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

Abstract

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 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. 1H and 13C NMR data are fully consistent with its assignment as a ruthenacyclobutane, but 1JCC values of 23 Hz for the CalphaH2-Cbeta bond and 8.5 Hz for the CalphaH-Cbeta 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.

Formation of 5-membered metallacycles at iPrPCPIr by C–H, O–H, and C–CO bond cleavage

“…For example, metallacycles have been determined as key intermediates in olefin metathesis reactions [3,4], a number of C-H functionalization reactions [5][6][7][8][9], and alkyne trimerisation [10,11]. The chemistry of iridacycles is quite diverse [12][13][14].…”

Ethenolysis: A Green Catalytic Tool to Cleave Carbon–Carbon Double Bonds

“…The mechanism of ethenolysis follows the general Chauvin mechanism proposed for the olefin metathesis reactions catalyzed by metal alkylidene complexes (Scheme ) . The formation of metallacyclobutane has been reported with various catalysts upon reaction with ethylene at low temperature . The presence of excess ethylene favors the formation of high amounts of metal methylidene species in the reaction medium that contributes to non‐productive metathesis processes but also shifts the equilibrium in the direction of ethenolysis.…”

ChemInform Abstract: Cyclization of 2‐Ethynylphenyl Vinyl Ether Catalyzed by a Ruthenium Complex: Mechanism of Catalytic Cyclization and Stoichiometric Cycloisomerization.

“…Nucleophilic attack of the enol moiety of F affords oxoniumi ntermediate G.T hen, G under-goes nucleophilic addition and demetalation to yield 2b.H owever,b ecause of the steric hindranceo ft he methyl group, the E isomer of F does not proceed in the catalytic cycleb yt he formation of an oxonium intermediate;i nstead, it takes the rearrangement pathway to yield 8b,t hat is, through ad ifferent [2+ +2] cycloaddition of the vinyl group with the Ru = Ca bond of F to give metallacycle I. [14] Protons hift by deprotonation gives J.E nsuingm etal migration is accompanied by cleavage of the CÀOb ond to form K.T hen, cleavageo ft he CÀCb ond near the vinyl group is accompanied by migration of the vinyl group to the oxygen atom, which leads to L.T his is followed by CÀCb ondf ormation between the central carbon atom of the new allenyl group and one of the vinyl carbon atoms to generate 8b.T he cycloaddition of the vinyl group with the vinylidene Ca = Cb bond of 1a mentioned above is probably deterred by lack of a b hydrogen.…”