Stable 5,5′-Substituted 2,2′-Bipyrroles: Building Blocks for Macrocyclic and Materials Chemistry

The preparation and
characterization of a family of stable 2,2′-bipyrroles
substituted at positions 5 and 5′ with thienyl, phenyl, TMS-ethynyl,
and vinyl groups is reported herein. The synthesis of these new bipyrroles
comprises three steps: formation of the corresponding 5,5′-unsubstituted
bipyrrole, bromination, and Stille or Suzuki coupling. The best results
in the coupling are obtained using the Stille reaction under microwave
irradiation. The new compounds have been fully characterized by UV–vis
absorption, fluorescence, and IR spectroscopies and cyclic voltammetry.
X-ray single-crystal analysis of four of the synthesized bipyrroles
indicates a trans coplanar geometry of the pyrrole rings. Furthermore,
the substituents at positions 5,5′ remain coplanar to the central
rings. This particular geometry extends the π-conjugation of
the systems, which is in agreement with a red-shifting observed for
the λmax of the substituted molecules compared to
the unsubstituted bipyrrole. All of these new compounds display a
moderate fluorescence. In contrast with unsubstituted bipyrroles,
these bipyrroles are endowed with a high chemical and thermal stability
and solubility in organic solvents.