Direct Coupling of Oxazolines and N-Heterocyclic Carbenes: A Modular Approach to a New Class of C−N Donor Ligands for Homogeneous Catalysis

Reaction of 1-mesityl imidazole with 2-bromo-4,4-dimethyloxazoline gave the 2-(4,4-dimethyl)oxazolinyl imidazolium salt 1, which was converted to the silver N-heterocyclic
carbene complex [AgI(carbene)Br] 2 by stirring 1 with Ag2O in dichloromethane at room
temperature. The crystal structure analysis confirmed the monomeric nature of complex 2,
the coordination around the metal being quasi-linear with a C(1)−Ag−Br bond angle of
169.4(1)° and a Ag−C(1) bond length of 2.093(4) Å. The silver complex 2 was reacted with
[PdCl2(COD)] to yield the corresponding mono-carbene-palladium complex 3, for which an
X-ray diffraction study established a distorted square planar configuration with the
imidazolyl and the oxazolinyl ring lying in the molecular plane. The palladium complex 3
was found to be an active catalyst for the Heck and Suzuki C−C coupling reactions. The
coupling of activated or deactivated bromoarenes proceeded rapidly even with a low catalyst
loading (0.02 mol %), while the reaction with activated chloroarenes required a higher catalyst
loading of 3 mol %.