.alpha.-Hydride vs. .beta.-hydride elimination. An example of an equilibrium between two tautomers

Fellmann, J. D.; Schrock, R. R.; Traficante, Daniel D.

doi:10.1021/om00063a013

Cited by 64 publications

(49 citation statements)

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“…Its 31 P{ 1 H} NMR spectrum shows two singlets at À8.0 and À9.3 ppm in a 10:1 ratio corresponding to coordinated PMe 3 of syn-(4m) and anti-(4m), respectively. The formation of 4m from 3m is in fact quite similar to what has been found for other bisalkyl complexes upon addition of PMe 3 , for which the extrusion of one neopentyl ligand is observed upon coordination of extra donor ligands [59][60][61][62]. This probably shows that the surface might play the role of this ligand in generating directly 2 at room temperature in place of 3.…”

Section: Development Of An Equivalent Homogeneous Olefin Metathesis Csupporting

confidence: 83%

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

Blanc

Chabanas

Copéret

et al. 2005

Journal of Organometallic Chemistry

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Section: Development Of An Equivalent Homogeneous Olefin Metathesis Csupporting

confidence: 83%

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

Blanc

Chabanas

Copéret

et al. 2005

Journal of Organometallic Chemistry

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“…Although β‐H elimination has long been considered to be much faster than α‐H elimination,28 Schrock et al. demonstrated that α‐ and β‐H elimination can in fact occur on the same time scale, and moreover that α‐H elimination can be faster than β‐H elimination in certain complexes 29–31. Therefore, for the reversible switching between ROMP and VIP, a ligand system that can reversibly abstract an α‐proton from the cationic species and re‐add a proton to the metal alkylidene was designed.…”

Section: Introductionmentioning

confidence: 99%

Group 4 Dimethylsilylenebisamido Complexes Bearing the 6‐[2‐(Diethylboryl)phenyl]pyrid‐2‐yl Motif: Synthesis and Use in Tandem Ring‐Opening Metathesis/Vinyl‐Insertion Copolymerization of Cyclic Olefins with Ethylene

Zou

Wang

Wurst

et al. 2011

Chemistry A European J

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Two novel Zr(IV)- and Hf(IV)-based bisamido complexes bearing the 6-[2-(diethylboryl)phenyl]pyrid-2-yl motif, that is, [ZrCl(2){Me(2)Si(DbppN)(2)}(thf)] (9) and [HfCl(2){Me(2)Si(DbppN)(2)}(thf)(2)] (10) (DbppN=6-[2-(diethylboryl)phenyl]pyridine-2-amido) have been prepared. Their reactivities have been compared with that of a model precatalyst that does not bear the aminoborane motif. Upon activation with methylalumoxane, precatalysts 9 and 10 are active in the homopolymerization of ethylene (E) yielding high-density polyethylene (HDPE). In the copolymerization of E with cyclopentene (CPE), for example by the action of 9, the presence of CPE resulted in a dramatic increase in the polymerization activity of E, while CPE incorporation remained close to or at zero. In the vinyl-insertion copolymerization of norborn-2-ene (NBE) with E by the action of 9, statistical cyclic olefin copolymers of these two monomers were obtained. At higher NBE concentrations, however, 9 gave rise to reversible ring-opening metathesis (ROMP)/vinyl-insertion polymerization (VIP) of NBE with E, resulting in the formation of multi-block copolymers of the general formula poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E). This particular feature of precatalyst 9, that is, the ability to induce a reversible α-H elimination/α-H addition reaction, is attributed to the unique role of the 6-[2-(diethylboryl)phenyl]pyrid-2-yl ligand. Accordingly, a model precatalyst lacking this ligand does not have the ability to induce α-H elimination/α-H addition reactions. The different (11)B NMR shifts of various diethylborylphenylpyrid-2-ylamines and -amides permit a ranking of the strengths of the B-N bonds in these compounds. This strength of the B-N bond is correlated with the propensity of 9/MAO to produce poly(NBE)(ROMP)-co-poly(NBE)(VIP)-co-poly(E) at different temperatures.

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“…For the W IV intermediate, the singlet state is calculated to be the thermodynamic ground state, lying by Δ G (Δ H ) 31.2 (33.1) kcal mol −1 below the corresponding triplet state. A similar process has been proposed to occur in the equilibrium between tantalum alkyl alkylidene and tantalum alkyl olefin complexes . From that W IV intermediate, the metathesis active W VI bis‐alkylidene can be generated by α‐H transfer, which occurs via a transition state energy of Δ G ≠ (Δ H ≠ ) of 42.8 (40.5) kcal mol −1 , making this pathway overall similar to the direct β‐H transfer mechanism.…”

Section: Resultsmentioning

confidence: 66%

Metal Alkyls with Alkylidynic Metal‐Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

Gordon

Copéret

2020

Angewandte Chemie

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The homologation of alkanes via alkane metathesis is catalyzed at low temperatures (150 °C) by the silica‐supported species (≡SiO)WMe5 and (≡SiO)TaMe4, while (≡SiO)TaMe3Cp* is inactive. The contrasting reactivity is paralleled by differences in the 13C NMR signature; the former display significantly more deshielded isotropic chemical shifts (δiso) and almost axially symmetric chemical shift tensors, similar to what is observed in their molecular precursors TaMe5 and WMe6. Analysis of the chemical shift tensors reveals the presence of a triple‐bond character in their metal‐carbon (formally single) bond. This electronic structure is reflected in their propensity to generate alkylidynes and to participate in alkane metathesis, further supporting the role of alkylidynes as key reaction intermediates. This study establishes chemical shift as a descriptor to identify potential alkane metathesis catalysts.

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.alpha.-Hydride vs. .beta.-hydride elimination. An example of an equilibrium between two tautomers

Cited by 64 publications

References 0 publications

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

Reactivity differences between molecular and surface silanols in the preparation of homogeneous and heterogeneous olefin metathesis catalysts

Group 4 Dimethylsilylenebisamido Complexes Bearing the 6‐[2‐(Diethylboryl)phenyl]pyrid‐2‐yl Motif: Synthesis and Use in Tandem Ring‐Opening Metathesis/Vinyl‐Insertion Copolymerization of Cyclic Olefins with Ethylene

Metal Alkyls with Alkylidynic Metal‐Carbon Bond Character: Key Electronic Structures in Alkane Metathesis Precatalysts

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