Pyrolysis of Aryl Azides. XI. Enhanced Neighboring Group Effects of Carbonyl in a Locked Conformation

Abstract

Rates of pyrolysis in nitrobenzene solution have been measured for 1-azido-9H-fluoren-9- one, 1-azido-9H-xanthen-9-one, 1-azidoacridin-9(10H)-one and 1-azidoanthracene-9,1O-dione; relative to azidobenzene at 120� these were respectively 5.68, 1750, 5090 and 18400. The lack of neighbouring group participation for the first azide is related to the large distance between carbonyl oxygen and the inner azido nitrogen atom, and the data argue against a published proposal that the transition state is stabilized by electrostatic attraction. In the remaining azides, the 'locked conformation' leads to much larger neighbouring group assistance than is observed for freely rotating ortho groups such as benzoyl (krel79). Only the last azide yields an isoxazole on pyrolysis , the second and third ones providing the first reported examples of neighbouring -group-assisted pyrolysis in which no cyclic product is obtained. These results are interpreted in terms of an electrocyclic mechanism in which the transition state is early and N---0 bond formation is less advanced than other changes in bonds. Much of the rate enhancement is attributed to an electron distribution which favours nitrogen loss. 1-Aminoanthracen-9(10H)-one, 1-amino-9H-fluoren-9-one and 1-amino-9H-xanthen-9-one do not yield the corresponding isoxazoles when oxidative cyclization is attempted.