ON THE TEMPERATURE-DEPENDENCE OF REGIOSELECTIVITY IN THE AMINATION OFBROMOBENZOTROPONES

The ipso/cine ratio in the amination of 5-bromo-2,3-benzo- or 2-bromo- 4,5-benzotropone shows a dependence upon the temperature at which the reaction is conducted, changing in favour of the ipso-product when the temperature is maintained high, ruling out an aryne-type mechanism. A comparison of independent mechanisms envisaged for the formation of t he two isomeric products suggests a two-part reason: (i) at a higher r eaction temperature, C-protonation, a step necessary for the formation of the cine-product, could be retarded when a direct internal mode is interfered with by a less efficient external one, and (ii) reketonisa tion by elimination of bromide, needed to form the ipso-product, is li kely to have a high temperature coefficient enabling the rate of its f ormation to overtake that of the cine-product.