Generation and spectroscopic characterization of ruthenacyclobutane and ruthenium olefin carbene intermediates relevant to ring closing metathesis catalysis

The reaction of phosphonium alkylidenes [(H(2)lMes)RUCl2=CHPR3](+)[A](-) (R = C6H11, A = OTf or B(C6F5)4, 1-Cy; R = i-C3H7, A = CIB(C6F5)(3) or OTf, 1-'Pr) with 1 equiv of ethylene at -78 degrees C, in the presence of 2-3 equiv of a trapping olefin substrate, yields intermediates relevant to olefin metathesis catalytic cycles. Dimethyl cyclopent-3-ene-1,1-dicarboxylate gives solutions of a substituted ruthenacyclobutane 3 of relevance to ring closing metathesis catalysis. H-1 and C-13 NMR data are fully consistent with its assignment as a ruthenacyclobutane, but (1)Jcc values of 23 Hz for the C alpha H2-C-beta bond and 8.5 Hz for the C alpha H-C-beta bond point to an unsymmetrical structure in which the latter bond is more activated than the former. In contrast, trapping with acenaphthylene leads to an olefin carbene complex (6) in which the putative ruthenacyclobutane has opened; this species was also fully characterized by NMR spectroscopy and compared to related species reported previously.