Double C-F/C-H bond activation

Superstrong C-F bonds don't readily lend themselves to activation and subsequent reaction, but that hasn't stopped chemists from edging closer to a general method for C-F activation. In one of the latest efforts, Kohei Fuchibe and Takahiko Akiyama of Gakushuin University, in Tokyo, report a "double activation" reaction in which a C-F bond and a C-H bond in close proximity in the same molecule are jointly activated, leading to ring-closing and formation of a new molecule (J. Am. Chem. Soc. 2006, 128, 1434). The researchers reacted o-phenyl-α,α,α-trifluorotoluene with NbCl₅ and LiAlH₄ to form fluorene in up to 82% yield (shown). They also synthesized a series of substituted fluorenes from various substituted phenyltrifluorotoluenes. "This is a pretty cool reaction," notes Oleg V. Ozerov of Brandeis University, Waltham, Mass., who also works on C-F activation chemistry. "This chemistry goes beyond what anyone else has been able to accomplish in that C-F activation is coupled with C-H activation in a C-C bond-forming step," he says. The substituted fluorenes are potential core structures for pharmaceuticals and for polymers used in molecular electronics.