1,2-Didehydro-3- and -4-(trifluoromethoxy)benzene: The “Aryne” Route to 1- and 2-(Trifluoromethoxy)naphthalenes

Upon treatment of 1-bromo-2-(trifluoromethoxy)benzene with lithium diisopropylamide (LIDA) at −100 °C, 3-bromo-2-(trifluoromethoxy)phenyllithium is generated. It can be trapped as such, but isomerizes to afford 2-bromo-6-(trifluoromethoxy)phenyllithium when the temperature is raised to −75 °C. The latter intermediate can be directly obtained from 1-bromo-3-(trifluoromethoxy)benzene. 1-Bromo-4-(trifluoromethoxy)benzene gives 5-bromo-2-(trifluoromethoxy)phenyllithium at −100 °C, but at −75 °C it slowly eliminates lithium bromide, thus setting free 1,2-dehydro-4-(trifluoromethoxy)benzene. In the same way, 1,2-dehydro-3-(trifluoromethoxy)benzene can be generated from 1-bromo-3-(trifluoromethoxy)benzene. Both “arynes” can be intercepted in situ with furan. The resulting [4+2] cycloadducts can be reduced with zinc powder, giving 1- and 2-(trifluoromethoxy)naphthalenes, they may be submitted to acid-catalyzed isomerization to produce trifluoromethoxy-1-naphthols, or they may be brominated to afford vic-dibromo derivatives. Base-promoted dehydrobromination of the latter compounds produces 2- or 3-bromo-1,4-epoxy-1,4-dihydro-5- or −6-(trifluoromethoxy)naphthalenes, which undergo regioselective ring-opening in acidic media and halogen/metal exchange when treated with butyllithium.