Both Schrock and Grubbs type alkene metathesis …
Synthesis of Aryloxide Tungsten (VI) Complexes and Their Application in Olefin Metathesis-- F.
Metathesis polymerization of olefins and polymerization …
The key challenge facing us in the course of investigations detailed herein originates from the availability of various pathways through which a catalyst might promote an enyne RCM reaction: a metal-based alkylidene may first either associate with the alkyne (pathways 1 and 2, ) or the alkene (pathway 3, ) unit of the substrate i. Transformation through complex ii gives rise to β-substituted metallacyclobutene iii, where the metal center forms a bond with the terminal carbon of the alkyne; conversion of iii to conjugated terminal alkylidene iv would then be followed by RCM to afford cyclic diene v (endo product). A metal alkylidene might alternatively coordinate to the alkyne in such a manner that, as depicted in pathway 2 in , generation of α-substituted metallacyclobutene vii is preferred; cycloreversion to disubstituted alkylidene viii culminates in the formation of the exo product ix. Pathway 3 () commences by initial interaction of the catalyst with the alkene unit x, affording terminal carbene xi, which through an intramolecular cycloaddition is converted to α,β-metallacyclobutene xii. Subsequent transformation gives rise to cyclopentene-substituted terminal carbene xiii, which through a cross-metathesis process with another enyne substrate (i) affords exo product ix and regenerates terminal carbene xi.
Stereogenic-at-Mo monoalkoxide and monoaryloxide complexes promote enyne ring-closing metathesis (RCM) reactions, affording the corresponding endo products with high selectivity (typically >98:endo:exo). All catalysts can be prepared and used in situ. Five-, six-, and seven-membered rings are obtained through reactions with enyne substrates that bear all-carbon tethers as well as those that contain heteroatom substituents. The newly developed catalytic protocols complement the related exo-selective Ru-catalyzed processes. In cases where Ru-based complexes deliver exo and endo products nondiscriminately, such as when tetrasubstituted cyclic alkenes are generated, Mo-catalyzed reactions afford the endo product exclusively. The efficiency of synthesis of N- and O-containing endo diene heterocycles can be improved significantly through structural modification of Mo catalysts. The modularity of Mo-based monopyrrolides is thus exploited in the identification of the most effective catalyst variants. Through alteration of O-based monodentate ligands, catalysts have been identified that promote enyne RCM with improved efficiency. The structural attributes of three Mo complexes are elucidated through X-ray crystallography. The first examples of catalytic enantioselective enyne RCM reactions are reported (up to 98:2 enantiomer ratio and >98% endo).
Metathesis polymerization of olefins and ..
Treatment of dienyne 43 with 10 mol % of 44a, prepared and used in situ, leads to complete consumption of the substrate (). The product from enantioselective RCM affords the endo cycloadduct 45 in 79:21 enantiomer ratio (er) and 30% yield after purification; a significant portion of the product mixture consists of oligomeric materials. We then surmised that if the RCM process is performed under an atmosphere of ethylene (vs N2), the efficiency with which the monomeric heterocycle is generated might be improved. This strategy, previously utilized in relation to Ru-catalyzed enyne metathesis reactions, is based on the principle that initial cross-metathesis of the alkyne unit with ethylene might deliver tetraene 46 (), which would undergo RCM to generate endocyclic product 45. Increased RCM efficiency would arise from the more expeditious removal of the terminal alkyne, largely responsible for the formation of oligomeric side products. When RCM of 43 is carried out under an atmosphere of ethylene (), unsaturated tosylpiperidine 45 is isolated in 60% yield (vs 30% under N2) in the same degree of enantiomeric purity (79:21 er). Enantioselectivity is improved with di-iodo complex 44b: the endo product is obtained in 85:15 er. Although unreacted substrate is recovered when 44b is used (69% conversion), there is minimal oligomerization: endo product 45 is formed in 63% yield after purification (). It should be noted that, despite repeated attempts, we did not detect the purported diene intermediate 46 (see below for an additional related observation).
Varying degrees of progress have been achieved in connection with enantioselective versions of several types of olefin metathesis reactions.- One subset of transformations that has not yet yielded to enantioselective catalysis is the RCM reactions of enynes. Based on the outcome of the investigations summarized above, and encouraged by the recent discoveries regarding the exceptional ability of stereogenic-at-Mo complexes to promote enantioselective RCM and ring-opening/cross-metathesis reactions of olefinic substrates, we set out to establish whether the same class of chiral alkylidenes can promote the formation of enantiomerically enriched cyclic endo dienes.
Olefin Metathesis - Chemistry LibreTexts
N2 - Herein we present the long-sought quantitative catalyst-substrate association relationships based on experimentally measured quantitative association preferences of diverse metathesis Mo and Ru catalysts (Mo-1, Schrock Mo; Mo-2, Schrock-Hoveyda Mo; Ru-1, Grubbs first generation Ru; Ru-2, Grubbs second generation Ru; Ru-3:, Grubbs-Hoveyda first generation Ru; and Ru-4, Grubbs-Hoveyda second generation Ru) to their substrates (alkenes, alkynes and allenes), determined directly by a general method based on FRET principle. The determined substrate preferences are proved to be dependent on the molecular identity of the catalyst, exhibiting the preference order of alkyne > alkene > allene for Mo-1 and Mo-2, allene > alkene > alkyne for Ru-1 and Ru-3, and alkyne > allene > alkene for Ru-2 and Ru-4. The results enable us to probe metathesis mechanisms by answering issues in metathesis reactions including the controversial reaction initiation in enyne or allenyne metathesis.
We began by examining the ability of representative Mo-based complexes to catalyze RCM of enyne 1 (). Use of Mo alkylidene 4 (5 mol %) does not lead to formation of either the endo or the exo cyclic diene isomer (2 and 3, respectively; 22 °C, 30 min). In contrast, under identical conditions, monopyrrolide–monoalkoxide 5 delivers endo RCM product 2 in 72% yield (exo isomer 3, detected by 400 MHz 1H NMR spectroscopy). As has been described previously, stereogenic-at-Mo complexes, represented by 5, are prepared and often used in situ through reaction of a bispyrrolide (e.g., 6) with an alcohol. As also shown in , the latter precursor complex does not exhibit any catalytic activity in this case (7 initiates enyne RCM, albeit less effectively than 5 (2 h vs 30 min of reaction time). It is only the exo isomer 3 that is isolated in the Ru-catalyzed process (). When the RCM promoted by carbene 7 is performed under an atmosphere of ethylene, after 1 h there is only 24% conversion to 3, which is obtained along with a byproduct that is derived from homo-cross-metathesis of the product (3, 26%; ~50% recovered substrate).
SCHROCK ALKYNE METATHESIS CATALYST(78234-36 …
CATALYST FOR ALKYNE METATHESIS - Technische, …
Olefin Metathesis: ..
10/07/2013 · CATALYST FOR ALKYNE METATHESIS ..
a chemical reaction in which alkanes are rearranged to give longer or shorter alkane products Alkyne metathesis …
Alkene metathesis catalyst development has ..
Brian Blunders schrock catalyst olefin metathesis intoxicated mishandled his gadding measurably
either by using catalytic amounts of the Schrock alkylidyne ..
We began our studies by treatment of 31 with 5 mol % 5, which resulted in the formation of pyran 32 in 46% yield and >98% endo selectivity; a significant amount of oligomer generation accounts for complete consumption of the substrate. With the more sterically hindered monoaryloxide 28, there is little or no competing oligomerization, but there is only 20% conversion to the desired product (>98% endo), and 32 is obtained in 15% yield. When enyne 34 is used, a similar trend in selectivity is observed (). The lower yield of the endo diene 35 versus that of 32 is likely because the sterically less hindered alkyne unit of 34 is more prone to undergo the intermolecular cross-metathesis reactions that deliver oligomeric products. As indicated in , when performed under an atmosphere of ethylene, RCM with Ru complex 7 affords the exo products with high efficiency; otherwise, only a complex mixture of products is obtained.
Alkene and Alkyne Metathesis Reactions
AB - Herein we present the long-sought quantitative catalyst-substrate association relationships based on experimentally measured quantitative association preferences of diverse metathesis Mo and Ru catalysts (Mo-1, Schrock Mo; Mo-2, Schrock-Hoveyda Mo; Ru-1, Grubbs first generation Ru; Ru-2, Grubbs second generation Ru; Ru-3:, Grubbs-Hoveyda first generation Ru; and Ru-4, Grubbs-Hoveyda second generation Ru) to their substrates (alkenes, alkynes and allenes), determined directly by a general method based on FRET principle. The determined substrate preferences are proved to be dependent on the molecular identity of the catalyst, exhibiting the preference order of alkyne > alkene > allene for Mo-1 and Mo-2, allene > alkene > alkyne for Ru-1 and Ru-3, and alkyne > allene > alkene for Ru-2 and Ru-4. The results enable us to probe metathesis mechanisms by answering issues in metathesis reactions including the controversial reaction initiation in enyne or allenyne metathesis.
using Mo-Schrock as metathesis catalyst.
Extensive previous research has established that when Ru-based carbene complexes are used to promote enyne RCM, the formation of exo products, represented by ix (), is strongly favored. Our interest in examining the ability of stereogenic-at-Mo complexes, recently developed in these laboratories to address a number of unresolved issues in selective catalytic olefin metathesis, arose from the principle that high oxidation state complexes should favor reaction through a pathway that commences with association with the alkyne site of an enyne substrate. The above considerations, together with the preference of the sterically demanding Mo center to form a bond with the terminal carbon of an alkyne, led us to surmise that β-metallacyclobutenes should prove to be favored intermediates, leading to selective formation of endo RCM products (pathway 1, ). We were also aware, however, that the strong preference for formation of Mo-alkyne complex could lead to competitive oligomerization processes, involving reaction of intermediate alkylidenes (e.g., iv or viii, ) with an alkyne of another substrate molecule. We will demonstrate that such complications can be addressed through modification of the catalyst structure so that intramolecular processes that lead to RCM products are preferred over intermolecular reactions that result in formation of oligomers.
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