Pd-complex-catalyzed synthesis of oligomers having a recurring benzodiazaborole unit in the main chain

BACKGROUND: Rigid-rod oligomers and polymers comprising a recurring benzodiazaborole unit show high thermal stability and intriguing optical and electrochemical properties due to the expanded electron system through the B-C and B-N bonds in the main chain by the use of the p. orbital on boron. However, the conventional method for the preparation of these oligomers and polymers often requires polycondensation under severe conditions. In this study, we report the synthesis of oligomers comprising a recurring benzodiazaborole unit using the Stille reaction under mild conditions. In addition, we describe their chemical properties and solid-state structures. RESULTS: The reaction of 3,3'-diaminobenzidine with 4-bromophenyl boronic acid and benzeneboronic acid yielded 2,2'bis(4-bromophenyl)-2,3,2',3'-tetrahydro-1H,1'H-[5,5']bi(benzo[1,3,2]diazaborolyl) (1)and 2,2'-biphenyl-2,3,2',3'-tetrahydro1H,1'H-[5,5']bi(benzo[1,3,2]diazaborolyl), respectively. Pd-complex-catalyzed polycondensation of 1 with bis(tributyltin) and bis(tributylstannyl)acetylene in N,N-dimethylformamide provided oligomers comprising a recurring benzodiazaborole unit in 98 and 97% yields, respectively. CONCLUSION: The oligomers comprising a recurring benzodiazaborole unit were obtained under mild reaction conditions in high yields. The expansion of the electron system through the B-C and B-N bonds of the oligomers was confirmed by UV-visible spectroscopy. The oligomers were photoluminescent in solution and electrochemically active in a film, and they assumed self-assembled stacked structures in the solid state. (C) 2008 Society of Chemical Industry