The metathesis of polyunsaturated fatty esters using the homogeneousW(O-2,6-C6H3X2)2Cl 4/Me4Sn catalytic systems

Article abstract of DOI:10.1016/j.molcata.2003.11.026

Promoted with a small amount of Me₄Sn, W(O-2,6-C ₆H₃X₂)₂Cl₄ (X=Ph, Cl) complexes are highly active metathesis catalysts for polyunsaturated fatty esters. Upon metathesis of methyl linoleate, substrate conversions of ca. 95 and 78% were obtained in 2h with the W(O-2,6-C₆H₃Ph ₂)₂Cl₄/Me₄Sn and W(O-2,6-C ₆H₃Cl₂)₂Cl₄/Me ₄Sn catalytic systems respectively. The lower activity of the W(O-2,6-C₆H₃Cl₂)₂Cl ₄/Me₄Sn catalytic system was attributed to the highly deactivating nature of the Cl substituents on the o,o′-position of the aryloxide ligand. Highly electron-withdrawing Cl substituents cause the tungsten atom to be electron-deficient and thus more susceptible to attack by the electron-rich ester groups. However, the W(O-2,6-C₆H ₃Cl₂)₂Cl₄/Me₄Sn system was more selective towards dicarboxylic esters. The latter are interesting starting materials for the synthesis of polyesters and polyamides. The W(O-2,6-C₆H₃X₂)₂Cl ₄/Me₄Sn catalytic systems were rapidly deactivated by bromine-containing olefinic esters. Deactivation was attributed to a side reaction between the Br-group and the electron-deficient W-centre.

Full text of DOI:10.1016/j.molcata.2003.11.026

Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
The metathesis of polyunsaturated fatty esters using the
homogeneousW(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic systems
B.B. Marvey, J.A.K. du Plessis, H.C.M. Vosloo∗
School of Chemistry and Biochemistry, Potchefstroomse Universiteit vir CHO, 2520 Potchefstroom, South Africa
Abstract
Promoted with a small amount of Me4Sn, W(O-2,6-C6H3X2)2Cl4 (X = Ph, Cl) complexes are highly active metathesis catalysts for
polyunsaturated fatty esters. Upon metathesis of methyl linoleate, substrate conversions of ca. 95 and 78% were obtained in 2 h with the
W(O-2,6-C6H3Ph2)2Cl4/Me4Sn and W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic systems respectively. The lower activity of the W(O-2,6-C6H3-
Cl2)2Cl4/Me4Sn catalytic system was attributed to the highly deactivating nature of the Cl substituents on the o,oꢀ-position of the aryloxide
ligand. Highly electron-withdrawing Cl substituents cause the tungsten atom to be electron-deficient and thus more susceptible to attack
by the electron-rich ester groups. However, the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn system was more selective towards dicarboxylic esters. The
latter are interesting starting materials for the synthesis of polyesters and polyamides. The W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic systems
were rapidly deactivated by bromine-containing olefinic esters. Deactivation was attributed to a side reaction between the Br-group and the
electron-deficient W-centre.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Metathesis; Polyunsaturated fatty esters; Methyl linoleate; W(O-2,6-C6H3X2)2Cl4 complexes
1. Introduction
cocatalyst. For a given cocatalyst and a given time of
precatalyst–cocatalyst interaction, metathesis activity in-
creased with the nature of the substituent in the o,oꢀ-position
on the aryloxide in the order: X = CH3 < C6H5 < F < Cl
< Br.
Self- and cross-metathesis of ethyl oleate, a monounsat-
urated fatty ester, was achieved using the W(O-2,6-C6H3-
X2)2Cl4 (X = Cl, Br) complexes promoted with Me4Sn or
Bu4Pb [7]. In most cases, the catalytic activity was better
than that reported for the conventional WCl6/R4Sn catalyst
system.
Tungsten(VI) aryloxide complexes were earlier reported
to be useful precatalysts for the metathesis of alkenes
[1–7]. These include W(OAr)6 [1,2], WO(OAr)4 [3] and
W(OAr)xCl6−x with x = 2–4 [2,4–7]. Aryloxide ligands
offer some advantages in that their electron-withdrawing
properties can be varied by changing the substituents on the
ring and their steric effect on tungsten can be modified by
varying the bulkiness of the substituents on the o,oꢀ-position
on the aryloxide [8].
Tungsten(VI) aryloxide complexes provide active
metathesis precatalysts for linear, cyclic and functionalised
alkenes. Quignard et al. [7] achieved metathesis of inter-
nal and terminal alkenes as well as olefinic esters using
the trans-W(O-2,6-C6H3X2)2Cl4 complexes 1 (X = Me,
Ph, Cl, F) promoted with a R4Sn compound. Parame-
ters which influenced metathesis activity upon metathesis
of cis-2-pentene were: the electron-withdrawing prop-
erty of the aryloxide ligand, the nature of the cocatalyst
and the interaction time between the precatalyst and the
Vosloo et al. [9,10] reported the catalysed metathesis
reaction of 1-alkenes using complex 1 (X = Cl, Ph) pro-
moted with a R4Sn compound. The W(O-2,6-C6H3X2)2Cl4
precatalysts (X = Cl, Ph) works optimally at 85 ◦C, a Sn:W
molar ratio of 3 and a precatalyst–cocatalyst interaction
time of 20 min. Secondary metathesis products resulting
from the double bond isomerisation of the 1-alkenes also
occur [9]. The W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic
systems deactivate in the presence of oxygenates with
W(O-2,6-C6H3Ph2)2Cl4 showing more resistance to deac-
tivation.
We have investigated the metathesis activity and selectiv-
ity of the W(O-2,6-C6H3X2)2Cl4 (X = Cl, Ph) precatalysts
promoted with Me4Sn on the methyl ester of linoleic acid
∗
Corresponding author. Tel.: +27-18-299-2358;
fax: +27-18-299-2350.
E-mail address: chehcmv@puk.ac.za (H.C.M. Vosloo).
1381-1169/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2003.11.026

152
B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
and a bromine-containing ester mixture. These catalytic
systems were found to be active with substrate conversions
exceeding 85% and high selectivity (>95%) towards the
primary metathesis products (i.e. very little double bond
isomerisation). A mixture of simple alkene, monoester and
diester metathesis products were obtained.
2.3. Catalytic reactions
The W(O-2,6-C6H3X2)2Cl4 precatalyst (X = Cl, Ph;
5.0 × 10−5 mol) was placed in a pear-shaped reaction flask
followed by the addition of chlorobenzene (1 ml). The pre-
catalyst mixture was heated to 85 ◦C after which Me4Sn
(15 × 10−5 mol) was added. After a precatalyst–cocatalyst
interaction time of 20 min, the substrate (1.5 × 10−3 mol
methyl linoleate or 0.3 ml brominated methyl linoleate mix-
ture) was added. The reaction mixture was allowed to stir
at 85 ◦C. Samples for GC analysis were collected at reg-
ular intervals and a few drops of methanol were added
to each sample to quench the reaction. After 4 h, the re-
action mixture was quenched with methanol, cooled and
analysed by GC/MS. In the case of the brominated mix-
ture, the metathesis products were debrominated prior to
analysis.
2. Experimental
2.1. Materials
Chlorobenzene (Aldrich) was dried by refluxing over
P2O5, distilled and stored under N2. Methyl linoleate (99%,
Aldrich) was purified from peroxides by pretreatment with
calcined SiO2–Al2O3–HA (24.3% Al2O3, AKZO) under a
N2 atmosphere. Tetramethyl tin (Me4Sn, Aldrich), WCl6,
CCl4 (Merck), 2,6-dichlorophenol and 2,6-diphenylphenol
(Aldrich) were used as purchased.
A similar procedure was used for the metathesis of methyl
linoleate in the presence of the WCl6/Me4Sn catalytic sys-
tem (W:Sn:ester molar ratio = 1:1:30).
2.4. Product analysis
2.2. Precatalysts
Samples were collected and analyzed on a GC equipped
with a DB-1 capillary column and FID after quenching the
reaction by adding a few drops of methanol. GC analyses
were performed on a Carlo Erba 6000 Vega series 3 GC
equipped with a cold on-column injector, a DB-1 fused silica
capillary column (J&W Scientific, 15 m×0.53 mm×0.25 m)
and FID using the following conditions: carrier gas (He), de-
tector temperature (300 ◦C), oven programmed from 100 to
150 ◦C at 10 ◦C/min, 150–160 ◦C at 1 ◦C/min, 160–280 ◦C
at 15 ◦C/min. A typical chromatogram is illustrated in
Fig. 1. The substrate conversions were calculated from the
peak areas of the GC analyses corrected for the molecular
The W(O-2,6-C6H3Cl2)2Cl4 and W(O-2,6-C6H3Ph2)2Cl4
complexes were prepared as described by Quignard et al. [8].
A solution of HO–2,6–C6H3X2 (2.5 × 10−3 mol) in CCl4
(20 ml) was added to a solution of WCl6 (5.0 × 10−3 mol)
in CCl4 (20 ml). The reaction mixture was stirred for 4 h
under reflux. The W(O-2,6-C6H3Ph2)2Cl4 complex was
filtered off and washed with hexane to give black micro-
crystals (79% yield). The W(O-2,6-C6H3Cl2)2Cl4 complex
was filtered off and recrystallized from ethanol to give
black microcrystals with green luster (80% yield). Both
complexes decompose slowly in air and were, therefore,
stored under N2 atmosphere.
Fig. 1. Gas chromatogram of the metathesis products of methyl linoleate: simple alkenes (peaks: 1–3, 5, 7), monocarboxylic esters (peaks: 4, 6, 8, 10,
12) and dicarboxylic esters (peaks: 9, 11, 13, 14).

B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
153
Table 1
response [11,12]:
Product spectrum resulting from the metathesis of methyl linoleate (*) in
the presence of the W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic systems
Cs
ꢀ
conversion (%) = 1 −
Ci
Carbon no.
Product
Yield (%)
X = Cl
X = Ph
where i = 1, 2, . . ., ξ; Cs is the corrected peak area for the
substrate ester; Ci the corrected peak area for the component
i in the product mixture with ξ components.
Metathesis products were characterized on a Fisons
GC/MS equipped with a DB-5 capillary column (J&W sci-
entific, 15 m × 0.32 mm × 0.25 m). Similar conditions as
mentioned above were used.
Simple alkenes
1
=
C12
C15
C18
C21
C6 C6
26.64
27.61
3.50
–
43.62
21.24
3.87
–
2
= =
C6 C3 C6
= =
C6 (C3)2 C6
= =
C6 (C3)3 C6
3
4
5
=
=
C24
C6 (C3)4 C6
–
–
Monoesters
1
=
C16
C19
C22
C25
C6 C8COOC
8.91
22.46
1.60
0.94
–
19.10
5.37
2.72
0.40
–
2
= =
C6 C3 C8COOC*
= =
C6 (C3)2 C8COOC
= =
C6 (C3)3 C8COOC
= =
C6 (C3)4 C8COOC
3
3. Results and discussion
4
5
C28
3.1. Metathesis of methyl linoleate with
W(O-2,6-C6H3X2)2Cl4/Me4Sn (X = Cl, Ph)
Diesters
1
=
C20
C23
C26
C29
COOCC8 C8COOC
= =
COOCC8 C3 C8COOC
= =
COOCC8 (C3)2 C8COOC
COOCC8 (C3)3 C8COOC
5.41
2.93
–
3.01
0.82
–
2
3
The metathesis activity of the W(O-2,6-C6H3X2)2Cl4/
Me4Sn (X = Cl, Ph) catalytic system was investigated for
polyunsaturated esters. As previously established by Quig-
nard et al. [7] and Vosloo et al. [9,10], a Sn/W molar ratio of
3, reaction temperature of 85 ◦C and a precatalyst–cocatalyst
interaction time of 20 min were employed. Upon activation
with a small amount of Me4Sn (substrate:W:Sn molar ra-
tio of 30:1:3), the W(O-2,6-C6H3X2)2Cl4 precatalysts were
active for the metathesis of methyl linoleate giving rise to
a product mixture consisting of simple alkenes, monoester
and diesters (Fig. 1 and Scheme 1 (hydrogens omitted for
clarity)). For both catalysts, metathesis equilibrium was at-
tained within 2 h reaction time.
4
=
=
–
–
Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. Reaction time = 2 h.
Table 1 shows the products and the yields obtained in the
presence of the W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic
systems. The simple alkenes detected were the monoene co-
mpound 6-dodecene (C121), the diene compound 6,9-pent-
adecadiene (C152) and the triene compound 6,9,12-octade-
catriene (C183). The esters detected were the monoesters
methyl 9-pentadecenoate (C161), methyl 9,12,15-heneicos-
atrienoate (C223) and methyl 9,12,15,18-tetracosatetraenoate
1
(C254), and the diesters dimethyl 9-octadecenoate (C20
)
and dimethyl 9,12-heneicosadienoate (C232). A methyl
linoleate conversion of ca. 95% to metathesis products
corresponding to a turnover number (TON) of 28.4 was
obtained with the W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic
system (Fig. 2). The W(O-2,6-C6H3Cl2)2Cl4/Me4Sn cat-
alytic system was slightly less active with a conversion of
ca. 78% (Fig. 3). Both the catalytic systems (X = Ph, Cl)
were very selective towards primary metathesis products
(>95%) with product selectivity increasing in the order
alkenes > monoesters > diesters. Figs. 2 and 3 also show
the formation of the simple alkenes, monoesters and di-
esters, and Figs. 4 and 5 the yields of monoenes, dienes,
trienes and tetraenes. In terms of unsaturation, selectivity
increased in the order monoenes > dienes > trienes >
tetraenes.
X
Cl
Cl
O
W
O
Cl
Cl
X
X
X
1
C6 C3 C8COOC
C6
C
C
6
p
+
[
]
3
The W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic sys-
tem gave higher yields for simple alkenes than the
W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic system. However,
the latter gave higher yields of diesters which are interest-
ing starting materials for the synthesis of polyesters and
polyamides [13]. The W(O-2,6-C6H3Ph2)2Cl4/Me4Sn cat-
alytic system also yielded more monoenes and trienes rela-
tive to the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic system
(simple alkenes)
C6 C3 C COOC
+
[
]
8
p
(monoesters)
COOCC8 C COOC
C
[
]
3
8
p
(diesters)
Scheme 1.

154
B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
100
80
60
40
20
0
100
80
60
40
20
0
0
60
120
180
240
0
60
120
180
240
Reaction time/min
Reaction time/min
Fig. 4. The yields of alkenes resulting from the metathesis of methyl
linoleate in the presence of the W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic
system. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. (᭺) monoenes;
(ᮀ) dienes; (ꢀ) trienes; (᭛) tetraenes.
Fig. 2. The yields of alkenes resulting from the metathesis of methyl
linoleate in the presence of the W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic
system. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. (᭹) Methyl
linoleate; (᭿) total metathesis; (᭡) simple alkenes; (᭢) monoesters; (᭜)
diesters.
cal homogeneous WCl6/Me4Sn catalyst [14,15]. The
W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic system was more
active than the conventional homogeneous WCl6/Me4Sn
catalytic system.
which on the other hand gave higher yields of dienes and
tetraenes.
Table 2 compares the activities of the tungsten ary-
loxide complexes with those reported for the classi-
100
80
60
40
20
0
100
80
60
40
20
0
0
60
120
180
240
0
60
120
180
240
Reaction time/min
Reaction time/min
Fig. 3. The yields of alkenes resulting from the metathesis of methyl
linoleate in the presence of the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic
system. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. (᭹) Methyl
linoleate; (᭿) total metathesis; (᭡) simple alkenes; (᭢) monoesters; (᭜)
diesters.
Fig. 5. The yields of alkenes resulting from the metathesis of methyl
linoleate in the presence of the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic
system. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. (᭺) Monoenes;
(ᮀ) dienes; (ꢀ) trienes; (᭛) tetraenes.

B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
155
Table 2
Table 3
Metathesis
activitities
of
W(O-2,6-C6H3Cl2)2Cl4/Me4Sn
and
Debrominated products resulting from the metathesis of 1:1 brominated
methyl linoleate (*) with the W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic sys-
tems
WCl6/Me4Sn catalytic systems with methyl linoleate as substrate
Catalyst
Solvent
Ester:W
Temp
(◦C)
Time
(h)
Conversion
(%)
Carbon no.
Product
Yield (%)
X = Cl
X = Ph
WCl6
PhCl
PhCl
PhCl
PhCl
None
30
30
30
12
31
70
70
85
60
80
2
2
2
2
4
77.5
94.6
81.1
83.2
84.0
X = Cl
X = Ph
Simple alkenes
1
=
a
C12
C15
C18
C21
C6 C6
5.4
6.9
3.0
–
9.0
7.1
3.4
–
WCl6
WCl6
2
= =
C6 C3 C6
= =
C6 (C3)2 C6
= =
C6 (C3)3 C6
b
3
a
4
Literature results by Ast et al. [14].
Literature results by Verkuijlen and Boelhouwer [15].
b
5
=
=
C24
C6 (C3)4 C6
–
–
Monoesters
1
=
3.2. Metathesis of brominated methyl linoleate with
W(O-2,6-C6H3X2)2Cl4/Me4Sn
C16
C19
C22
C25
C6 C8COOC
11.7
70.5
0.7
0.4
–
8.7
68.0
0.9
0.5
–
2
= =
C6 C3 C8COOC*
= =
C6 (C3)2 C8COOC
= =
C6 (C3)3 C8COOC
= =
C6 (C3)4 C8COOC
3
4
A mixture of brominated esters resulting from a 1:1
bromination of methyl linoleate (Scheme 2 (hydrogens
omitted for clarity)) was metathesized in the presence
of the W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic systems
(X = Ph, Cl). Although a catalytic conversion was ob-
served, there was generally a rapid deactivation of the
W(O-2,6-C6H3X2)2Cl4/Me4Sn catalytic systems. The
metathesis products obtained after debromination are shown
in Table 3. The selectivity for primary metathesis products
was >95% and the W(O-2,6-C6H3Ph2)2Cl4/Me4Sn cat-
alytic system had more resistance to deactivation than the
W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic system.
Fig. 6 compares the yields for simple alkenes, monoesters
and diesters for the two systems. The W(O-2,6-C6H3Ph2)2
Cl4/Me4Sn catalytic system displayed a higher selectivity
towards simple alkenes and diesters (Fig. 6), and towards
monoene, diene and triene compounds (Fig. 7).
5
C28
Diesters
1
=
C20
C23
C26
C29
COOCC8 C8COOC
= =
COOCC8 C3 C8COOC
= =
COOCC8 (C3)2 C8COOC
COOCC8 (C3)3 C8COOC
1.2
0.2
–
1.6
0.8
–
2
3
4
=
=
–
–
Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. Reaction time = 6 h.
relatively easy deactivation of the W(O-2,6-C6H3Cl2)2Cl4
precatalyst was attributed to the high electron-with-
drawing nature of the aryloxide ligand. With a more
electron-withdrawing aryloxide ligand, the tungsten metal
centre becomes more electrophilic and causes electron-rich
oxygenates, to bind more easily with the tungsten atom [9].
However, the phenyl substituent is less electron-withdrawing
thus it minimises the competition between the electron-rich
3.3. Deactivation
20
15
10
5
Previous studies have shown that systems derived
from the W(O-2,6-C6H3Ph2)2Cl4 precatalyst are more
resistant to deactivation by oxygenates, e.g. butyl ac-
etate, 1-butanol, acetic acid, 2-pentanone and water, than
the W(O-2,6-C6H3Cl2)2Cl4 system [9]. In an air at-
mosphere its metathesis activity increased, whereas the
W(O-2,6-C6H3Cl2)2Cl4 precatalyst was deactivated. The
C6 C3 C8COOC
+
Br2
0
C6Br2C3 C8COOC
+
X = Cl
X = Ph
W(O-2,6-C H3X2)2Cl4
C6 C3Br2C8COOC
+
6
C6Br2C3Br2C8COOC
+
Fig. 6. The yields of alkenes resulting from the metathesis of brominated
methyl linoleate in the presence of the W(O-2,6-C6H3X2)2Cl4/Me4Sn
catalytic systems. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. Reaction
time = 6 h. (᭿) Simple alkenes; monoesters; (ᮀ) diesters.
C6 C3 C8COOC
Scheme 2.

156
B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
R1
20
15
10
5
R1
R1
R2
H
LnWn+
Br
LnW
Br
LnWn+
Br
+
+
( )n
( )n
( )n
R2
(1)
R2
LnW(n-2)+
HBr
R1
Br
( )n
( )n
R1
Br
R2
LnWn+
( )n
LnW
Br
R2
Br
R2
R1
( )n
R1
R2
(2)
0
Br
( )n
( )n
X = Cl
X = Ph
R1
H
LnWn+
W(O-2,6-C H3X2)2Cl4
6
R2
+
Fig. 7. The yields of alkenes resulting from the metathesis of brominated
methyl linoleate in the presence of the W(O-2,6-C6H3X2)2Cl4/Me4Sn
catalytic systems. Reaction temp. = 85 ◦C. Ester:Sn:W = 30:1:3. Reaction
time = 6 h. (᭿) Monoenes; ; dienes; ; trienes; (ᮀ) tetraenes.
R2
Br
R1
Scheme 3.
R1
R1
R2
compounds and the carbon–carbon double bond for the
metal centre. As a result, less deactivation is obtained with
the phenyl substituent than with the chloride substituent.
Since unsaturated fatty esters have both the olefinic bond
and the ester group in the same molecule, deactivation by
the ester group is inevitable. Indeed deactivation of the
W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic system was greater
compared to that of the W(O-2,6-C6H3Ph2)2Cl4/Me4Sn cat-
alytic system. The observed difference in the catalytic ac-
tivity of tungsten-based catalysts could be attributed to the
difference in the electron-withdrawing properties of the sub-
stituents in the o,oꢀ-position of the oxoaryl ligands. The
highly electron-withdrawing Cl substituent strongly acti-
vates the metal center for attack by the ester group. This
kind of metal activation is less pronounced when X = Ph.
In the case of brominated alkenes, rapid deactivation could
be attributed to the increased interaction between the tung-
sten atom and the electron-rich bromides, in addition to a
side reaction with the ester group. In this regard, two deacti-
vation pathways seem possible, namely, intramolecular and
intermolecular coordination of Br and W atom (Scheme 3).
Kawai et al. [16] previously proposed similar deactivation
pathways for the reaction of bromine-containing alkenes
with the Re2O7/Al2O3 catalytic system.
R1
(4)
(3)
C
+
LnWn+
LnW
LnW
R2
R2
R1
(6)
(5)
H
LnWn+
LnW(n-2)+
+
R1
R2
R2
LnW(n-2)+
R1
+
R2
Scheme 4.
ent molecule and the metal to form an intermediate followed
by ␤-elimination of a hydrogen atom.
Deactivation could also occur by reductive elimination
of the metal carbene as shown in Scheme 4 [12,17]. Once
the metal carbene has formed (3), it could either split pro-
ductively to form a new alkene (4) or could undergo re-
ductive elimination to form a cycloalkane (5) or an alkene
(6).
In the first mechanistic pathway: (1) the bromine atom
interacts with the metal center intramolecularly and blocks
the coordination site needed for metathesis. Such an interac-
tion leads to the formation of an intermediate that undergoes
␤-elimination of a hydrogen atom to form a cycloalkene and
H–Wn+–Br complex. The H–Wn+–Br complex loses HBr
by reductive elimination to form W(n−2)+. In the second
mechanism: (2) deactivation occurs as a result of an inter-
molecular interaction between the bromine atom of a differ-
4. Conclusions
Promoted with a small amount of Me4Sn, the W(O-2,6-
C6H3X2)2Cl4 (X = Ph, Cl) complexes are active metathe-

B.B. Marvey et al. / Journal of Molecular Catalysis A: Chemical 213 (2004) 151–157
157
sis catalysts for polyunsaturated fatty esters. These catalytic
systems are highly selective towards primary metathesis
products (>95%). In the presence of the ester group, the
W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic system shows
more resistance against deactivation compared to the
W(O-2,6-C6H3Ph2)2Cl4/Me4Sn catalytic system. The rela-
tively lower activity of the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn
catalytic system, in the presence of the ester group, re-
sult from the highly deactivating nature of the Cl sub-
stituents on the o,oꢀ-position of the aryloxide ligand.
Highly electron-withdrawing Cl substituents cause the
tungsten atom to be electron-deficient and thus more
susceptible to attack by the electron-rich ester groups.
However, the W(O-2,6-C6H3Cl2)2Cl4/Me4Sn catalytic
system is relatively more selective towards dicarboxylic
esters which are interesting starting materials for the
synthesis of polyesters and polyamides. Generally, the
W(O-2,6-C6H3X2)2Cl4/Me4Sn (X = Ph, Cl) catalytic sys-
tems are rapidly deactivated by the bromine-containing
olefinic esters. Deactivation is the result of a side reac-
tion between the bromine group and the electron-deficient
tungsten-centre.
of the North–West and the Potchefstroom University for
CHE.
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For financial support, the authors wish to thank the Na-
tional Research Foundation of South Africa, the University