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Niobium-catalyzed activation of aryl trifluoromethyl groups and functionalization of C-H bonds: an efficient and convergent approach to the synthesis of N-heterocycles.

The functionalization of carbon–fluorine bonds is an area of current interest because of the toxic and persistent nature of perfluorinated compounds and because the C F bond represents one of the most inert functionalities in chemistry. While substantial strides have been made in the transitionmetal-catalyzed activation of aryl C F bonds as well as the hydro-defluorination of alkyl C F bonds, methods to synthesize complex, functionalized molecules by the manipulation of a C F bond are relatively underdeveloped. In comparison to other organofluorine functionalities, the aryl trifluoromethyl group is particularly difficult to activate. Its inertness originates from both thermodynamic and kinetic considerations. The strength of the benzylic C F bond increases with additional fluorination and its length decreases to result in increased steric shielding at the carbon center. Recently though, two promising strategies have emerged to activate this group. One approach uses a cation to abstract a fluoride anion to produce defluorinated products (Scheme 1a). Orthogonally, a,a,a-trifluorotoluene derivatives can be reduced electrochemically or with a low-valent metal to directly access functionalized products (Scheme 1b). Significant progress has been made in the abstraction of a benzylic fluorine atom by a cation (Scheme 2). In 1997, Lectka and co-workers reported that aryl carbocations successfully abstract a neighboring fluoride ion to produce biaryl 2 in Et2O or fluorenone 3 if C6F6 was used as the