Dehalogenation of Monohalopyridines Catalyzed by Group 4 Metallocene Reagent

Abstract

The Cp'2MCl2/hydride (Cp' = Cp or Cp * : Cp = η 5 -C5H5, Cp * = η 5 -C5Me5; M = Ti, Zr, Hf) reagent catalyzes the dehalogenation of monohalopyridines at room temperature to give pyridine. The catalytic activity decreases in the order of M = Ti > Zr > Hf: M-F ≈ M-Cl ≈ M-Br ≈ M-I; Cp2M > CpCp * M>C p * 2M; Red-Al > N-Selectride > K-Selectride > L-Selectride >> DIBAL-H, MeLi, n-BuLi; 2-fluoropyridine > 3-fluoropyridine; C-F > C-Cl > C-Br. The rate of dehalogenation is boosted by adding 4 A molecular sieve. The catalyst combinations are all ineffective on the defluorination of fluorobenzenes and perfluorodecalin at the reaction condition. The possible mechanisms are suggested.