Aromatic versus Benzylic CH Bond Activation of Alkylaromatics by a Transient η2-Cyclopropene Complex

The methyl cyclopropyl hydrotris(3,5-dimethyl-pyrazolyl)borate complex Tp(Me2)NbMe(c-C3H5)(MeCCMe) reacts smoothly with different alkylaromatics XH at 308 K to yield methane and Tp(Me2)NbX(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(Me2)Nb(CH2Ar')(c-C3H5)(MeCCMe) (Ar' = 3,5-Me2C6H3, 4-MeC6H4, respectively). Selective arene CH bond activation is observed with 1,2-dimethylbenzene, yielding Tp(Me2)Nb(3,4-Me2C6H3) (C-C3H5) (MeCCMe). With 1,3-dimethylbenzene,a 3:1 mixture of arene and benzylic CH activated prcidutts, Tp(Me2)Nb(3,5-Me2C6H3)(c-C3H5)(MeCCMe) and Tp(Me2)Nb-(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 beta-H abstraction of methane to yield a transient unsaturated eta(2)-cyclopropene intermediate. This intermediate reacts rapidly with the aromatic or benzylic CH bond of the arene via 1,3-addition across a Nb(eta(2)-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.