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 pi-conjugated bipyrroles by Pd-II-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 alpha-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 alpha-oxygenation of bipyrroles is found to be a stepwise process, involving alpha-acetoxylated intermediates.