Generation and Spectroscopic Characterization of Ruthenacyclobutane and Ruthenium Olefin Carbene Intermediates Relevant to Ring Closing Metathesis Catalysis

The reaction of phosphonium alkylidenes [(H2IMes)RuCl2CHPR3]+[A]- (R = C6H11, A = OTf
or B(C6F5)4, 1-Cy; R = i-C3H7, A = ClB(C6F5)3 or OTf, 1-iPr) with 1 equiv of ethylene at −78 °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. 1H and 13C NMR data are fully consistent
with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CαH2−Cβ bond and 8.5 Hz
for the CαH−Cβ 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.