Isocyanate- and isothiocyanate-derived Ru(IV)-based alkylidenes: synthesis, structure, and activity.

The synthesis and characterization of a series of isocyanate- and isothiocyanate-derived second generation Grubbs-Hoveyda-type ruthenium-alkylidene complexes, that is, [Ru(N=C=O)(2)(IMesH(2))(=CH-2-(2-PrO)-C(6)H(4))] (1), [Ru(N=C=O)(2)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(=CH-2-(2-PrO)-C(6)H(4))] (2), [Ru(N=C=S)(2)(IMesH(2))(=CH-2-(2-PrO)-C(6)H(4))] (3), and [Ru(N=C=S)(2)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(=CH-2-(2-PrO)-C(6)H(4))] (4), and their activity in various metathesis reactions are described. Compounds 1-4 were prepared by reaction of the parent complexes [RuCl(2)(IMesH(2))(=CH-2-(2-PrO)C(6)H(4))] (5) (IMesH(2)=1,3-bis-(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene) and [RuCl(2)(1,3-dimesityl-3,4,5,6-tetrahydropyrimidin-2-ylidene)(=CH-2-(2-PrO)-C(6)H(4))] (6) with silver cyanate and thiocyanate, respectively. The X-ray structure of 1 was determined, confirming the isocyanate-type bonding of the ligand. The isothiocyanate-type bonding in 3 and 4 was unambiguously confirmed by IR and (13)C NMR spectroscopy. The isocyanate-derived complexes 1 and 2 were found to be excellent catalysts for the ring-opening metathesis polymerization (ROMP) of cis-cycloocta-1,5-diene (COD). Both 1 and 2 yielded poly(COD) with a trans-content of about 80 %. First-order kinetics with unprecedentedly high rate constants of polymerization (k(p)=0.068 and 0.26 s(-1), respectively) were observed. Compounds 3 and 4 were also active initiators for the ROMP of COD, however, they generated poly(COD) with a cis-content of 80 and 67 %, respectively. Complexes 1 and 2 also showed good catalytic activity in cross-metathesis (CM) reactions. Finally, 1-4 were also found to be excellent catalysts for the regioselective cyclopolymerization of diethyl 2,2-dipropargylmalonate (DEDPM), resulting in poly(DEDPM) almost entirely based on five-membered repeat units, that is, cyclopent-1-ene-1,2-vinylenes.