Meta-substituted thienyl benzenes: a comparative synthetic, structural and computational study.

A selection of metal-catalyzed C-C bond-forming strategies has been evaluated in the synthesis of a series of meta-substituted thienylbenzenes, (T(n))(2)C(6)H(4) and (T(n))(3)C(6)H(3) (n = 1, 2; T(1) = 2-thienyl; T(2) = 2,2'-bithien-5-yl). Kumada coupling reactions catalyzed by PdCl(2)(dppf) between the appropriate thienyl Grignard and either 1,3- or 1,3,5-bromo- or iodobenzenes were found to be the most reliable in terms of convenience, selectivity and yield (dppf = 1,1'-bis(diphenylphosphino)ferrocene). These conditions also allowed the optimized syntheses of mixed (thienyl)(halo)benzenes, (T(n))C(6)H(3)X(2) and (T(n))(2)C(6)H(3)X (X = Br, I); the latter bromides could be further elaborated in subsequent Stille, Sonogashira, or Kumada reactions to furnish bis(thienyl) compounds bearing electron-donating or -withdrawing groups in the third meta position, (T(n))(2)C(6)H(3)R (R = Ph, p-MeOC(6)H(4), p-FC(6)H(4); n = 1, 2) and (T(1))(2)C(6)H(3)R' (R' = Me, C[triple bond]CPh, Fc; Fc = ferrocenyl). The relative effects of R, R', and n were evaluated by electronic spectroscopy, cyclic voltammetry and calculation. The absorption and emission characteristics and calculated ionization potentials and HOMO-LUMO gaps of these compounds were strongly dependent on n and largely insensitive to R/R'. These measured and calculated properties were also found to be largely invariant with respect to the degree of substitution about the central ring in the meta-substituted benzenes (T(n))(m)X(3-m)C(6)H(3) (m = 1-3; n = 1, 2; X = Br, H), although in the case of n = 1, there was a smooth, albeit small, increase in the emission maximum with increasing thienyl substitution. These findings essentially confirmed earlier theoretical predictions that the thienyl "arms" of meta-substituted phenyl-cored dendrimers were predominantly responsible for absorption and that excitons were localized to the "arms" without any electronic coupling between them, but also introduced the caveat that there was a minimum of two contiguous thiophene rings required for strict exciton localization to the arms. The oxidation potentials of the compounds in solution ranged from 0.9 to 1.6 V and were not rationally dependent on the degree of substitution or the nature of R/R'.