The first example of tungsten-based carbene generation from WCl6 and atomic carbon and its use in olefin metathesis

Article abstract of DOI:10.1016/j.tetlet.2006.05.054

We describe a new route for the synthesis of tungsten-based carbenes generated by the reaction of WCl₆ with atomic carbon in a carbon arc reactor. The active species formed under these conditions, [W] = CCl₂, was found to catalyze olefin metathesis reactions of 1-octene, 2-octene and 1-heptene. We also evaluated the mechanism of formation of [W] = CCl₂ within the WCl₆/C system at the DFT level.

Full text of DOI:10.1016/j.tetlet.2006.05.054

Tetrahedron Letters 47 (2006) 5167–5170
The first example of tungsten-based carbene generation
from WCl6 and atomic carbon and its use in olefin metathesis
Bulent Duz,* Dilek Yuksel, Abdulilah Ece and Fatma Sevin*
¨
¨
¨
Department of Chemistry, Hacettepe University, Beytepe, Ankara 06532, Turkey
Received 11 January 2006; revised 1 May 2006; accepted 10 May 2006
Available online 5 June 2006
Abstract—We describe a new route for the synthesis of tungsten-based carbenes generated by the reaction of WCl6 with atomic car-
bon in a carbon arc reactor. The active species formed under these conditions, [W] = CCl2, was found to catalyze olefin metathesis
reactions of 1-octene, 2-octene and 1-heptene. We also evaluated the mechanism of formation of [W] = CCl2 within the WCl6/C
system at the DFT level.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Olefin metathesis is an extremely useful reaction which
has recently found a very wide range of application in
the development of new medicines, biologically active
compounds, polymeric materials and in industrial syn-
theses.1 Catalyst systems for olefin metathesis reactions
almost always contain a transition metal compound.
There is much evidence to show that these catalyst sys-
tems act through the initial formation of metal carbene
complexes which initiate and then propagate the reac-
tion. The accepted metal carbene metallacyclobutane
mechanism for olefin metathesis was first proposed by
tions, via combination of a high energy C atom with
WCl6. Thus, the carbon arc reactor described by Skell
and co-workers was used.4 The ends of two high purity
graphite rods were drilled and filled with powdered
WCl6. Then, an arc was struck intermittently between
the graphite rods and the tungsten carbene intermedi-
ates were condensed on the walls of the reactor which
was cooled by liquid nitrogen at 77 K. When the graph-
ite rods had been used up, vaporized olefin was trans-
ferred into the reactor (without arcing to prevent the
reaction of atomic carbon with the olefin). Liquid nitro-
gen was removed and the reactor was allowed to warm
to room temperature. At room temperature, the olefin
and tungsten carbene reacted. After 30 min, the contents
of the reactor were dissolved in chloroform and the solu-
tion was concentrated by rotary evaporation and then
analyzed by GC/MS.5
´
Yves Chauvin and his student Jean-Louis Herisson in
1971.2 From 1980 onwards, well-defined carbene com-
plexes of Ta, Mo, W, Re, Ru have been reported which
act as initiators without the need for activation by heat,
light or co-catalyst.1
On the other hand, the reaction of carbon vapor with
organic substrates in a carbon arc reactor can lead to
the formation of carbenes via several pathways includ-
ing single bond insertion and/or double bond addition.3
These reactions involve a great deal of excess energy and
are thus highly exothermic.
The results are given in Table 1 (see also the Supplemen-
tary data). The main metathesis product obtained from
1-heptene and 2-octene was 6-dodecene and that from 1-
octene was 7-tetradecene. The metathetic activity ob-
tained with the WCl6/C system led to the formation of
these products. When we repeated the same procedure
without using WCl6 we did not obtain any metathesis
products.
In this work we intended to generate a tungsten carbene
species, which would be active in olefin metathesis reac-
Previously, information regarding the nature of the ini-
tiating metal carbene has been obtained in many cases1
by examining the first-formed products in the reactions
of olefins. The terminal groups in polymers formed by
ROMP (ring opening metathesis polymerization) also
Keywords: Carbon arc; Olefin metathesis; WCl6; DFT.
*
Corresponding authors. Tel.: +90 312 2977952; fax: +90 312 2992163
(B.D.); tel.: +90 312 2977959; fax: +90 312 2992163 (F.S.); e-mail
addresses: bduz@hacettepe.edu.tr; sevin@hacettepe.edu.tr
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.054

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B. Du¨z et al. / Tetrahedron Letters 47 (2006) 5167–5170
Table 1. GC/MS analyses of the metathesis products obtained
Reactants
Relative area of products (%)
Side products
Isomerizationa
Metathesis product
Chlorinated compounds
Othersb
13.82
1-Octene
7-Tetradecene
31.86
2-Chlorooctane
29.66
2-Octene (Z/E)
13.82/4.13
3-Octene (Z or E)
3.96/2.73
2-Octene
6-Dodecene
2.85
2-Chlorooctane
66.41
2- or 3-Chlorooctene
10.17
—
20.57
8.79
1-Heptene
6-Dodecene
6.57
2-Chloroheptane
64.43
2-Heptene
3.74
2-Chloroheptane
12.61
3-Heptane
3.84
a Cis and trans-isomers.
b Oxygenated compounds which could not be exactly clarified.
carry information about the initiating carbene. Further-
more, it has been shown that the initiating species has a
dichlorocarbene ligand, [W] = CCl2, since the dichloro-
carbene moiety has been identified in the products of
reaction of preirradiated W(CO)6/CCl4/hm with 1-eth-
oxynorbornene.6 However, in the reaction with 4-octene
and 7-tetradecene, no RCH = CCl2 moiety was detected
from cross-metathesis7 and in the reaction with 2-pen-
tene there was again no evidence for metathesis initia-
sition states were optimized (see also Supplementary
data). Geometries, relative energies, W–C bond lengths
and the charges on W and C atoms for the structures
found on the potential energy surface of the WCl6/C
atom system are given in Scheme 2.
In the triplet state, complex 1t was transformed into the
more stable structure 3t with trigonal bipyramidal
˚
geometry and a 2.01 A W@C bond, which is a double
bond within experimental results,14 through TS1t with
an activation energy of 8.28 kcal molꢀ1. From the struc-
ture 1t to the structure 3t, the W–C bond distance de-
8
tion by [W] = CCl2. In the last case it appeared that
[W] = CCl2 was destroyed before it could react with
the substrate and the metathesis reaction was initiated
from the surface of a precipitate; the nature of the initi-
ating species is not clear. In the presence of benzalde-
hyde as a carbene trap, polymerization was inhibited
and the main product was 2-benzylidenenorbornene.
Similarly, the carbene ligands generated in the systems
W(CO)6/CCl4/hm and W(CO)6/Ph2CCl2/hm have been
identified as @CCl2 and @CPh2, respectively, from the
products of reaction with 2-ethoxynorbornene.9
˚
creased steadily from 3.46 to 2.01 A and a W@C
double bond formed. The charge on the W atom in-
creases from ꢀ0.037 to +0.592 and the charge on the
C atom decreases from 0.00 to ꢀ0.473. Another path-
way leading to 3t starts with structure 2t. Since we could
not localize any complex, which could lead to the struc-
ture 2t, we assumed that it had formed directly from
WCl6 and a C atom via insertion of the highly energetic
C atom into the W–Cl bond. Then, through TS2t with
an activation energy of 7.41 kcal molꢀ1, it has formed
3t and with a geometric isomerization and an activation
energy of 0.53 kcal molꢀ1, the tungsten–carbene 3t
adopted its least energetic form, 3t0, through TS3t. In
the singlet state, neither 1s, nor 2s could be localized.
All calculations gave only one significant zero order sta-
tionary point corresponding to the structure 3s, which is
very similar to 3t0. When we compare 3s with 3t0, we can
see that the geometries are nearly the same and are
square pyramidal, but the W@C bond distance is shorter
and polarization of the W@C bond is higher for the sin-
glet state. According to these calculations, we assumed
that the active species, [W] = CCl2, has a five coordinate
square pyramidal geometry as in 3s, which has a nucleo-
philic carbon atom bonded to the electrophilic metal.
With respect to this work, we presumed that the active
species for our reaction was [W] = CCl2 and we thus
used benzaldehyde as a carbene trap to catch the tung-
sten–carbene species. As seen in Scheme 1, the forma-
tion of b,b-dichlorostyrene was detected by GC/MS
analysis and this confirmed our supposition on the
structure of tungsten carbene.
To gain insight into the mechanism of formation of
[W] = CCl2 from WCl6 and a C atom, we performed
molecular modeling calculations using GAUSSIAN
03.10 The B3LYP11 density-functional method12 was
used in combination with the LANL2DZ13 basis set.
The structures of all the carbene intermediates and tran-
O
CCl2
In summary, we have found a new route for the synthe-
sis of metathetic active tungsten based carbene interme-
diates in the reaction of a C atom with WCl6. Current
studies using other olefins are under way and will be
reported in due course.
H
H
77 K
with arc
WCl6 + C
[W]=CCl2
[W]=O
+
without arc,
room temp.
Scheme 1.

B. Du¨z et al. / Tetrahedron Letters 47 (2006) 5167–5170
5169
Scheme 2.
5. Shimadzu GCMS-QP5050A using an Optima column, 5–
1.0 lm (50 m · 0.32 mm), a temperature range of 80–
250 ꢁC (20 ꢁC minꢀ1) and the carrier gas was helium at
Acknowledgement
This research was supported by the TUBITAK (Scien-
tific and Technical Research Council of the Turkish
Republic) under Grant 104T402.
1 mL minꢀ1
.
6. Rudler, H. J. Mol. Cat. 1980, 8, 53–72.
7. Karan, B.; Imamoglu, Y. The presence of dichlorotung-
sten carbenes in photocatalytic olefin metathesis reactions.
In Advances in Metal Carbene Complexes; Shubert, U. S.,
Ed.; NATO ASI Series; Kluwer Academic: Dordrecht,
1989; Vol. 269, pp 347–349.
Supplementary data
Detailed spectral data for GC/MS analyses and compu-
tational data for DFT calculations are available. Sup-
plementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2006.05.054.
8. Schilder, P. G. M.; Stufkens, D. J.; Oskam, A.; Mol, J. C.
J. Organomet. Chem. 1992, 426, 351–359.
9. Garnier, F.; Krausz, P.; Rudler, H. J. Organomet. Chem.
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M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.;
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Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.;
Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.;
Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
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