Alkenylation of 2-methylpyridine via pyridylidene-osmium complexes

Abstract

Complex [OsTp(κ1-OCMe2)2(PiPr3)]BF4 (1; Tp = hydridotris(pyrazolyl)borate) reacts with 2-methylpyridine and pyridine, in acetone, at room temperature to give the N-coordinated heterocyclic derivatives [OsTp{κ1-N[NC5H4Me]}(κ1-OCMe2)(PiPr3)]BF4 (2) and [OsTp{κ1-N[NC5H5]}(κ1-OCMe2)(PiPr3)]BF4 (3). However, in fluorobenzene, at 100 °C, the reaction of 1 with 2-methylpyridine leads to the pyridylidene compound [OsTp{κ1-C[HNC5H3Me]}(κ1-OCMe2)(PiPr3)]BF4 (4). The addition of phenylacetylene and cyclohexylacetylene to the fluorobenzene solutions of the latter affords the pyridylidene-vinylidene complexes [OsTp{κ1-C[HNC5H3Me]}(═C═CHR)(PiPr3)]BF4 (R = Ph (5), Cy (6)), which undergo selective deprotonation at the Cβ atom of the vinylidene ligand to generate the alkynyl-pyridylidene derivatives OsTp(C≡CR){κ1-C[HNC5H3Me]}(PiPr3) (R = Ph (7), Cy (8)). Heating of toluene solutions of 7 and 8 at temperatures higher than 50 °C produces the hydrogen transfer from the nitrogen atom to the Cβ atom of the alkynyl ligands to afford the pyridyl-vinylidene intermediates [OsTp{κ1-C[NC5H3Me]}(═C═CHR)(PiPr3)]BF4 (R = Ph (7a), Cy (8a)), which evolve by 1,2-migratory insertion of the vinylidene ligands into the Os–pyridyl bond to give the alkenylation products OsTp{κ2-C,N[C(═CHR)C5(Me)H3N]}(PiPr3) (R = Ph (9), Cy (10)). Protonation of 9 and 10 with HBF4·EtO2 yields [OsTp{κ2-C,N[═C(CH2R)C5(Me)H3N]}(PiPr3)]BF4 (R = Ph (11), Cy (12)). Complexes 7–9 have been characterized by X-ray diffraction analysis.