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

The reaction of diethyl 2,2'-diiodo-4,4'-biphenyldicarboxylate (7) with diarylacetylenes in the presence of 3, 5-(NO(2))(2)C(6)H(3)CO(2)Me (MeDNB) or 4-(NO(2))C(6)H(4)CO(2)Me (MePNB), Pd(OAc)(2) (10 mol %), K(2)CO(3), and Bu(4)NBr, in DMF at 100 degrees C, gives 4-methyl-9,10-diaryl-2, 7-phenanthrenedicarboxylic acid diethyl esters in good yields. The methyl group at position 4 originates from the electron-deficient methyl nitrobenzoates. High regioselectivity for the annulation of 7 with nonsymmetrical diarylalkynes was observed with the selectivity controlled mainly by electronic factors rather than by steric factors. A competitive kinetic isotope study was carried out using a 1:1 mixture of MePNB and MePNB-d(3) in the annulation reaction of 7 which gave 4-methyl- and 4-(methyl-d(3))phenanthrene in a 67:33 ratio, leading to an apparent value of k(H)/k(D) = 1.26. Possible mechanisms for the methyl transferring process are discussed.