The reduction of fluorine-containing triarylmethanols by formic acid

Triarylmethanols containing one or more fluorine substituants in the para-positions are converted by 90% formic acid into a mixture of the fluorine-containing triarylmethane, and a second component in which one fluorine has been replaced by a hydroxy-group. A mechanism is proposed which involves nucleophilic attack by water on the initial carbonium ion (4) with subsequent loss of hydrogen fluoride, to give the diphenyl-p-hydroxyphenylmethyl carbonium ion (6) which undergoes hydride transfer with formate to give the product. This mechanism was supported by the isolation of 4-hydroxytriphenylmethanol from the reaction involving 4-fluorotriphenylmethanol. Pentafluorophenyldiphenylmethanol on similar treatment gave 9-(pentafluorophenyl)fluorene, and 2-(trifluoromethyl)triphenylmethanol gave 9-(2-trifluoromethylphenyl)fluorene. Both reactions presumably occur via an intramolecular electrophilic aromatic substitution. The ¹⁹F and ¹³C n.m.r. spectra of the triarylmethanes and 9-aryl-fluorenes are reported and discussed.