotization of rotameric primary amines 27 with 1,4-dimethyl substituents.

Scheme 21.

A Newman-type projection of 1-tert-butyl-1-halotriptycene.

Scheme 22.

Halogenation of 9-tert-butyl-1,2,3,4-tetrahalotriptycene.