Synthetic and Mechanistic Evaluation of Palladium(II) Bis(Arylazoformamide) Precatalysts in the Sonogashira Reaction

The palladium-catalyzed sp-sp2 C–C
bond forming
the Sonogashira reaction has been both extensively studied mechanistically
and widely used in organic synthesis. Herein, we describe an investigation
into how a palladium(II) complex with arylazoformamide (AAF) ligands
mediates these transformations. When mixed, two AAFs coordinate in
a κ1-fashion with an equivalent of PdCl2, creating complexes of the form PdCl2(AAF)2. Under typical and optimized copper(I)-cocatalyzed Sonogashira conditions,
using phenylacetylene and iodobenzene as reagents, these complexes
(precatalysts) reduce to Pd(0) and afford the coupled diphenylacetylene
product in high yields (i.e., 99%). A substrate scope explored the
substitution on both rings, yielding 18 examples with yields varying
from 38 to 99%. Mechanistically, from DFT studies, a formal Pd(I)
open-shell singlet complex is suggested, along with an explanation
of the need for DBU when employing CuI in toluene. Further DFT exploration
provides insight into the copper-free Sonogashira reaction when utilizing
Pd(AAF)2 complexes.