▁di h ydro phen anth ren es . ▁ ▁Most ▁of ▁the ▁internal ▁rings ▁formed ▁are ▁limited ▁to ▁ 5 ▁or ▁ 6 ▁members , ▁however ▁some ▁cases ▁of ▁macro cy cl ization ▁have ▁been ▁reported . ▁ ▁Un like ▁a ▁normal ▁St ille ▁coupling , ▁ch lor ine ▁does ▁not ▁work ▁as ▁a ▁hal ogen , ▁possibly ▁due ▁to ▁its ▁lower ▁react ivity ▁in ▁the ▁hal ogen ▁sequence ▁( its ▁shorter ▁bond ▁length ▁and ▁stronger ▁bond ▁dis soci ation ▁energy ▁makes ▁it ▁more ▁difficult ▁to ▁break ▁via ▁ox id ative ▁addition ). ▁ ▁Starting ▁in ▁the ▁middle ▁of ▁the ▁scheme ▁below ▁and ▁going ▁clock wise , ▁the ▁p all ad ium ▁catal yst ▁( 1 ) ▁ox id atively ▁adds ▁to ▁the ▁most ▁re active ▁C - X ▁bond ▁( 1 3 ) ▁to ▁form ▁ 1 4 , ▁followed ▁by ▁trans met al ation ▁with ▁dist ann ane ▁( 1 5 ) ▁to ▁yield ▁ 1 6 ▁and ▁re duct ive ▁elim ination ▁to ▁yield ▁an ▁ar yl st ann ane ▁( 1 8 ). ▁ ▁The ▁reg ener ated ▁p all ad ium ▁catal yst ▁( 1 ) ▁can ▁ox id ative ▁add ▁to ▁the ▁second ▁C - X ▁bond ▁of ▁ 1 8 ▁to ▁form ▁ 1 9 , ▁followed ▁by ▁intr am ole cular ▁trans met al ation ▁to ▁yield ▁ 2 0 , ▁followed ▁by ▁re duct ive ▁elim ination ▁to ▁yield ▁the ▁couple d ▁product ▁( 2 2 ). ▁ ▁J ie ▁Jack ▁Lie ▁et ▁al