Selective palladium-catalyzed cocyclotrimerization of arynes with dimethylacetylenedicarboxylate: A versatile method for the synthesis of polycyclicaromatic hydrocarbons

Benzyne (1a) and the substituted derivatives 4,5-difluorobenzyne (1b) and 3 -methoxybenzyne (2) undergo chemoselective palladium-catalyzed [2 + 2 + 2]- cocyclotrimerization with dimethyl acetylenedicarboxylate (DMAD) to afford the corresponding phenanthrenes and/or naphthalenes. The major products are phenanthrenes if Pd(PPh3)(4) is used as the,catalyst, naphthalenes if Pd-2 (dba)(3) is used. When the method is,applied to polycyclic arynes 3-6, whic h are generated from the corresponding o-trimethylsilylaryl triflates, the same reactivity pattern is observed: the reaction can be selectively direct ed either toward the cocyclization of one molecule of aryne and two molecul es of alkyne or to the reaction of two molecules of aryne with one molecule of alkyne, by appropriate choice of the palladium catalyst. The synthesis of polycyclic aromatic compounds 33-39 using this methodology is reported.