AN INTRAMOLECULAR ARENE TRIFLATE COUPLING REACTION FOR THE REGIOSPECIFIC SYNTHESIS OF SUBSTITUTED BENZOFLUORANTHENES

An intramolecular triflate-arene coupling reaction mediated by bis(triphenylphosphine)palladium(II) chloride has been developed for the synthesis of each of the isomeric benzofluoranthenes. This reaction, which results in formation of a new five-membered ring, proceeds in highest yield when performed using 0.1 equiv of the palladium catalyst, 3 equiv of lithium chloride, and 1.2 equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene in N,N-dimethylformamide at 140-degrees-C. The biaryl precursors needed for the coupling reaction can be prepared by [1,2-bis(diphenylphosphino)ethane]nickel(II) chloride catalyzed coupling of an aryl bromide with an [o-(methoxymethoxy)aryl]magnesium bromide (prepared by ortho-lithiation of an aryl methoxymethyl ether followed by transmetalation with magnesium bromide). Using this procedure benzo[a]fluoranthene, benzo[b]fluoranthene, benzo[j]fluoranthene, and benzo[k]fluoranthene were prepared in yields of 84%, 85%, 93%, and 64%, respectively. The reaction to prepare benzo[j]fluoranthene was regiospecific and afforded none of the six-membered ring product, perylene. The method was extended to the preparation of benzo[b]fluoranthene (BbF) derivatives with fluoro or methoxy groups on the benzo ring. The cyclization of compounds possessing a methoxy group on the same ring as the triflate required the addition of 0.4 equiv of triphenylphosphine to the reaction mixture. Strategies are reported for the regiospecific preparation of 4-, 5-, 6-, and 7-substituted benzo[b]fluoranthenes. Evidence is presented which suggests the intermediacy of radicals in the oxidative-addition of aryl triflates to the palladium catalyst.