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.
Mandal
AB
Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, Republic of China.
Lee
GH
Liu
YH
Peng
SM
Leung
Mk