Diastereoselective Synthesis of Sulfoxide-Functionalized N-Heterocyclic Carbene Ruthenium Complexes: An Experimental and Computational Study.

Abstract

The synthesis of sulfoxide-functionalized NHC ligand precursors were carried out by direct and mild oxidation from corresponding thioether precursors with high selectivity. Using these salts, a series of cationic [Ru(II)(η6-p-cymene)(NHC-SO)Cl]+ complexes were obtained in excellent yields by the classical Ag2O transmetallation route. NMR analyses suggested a chelate structure for the metal complexes, and X-ray diffractometry studies of complexes 4 b, 4 c, 4dBArF and 4 e unambiguously confirmed the preference for the bidentate (κ2-C,S) coordination mode of the NHC-SO ligands. Interestingly, only one diastereomer, in the form of an enantiomeric pair, was observed both in 1H NMR and in the solid state for the complexes. DFT calculations showed a possible intrinsic energy difference between the two pairs of diastereomer. The calculated energy barriers suggested that inversion of the sulfoxide is only plausible from the higher energy diastereomer together with bulky substituents. Inverting the configuration at the Ru center instead shows a lower and accessible activation barrier to provide the most stable diastereomer through thermodynamic control, consistent with the observation of a single species by 1H NMR as a pair of enantiomers. All these complexes catalyse the β-alkylation of secondary alcohols. Complex 4dPF6 bearing an NHC-functionalised S-Ad group has been further studied with different primary and secondary alcohols as substrates, showing high reactivity and high to moderate β-ol-selectivities.