A cyclometalated
aryloxy(chloro)neopentylidenetungsten complex: a
highly active and stereoselective catalyst for metathesis
of cis- or trans- 2-pentene, norbornene,
1-methylnorbornene and ethyloleate

J.L. Couturier, C. Paillet, M. Leconte, J.M. Basset, K. Weiss

► To cite this version:

J.L. Couturier, C. Paillet, M. Leconte, J.M. Basset, K. Weiss. A cyclometalated aryloxy(chloro)neopentylidenetungsten complex: a highly active and stereoselective catalyst for metathesis of cis- or trans- 2-pentene, norbornene, 1-methylnorbornene and ethyloleate. Angewandte Chemie International Edition, 1992, 31, pp.628-631. 10.1002/anie.199206281. hal-00006230

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Distributed under a Creative Commons CC BY 4.0 - Attribution - International License
A Cyclometalated Aryloxy(chloro)neopentylidene-
tungsten Complex: A Highly Active and
Stereoselective Catalyst for the Metathesis of
cis- and trans-2-Pentene, Norbornene, 1-Methyl-
norbornene, and Ethyl Oleate**

By Jean-Luc Couturier, Christophe Paillet, Michel Leconte,
Jean-Marie Basset,* and Karin Weiss

The design of versatile, highly active, and well-character-
ized catalysts remains one of the main objectives of the re-
search in olefin metathesis.[1] In the past few years, it has
been shown that some neopentylidene-tungsten(vi) com-
plexes with alkoxide,[2, 3] imido,[3] or aryloxide ligands[4] are
efficient homogeneous metathesis catalysts. Aryloxides
prove to be particularly useful ligands since their steric and
electronic properties can be varied substantially by changing
the nature, the number, and the position of the substituents
on the phenoxide ligand. With these ligands, it has been
possible to control both the activity and the stereoselectivity

<footnote>[*] Dr. J.-M. Basset, J.-L. Couturier, Dr. C. Paillet, Dr. M. Leconte
Institut de Recherches sur la Catalyse
Laboratoire Propre du CNRS, conventionné à l'Université Claude
Bernard-Lyon 1 and
Ecole Supérieure de Chimie Industrielle de Lyon
2, Avenue Albert Einstein, 69626 Villeurbanne Cédex (France)
Dr. K. Weiss
Laboratorium für Anorganische Chemie der Universität Bayreuth
Postfach 10 12 51, D-W-8580 Bayreuth (FRG)</footnote>

<footnote>[**] This work was supported by the Société Nationale Elf-Aquitaine and by the Commission of the European Communities. We thank Prof. R. R. Schrock (Massachusetts Institute of Technology) for giving us a preprint of [6b].</footnote>
of the metathesis reaction.[4, 5] We report here the synthesis and the catalytic properties of a new 2,6-diphenylphenoxy (neopentylidene)tungsten derivative, which has the cyclo-metalated structure 1a/1b (Scheme 1).

<figure><img src="image_2.png" /><figcaption>Scheme 1. Two routes for the synthesis of the cyclometalated complexes 1a and 1b. Because X-ray structure data are not available, the structures are still hypothetical and are derived from that proposed by R. R. Schrock et al. for the starting complex [WCl₃(CCMe₃)(dme)] [6] and from the known complex [WCl₄(O-2,6-C₆H₃Ph₂)₂] [7].</figcaption></figure>

The tungsten complexes 1a/1b can be obtained by two independent routes. Reaction of [WCl₃(CCMe₃)(dme)][6] (dme = dimethoxyethane) with two equivalents of LiOAr (OAr = O-2,6-C₆H₃Ph₂) in diethyl ether leads to the orange-brown solid 1a, which likely arises from path a). The reaction of [WCl₄(O-2,6-C₆H₃Ph₂)₂][7] in diethyl ether with one equivalent of MgNp₂ · dioxane (Np = neopentyl) leads, after purification by exchange of diethyl ether with diisopropyl ether, to the orange-brown solid 1b.[8] The probable sequence of reactions leading to 1b is path b). 1a and 1b were characterized by elemental analyses and ¹H and ¹³C NMR spectroscopy.[9]

The key step of the two syntheses is the different intramolecular activation of the C-H bond of the arene substituent on the d⁰ metal which leads to a stable cyclometalated structure.[10-12] In path a) C-H activation leads to the addition of a hydrogen atom to the carbonyl ligand forming a carbene and simultaneous cyclometalation. In path b) the C-H activation leads to the elimination of HCl. The role of the weakly coordinated ether is probably crucial in path a) in which a pentacoordinated W VI center with a possible agostic C-H bond is the precursor of the carbene complex formed via a four-center transition state; this type of sequence is

often discussed for the electrophilic activation of C-H bonds.[13]

Complexes 1a and 1b are excellent catalysts for olefin metathesis. For example, with 1a the metathesis equilibrium of 500 equivalents of cis-2-pentene or trans-2-pentene is reached in 1 min at 25 °C (the equilibrium mixture consists of 2-butene, 2-pentene, and 3-hexene in the approximate ratio of 1:2:1).[1a] 1a is also highly stereoselective: Extrapolation to 0% conversion shows that the trans/cis ratios of 2-butene (C₄) and 3-hexene (C₆) in the metathesis of cis-2-pentene are 0.01 and 0.00, respectively[14] (at equilibrium, trans/cis C₄ ≈ 3 and trans/cis C₆ ≈ 6). In the metathesis of trans-2-pentene, the cis/trans ratios of both C₄ and C₆ are equal to 0.004 at 0% conversion. These high stereoselectivities with retention of configuration of the starting olefin are maintained even at high conversions (Figs. 1 and 2). When the

<figure><img src="image_3.png" /><figcaption>Fig. 1. Metathesis of cis-2-pentene catalyzed by 1a: Plot of the ratio of trans-to-cis-2-butene (t/c) versus the yield of 2-butene (C₄). trans-2-Pentene/1a molar ratio = 500/1, T = 25 °C, solvent C₆H₅Cl.</figcaption></figure>

equilibrium of productive metathesis is nearly reached (roughly 25% yield in 2-butene), there is a dramatic change of the configuration of the products due to secondary metathetical isomerization reactions that progressively lead to the ultimate equilibrium trans/cis ratios.

<figure><img src="image_4.png" /><figcaption>Fig. 2. Metathesis of trans-2-pentene catalyzed by 1a: Plot of the ratio of cis-to-trans-2-butene (c/t) versus the yield of 2-butene (C₄). trans-2-Pentene/1a molar ratio = 500/1, T = 25 °C, solvent C₆H₅Cl.</figcaption></figure>

To our knowledge such high stereoselectivities observed almost up to thermodynamic metathesis equilibrium have never been reported in metathesis of 2-pentene with any highly active tungsten-based catalyst. The extremely high retention of configuration of the starting olefin is easily explained on the basis of the favored configuration of the tungstacyclobutane intermediate with two equatorial (e) alkyl groups in the 1 and 3 positions (Scheme 2).[15] One can
<figure><img src="image_5.png" /><figcaption>Scheme 2. The two favored tungstacyclobutane intermediates with 1,3-diequatorial alkyl substituents that lead to cis-2-butene (in the metathesis of cis-2-pentene) and to trans-2-butene (in the metathesis of trans-2-pentene).</figcaption></figure>

assume that these favored conformations are particularly stabilized by the bulky ligands on tungsten and/or by the rigidity generated by the cyclometalation.

With 1a, 500 equivalents of norbornene are polymerized at 25 °C in less than one minute.[16] ¹H and ¹³C NMR spectra (CDCl₃, 25 °C) indicate that 70% of the C=C bonds have cis configuration. This value is in agreement with earlier results found in norbornene metathesis with other stereoselective aryloxy(chloro)carbene-tungsten catalysts.[17]

Polymerization of 1-methylnorbornene with 1a or 1b provides a completely head-tail, predominantly syndiotactic poly-1-methylnorbornene with 100% cis C=C bonds.[18] Such extremely high stereoselectivity can also be explained on the basis of the favored configuration of the tungstacyclobutane intermediate with the two bulkiest alkyl substituents in 1,3-diequatorial positions (Scheme 3).[20]

<figure><img src="image_6.png" /><figcaption>Scheme 3. In the ring-opening metathesis polymerization of 1-methylnorbornene with 1a or 1b the favored reaction path leads to the formation of a head-tail, cis polymer.</figcaption></figure>

Very promising preliminary results were obtained in the metathesis of ethyl oleate (ethyl-9-octadecenoate), an olefin bearing a functional group (Scheme 4). 1a can convert selec-

<figure><img src="image_7.png" /><figcaption>Scheme 4. The equilibrium metathesis reaction of ethyl oleate leads to 9-octadecene and diethyl 9-octadecenedioate.</figcaption></figure>

tively roughly 50% of 500 equivalents of ethyl oleate[21] into 9-octadecene and diethyl 9-octadecenedioate in 60 min at 25 °C (Fig. 3; the equilibrium mixture consists of ethyl oleate, 9-octadecene, and the diester in the approximate ratio of 2:1:1). To our knowledge, this is the highest activity re-

<figure><img src="image_8.png" /><figcaption>Fig. 3. Metathesis of ethyl oleate catalyzed by 1a: Plot of the yield of 9-octadecene (C₁₈) versus the reaction time. Ethyl oleate/1a molar ratio = 500/1, T = 25 °C, solvent C₆H₅Cl.</figcaption></figure>

ported so far for homogeneous catalysts in the metathesis of this type of substrate.[3, 22]

<footnote>[1] a) K. J. Ivin, Olefin Metathesis, Academic Press, London, 1983; b) K. J. Ivin in Olefin Metathesis and Polymerization Catalysts (Eds.: Y. Imamoglu, B. Zumreoglu-Karan, and A. J. Amass), Kluwer Academic Publishers, Dordrecht, 1990, p. 1.</footnote>

<footnote>[2] a) J. Kress, M. Wesolek, J. A. Osborn, *J. Chem. Soc. Chem. Commun.* **1982**, *514–516*; b) J. Kress, A. Aguero, J. A. Osborn, *J. Mol. Catal.* **1986**, *36*, 1–12.</footnote>

<footnote>[3] C. J. Schaverien, J. C. Dewan, R. R. Schrock, *J. Am. Chem. Soc.* **1986**, *108*, 2771–2773.</footnote>

<footnote>[4] F. Quignard, M. Leconte, J.-M. Basset, *J. Chem. Soc. Chem. Commun.* **1985**, 1816–1817.</footnote>

<footnote>[5] a) J.-M. Basset, M. Leconte, J. Ollivier, F. Quignard, Fr. Pat. 8309876, **1983**, SNEA, US-Pat. 4550216, **1985**, SNEA; b) Fr. Pat. 8501930, **1985**, SNEA, US-Pat. 4654462, **1987**, SNEA; c) Fr. Pat. 8611978, **1986**, SNEA, US-Pat. 4861848, **1989**, SNEA; d) F. Quignard, M. Leconte, J.-M. Basset, *J. Mol. Catal.* **1985**, *28*, 27–32; e) *ibid.* **1986**, *36*, 13–29.</footnote>

<footnote>[6] a) R. R. Schrock, D. N. Clark, J. Sancho, J. H. Wengrovius, S. M. Rocklage, S. F. Pedersen, *Organometallics* **1982**, *1*, 1645; b) R. R. Schrock, J. Sancho, S. F. Pedersen, *Inorg. Synth.* **1989**, *26*, 44–51.</footnote>

<footnote>[7] F. Quignard, M. Leconte, J.-M. Basset, L.-Y. Hsu, J. J. Alexander, S. G. Shore, *Inorg. Chem.* **1987**, *26*, 4272–4277.</footnote>

<footnote>[8] In our first study [4] we found that reaction of one equivalent of MgNp₂ with [WCl₄(O-2,6-C₆H₃Ph₂)₂] in Et₂O led to a compound A1 whose ¹H NMR spectrum (CDCl₃) showed a signal at δ = 8.7 for a hydrogen atom bound to a carbene; A1 proved essentially inactive in the metathesis of cis-2-pentene. We have now some indications that suggest that A1 is a dimeric neopentylidene species (not yet totally identified) resulting from the dimerization of 1a or 1b. This dimerization seems to be rather fast in solution (especially in CHCl₃ and CH₂Cl₂) but can also be observed in the solid state where the reaction is slower.</footnote>

<footnote>[9] Characteristic ¹H (100 MHz) and ¹³C (25 MHz) NMR data for 1a (C₆D₆, 25 °C): δ(W = CHCMe₃) = 10.11 (Jw,HH = 14 Hz), δ(W = CHCMe₃) = 0.57; δ(W = CH) = 296.66 (Jc,HH = 130.5, Jc,w = 166 Hz), δ(W-Cipss) = 183.15 (Jc,w = 115 Hz), δ(W = CHCMe₃) = 45.24, δ(W = CHCMe₃) = 31.82. The corresponding data for 1b are practically identical.</footnote>

<footnote>[10] M. R. Churchill, J. W. Ziller, J. H. Freudenberger, R. R. Schrock, *Organometallics* **1984**, *3*, 1554–1562.</footnote>

<footnote>[11] R. R. Schrock, R. T. DePue, J. Feldman, K. B. Yap, D. C. Yang, W. M. Davis, L. Park, M. DiMare, M. Schofield, J. Anhaus, E. Walborsky, E. Evitt, C. Krüger, P. Betz, *Organometallics* **1990**, *9*, 2262–2275.</footnote>
[12] a) J. L. Kerschner, P. E. Fanwick, I. P. Rothwell, *J. Am. Chem. Soc.* **1987**, *109*, 5840–5842; b) J. L. Kerschner, I. P. Rothwell, J. C. Huffman, W. E. Streib, *Organometallics* **1988**, *7*, 1871–1873; c) J. L. Kerschner, P. E. Fanwick, I. P. Rothwell, J. C. Huffman, *ibid.* **1989**, *8*, 1431–1438.

[13] M. E. Thompson, S. M. Baxter, A. R. Bulls, B. J. Burger, M. C. Nolan, B. D. Santarsiero, W. P. Schaefer, J. E. Bercaw, *J. Am. Chem. Soc.* **1987**, *109*, 203–219.

[14] For the metathesis of cis-2-pentene, the stereoselectivity at 0% conversion is accurately determined by plotting the *trans/cis* ratio of 2-butene vs. the *trans/cis* ratio of 2-pentene and by extrapolating at *trans/cis-2-pentene* = 0 (in the case of *trans-2-pentene*, *cis/trans C*₄ is plotted vs. *cis/trans C*₅).¹⁵⁾

[15] M. Leconte, J.-M. Basset, *J. Am. Chem. Soc.* **1979**, *101*, 7296–7302.

[16] The catalyst is deactivated by addition of benzaldehyde; polynorbornene is recovered after addition of THF to the reaction mixture, precipitation with methanol, filtration, and drying.

[17] Dj. Boutarfa, F. Quignard, M. Leconte, J.-M. Basset, J. G. Hamilton, K. J. Ivin, J. J. Rooney in *Transition Metal Catalyzed Polymerizations: Ziegler-Natta and Metathesis Polymerizations* (Ed.: R. P. Quirk), Cambridge University Press, Cambridge, **1988**, p. 695.

[18] The 100% head-tail, 100% *cis* structure of the poly-1-methylnorbornene obtained with *1a* or *1b* was deduced from ¹H and ¹³C NMR spectra of the polymer:¹¹⁹⁾ ¹³C{¹H} NMR (62.8 MHz, CDCl₃): δ = 29.29 (C-8), 33.02 (C-5), 38.14 (C-4), 41.82 (C-6), 44.13 (C-1), 50.02 (C-7), 135.08 (C-3), 139.47 (C-2) (see Scheme 3 for the numbering of the carbon atoms). The predominantly syndiotactic structure was deduced from the ¹³C NMR spectrum of the hydrogenated polymer [19].

[19] J. G. Hamilton, K. J. Ivin, J. J. Rooney, *Br. Polym. J.* **1984**, *16*, 21–33.

[20] J.-M. Basset, Dj. Boutarfa, E. Custodero, M. Leconte, C. Paillet in *Olefin Metathesis and Polymerization Catalysts* (Eds.: Y. Imamoglu, B. Zureoglu-Karan, A. J. Amass), Kluwer Academic Publishers, Dordrecht, **1990**, p. 45.

[21] Ethyl oleate (Sigma, 99%) was used immediately after the ampules were opened and the contents purified by passage through a column of activated alumina.

[22] J. C. Mol., *J. Mol. Catal.* **1991**, *65*, 145–162.