1,2-CH Bond Activation of Pyridine across a Transient Titanium Alkylidene Radical and Re-Formation of the Ti═CHtBu Moiety

Abstract

Reduction of the titanium alkylidene [(PNP)Ti═CHtBu(OTf)] (PNP– = N[2-PiPr2-4-methylphenyl]2–) with KC8 in the presence of pyridine results in formation of the transient titanium(III) alkylidene radical [(PNP)Ti═CHtBu)] (A) or the adduct [(PNP)Ti═CHtBu)(NC5H5)] (B), which activates the C–H bond of pyridine to form the titanium(III) pyridyl alkyl complex [(PNP)Ti(CH2tBu)(η2-NC5H4)] (1) in 64% yield as brown microcrystals. Low-temperature X-band EPR spectroscopy and single-crystal X-ray diffraction studies confirm the identity of 1 as a d1 metal-centric radical with superhyperfine coupling to one nitrogen atom and having a side-on pyridyl moiety, which results in formation of the two isomeric forms 1a,b. Oxidation of 1 with [FeCp*2][OTf] cleanly promotes α-hydrogen abstraction to re-form [(PNP)Ti═CHtBu(OTf)] with concurrent elimination of pyridine and FeCp*2. Re-formation of the alkylidene moiety most likely stems from an intermediate such as [(PNP)Ti(CH2tBu)(η2-NC5H4)(OTf)] (C).