An Efficient Synthetic Approach to trans-(NHC)₂Pd(R)Br Type Complexes and Their Use in Suzuki–Miyaura Cross-Coupling Reactions

Mixed organohalidopalladium complexes of the type trans-(NHC)₂Pd(R)Br were conveniently obtained from trans-(NHC)₂PdBr₂ type complexes by a ligand-substitution reaction. In particular, the trans-[1-(1S)-menthyl-4-(R¹)-1,2,4-triazol-5-ylidene]₂Pd(R²)Br [R¹ = Et, R² = CH₂Ph (3a₁); R¹ = Et, R² = o-OMeC₆H₄ (3a₂); R¹ = R² = CH₂Ph (3b₁); R¹ = CH₂Ph, R² = o-OMeC₆H₄ (3b₂)] complexes were obtained from the corresponding palladium(II) precursor complexes, 2a,b, by reaction with the respective Grignard reagents, in good to excellent yields (74–93 %). Three of the four mixed organohalidopalladium complexes, 3a₁,a₂,b₁, have been structurally characterized by single-crystal X-ray diffraction, which revealed the trans disposition of the NHC ligands around the palladium center. The use of these mixed organohalidopalladium complexes in the Suzuki–Miyaura cross-coupling reaction was established for all of the complexes, 3a₁–b₂, which yielded the desired cross-coupled products upon treatment with various ArB(OH)₂ [Ar = 1-naphthyl, 4-(1,1′-biphenyl), 9-phenanthrenyl, 4-FC₆H₄, and 2,6-Me₂C₆H₃] compounds, in the presence of NaOH as a base, in CH₃CN, in three hours, under reflux conditions.