The trifluoromethoxy group:- a long-range electron-withdrawing substituent

Abstract

Judged by its capacity to promote a hydrogen/metal permutation at an ortho position, the trifluoromethoxy group is superior to both the methoxy and trifluoromethyl groups. Moreover, like CF3 and unlike OCH3, OCF3 exerts a long-range effect that still considerably lowers the basicity of arylmetal compds. when located in a more remote meta or even para position. As a consequence, 4-(trifluoromethoxy)anisole is deprotonated by sec-butyllithium mainly, and by tert-butyllithium exclusively, at a position adjacent to the OCH3 group rather than next to the strongly electron-withdrawing CF3O group. 1,3-Benzodioxole undergoes ortho lithiation only six times faster than anisole, whereas 2,2-difluoro-1,3-benzodioxole reacts about 5000 times faster, as evidenced by competition expts. The structure and distance dependence of substituent effects can be rationalized by assuming superposing s- and p-polarizing interactions. [on SciFinder (R)]