Kinetic and other studies of di-and tri-arylmethyl compounds

Abstract

The steric requirements of the three aromatic rings in the triphenylmethyl radical impose non-planarity upon the system, and a three-bladed propeller shape, where the blades are slightly feathered out of plane is thought to be a good approximation to the actual structure. Such a species, in appropriate circumstances could in principle exist in two geometric modifications, termed a symmetrical helix and an unsymmetrical helix.

The isolation of a triarylmethyl derivative in an optically active modification, which possesses two identical aromatic rings, and retains its optical properties upon conversion to a free radical, would provide support for this conception. Accordingly, tri-1,1',1"-naphthylmethanol, some 4-substituted tri-1,1',1"-naphthylmethanols and 5-dimethylamino-tri-2,2',2"-tolylmethanol were prepared.

With the objective of preparing triarylmethyl chlorides from which a radical should be accessible, the behaviour of these alcohols with some acid chlorides was studied. However, tri-arylmethyl chlorides were not isolated as dehydration processes seemed to predominate.

The publication of evidence against the hexaphenylethane formulation for the triphenylmethyl radical dimer, suggested that a quantitative investigation of the effects of crowding upon polyarylmethyl systems would be of interest. The measurement of rate coefficients for formation of polyarylmethyl carbonium ions from their respective chlorides is a convenient way to achieve this.

The diphenylmethyl skeleton was studied, and variations in the. extent of crowding of the central carbon atom were achieved by the successive replacement of the four ortho-hydrogen atoms by methyl groups.

The preparation and measurement of rates of ethanolysis of diphenylmethyl, 2-methyldiphenylmethyl, 2,2'-dimethyldiphenylmethyl 2,6-dimethyldiphenylmethyl, 2,2',6-trimethyldiphenylmethyl and 2,2',6,6f-tetramethyldiphenylmethyl chlorides were carried out. The results showed the intervention of a large steric acceleration to ethanolysis with the two most highly substituted molecules.

The mode of transmission of electronic effects through aromatic systems with different dihedral angles was also studied by investigating the stabilisation imparted by 4-methyl and 4-t-butyl groups in the systems diphenylmethyl, 2,6-dimethyl-diphenylmethyl and 2,2',6-trimethyldiphenylmethyl. The resultsindicate that the predominant contributor to the activation produced by these groups is T-electron-transmitted, probably a hyperconjugative interaction. Steric inhibition of the hyperconjugation is also demonstrated.