Synthesis Of Pyridyl-Supported N-Heterocyclic Ligands And Their Applications In Homogeneous Catalysis

2-Bromo-6-(N-pyrazol-1-yl) pyridines and 2,6-bis(pyrazol-1-yl)pyridineswere selectively synthesized by reactionof 2,6-dibromopyridine and pyrazoles. Abase effect and inhibition of catalytic activity for Pd (OAc)₂ were observed. Usingthe catalyst system Pd(OAc)₂/PPh₃, coupling of 2,6-dibromopyridine andpyrazoles in a molar ratio of 2,6-dibromopyridine:pyrazole:base = 1:2.4:2.5afforded the mono-substituted compounds 2-bromo-6-(N-pyrazol-1-yl)pyridinesas the major products when KOtBu was used as the base. Without using thecatalyst or with the catalyst and using NaOtBu as the base the disubstitutedcompounds 2,6-bis(pyrazol-1-yl) -pyridines were formed as the major products inyields up to 93%. Without using the catalyst or with the catalyst, reactions of2,6-dibromopyridine and pyrazoles in a molar ratio of2,6-dibromopyridine:pyrazole:base = 1:1:1 selectively produced monosubstitutedcompounds in yields up to 82% when KOtBu or NaOtBu was used as the base.The complex formed from the reaction of 2-bromo-6-(N-pyrazol-1-yl)pyridineand Pd(OAc)₂ did not undergo further reaction with a pyrazole to form thedisubstituted compound in the presence of the base.A new, neutral ruthenium(â…¡) complex RuCl₂(PPh₃)(iBu-BTP) wassynthesized by the reaction of RuCl₂(PPh₃)₃ with 2,6-bis(5,6-bis(iso-butyl)-1,2,4-triazin-yl)pyridine(iBu-BTP), and its molecular structure has beencomfirmed by X-ray crystallography. The catalyst was applied to transferhydrogenation of ketones. When acetophenones with electron-donoringsubstitutents (Me, or MeO) were used, yields of the corresponding alcohols are upto 100% over a period of 6-10 hr. When acetophenones withelectron-withdrawing substitutent such as Cl were used, yields of thecorresponding alcohols are only up to 60% for 30 hr. When alkyl ketone andcycloalkyl ketones were used, yields of the conrresponding alcohols can bereached 100%.