The intramolecular sp2 and sp3 C-H bond activation of (p-cymene)ruthenium(II) N-heterocyclic carbene complexes.

The intramolecular sp2 and sp3 C-H activated products, as well as the monometalated products, based on the "(p-cymene)Ru(NHC)" framework were synthesised by treatment of a series of NHCs (1-R-3-methylimidazol-2-ylidene [R=Ph (1), Bn (2), t-Bu (3), i-Pr (4), Mes (5), Cy (6)] and 1,3-bis(isopropyl)imidazol-2-ylidene (7)) with [(p-cymene)RuCl2]2 under mild conditions. A new NHC precursor (1-tert-butyl-3-phenyl-4,5-dihydro-imidazol-2-ylidene) was also designed to compare the reactivity of sp2 C-H and sp3 C-H bonds upon cyclometalation, and only the sp3 C-H activated product (8) was observed. The factors that possibly determine the selectivity of intramolecular sp2 or sp3 C-H activation are elucidated by a series of experiments. In the cases where activation of both sp2 C-H and sp3 C-H is possible, steric factors overrode the others to dominate the regioselectivity of activation. All complexes were characterised by 1H NMR, 13C NMR and HRMS spectra. The molecular structures of 1, 3, 5, 6, 7, and 8 were confirmed by X-ray diffraction.