High Oxidation State Molybdenum Imido Heteroatom-Substituted Alkylidene Complexes

Townsend, Erik M.; Kilyanek, Stefan M.; Schrock, Richard R.; Müller, Péter; Smith, Stacey J.; Hoveyda, Amir H.

doi:10.1021/om400584f

Cited by 38 publications

(28 citation statements)

References 32 publications

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“…The catalytically active halo-substituted alkylidenes derived from Mo-4c can thus deliver the necessary activity (e.g., vs. Ru - 2-4 or W-1 ) but not at the expense of catalyst lifetime [e.g., vs. bis(alkoxide) Mo-1 ]. The Mo center in a MAP system is likely electron-deficient enough to prevent metal-carbide formation 34 , and yet, unlike Ru carbenes, π-electron donation by a halide alkylidene substituent 50 does not hamper reactivity. Another noteworthy aspect is the design of reactions where the use of a dissymmetric Z -bromo-fluoroethene leads to the predominant or exclusive formation of fluoro-substituted alkenes (vs. the bromo derivatives); this way, an easy-to-handle and readily accessible reagent can be used instead of the costly and impractical fluoro-olefin alternatives (e.g., vinyl fluoride or Z -1,2-difluoroethene).…”

Section: Discussionmentioning

confidence: 99%

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Koh

Nguyen

Zhang

et al. 2016

Nature

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167

117

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Olefin metathesis has made a significant impact on modern organic chemistry, but important shortcomings remain: for example, the lack of efficient processes that can be used to generate acyclic alkenyl halides. Halo-substituted ruthenium carbene complexes decompose rapidly or deliver low activity and/or minimal stereoselectivity, and our understanding of the corresponding high-oxidation-state systems is very limited. In this manuscript, we show that previously unknown halo-substituted molybdenum alkylidene species are exceptionally reactive and are able to participate in high-yielding olefin metathesis reactions that afford acyclic 1,2-disubstituted Z-alkenyl halides. Transformations are promoted by small amounts of an in situ-generated catalyst with unpurified, commercially available and easy-to-handle liquid 1,2-dihaloethene reagents and proceed to high conversion at ambient temperature within four hours. Many alkenyl chlorides, bromides and fluorides can be obtained in up to 91 percent yield and complete Z selectivity. This method can be used to easily synthesize biologically active compounds and to perform the site- and stereoselective fluorination of other organic compounds.

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Section: Discussionmentioning

confidence: 99%

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Koh

Nguyen

Zhang

et al. 2016

Nature

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167

117

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“…It is believed that the chloro-alkylidene generated in the metathesis is of low stability, and because mRCM's are performed under dilute conditions, the alkylidene decomposes before it can react with another olefin. Previous studies showed that pinacolatoboryl-(B(pin)) substituted alkylidenes were less reactive than their carbon counterparts, [110] so these were …”

Section: 1002/anie201704686 Angewandte Chemie International Editionmentioning

confidence: 99%

Stereoretentive Olefin Metathesis: An Avenue to Kinetic Selectivity

2017

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Abstract:Olefin metathesis is an incredibly valuable transformation that has gained widespread use in both academic and industrial settings. Lately, stereoretentive olefin metathesis has garnered much attention as a method for the selective generation of both Eand Z-olefins. Early studies employing ill-defined catalysts showed evidence for retention of the starting olefins at early conversion. However, thermodynamic ratios were reached as the reaction proceeded to equilibrium. Recent studies in olefin metathesis have focused on the synthesis of catalysts that can overcome the inherent thermodynamic preference of an olefin, providing synthetically useful quantities of a kinetically favored olefin isomer. These reports have led to the development of stereoretentive catalysts that not only generate Z-olefins selectively, but also kinetically produce E-olefins, a previously unmet challenge in olefin metathesis. Advancements in stereoretentive olefin metathesis using tungsten, ruthenium, and molybdenum catalysts are presented.

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“…The vast majority of metathesis reactions involve M=CHR intermediates in which R is carbon-based or H. Some alkylidenes in which R is a heteroatom have now been isolated and appear to react with olefins in a metathesis fashion. 35 Much remains to be done in terms of developing catalysts for metathesis reactions that yield directly functionalized olefins, especially those that can be employed subsequently in catalytic reactions that are not metathesis-based. 36 …”

Section: Organic Synthesismentioning

confidence: 99%

Metathesis by Molybdenum and Tungsten Catalysts

Schrock¹

2015

Chimia

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IntroductionIn the early 1970's organometallic, organic, and polymer chemists were attracted to a new type of catalytic reaction called Olefin Disproportionation by Banks and Bailey, who published results in the open literature in 1964; 1 the reaction also was reported by Natta in 1964 2 and had appeared in patents filed at duPont 3 and Standard Oil 4 several years earlier. The basic process, which was shown to be catalyzed by various molybdenum or tungsten, and later rhenium, compounds of unknown type, resulted in an "exchange" of alkylidene (usually CHR, where R = H or alkyl) units in alkenes with one another, e.g., propylene could be equilibrated with ethylene and cis and trans-2-butenes (equation 1) or norbornene could be polymerized through a "ringopening" of its double bonds (equation 2). Although several mechanisms were proposed, the correct one appeared first in a publication by Hérrison and Chauvin in 1971, 5 namely the reversible reaction between a metal complex that contains a metal-carbon double bond (M=CHR) and a C=C bond to yield an intermediate that contains a metallacyclobutane (MC 3 ) ring. A related reaction that involves alkynes was discovered to be catalyzed by a heterogeneous catalyst in 1968 6 and a homogeneous catalyst in 1972. 7 This "alkyne disproportionation" reaction was proposed to consist of a reversible reaction between a M≡CR bond (R≠H) and a C≡C bond to give all possible alkynes via a metallacyclobutadiene intermediate. 8 Neither process resulted in any positional isomerization of the C=C or C≡C bonds. Although Fischer-type "low oxidation state" carbene 9 complexes were known at the time, and the synthesis of carbyne complexes soon followed, 10 they did not promote what came to be known as alkene metathesis 11 and alkyne metathesis, respectively, in the rapid manner often observed for what are now known as "classical" alkene and alkyne metathesis catalysts. However, experiments first published in 1976 showed that polymerization of cyclic olefins, metathesis of linear olefins, enyne metathesis, and polymerization of acetylenes could be initiated by certain Fischer-type carbene complexes, although new alkylidenes of the same type as the initiator were not observed. 12 Classical metathesis catalysts often are formed from metal oxides on silica or alumina 13 or in solution from a variety of metal complexes. An alkylating agent is often required, but simply exposing the supported metal oxide to alkenes or alkynes at high temperatures will generate the catalyst through some unknown mechanism of mechanisms.

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High Oxidation State Molybdenum Imido Heteroatom-Substituted Alkylidene Complexes

Cited by 38 publications

References 32 publications

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Direct synthesis of Z-alkenyl halides through catalytic cross-metathesis

Stereoretentive Olefin Metathesis: An Avenue to Kinetic Selectivity

Metathesis by Molybdenum and Tungsten Catalysts

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