Neighbouring-group participation in pyrolysis of aryl azides

Abstract

Kinetic studies of the pyrolysis of a series of substituted phenyl azides in decalin or di-n-butyl phthalate solutions have revealed substantial anchimeric assistance when a phenylazo-, nitro-, acetyl, or benzoyl group is the ortho-substituent. E_act Values for these four azides are respectively 22·4, 26·2, 25·8, and 27·2 kcal. mole^–1, while for 4-nitrophenyl azide, it is 40·6. Corresponding 10^–11A values are 3·0, 19, 5·8, 5·3, and 690,000. Driving-force values are calculated, and constitute the first set of quantitative data available on neighbouring-group abilities in reactions involving concerted π-bond reorganization.

E _act Values for 4-methyl-, 3-methyl-, and 6-methyl-2-nitrophenyl azide are 27·3, 29·0, and 28·8 kcal. mole^–1, with corresponding 10^–11A values of 42, 44, and 10. These results demonstrate the requirement that the azido- and nitro-groups both lie in the plane of the aromatic ring for ready cyclization.