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

Abstract

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.