Bulky picolyl substituted NHC ligands and their Pd-0 complexes

Heterobidentate N-heterocyclic carbene-picolyl ligands with various substitution patterns and their palladium(0) complexes have been synthesized in excellent yields via their corresponding silver(I) complexes. These complexes are among the first examples where substitution next to the coordinating nitrogen is evaluated systematically. The complexes have been studied by NMR and X-ray diffraction, confirming their bidentate nature. The complexes are active precatalysts in the transfer semihydrogenation of alkynes to Z-alkenes, with activity and selectivity depending on the picolyl substituent to a high degree. Selectivities as high as 92 % were observed.