COPLANARITY IN 2,6-DIMETHYLAZOBENZENES - A C-13 NMR-STUDY

A number of 3- and 4-X-2',6'-dimethylazobenzenes and 4-X-2,6-dimethyla zobenzenes have been prepared, and their C-13 n.m.r. spectra have been measured. Comparison of the effect of X on the C-13 n.m.r. chemical s hifts for C 4' with that for the corresponding azobenzenes has been us ed as a probe for exploring the influence of the two introduced ortho- methyl groups on the degree of coplanarity of the azobenzene system an d the efficiency of transmission of electronic effects from one ring t o the other. The results support previous studies that have suggested that the methyl groups have a substantial effect on the planarity of t he system, but, surprisingly, suggest that such loss of planarity has relatively little effect on the efficiencies of transmission of both p olar and resonance effects between the two rings.