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 PdII-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.