The trifluoromethoxy group: a long-range electron-withdrawing substituent.
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 CF(3) and unlike OCH(3), OCF(3) exerts a long-range effect that still considerably lowers the basicity of arylmetal compounds 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 OCH(3) group rather than next to the strongly electron-withdrawing CF(3)O 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 experiments. The structure and distance dependence of substituent effects can be rationalized by assuming superposing sigma- and pi-polarizing interactions.
Castagnetti
Eva
E
Institut de Chimie Organique Ecole Polytechnique Fédérale de Lausanne 1015 Lausanne, Switzerland.
Schlosser
Manfred
M