Selective isomerization of 1-alkenes by binary metal carbonyl compounds

Article abstract of DOI:10.1016/j.poly.2008.02.026

An efficient and selective isomerization of 1-alkenes to their corresponding 2-alkenes is achieved by using binary metal carbonyls such as Ru₃(CO)₁₂ as catalysts. Possible mechanisms are discussed. Substituents on the 1-alkene have a significant effect on the isomerization.

Full text of DOI:10.1016/j.poly.2008.02.026

Polyhedron 27 (2008) 1911–1916
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Selective isomerization of 1-alkenes by binary metal carbonyl compounds
Akella Sivaramakrishna a, Paul Mushonga a, Ian L. Rogers a, Feng Zheng a, Raymond J. Haines a,
Ebbe Nordlander b, John R. Moss a,
*
a Department of Chemistry, University of Cape Town, Rondebosch, 7701 Cape Town, South Africa
b Department of Inorganic Chemistry, Center for Chemistry and Chemical Engineering, Lund University, Box 124, SE-221 00 Lund, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient and selective isomerization of 1-alkenes to their corresponding 2-alkenes is achieved by
using binary metal carbonyls such as Ru3(CO)12 as catalysts. Possible mechanisms are discussed. Substit-
uents on the 1-alkene have a significant effect on the isomerization.
Received 18 October 2007
Accepted 25 February 2008
Available online 9 April 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Isomerization
1-Alkenes
Metal carbonyls
Clusters
Substituent effects
1. Introduction
nistic pathways have been discussed previously [1,10,16]. Since
the isomerization plays an important role in the reactions of al-
1-Alkene isomerization is a key step in many industrial pro-
cesses, particularly in petrochemical refining, which involves vari-
ous heterogeneous and homogeneous catalysts [1–3]. Thus,
selective olefin isomerization under mild conditions is an impor-
tant goal. Double bond isomerization has been exploited as a de-
sired reaction in organic synthesis [4,5], SHOP technology [1],
polymerization reactions [6], DuPont butadiene to adiponitrile
synthesis [1], BASF synthesis of vitamin A1, asymmetric isomeriza-
tion [7], etc.
kenes catalyzed by transition metal complexes, we now describe
an investigation on this isomerization in the presence of binary
metal carbonyl clusters to evaluate the role of these compounds
for this alkene transformation.
2. Experimental
2.1. Materials and methods
It has been reported that organotitanium [8] or titanocene [9]
derivatives are useful catalysts for the selective isomerization of
1-alkenes to their corresponding 2-alkenes. Most of the catalysts
reported for alkene isomerization and subsequent hydrogenation
are metal carbonyl hydride clusters and their derivatives [10],
Ru(I) and Ru(II) phosphine substituted carbonyl derivatives [11]
and Fe3(CO)12 [12]. Photochemical catalysis of Ru3(CO)12 was also
briefly reported for 1-alkene isomerization reactions [13]. Com-
plexes such as M(CO)2(PPh3)3 (M = Fe, Ru) were found [14] to be
active, but air-sensitive, catalysts for alkene isomerization. Olefin
isomerization was also observed with some metathesis catalysts,
sometimes prior to olefin metathesis [15]. Depending upon the
particular catalyst involved, the isomerization can occur by either
a metal hydride addition–elimination mechanism or by a mecha-
nism involving a p-allyl metal hydride intermediate. These mecha-
All reactions and manipulations were carried out under an inert
atmosphere of dry nitrogen using standard Schlenk and vacuum-
line techniques. Commercially available solvents were distilled
from Na metal/benzophenone ketyl before use.
2.2. Nuclear magnetic resonance spectroscopy
1H, 13C and 31P NMR spectra were recorded on a Bruker DMX-
400 spectrometer and all 1H chemical shifts are reported relative
to the residual proton resonance in deuterated solvents (all at
298 K).
2.3. Elemental analysis
Microanalyses were conducted with a Thermo Flash 1112 Series
CHNS-O Elemental Analyzer instrument.
* Corresponding author. Tel.: +27 21 650 2535; fax: +27 21 689 7499.
E-mail address: John.Moss@uct.ac.za (J.R. Moss).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.02.026

1912
A. Sivaramakrishna et al. / Polyhedron 27 (2008) 1911–1916
Table 2
2.4. GC–MS analysis
Various catalysts tested for the isomerization of 1-pentene to 2-pentene
GC analyses were carried out using a Varian 3900 gas chro-
matograph equipped with an FID and a 30 m ꢀ 0.32 mm CP-
Wax 52 CB column (0.25 lm film thickness). The carrier gas
was helium at 5.0 psi. The oven was programmed to hold at
32 °C for 4 min and then to ramp to 200 °C at 10°/min and hold
5 min. GC–MS analyses for peak identification were performed
using an Agilent 5973 gas chromatograph equipped with MSD
and a 60 m ꢀ 0.25 mm Rtx-1 column (0.5 lm film thickness).
The carrier gas was helium at 0.9 ml/min. The oven was pro-
grammed to hold at 50 °C for 2 min and then ramp to 250 °C
at 10°/min and hold for 8 min.
Catalyst used
Experimental conditions:
80 °C, toluene
% Conversion (a mixture
of cis and trans isomers)
1
2
3
4
5
6
3 h
8 h
10 d
5 d
10 d
10 d
100
100a
66
100
35
0
Os3(CO)12
Re2(CO)10
Co2(CO)8
W(CO)6
10 d
10 d
3 d
10 d
10 d
100
4
100a
0
Mo(CO)6
80a
a
2.5. Infrared spectroscopy
Catalyst decomposed under these experimental conditions.
Infrared spectra were recorded on a Perkin–Elmer Spectrum
One FT-IR spectrometer.
H, 2.21. Found: C, 38.69; H, 2.32%. IR data in CH2Cl2: m (cmꢁ1
)
2060 (s), 2045 (s), 2028 (broad), 2012 (broad), 1966 (broad).
Ru3(CO)9[PPh2CH2-CH2CH@CHCH3]3 was also isolated as a sec-
ondary product (12%) from the orange yellow band on prepara-
tive TLC. Anal. Calc. for C60H57O9P3Ru3: C, 54.67; H, 4.36. Found:
C, 55.04; H, 4.21%.
2.6. Materials
Ru3(CO)12 (1), Os3(CO)12, Re2(CO)10, Co2(CO)8, Mo(CO)6, and
W(CO)6 were purchased from Aldrich and used without further
purification. PPh2CH2CH2CH2CH@CH2 [17] was prepared as previ-
ously described. The solvents were commercially available and dis-
tilled from dark purple solutions of sodium/benzophenone ketyl.
1H and 31P NMR spectra were recorded on a Bruker DMX-400 spec-
trometer and all 1H chemical shifts are reported relative to the
residual proton resonance in the deuterated solvents. The com-
pounds 3–6 were prepared by the reported literature procedures
[18,19].
2.6.2. General procedure for 1-alkene isomerization reactions
All the isomerization reactions were carried out in sealed John-
Young NMR tubes (Aldrich). The reaction mixture containing sol-
vent, catalyst and olefin was cooled, evacuated and heated in an
oil bath thermostated at the prescribed temperature ( 1 °C). The
reaction progress was monitored by 1H NMR and then the products
analyzed by GC and GC–MS to identify and quantify the final or-
ganic products. The results of these catalytic experiments are re-
ported in Tables 1 and 2.
2.6.1. Ru3(CO)11[PPh2(CH2CH2CH@CHCH3)] (2)
In a Schlenk flask, PPh2CH2CH2CH2CH@CH2 (248 mg, 0.975
mmol) was added to Ru3(CO)12 (750 mg, 1.173 mmol) in 50 ml
acetonitrile. Slowly, the colour of the solution changed to intense
red. The solution was heated 80 °C for 72 h under constant stir-
ring to yield 2 as a product and the reaction was monitored by
31P NMR. The product was isolated by using preparative TLC
using dichloromethane as eluent. The major red band was ex-
tracted using dichloromethane. After removing the solvent, the
product was dried under high vacuum for 6 h to give red crystals
of 2; m.p. 156–158 °C (with decomposition); yield 47%; 1H NMR
d 7.28–7.46 (m, 10H, Ph); 5.34–5.39 (m, 2H, @CH); 2.41–2.54 (m,
2H, P–CH2); 1.81–1.99 (m, 2H, CH2); 2.17 (s, 3H, CH3); 31P{1H}
26.70 (s) and 27.36 (s); Anal. Calc. for C28H19O11PRu3: C, 38.85;
3. Results and discussion
Ru3(CO)12 (1), Ru3(CO)11L [L = (2-pentenyl)diphenylphosphine]
(2), [Ru(CO)2(MeCO2)2]n (3), [Ru2(CO)4(l-MeCO2)2(CH3CN)2] (4),
[Ru2(CO)4(l-MeCO2)(l-dppm)2][PF6]
(5)
[where
dppm =
(6),
bis(diphenylphosphino)methane],
[Os2(CO)6(l-MeCO2)2]
Os3(CO)12, Re2(CO)10, Co2(CO)8, Mo(CO)6, and W(CO)6 were all
tested as homogeneous catalysts for the 1-alkene isomerization
reactions (see Tables 1 and 2). Complexes 1 and 2 are catalyti-
cally active for olefin isomerization in a broad range of solvents
(see below) and at temperatures above 60 °C. Our attempts to
carry out the isomerization reaction of (1-pentenyl)diphenyl-
phosphine with 1 on heating in order to examine the effect of
the bulky phosphine group on the alkenyl chain, yielded 2 (Ru3
(CO)11L) (Chart 1). Compound 2 was obtained as an intense red
solid in good yield after the reaction of Ru3(CO)12with (1-pente-
nyl)diphenylphosphine [17] [L] as ligand in a molar ratio of 1:1.
It was recrystallized from CH2Cl2/diethylether and was charac-
terized spectroscopically and analytically. The studies with the
phosphine derivative Ru3(CO)11L (2) were carried out in order
to explore the influence of this functionalized phosphine ligand
on the catalytic activity in the isomerization reactions. The other
compounds (3–5) were synthesized from Ru3(CO)12 (1) using lit-
erature procedures [19] to explore the catalytic activity towards
1-alkene isomerization reactions. In a similar way, compound 6
was obtained from the reaction of acetic acid with Os3(CO)12
[20]. Various 1-alkenes were tested for the isomerization reac-
tions to give their corresponding 2-alkenes as shown in Eq. (1).
In a similar way, the isomerization of 1,5-hexadiene yielded
2,4-hexadiene quantitatively (Eq. (2)).
Table 1
Various substrates tested for the isomerization with Ru3(CO)12
Substrate
Experimental
conditions
Product
% Conversion
(a mixture of cis
and trans isomers)a
1-Pentene
1-Hexene
1-Heptene
1-Octene
3 h, 80 °C
3 h, 80 °C
3 h, 80 °C
3 h, 80 °C
3 h, 80 °C
10 d, 80 °C
16 d, 80 °C
3 h, 80 °C
6 h, 80 °C
6 h, 80 °C
2-pentene
2-hexene
2-heptene
2-octene
100
100
100
100
100
84
1-Decene
2-decene
5-Bromo-1-pentene
6-Chloro-1-hexene
1,5-Hexadiene
5-Hexen-1-ol
7-Hexene-1,2-diol
5-bromo-2-pentene
6-chloro-2-hexene
2,4-hexadiene
4-hexene-1-ol
6-hexene-1,2-diol
93
100
100
100
a
Analyzed by GC–MS.

A. Sivaramakrishna et al. / Polyhedron 27 (2008) 1911–1916
1913
Ph2P
Ru
Ru
Ru
Ru
Ru
Ru
1
2
+
H2
C
Me
Me
Ph2P
PPh2
CO
CO
-
O
O
O
O
(PF6)
OC Ru
O
Ru CO
CO
CO
n
CO
CO
O
Ru
Ru
MeCN Ru
Ru NCMe
Me
PPh2
Ph2P
O
O
O
O
C
CO
CO
CO
CO
Me
H2
H3C
3
4
5
Me
O
O
CO
CO
OC Os
O
Os CO
O
CO
CO
H3C
6
Chart 1. Catalysts used in olefin isomerization reactions.
polymeric compound 3 is decreased considerably relative to 4,
which might be due to the insoluble nature of the compound in
X
1
X
n
n
Toluene,
ð1Þ
2-alkene
(cis- + trans- isomers)
1-alkene
a) when X = CH3, n = 1-6
X = CH3, Cl or Br, OH,
PPh2
100%
aromatic solvents. In contrast, the compound 4, which was derived
from 3 by reaction with acetonitrile, catalyzes the isomerization of
1-pentene to 2-pentene quantitatively. The other metal carbonyl
compounds such as Os3(CO)12, Mo(CO)6 and Co2(CO)8 showed sig-
nificant activity under similar experimental conditions. Ru3(CO)12
however showed the best results (the turnover number for
Ru3(CO)12 was found to be 10000 hꢁ1) among the other catalysts,
while the activity is quite dependent on the nature of the metal
at 80 °C. The rate of the isomerization reaction decreases along
the series: Ru3(CO)12 > Os3(CO)12 > Mo(CO)6 > Re2(CO)10 > W(CO)6
(see Table 2). It was also found that the di-ruthenium compounds
L2PtRu2(CO)8 [20] and [Cp*Ru(CO)2]2 were inactive towards the
olefin isomerization reactions, which could be due to the steric,
and electronic properties of these compounds. At high tempera-
tures, the isomerization of 1-alkenes to their internal alkenes is
1
Toluene,
2, 4 -hexadiene
100%
1, 5 -hexadiene
ð2Þ
Although we find that many mononuclear, binuclear and trinu-
clear metal carbonyl compounds can, in fact, isomerize olefins, sig-
nificant differences in the activity of the various complexes are
formed (see Table 2): W(CO)6 and 6 are mainly inactive in isomer-
ization reactions of aliphatic alkenes and dienes. In case of
Re2(CO)10, the catalytic activity was extremely low. The activity
of 5 was decreased relative to 4 due to the steric as well as chelat-
ing nature of diphosphine ligands. The catalytic activity of the

1914
A. Sivaramakrishna et al. / Polyhedron 27 (2008) 1911–1916
feasible with the other metal carbonyl compounds but the catalyst
recovery is difficult. In all of these isomerizations of 1-olefins, the
trans-2-olefin is the major product. Tables 1 and 2 summarize
the results of the isomerization of aliphatic olefins. We find
that the order of activity of the various catalysts we tested is as fol-
lows: Ru3(CO)12 > Ru3(CO)11L > Co2(CO)8 > Os3(CO)12 > Mo(CO)6 ꢂ
Re2(CO)10 ꢂ W(CO)6.
(vs), 2028 (vs) and 1985 (vw) cmꢁ1]. Most striking bands are at
1825 (m) and 1795 (m) which could be due to bridging carbonyls
of a ruthenium intermediate such as B in Scheme 1a.
A key step in the alkene isomerization reaction could be the dis-
sociation of a CO ligand and olefin coordination to give the inter-
mediate species A. That CO dissociation occurs during the
isomerization reaction was supported by the observation that 1-al-
kene isomerization is inhibited when the reaction was carried out
under a CO atmosphere. The oxidative addition of a C–H bond to
the metal centre could lead to the formation of g3-allyl metal hy-
dride complex B which then decomposes to give the 2-alkene
product and 1 as shown in Scheme 1. An alternative mechanism
could involve mononuclear ruthenium species as shown in Scheme
1b. Both the mechanisms could be expected to be inhibited by CO,
as is found.
3.1. Mechanistic pathways
We have carried out a number of experiments to try and shed
light on the mechanism of these isomerization reactions. Thus,
we have carried out IR and NMR spectral studies of the reaction
of Ru3(CO)12 with 1-octene in CDCl3. We have made attempts using
1H NMR to observe an g3-allyl ruthenium hydride intermediate B
in Scheme 1a, which is an established mechanism for other metal
complexes [10,16]. However, these attempts were unsuccessful.
We also used IR to obtain new information thus after heating the
reaction mixture for 20 min at 80 °C, the reaction mixture was
cooled and the IR spectrum was recorded [bands were now seen
at 2100 (vw), 2061 (vs), 2028 (vs), 1985 (vw), 1825 (m), 1795
(m), 1652 (vs) cmꢁ1] (Fig. 1). Thus, new bands are seen which are
not present in Ru3(CO)12 [the bands of Ru3(CO)12 are: m(CO) 2061
We could not detect any ruthenium hydride species in the 1H
NMR spectra of the reaction mixtures even at temperatures up to
90 °C in CD3C6D5. After heating at 80 °C for 3 h, the reaction was
complete as the IR showed the disappearance of the new bands
of the intermediate and indicated that the trinuclear ruthenium
catalyst is restored (see Fig. 1). The GC analyses of the solution at
the end of reaction showed mainly, the presence of cis-oct-2-ene
(14%) and trans-oct-2-ene (86%). As the isomerization activity is
high at high temperatures (>120 °C), there is a significant increase
in the formation of oct-3-ene (>5%). The reactions of olefins in the
presence of other catalysts showed similar trends in the isomeriza-
tion reactions and analogous behaviour was noticed with 2, which
could go via a similar mechanism. These suggestions are also in
agreement with previous studies reported where Fe3(CO)12 was
used as catalyst [12].
(a) Involving Ru3 cluster
1-alkene
(OC)4Ru
Ru(CO)4
(CO)3Ru
Ru(CO)4
-CO
4. Factors affecting the isomerization reaction
Ru
Ru
(CO)4
(CO)4
4.1. Solvent effects
1
- 2-alkene
O
The factors influencing the rate of the alkene isomerization
reactions are the solvent system, temperature, remote substituent
on the alkene, and nature of catalyst used. The polarity of the sol-
vent using either methanol or hexane did not affect the isomeriza-
tion reaction. However, catalyst recovery was not possible when
halogenated solvents were used as these solvents make the cata-
lyst inactive. In the other solvent systems, the catalyst recovery
was more than 60% in case of Ru3(CO)12. The absence of solvent
increased the rate of reaction significantly, for example the isomer-
ization of 1-octene to 2-octene occurred quantitatively within 60 s
at 80 °C, in the presence of Ru3(CO)12. It was also found that these
reactions do not involve heterogeneous catalysts as the dilute
solutions (where catalytic amount of catalysts used) also perform
the isomerization reaction in a similar way to concentrated
solutions.
A
-CO
+ 2CO
CO
H
O
Ru
OC
OC
CO
Ru
Ru
CO
OC
CO
O
B
(b) Involving mononuclear Ru fragment
1-alkene
3 Ru(CO)4
4.2. Effect of temperature
Ru(CO)4
1
A series of experiments on the alkene isomerization reactions,
using Ru3(CO)12, as a function of temperature were carried out.
The ideal temperature range was found to be 80–100 °C. When
the temperature was >120 °C, the catalyst recovery was not possi-
ble even though the rate of reaction was increased dramatically.
With compound 2, the recovery of the catalyst was not successful.
It was observed however that this monosubstituted phosphine
derivative at high temperatures was converted to the stable
tris(phosphine) derivative, i.e. Ru3(CO)9(PPh2R)3. Also at high
temperatures (>150 °C), the formation of traces of 3-alkenes is
observed with the longer alkenyl chain, i.e. PC7.
+ CO
-CO
- 2-alkene
Ru(CO)3
H
Scheme 1. Two possible reaction mechanisms for the catalytic 1-alkene isomer-
ization. (a) Involving Ru3 cluster. (b) Involving mononuclear Ru fragment.

A. Sivaramakrishna et al. / Polyhedron 27 (2008) 1911–1916
1915
107.7
100
90
80
%T
(a)
2027.11
70
130.0
120
2059.32
62.2
110
2000
1900
1800
1700
1620.0
2200.0
-1
100
cm
90
80
70
60
50
40
%T
1986.00
(b)
1795.15
1826.10
30
20
10
1640.42
2028.14
2059.25
-5.0
104.8
104
2200.0
2100
2000
1950
1900
1850
cm
1800
1750
1700
1650
1580.0
-1
102
100
98
96
94
%T
1604.67
(c)
92
90
88
2029.44
2062.36
85.3
2220.0
2000
1900
1800
-1
1700
1600
1500.0
cm
Fig. 1. IR spectra showing the intermediate formed during the olefin isomerization (spectra recorded in CHCl3). (a) Ru3(CO)12. (b) The reaction mixture; after heating with 1-
pentene for 20 min at 80 °C. (c) After the complete isomerization reaction after 3 h at 80 °C.
4.3. Substituent effect
also possible that both pathways involving either CO loss or metal–
metal bond cleavage could operate for the cluster compounds (see
Scheme 1a and b).
Substituents on the alkenyl chains show a significant effect on
the isomerization reaction (see Table 1). When X = Cl, Br, the rate
of reaction decreases drastically and the formation of different iso-
meric species was also observed. This could be due to the electron
withdrawing effect of the halogens in the alkenyl chain and com-
peting reactions of catalysts with the haloalkenes. Mass spectral
studies of the reaction of 1 with 6-chloro-1-hexene indicated the
formation of [Ru(CO)3Cl]2 as one of the products. The presence of
hydroxyl groups (X = OH or (OH)2) did not have any significant ef-
fect on the isomerization (Eq. (1)). When X = PPh2, the rate of reac-
tion decreased sharply, which may be due to the steric bulk of the
phosphine as well as the phosphine coordinating to the metal and
positioning the alkenyl chain away from the cluster. However, on
long heating at 80 °C, in acetonitrile for 72 h the corresponding
complex 2 was formed. The 31P NMR spectrum showed two singlets
close together at d 26.70 and 27.36 for this compound which could
be due to the presence of possible alkene isomers of the compound.
4.5. Addition of excess ligands
As expected, the addition of excess ligands such as (diphenyl-
phosphino)methane (dppm) or acetonitrile to the reaction mixture
(Ru3(CO)12 and 1-pentene in toulene) resulted in a dramatic
decrease in isomerization activity. There was no isomerization
observed in the presence of excess of dppm ligand even after the
long heating at 80 °C. This is probably due to the phosphine ligand
blocking the coordinating sites at ruthenium. Similarly, the isomer-
ization was also retarded when the experiment was carried out un-
der an atmosphere of CO. This suggests that the dissociation of CO
from the metal carbonyls is a key step in these catalytic reactions.
5. Conclusions
Some ruthenium carbonyl cluster complexes are efficient and
selective catalysts for the isomerization of 1-alkenes to their corre-
sponding 2-alkenes under the mild conditions. The turnover num-
ber for the Ru3(CO)12 was found to be 10000 hꢁ1 and possible
mechanistic pathways for these isomerization reactions are dis-
cussed. Other metal carbonyl compounds also show some activity
for this reaction.
4.4. Effect of catalyst
While Ru3(CO)12 was found to be the most active catalyst (see
Table 2), some mononuclear carbonyls, notably Mo(CO)6, were also
found to be active. This may suggest that the first step in the mech-
anistic pathway can be CO loss (as for Mo(CO)6 or Ru3(CO)12). It is

1916
A. Sivaramakrishna et al. / Polyhedron 27 (2008) 1911–1916
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