Pd-Induced Double C-H Bond Activation in Annulative Syntheses of Bipyrrole Boomerangs: Mechanistic Insights from NMR Spectroscopy and Computation.

1, n-Dipyrrolylalkanes can be efficiently converted into extensively π-conjugated bipyrroles by Pd-mediated annulative double C-H activation, and this approach might be further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. Herein, the mechanism of these transformations is explored using NMR spectroscopy and DFT calculations. The kinetics of the annulation are found to depend on the conjugation extent and donor-acceptor character of the pyrroles, as well as on substitution and the linker length. Combined experimental and theoretical evidence indicates that a change of the rate-determining step occurs for the most electron-deficient substrates. The unprecedented double α-oxygenation of bipyrroles is found to be a stepwise process, involving α-acetoxylated intermediates.