Alkenylation of 2‑Methylpyridine via Pyridylidene–Osmium Complexes

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.