REAGENT-MODULATED OPTIONAL SITE SELECTIVITIES - THE METALATION OF O-HALOBENZOTRIFLUORIDE, M-HALOBENZOTRIFLUORIDE AND P-HALOBENZOTRIFLUORIDE

Chloro(trifluoromethyl)benzenes and bromo(trifluoromethyl)benzenes und ergo deproronation at a position adjacent to the single halogen substi tuent when treated with alkyllithiums (at -75 degrees C) and respectiv ely, lithium 2,2,6,6-tetramethylpiperidide (at -100 degrees C) in tetr ahydrofuran. Positional ambiguities, if existing, on be exploited to e stablish optional site selectivities. Thus, butyllithium reacts with 1 -chloro-3-(trifluoromethyl)benzene under hydrogen/metal interconversio n at the 2-position whereas sec-butyllithium attacks exclusively the 6 -position. The latter mode of regioselectivity is also exhibited by 1- bromo-3-(trifluoromethyl)benzene in the presence of lithium 2,2,6,6-te tramethylpiperidide, only 2-Bromo-4-(trifluoromethyl)phenyllithium bei ng produced. 2-Bromo-6-(tiinuoromethyl)phenyllithium is directly inacc essible, but is formed when 2-bromo-3-(trifluoromethyl)phenyllithium, generated at -100 degrees C, is allowed to isomerize at -75 degrees C. Copyright (C) 1996 Elsevier Science Ltd.