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)C6H4 and (T-n)(3)C6H3 (n = 1, 2; T-1 = 2-thienyl; T-2 = 2,2'-bithien-5-yl). Kumada coupling reactions catalyzed by PdCl2(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)C6H3X2 and (T-n)(2)C6H3X (X = Br, 1); 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)C6H3R (R = Ph, p-MeOC6H4, p-FC6H4; n = 1, 2) and (T-1)(2)C6H3R' (R' = Me, C 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 it 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)X3-mC6H3 (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'.