Aromatic versus benzylic CH bond activation of alkylaromatics by a transient η2-cyclopropene complex

The methyl cyclopropyl hydrotris(3,5-dimethylpyrazolyl)borate complex Tp^Me2NbMe(c-C₃H₅)(MeCCMe) reacts smoothly with different alkylaromatics XH at 308 K to yield methane and Tp^Me2NbX(c-C₃H₅)(MeCCMe). NMR data show that for mesitylene and 1,4-dimethylbenzene, selective benzylic CH bond activation is observed, giving the benzyl cyclopropyl complexes Tp^Me2Nb(CH₂Ar′)(c-C₃H₅)(MeCCMe) (Ar′ = 3,5-Me₂C₆H₃, 4-MeC₆H₄, respectively). Selective arene CH bond activation is observed with 1,2-dimethylbenzene, yielding Tp^Me2Nb(3,4-Me₂C₆H₃)(c-C₃H₅)(MeCCMe). With 1,3-dimethylbenzene, a 3:1 mixture of arene and benzylic CH activated products, Tp^Me2Nb(3,5-Me₂C₆H₃)(c-C₃H₅)(MeCCMe) and Tp^Me2Nb(CH₂-3-MeC₆H₄)(c-C₃H₅)(MeCCMe), is observed, translating to a 18:1 preference for aromatic versus benzylic CH bond activation on a per CH bond basis. Kinetic studies are consistent with rate-limiting intramolecular β-H abstraction of methane to yield a transient unsaturated η²-cyclopropene intermediate. This intermediate reacts rapidly with the aromatic or benzylic CH bond of the arene via 1,3-addition across a Nb(η²-cyclopropene) bond. DFT calculations suggest that the observed selectivities are a result of the steric influence of the methyl groups on the arene ring, which blocks activation of an ortho C-H bond.