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)(2)Pd(R)Br were conveniently obtained from trans-(NHC)(2)PdBr2 type complexes by a ligand-substitution reaction. In particular, the trans-[1-(1S)-menthyl-4-(R-1)-1,2,4-triazol-5-ylidene](2)Pd(R-2)Br [R-1 = Et, R-2 = CH2Ph (3a(1)); R-1 = Et, R-2 = o-OMeC6H4 (3a(2)); R-1 = R-2 = CH2Ph (3b(1)); R-1 = CH2Ph, R-2 = o-OMeC6H4 (3b(2))] 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, 3a(1),a(2),b(1), 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, 3a(1)-b(2), 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.