Metallacyclopentane Formation:  A Deactivation Pathway for a Tungsten(VI) Alkylidene Complex in Olefin Metathesis Reactions

Wang, Shu-Yu S.; VanderLende, Daniel D.; Abboud, Khalil A.; Boncella, James M.

doi:10.1021/om9708283

Cited by 51 publications

(40 citation statements)

References 36 publications

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“…The formation of propene byproducts and the observation of a metallacyclopentane intermediate is reminiscent of the well documented (for molecular complexes) formation of metallacyclopentane intermediates from M(IV) olefin adducts (M = Ta, Mo, W) − and their ring contraction to form carbene species (Figure F) . This suggests that the formation of the carbene active sites in our material occurs by a similar mechanism.…”

Section: Resultssupporting

confidence: 75%

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

et al. 2016

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Alkene metathesis is a widely and increasingly used reaction in academia and industry because of its efficiency in terms of atom economy and its wide applicability. This reaction is notably responsible for the production of several million tons of propene annually. Such industrial processes rely on inexpensive silica-supported tungsten oxide catalysts, which operate at high temperatures (>350 °C), in contrast with the mild room temperature reaction conditions typically used with the corresponding molecular alkene metathesis homogeneous catalysts. This large difference in the temperature requirements is generally thought to arise from the difficulty in generating active sites (carbenes or metallacyclobutanes) in the classical metal oxide catalysts and prevents broader applicability, notably with functionalized substrates. We report here a low temperature activation process of well-defined metal oxo surface species using organosilicon reductants, which generate a large amount of active species at only 70 °C (0.6 active sites/W). This high activity at low temperature broadens the scope of these catalysts to functionalized substrates. This activation process can also be applied to classical industrial catalysts. We provide evidence for the formation of a metallacyclopentane intermediate and propose how the active species are formed.

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Section: Resultssupporting

confidence: 75%

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

et al. 2016

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“…The C1 C2 bond distance of the coordinated ethylene molecule is 1.41 Å, indicating a significant back-donation from the metal center to the π* orbital of ethylene. 41 In the presence of 3 equiv of B(C 6 F 5 ) 3 , the reaction of 2a and 13 C-dilabeled ethylene yields the py-B(C 6 F 5 ) 3 adduct, 3a-C 2 H 4 , and a small amount of the metallacyclopentane 4a (Scheme 4) at room temperature. Heating the reaction mixture to 50 °C leads to complete conversion of 3a-C 2 H 4 and removal of most of the coordinated pyridine from W centers to give py-B(C 6 F 5 ) 3 (ca.…”

Section: ■ Results and Discussionmentioning

confidence: 70%

“… , 3a-C 2 H 4 can be isolated by recrystallization in toluene at −40 °C, and its structure is confirmed by single-crystal X-ray diffraction studies. The C1C2 bond distance of the coordinated ethylene molecule is 1.41 Å, indicating a significant back-donation from the metal center to the π* orbital of ethylene . In the presence of 3 equiv of B(C 6 F 5 ) 3 , the reaction of 2a and 13 C-dilabeled ethylene yields the py-B(C 6 F 5 ) 3 adduct, 3a-C 2 H 4 , and a small amount of the metallacyclopentane 4a (Scheme ) at room temperature.…”

Section: Resultsmentioning

confidence: 99%

C–H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)–Oxo Complex

et al. 2018

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In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species-generating alkylidenes-is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)py (R = CMe(CF) (2a), R = Si(O tBu) (2b), and R = C(CF) (2c); py = pyridine), and show that upon activation with B(CF) they display alkene metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the alkene reactant is crucial for initiating alkene metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl "hydride" involves a proton transfer facilitated by the coordination of a Lewis acid (B(CF)) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the metathesis active species.

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“…Reference is made to the isolation by Boncella et al of an imido-tungstanacyclopentane complex, which whilst confirming the existence of metallacyclic species for tungsten, was the deactivation by-product from metathesis. 25 This, coupled with the ability to isolate and characterise this species suggests that it is kinetically inert, resisting breakdown of the metallacycle and hence perhaps not a This journal is © The Royal Society of Chemistry 2010 catalytically important intermediate. Nonetheless, computational studies have suggested that a metallacyclic pathway may be energetically feasible for tungsten-imido based catalysts.…”

Section: Mechanistic Considerationsmentioning

confidence: 99%

Selective dimerisation of α-olefins using tungsten-based initiators

et al. 2010

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The selective dimerisation of the alpha-olefins 1-pentene through to 1-nonene is reported using an in situ-generated catalyst derived from tungsten hexachloride, aniline, triethylamine and alkylaluminium halide. The influence of reagent identity and reaction stoichiometry, along with activator, solvent and alpha-olefin substrate choice are probed. The catalyst is found to be highly selective towards dimerisation, minimising the formation of undesired heavier oligomers. Notably, the selectivity within the dimer fraction is found to favour the formation of products with methyl branches. The selectivity towards individual olefin isomers has been determined and the system is found to also produce trace levels of dienes and alkanes. A kinetic study of the system reveals a second order dependence on substrate. Comparison of the products observed, with those expected for metallacyclic and Cossee-type mechanisms, suggests that the latter is in operation, something confirmed by the results of a C(2)H(4)/C(2)D(4) co-dimerisation experiment which showed full isotopic scrambling in the products. Thus a mechanistic proposal is made to account for the observed behaviour of the system, including the diene and alkane formation.

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Metallacyclopentane Formation: A Deactivation Pathway for a Tungsten(VI) Alkylidene Complex in Olefin Metathesis Reactions

Cited by 51 publications

References 36 publications

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

Low Temperature Activation of Supported Metathesis Catalysts by Organosilicon Reducing Agents

C–H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)–Oxo Complex

Selective dimerisation of α-olefins using tungsten-based initiators

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