Formation of 4-methylphenanthrenes in palladium-catalyzed annulation of diethyl 2,2 '-diiodo-4,4 '-biphenyldicarboxylate with internal alkynes, usingmethyl nitrobenzoates as the methylating agent

The reaction of diethyl 2,2'-diiodo-4,4'-biphenyldicarboxylate (7) with dia rylacetylenes in the presence of 3,5-(NO2)(2)C6H3CO2Me (MeDNB) or 4-(NO2)C6 H4CO2Me (MePNB), Pd(OAc)(2) (10 mol %), K2CO3, and Bu4NBr, in DMF at 100 de grees C, gives 4-methyl-9,10-diaryl-2,7-phenanthrenediccarboxylic acid diet hyl esters in good yields. The methyl group at position 4 originates from t he electron-deficient methyl nitrobenzoates. High regioselectivity for the annulation of 7 with nonsymmetrical diarylalkynes was observed with the sel ectivity controlled mainly by electronic factors rather than by steric fact ors. A competitive kinetic isotope study was carried out using a 1:1 mixtur e of MePNB and MePNB-d(3) in the annulation reaction of 7 which gave 4-meth yl- and 4-(methyl-d(3))phenanthrene in a 67:33 ratio, leading to an apparen t value of k(H)/k(D) = 1.26. Possible mechanisms for the methyl transferrin g process are discussed.