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Relevant academic research and scientific papers

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