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Relevant academic research and scientific papers

Catalytic Alkyne and Diyne Metathesis with Mixed Fluoroalkoxy-Siloxy Molybdenum Alkylidyne Complexes

The reaction of the molybdenum alkylidyne complex [MesC≡Mo{OC(CF3)3}3] (MoF9, Mes = 2,4,6-trimethylphenyl) with the potassium siloxides KOSi(OtBu)3and KOSi(OtBu)2(OMes) furnished the mixed fluoroalkoxy-siloxy alkylidyne complexes [MesC≡Mo{OC(CF3)3}2{OSi(OtBu)3}] (MoSiF9) and [MesC≡Mo{OC(CF3)3}2{OSi(OtBu)2(OMes)}] (MoSi*F9). A treatment ofMoF9MoSiF9, andMoSi*F9with an excess of 3-hexyne (EtC≡CEt) afforded labile metallacyclobutadiene (MCBD) complexes with a (C3Et3)Mo core, which are in equilibrium with the corresponding propylidyne (EtC≡Mo) complexes in solution. Thermodynamic parameters for these [2 + 2]-cycloaddition/cycloreversion reactions were determined by van’t Hoff plots, revealing that the nature of the ancillary siloxide ligand exerts a significant effect on the MCBD stability. An X-ray diffraction analysis of MoSi*F9-MCBDprovided one of the first accurate crystal structures of a molybdenacyclobutadiene. MoF9MoSiF9, andMoSi*F9proved active catalysts for the metathesis of internal alkynes and diynes, withMoSi*F9showing unprecendented selectivity in the conversion of sterically encumbered 1,3-pentadiynes into symmetrical 1,3,5-triynes and 2-butyne.

Tuning the Catalytic Alkyne Metathesis Activity of Molybdenum and Tungsten 2,4,6-Trimethylbenzylidyne Complexes with Fluoroalkoxide Ligands OC(CF3)nMe3-n (n = 0-3)

The molybdenum and tungsten 2,4,6-trimethylbenzylidyne complexes [MesC=M{OC(CF3)nMe3-n}3] (M = Mo: MoF0, n = 0; MoF3, n = 1; MoF6, n = 2; MoF9, n = 3; M = W: WF3, n = 1; Mes = 2,4,6-trimethylphenyl) were prepared by the reaction of the tribromides [MesC - MBr3(dme)] (dme = 1,2-dimethoxyethane) with the corresponding potassium alkoxides KOC(CF3)nMe3-n. The molecular structures of all complexes were established by X-ray diffraction analysis. The catalytic activity of the resulting alkylidyne complexes in the homometathesis and ring-closing alkyne metathesis of internal and terminal alkynes was studied, revealing a strong dependency on the fluorine content of the alkoxide ligand. The different catalytic performances were rationalized by DFT calculations involving the metathesis model reaction of 2-butyne. Because the calculations predict the stabilization of metallacyclobutadiene (MCBD) intermediates by increasing the degree of fluorination, MoF9 was treated with 3-hexyne to afford the MCBD complex [(C3Et3)Mo{OC(CF3)3}3], which was characterized spectroscopically.

Efficient catalytic alkyne metathesis with a fluoroalkoxy-supported ditungsten(III) complex

The molybdenum and tungsten complexes M2(OR)6 (Mo2F6, M = Mo, R = C(CF3)2Me; W2F3, M = W, R = OC(CF3)Me2) were synthesized as bimetallic congeners of the highly active alkyne metathesis catalysts [MesC-M{OC(CF3)nMe3-n}] (MoF6, M = Mo, n = 2; WF3, M = W, n = 1; Mes = 2,4,6-trimethylphenyl). The corresponding benzylidyne complex [PhC-W{OC(CF3)Me2}] (WPhF3) was prepared by cleaving the W-W bond in W2F3 with 1-phenyl-1-propyne. The catalytic alkyne metathesis activity of these metal complexes was determined in the self-metathesis, ring-closing alkyne metathesis and cross-metathesis of internal and terminal alkynes, revealing an almost equally high metathesis activity for the bimetallic tungsten complex W2F3 and the alkylidyne complex WPhF3. In contrast, Mo2F6 displayed no significant activity in alkyne metathesis.

Increasing the structural span of alkyne metathesis

A new generation of alkyne metathesis catalysts, which are distinguished by high activity and an exquisite functional group tolerance, allows the scope of this transformation to be extended beyond its traditional range. They accept substrates that were previously found problematic or unreactive, such as propargyl alcohol derivatives, electron-deficient and electron-rich acetylenes of various types, and even terminal alkynes. Moreover, post-metathetic transformations other than semi-reduction increase the structural portfolio, as witnessed by the synthesis of a annulated phenol derivative via ring-closing alkyne metathesis (RCAM) followed by a transannular gold-catalyzed Conia-ene reaction. Further examples encompass a post-metathetic transannular ketone-alkyne cyclization with formation of a trisubstituted furan, a ruthenium-catalyzed redox isomerization, and a Meyer-Schuster rearrangement/oxa-Michael cascade. These reaction modes fueled model studies toward salicylate macrolides, furanocembranolides, and the cytotoxic macrolides acutiphycin and enigmazole A; moreover, they served as the key design elements of concise total syntheses of dehydrocurvularin (27) and the antibiotic agent A26771B (36). Ask for more! Many possibilities exist to take advantage of alkyne metathesis. In addition to the approved semi-reduction to stereodefined olefins, it is shown how to convert the products primarily formed into aromatic or heteroaromatic rings, enones, Michael adducts, or β-ketolactones. At the same time, new types of substrates were engaged, including electron-rich, electron-poor, and terminal alkynes, as well as propargyl alcohol derivatives. Copyright

Efficient metathesis of terminal alkynes

Despite remarkable recent advances in the development of well-defined homogeneous catalysts for alkyne metathesis with regard to their activity, functional-group tolerance, and required reaction temperature,[1] this method is largely limited to the use of internal alkynes, RC≡CR' (R=alkyl, aryl; R'≠6 H). Commonly, methyl-capped alkynes (R'=Me) are employed,[2] resulting in the formation of RC≡CR and volatile 2-butyne, which can be removed by evaporation (b.p.=278C) or adsorption on molecular sieves (MS 5 A)[3,4] to drive these equilibrium reactions to completion.[5] In contrast, terminal alkyne metathesis (TAM) has rarely been achieved,[2c, 6] since many high-oxidation-state, Schrock-type alkylidyne complexes are known to degrade in the presence of terminal alkyne substrates. For most cases, it was suggested that deactivation occurs through deprotonation of intermediate metallacyclobutadiene species with formation of " deprotiometallacycles" (DMCs),[7, 8] which can be stabilized and made isolable by addition of donor (Don) molecules (Scheme 1).[9] These DMCs can be regarded as containing a chelating alkynylalkylidene ligand, and it was proposed that such carbene complexes are responsible for the observed high activity towards the polymerization of terminal alkynes.[8b, 10] In addition, dimerization of methylidyne complexes [HC-MX3] and formation of dimetallatetrahedrane species [X3M- (m-C2H2)MX3] should be considered as an alternative deactivation path.

Preparation of iniidazolin-2-iminato molybdenum and tungsten benzylidyne complexes: A new pathway to highly active alkyne metathesis catalysts

The reaction of [PhC=MBr3(dme)] (dme = 1,2-dimethoxyethane) with the hexafluoro-fert-butoxides LiJ. or KX [X = OC(CF3)2Me] afforded the benzylidyne complexes [PhC=MX3(dme)] (2a: M = W, 2b: M = Mo), which further