Improved cross-metathesis of acrylate esters catalyzed by 2nd generation ruthenium carbene complexes

Article abstract of DOI:10.1016/j.jorganchem.2005.07.107

The performance of cross-metathesis reactions between acrylate esters and olefins catalyzed by Grubbs catalysts have been enhanced by the simple addition of p-cresol. For example, the efficiency of the cross metathesis reaction between methyl acrylate and

Full text of DOI:10.1016/j.jorganchem.2005.07.107

Journal of Organometallic Chemistry 690 (2005) 5863–5866
www.elsevier.com/locate/jorganchem
Improved cross-metathesis of acrylate esters catalyzed
by 2nd generation ruthenium carbene complexes
*
Grant S. Forman , Robert P. Tooze
Sasol Technology Research Laboratory, Sasol Technology (UK) Limited, Purdie Building, North Haugh, St. Andrews, Fife KY16 9ST, Scotland, UK
Received 26 May 2005; accepted 15 July 2005
Available online 1 September 2005
Abstract
The performance of cross-metathesis reactions between acrylate esters and olefins catalyzed by Grubbs catalysts have been
enhanced by the simple addition of p-cresol. For example, the efficiency of the cross metathesis reaction between methyl acrylate
and 1-decene catalyzed by 2 was significantly increased by addition of p-cresol to the reaction mixture, resulting in increased product
yields and E/Z ratios.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Cross-metathesis; Grubbs catalyst; Acrylate; Carbene
N-heterocyclic carbenes (NHCs) have emerged as an
important class of ligands for transition metals and are
beginning to play a role in transition metal catalysis
[1]. Nucleophilic carbene ligands are neutral two-elec-
tron donor ligands that have a negligible tendency to-
wards p-back bonding, and as such they can be viewed
as alternatives to phosphine ligands [2]. When com-
plexed to middle or late transition metals, NHCÕs are
relatively robust ligands in homogeneous catalysis [3],
although reductive elimination of transition metal car-
bene complexes containing hydrocarbyl groups to form
hydrocarbylimidazolium salts is also well documented
[4]. A range of very active NHC containing catalysts
have been reported in recent years [5a–i], the most
prominent example being Grubbs 2nd generation olefin
metathesis catalysts (2). Until the advent of this latter
catalyst, the preparation of substituted olefins contain-
ing an electron-withdrawing functionality, such as
a,b-unsaturated carbonyl compounds, through cross-
metathesis (CM) with terminal alkenes was a difficult
task. These limitations were dramatically alleviated with
the sterically demanding NHC containing complex 2
[6,7]. Despite these advances, many CM reactions be-
tween substituted olefins and terminal olefins require
high catalyst loadings, and typically 1–5 mol.% catalyst
has been employed in these reactions [7]. In these in-
stances only 20–100 catalyst turnovers are possible,
meaning the true efficiency of Ru–carbene 2 in these
reactions remains uncertain.
N
N
PCy3
Cl
Cl
Ru
Ru
Ph
Ph
Cl
Cl
PCy3
PCy3
1
2
We recently reported the addition of relatively low
concentrations of phenol had a significant beneficial ef-
fect on self-metathesis (SM) reactions catalyzed by the
1st generation catalyst 1. In addition, significant
improvements in catalyst performance were observed
*
Corresponding author. Tel.: +44 1334 460938; fax: +44 1334
460939.
E-mail address: grant.forman@uk.sasol.com (G.S. Forman).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.107

5864
G.S. Forman, R.P. Tooze / Journal of Organometallic Chemistry 690 (2005) 5863–5866
when phenol was added to CM reactions of methyl acry-
late with 1-decene catalyzed by the NHC containing 2nd
generation catalyst 2 [8]. Herein we report the efficiency
of CM reactions between acrylate esters and terminal
olefins catalyzed by 2 is increased by phenol addition,
which allows reactions to be performed at lower catalyst
loadings, a natural consequence of which is more eco-
nomical processes.
The utility of olefin metathesis in forming new C–C
bonds has now been widely demonstrated in organic
synthesis [9]. In particular, CM of acrylate esters with
terminal olefins is a potential alternative to traditional
Wittig and Heck-type C–C bond formation reactions
[6,7]. Despite this, currently CM between a-carbonyl
compounds, such as acrylate esters, with terminal olefins
is relatively inefficient, requiring high catalyst loadings
and extended reaction times. Recently Grubbs defined
an acrylate ester as a Type II olefin, because it is capable
of undergoing slow homodimerization [7]. In our hands
however we were unable to detect (GC, GCMS) any
homodimers of methylacrylate in metathesis reactions
catalyzed by 2. This observation coupled with the lack
of reactivity of methyl fumarate and the known [6] insta-
bility of b-carbonyl-carbene species of the type
[Ru] = CH(CO)R suggests acrylate esters are more suit-
ably classified as Type III olefins.
In an effort to make the CM reaction of acrylate es-
ters with terminal olefins more efficient, a series of CM
reactions between acrylate esters and olefins involving
catalyst 2 and p-cresol (4-methylphenol) were per-
formed, and the results are shown in Table 1. In this in-
stance p-cresol (500 equivalents) was used for ease of
handling, but results were found to be similar when phe-
nol was used. Most reactions were performed at 50 °C
and employed 2 equivalents of acrylate as cross-partner
[10]. These results indicate that addition of phenol to
acrylate CM reactions catalyzed by 2 has a positive ef-
fect on end conversions. For example, with 0.025
mol.% of catalyst 2, the CM of methyl acrylate with 1-
decene was achieved in 96% conversion after only 2 h
Table 1
% Conv.
%
CM
CM
Cat
(Mol %)
Ratio
(Y:Z)
E/Z
with
E/Z
Ratio
Convc
Entry
Product(s)b
Partner Ya
Partner Za
Ratio
500 eq
cresolc
O
O
0.10d
0.05
2:1
2:1
2:1
2:1
2:1
2:1
2:1
2:1
24
56
58
47
43
24
16
8
13:1
15:1
100
97
96
71
95
66
57
40
35:1
26:1
1
2
3
4
5
6
7
8
6
6
6
6
O
O
O
6
O
O
O
O
6
O
0.025
0.00625
0.05
13:1
26:1
O
O
O
6
O
15:1
16:1
O
O
O
6
O
14:1
22:1
6
O
O
6
O
O
Ph
0.025
0.00625
0.025
>50:1
>50:1
>50:1
11:1
>50:1
>50:1
>50:1
25:1
Ph
O
O
O
O
O
O
Ph
O
Ph
Ph
O
Ph
O
O
O
O
O
O
6
O
7
7
0.0125
4:1
45
82
9
O
O
O
10:1
32:1
O
6
O
O
O
O
0.10
10:1
4:1
11
5e
>50:1
18
>50:1
13:1
10
11
O
O
O
O
0.025
11:1
24f
4
O
3
a Unless otherwise stated, reagents were purified bydistillation and/or passing through a neutral alumina column.
b Reactions performed at 50 ˚C for 2 h.
c Determined by GC/GCMS.
d Untreated 1-decene used.
e
29% cyclohexene formed.
66% cyclohexene formed.
f

G.S. Forman, R.P. Tooze / Journal of Organometallic Chemistry 690 (2005) 5863–5866
5865
with added p-cresol (500 equiv.), whereas only 58% con-
version was obtained in the absence of p-cresol (Entry
3). A similar effect was observed for the CM of methyl
acrylate with styrene (Entries 6–8). Additionally, the
efficiency of CM reactions between methyl oleate and
methyl acrylate was also increased with p-cresol addition
(Entry 9). In certain instances the combination of cata-
lyst 2 and phenols also appear to produce greater selec-
tivity towards the desired product. For example, for the
CM reaction between methyl oleate and methyl acrylate
(Entry 9), in the absence of added p-cresol, 45% conver-
sion was obtained with 87% selectivity towards end
products, while with added p-cresol, 82% conversion
was obtained with 95% selectivity towards products.
In most instances the E/Z ratio in reactions with
added p-cresol is often significantly higher than for reac-
tions with only catalyst 2. For example, with 0.025
mol.% of catalyst 2, the CM of methyl acrylate with 1-
decene produced an E/Z ratio of 26:1 after only 2 h with
added p-cresol (500 equiv.), whereas an E/Z ratio of
only 13:1 was obtained in the absence of p-cresol (Entry
3). In this context, these results can be rationalized in
terms of faster decomposition of catalyst 2 in the ab-
sence of p-cresol, preventing conversion of the sterically
more accessible cis-isomer to the trans-product [8].
Addition of p-cresol appears to impart greater catalyst
lifetime, which allows for continued reprocessing of
the cis-product to the trans-product.
In summary, this report has shown that the activity
and lifetime of CM reactions between acrylate esters
and terminal olefins catalyzed by 2 can be enhanced
by the simple addition of phenols such as p-cresol. This
system gives rise to higher conversions and greater E/Z
selectivity compared to reactions where only 2 is used,
and a natural consequence of the greater catalyst effi-
ciency described above is lower catalyst loadings and
more economical metathesis processes. Studies aimed
at further increasing the efficiency of Ru–carbene
metathesis catalysts and applying them to industrially
relevant applications are ongoing.
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1
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