Benign catalysis with iron: Unique selectivity in catalytic isomerization reactions of olefins

Article abstract of DOI:10.1002/cssc.201100404

The use of noble metal catalysts in homogeneous catalysis has been well established. Due to their price and limited availability, there is growing interest in the substitution of such precious metal complexes with readily available and bio-relevant catalysts. In particular, iron is a "rising star" in catalysis. Herein, we present a general and selective iron-catalyzed monoisomerization of olefins, which allows for the selective generation of 2-olefins. Typically, common metal complexes give mixtures of various internal olefins. Both bulk-scale terminal olefins and functionalized terminal olefins give the corresponding products under mild conditions in good to excellent yields. The proposed reaction mechanism was elucidated by in situ NMR studies and supported by DFT calculations and extended X-ray absorption fine structure (EXAFS) measurements.

Full text of DOI:10.1002/cssc.201100404

DOI: 10.1002/cssc.201100404
Benign Catalysis with Iron: Unique Selectivity in Catalytic
Isomerization Reactions of Olefins
[
a]
[a]
[a]
[b]
[a]
Reiko Jennerjahn, Ralf Jackstell, Irene Piras, Robert Franke, Haijun Jiao,
[c]
[a]
Matthias Bauer, and Matthias Beller*
The use of noble metal catalysts in homogeneous catalysis has
been well established. Due to their price and limited availabili-
ty, there is growing interest in the substitution of such pre-
cious metal complexes with readily available and bio-relevant
catalysts. In particular, iron is a “rising star” in catalysis. Herein,
we present a general and selective iron-catalyzed monoisome-
rization of olefins, which allows for the selective generation of
2-olefins. Typically, common metal complexes give mixtures of
various internal olefins. Both bulk-scale terminal olefins and
functionalized terminal olefins give the corresponding prod-
ucts under mild conditions in good to excellent yields. The
proposed reaction mechanism was elucidated by in situ NMR
studies and supported by DFT calculations and extended X-ray
absorption fine structure (EXAFS) measurements.
Introduction
[
20]
The development of organometallic catalysis has been a real
success story in the past decades, and the introduction of mo-
lecular-defined organometallic complexes has modernized or-
ganic synthesis and the industrial production of fine and bulk
Table 1). Other precious metal complexes based on palladi-
um or ruthenium isomerize 1-octene to similar mixtures
[23–27]
(Table 1).
As early as 1966, Frankel et al. described the iso-
[28]
merization activity of simple iron pentacarbonyl complexes.
[
1]
chemicals. Clearly, most of the work in organometallic cataly-
sis has been performed by applying noble metals based on
Again, mixtures of olefins were obtained. Further problems of
known isomerization catalysts are their limited functional
[2–5]
palladium, rhodium, iridium, and ruthenium complexes.
Due to economic constraints, limited availability, and some-
times sensitivity and toxicity of precious metal complexes,
there is growing interest in substituting such catalysts with
less expensive bio-relevant metals. In this respect, homogene-
ous catalysis with iron complexes offers a highly attractive sub-
stitute. Hence, this area has become one of the “hot topics” in
Table 1. Isomerization of 1-octene in the presence of different metal
complexes.
Isomer
Yield [%]
Rh
[
a]
[b]
[c]
[d]
[e]
Pd
Fe
Rh
Ru
2
11
28
2
44
36
18
16
36
33
15
4
68
24
4
2
37
43
18
[
6–17]
catalysis.
To date, most of the work in this field has demon-
strated that iron complexes can be used in the same way as
noble metal catalysts. Although this is an important goal, little
is known about the development of organometallic catalysis
using iron complexes. Herein, we demonstrate that a trinuclear
iron carbonyl cluster in the presence of water enables a highly
selective olefin isomerization reaction.
59
[a] 0.5 mol% [Rh(acac)(CO)
.8 mol% 2-(dicyclohexyl-phosphino)-1-[2-(dicyclohexylphosphino)naph-
thalen-1-yl]-1H-pyrrole, 0.8 mol% p-toluenesulfonic acid, 1 h. [c] 5 mol%
2 2 2
], 0.5 MPa H , 30 min. [b] 0.2 mol% [Pd(acac) ],
0
[
[
Fe(CO)
5
], 1 h, 1908C. [d] 1 mol% [HRhCO(PPh
], 6 h, 1008C.
3 3
) ], 6 h, 1008C. [e] 1 mol%
H
2
RuCO(PPh )
3 3
Olefin functionalization is a fundamental catalytic process
that is important for organic synthesis on the laboratory scale,
[
18,19]
as well as for large-scale industrial applications.
ple, in the production of plasticizers and detergent alcohols
Shell higher olefin process), isomerization processes play a cen-
For exam-
[a] Dr. R. Jennerjahn, Dr. R. Jackstell, Dr. I. Piras, Dr. H. Jiao, Prof. M. Beller
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
Fax: (+49)381-1281-5000
(
tral role on the million-ton scale. Additionally, these methods
are often used in the pharmaceutical and fragrance industry. In
general, classic acidic and organometallic isomerization cata-
lysts yield the thermodynamically most stable internal prod-
ucts. Hence, mixtures of olefins are obtained, and the selective
functionalization of such mixtures continues to be a highly
E-mail: matthias.beller@catalysis.de
[
b] Prof. Dr. R. Franke
Evonik Oxeno GmbH, 45772 Marl (Germany)
and Lehrstuhl fꢀr Theoretische Chemie
Ruhr-Universitꢁt Bochum, 44780 Bochum (Germany)
[
20–22]
[c] Dr. M. Bauer
challenging task.
For example, in the presence of a hydro-
Fachbereich Chemie, Technische Universitꢁt Kaiserslautern
Erwin-Schrçdinger-Straße 52, 67663 Kaiserslautern (Germany)
gen-activated [Rh(acac)(CO) ] (acac=acetylacetone) complex,
2
1
-octene provides an olefin mixture consisting of 1-octene
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100404.
(2%), 2-octenes (11%), 3-octenes (28%), and 4-octenes (59%,
7
34
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 734 – 739

Benign Catalytic Isomerization of Olefins with Iron
group tolerance and the necessity to use high pressure and/or
temperature. In this respect, the development of novel rutheni-
Table 2. Isomerization of 1-octene in the presence of different iron
complexes.
[a]
[29–34]
um complexes that perform isomerization are noteworthy.
Based on our general interest in catalysis with iron carbonyl
[
b]
Entry
Iron source
Additive
t
Yield of octene [%]
[
35–37]
[h]
1-
2-
3-
4-
complexes,
we recently investigated the iron-catalyzed hy-
1
6
1
6
1
5
1
6
1
6
1
6
6
6
1
6
6
6
6
27.7
16.1
4
1.7
4.2
2.6
4.2
2.8
5.6
2.7
6.6
2.7
95.1
99.7
2.7
31
36
28.4
33.1
2.8
5.3
2.8
10.6
7.8
11.3
2.1
7.3
3.3
8.2
0.1
0.1
3.4
18.7
0.1
0.1
13.2
15
0.4
0.5
0.3
0.8
1.3
1.6
0.2
0.5
0.7
0.8
0
droformylation of 1-octene with synthesis gas (CO/H =1:1)
[d]
2
1
2
3
4
5
6
[Fe (CO) ]
–
3
3
3
12
under various conditions. Unfortunately, only low yields in al-
dehydes (<12%) and mixtures of octenes were obtained. To
our surprise, in the absence of carbon monoxide, selective iso-
merization to 2-octene occurred (Figure 1). Apart from work in
the early days of transition-metal catalysis, iron has been rarely
92.8
92.5
92.7
85.9
86.7
84.4
92.0
89.6
89.4
88.3
4.8
[Fe
[Fe
(CO)12
]
]
KOH
NaOH
KCl
[
[
c]
c]
(CO)12
[Fe (CO) ]
3
12
[
38,39]
investigated for catalytic isomerization processes.
Notably,
[Fe(CO)
5
]
KOH
KOH
Periasamy et al. described the selective conversion of aliphatic
olefins in the presence of stoichiometric amounts of iron car-
bonyl complexes and an excess of copper salts (2 equiv) or
2 9
[Fe (CO) ]
[
d]
7
8
Na
Na
2
[Fe(CO)
[Fe(CO)
4
4
]
]
[
40]
1,2-dibromoethane.
In addition, iron carbonyl complexes
[c]
2
2
HCl
0.2
0
have been used for the isomerization of alkenols to alde-
93.7
77.4
0.2
0.2
0.2
0.1
1.8
0
0
0
[
d]
9
Na
[Fe(CO)
4
]
2
H O
[
41]
2.1
hydes.
More recently, Harris et al. have demonstrated in
10
[Fe(BF
[CpFe(CO)
Na [Fe(CN)
4
)
2
]
99.7
99.7
99.7
detail the mechanism of photoactivating iron carbonyl com-
1
1
2
]
[
42]
plexes in the presence of alkenes. Nevertheless, a general
and selective iron-catalyzed olefin isomerization methodology
has yet to be formulated, and the compatibility of functional
groups has not yet been explored.
12
2
5
NO]
0.1
[
[
a] [Fe
3
(CO)12] (1 mol%), diglyme (2 mL), aq. KOH (0.16 mL, 3n), 808C.
b] Determined by performing GC. [c] Additive used instead of KOH.
[d] Without aq. KOH.
trum, d=À8.95 ppm) obtained by hydrolysis of Collman’s re-
1
agent was slowly converted into the trinuclear anion ( H NMR
[45]
spectrum, d=À14.92 ppm), which showed activity compara-
ble with the catalyst system generated in situ (Table 2, entry 9).
When the isomerization process was started with [Fe(CO) ], the
5
À
same monomeric iron anion [HFe(CO)4] was formed upon re-
action with 3 equiv of NaOH or KOH. After a few hours, this
species was converted to the same red-colored trinuclear
[43,44]
anion.
When using [Fe (CO) ] instead of [Fe(CO) ], the reac-
2 9 5
tion solution turned red after 2 h. Interestingly, [Fe (CO) ] pro-
3
12
Figure 1. Selective isomerization of 1-octene in the presence of [Fe
O/KOH. ^: 1-Octene;
: 2-octene; ~: 3-octene; *: 4-octene.
3
(CO)12]/
duced the red active-catalyst solution after a few minutes. This
explains the higher activity of this precatalyst relative to all the
other iron carbonyl complexes. No isomerization was achieved
H
2
&
À
using [Fe(BF ) ] [CpFe(CO)2] (Cp=cyclopentadienyl) and
4
2 ,
Results and Discussion
Na [Fe(CN) NO] (Table 2, entries 10–12). Investigations into the
2
5
We initially studied the reactivity of different iron hydride car-
bonyl complexes with 1-octene as the model system (Table 2).
For the selective generation of hydride species through
formation of the active catalyst with the aid of deuterium-la-
beling studies showed that the active iron hydride complex
was formed by a classic Hieber base reaction and not by pho-
toactivation. Hence, using a stoichiometric amount of [Fe(CO)5]
[
43,44]
a water-gas shift reaction,
adding water and base proved
to be essential. Using only iron carbonyl complexes without
water and base resulted in a mixture of internal olefins
in the presence of KOD and D O led to approximately 30%
2
deuteration of 2-octene (see the Supporting Information). This
partial deuteration can be explained by the conversion of
(Table 2, entry 1). However, applying [Fe(CO) ], [Fe (CO) ], or
5 2 9
À
[
Fe (CO) ] in the presence of water and KOH immediately cre-
100 mol% iron pentacarbonyl to 33 mol% of [DFe (CO) ] .
3
12
3
x
À
ated an active isomerization species, and selective isomeriza-
tion of the terminal olefin to the corresponding 2-octene took
place within 1 h in>92% yield (Table 2, entry 2). Then, for the
next 5 h the composition of this mixture basically did not
change. Using Collman’s reagent or a combination of HCl and
Collman’s reagent, which is known to form the dihydride com-
plex [H Fe(CO) ], no activity at all was observed (Table 2, en-
After one catalytic isomerization cycle, [HFe (CO) ] was gener-
3 x
ated by b-hydride elimination from the olefin, and further
deuteration did not occur.
A remarkable feature of this reaction is the selective mono-
isomerization process. At first glance, there is no reason why
the isomerization should stop at the b-position. Thermody-
namic control would lead to a mixture of internal olefins simi-
lar to that of entry 1 in Table 2. To investigate the nature of the
selective a-to-b isomerization, we performed reactions with
2
4
tries 7–8). However, upon standing in solution at room temper-
À
1
ature, the monohydride complex [HFe(CO)4] ( H NMR spec-
ChemSusChem 2012, 5, 734 – 739
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
735

M. Beller et al.
À1
stereochemically pure 2-octenes and a E/Z mixture of 2-octene
see the Supporting Information, Table S2). The different be-
havior of these olefins is shown in Scheme 1. Beginning with
5.4 kcalmol , respectively. Dissociation of these adducts gave
the 2-butene species and the active catalytic complex
(
À
[HFe (CO) ] . Both Fe(2-butyl)-1 and Fe(2-butyl)-2 have a triply
3
8
bridging CO ligand over three metal centers and a terminal CO
ligand on the face site of the three iron centers. Based on the
exploration of the energy hypersurface in the neighborhood of
these stationary points, we presume that these specific geo-
metric configurations cause rotation of both H and CH groups
3
without significant steric hindrance. For alkyl groups larger
than CH (olefins with a double bond located more internally
3
3 2
Scheme 1. Isomerization of Z- and E-2-octene with [Fe (CO)12]/H O/KOH.
than position 2), however, this steric hindrance becomes signif-
icant and suppresses the rotation of these alkyl groups. The
observed selectivity was determined by using the 2-butyl rota-
tion between Fe(2-butyl)-1 and Fe(2-butyl)-2. Because the rota-
tion of 2-butyl in TS5 needs a higher barrier than in TS4, the
transformation from Fe(2-butyl)-1 to Fe(2-butyl)-2 is easier
than the back transformation. As for the equilibrium between
Fe(2-butyl)-1 and Fe(2-butyl)-2, the expected isomer ratio
based on calculations should be about 91% for 2-E-butene
and 9% for 1-butene. This computed isomer ratio agrees rea-
sonably well with the observed isomer distribution (Table 2,
entry 1).
pure E-2-octene, almost no conversion was observed. However,
in the case of the E/Z mixture with 79% Z-olefin, selective con-
version took place from Z-2-octene into E-2-octene. Apparently,
the selectivity for the terminal olefin is based on the steric
demand of the iron complex. To understand this peculiar be-
havior in more detail, we carried out a comprehensive DFT
study into the reaction mechanism using 1-butene as the
model olefin. On the basis of the observed change in reaction
rate and solution color with different precatalysts in our experi-
mental work, we assumed that the active catalyst should be
À
triiron carbonyl clusters, [HFe (CO) ] . Nevertheless, our com-
In summary, the proposed isomerization mechanism starts
from the 1-alkene complex and forms either 2-E-alkene or 2-Z-
alkene. The important step is the formation of Fe(2-butyl)-
1 with a CH agostic interaction after H–Fe insertion. Because
the iron center contains only one free coordination site, the
3
x
putational search for the catalytically active species comprised
monoiron and diiron carbonyl clusters, as well as complexes
À
such as [HFe (CO) ] with x=11–8.
3
x
À
8
Unexpectedly, only in the case of [HFe (CO) ] the free
3
energy and enthalpy of binding
of
1-butene were not positive:
Figure 2 shows the calculated
energy profile and selected opti-
mized structures for the pro-
posed reaction mechanism. The
binding energy of 1-butene, 2-Z-
butene, and 2-E-butene is À12.0,
À1
À10.7, and À10.5 kcalmol , re-
spectively. The H–Fe insertion
into the 1-butene double bond
starting from HFe(1-butene) via
TS1 corresponds to a barrier of
À1
1
0.6 kcalmol . Our hitherto con-
ducted calculations of inter-
mediate and transition-state-like
structures indicate that the reor-
ientation in the Fe(2-butyl)-
1
complex towards the Fe(2-
butyl)-2 complex proceeded via
two adjacent transition states.
This type of pathway is known
as a two-step no-intermediate
mechanism and has been report-
ed for isomerization reactions
[
46]
amongst others.
between Fe(2-butyl)-2
butene adducts HFe(2-E-butene)
The barrier
and
and HFe(2-Z-butene) is 5.8 and Figure 2. Energy profiles (associated optimized structures are shown in the Supporting Information).
7
36
www.chemsuschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2012, 5, 734 – 739

Benign Catalytic Isomerization of Olefins with Iron
coordinated alkyl group has to
Table 3. Results of fitting of experimental EXAFS data with theoretical models.
rotate for isomerization. The de-
cisive factor is the detachment
of the CÀH agostic interaction
with subsequent rotation over
the sterically demanding triply
bridged CO. This rotation is the
governing force for 2-alkene se-
[
a]
[b]
[c]
[d]
[e]
4[f]
Sample
Abs–Bs
N(Bs)
R(Abs–Bs)
ꢁ]
s(Abs–Bs)
E
f
FIꢂ10
À1
[
[ꢁ
]
[eV]
[
g]
[Fe (CO) ] (solid)
3
3
3
12
FeÀC
4.7
1.81Æ0.02
2.56Æ0.02
1.78Æ0.02
2.53Æ0.02
1.79Æ0.02
2.52Æ0.02
0.063Æ0.006
0.089Æ0.018
0.050Æ0.005
0.077Æ0.015
0.055Æ0.006
0.074Æ0.015
6.86
14.51
9.48
15.98
6.97
16.78
3.3
11.1
4.9
8.0
6.6
[
g]
FeÀFe
FeÀC
2
[
h]
[
Fe
Fe
(CO)12] (1 mol%)
4.9Æ0.5
1.8Æ0.4
5.0Æ0.5
1.7Æ0.3
FeÀFe
FeÀC
[
(CO)12] (1 mol%)
[
i]
lectivity: a small CH group can
and 1-octene
FeÀFe
7.5
3
rotate in one direction [Fe(2-
butyl)-1 to TS4], whereas the ro-
tation of a group larger than CH3
is more hindered by the triply
bridging CO ligand [Fe(2-butyl)-2
to TS5].
[
a] Abs=X-ray absorbing atom, Bs=backscattering atom. [b] Number of backscattering atoms. [c] Abs—Bs dis-
tance. [d] Debye–Waller-like factor. [e] Fermi energy to account for phase shift between experiment and theory.
[f] Quality of fit represented by the fit index (FI). [g] Value fixed to the crystallographic number. [h] In diglyme/
KOH at RT. [i] In diglyme/KOH at 808C.
Then, extended X-ray absorption fine structure (EXAFS) stud-
ies were performed to prove the existence of a trinuclear iron
cluster in solution. The obtained spectra for the pre-catalyst
stable calculated complexes HFe(1-butene) and HFe(2-E-
butene). Notably, no fragmentation of the Fe cluster is ob-
3
served because the FeÀFe coordination number remains at
two within the error bar. Within the framework of X-ray ab-
sorption experiments, mono- and diiron clusters could there-
fore also be excluded as catalysts.
[
Fe (CO) ], and the progress of its reaction within 30 min in di-
3 12
glyme/KOH at room temperature are presented in the Sup-
porting Information (Figure S2). FeÀC and FeÀFe distances ob-
[
47]
tained for [Fe (CO) ] agree well with the literature, although
Finally, we investigated the scope of this unusual mono-iso-
merization process. As shown in Table 4, various a-olefins are
transformed highly selectively to the corresponding b-olefins.
3
12
a weighting of FeÀC contribution to the shorter distances was
again observed.
[
48]
In the diglyme/KOH solution,
the FeÀC, coordination number
[
a]
Table 4. Iron-catalyzed isomerization: scope and limitations.
is slightly higher than in the
À
[b]
[
HFe CO ] complex of the calcu-
Entry
Substrate
Conv.
T
[8C]
t
[h]
Yield
[%]
Product
E/Z
3
8
[
b]
[
%]
lations (Table 3), but changes in
the electronic structure are indi-
cated by the reduced FeÀC and
FeÀFe bond distances and X-ray
absorption near edge structure
6
51
6
20
20
80
1
4
49
93
5
3.2
3.1
2.9
3.0
4.9
1
400
1
9
[
[
c]
2
3
4
97
97
87
100
100
100
6
1
6
96
97
81
c,d]
(XANES) spectra (see the Sup-
[c]
porting Information, Figure S3),
in which it is apparent that the
shape is modified. Oxidation-
state changes hardly affect the
[
c,d]
5
93
100
6
90
6.1
[
[
c]
c]
6
7
8
92
97
100
100
80
6
22
6
77
97
2.4
–
[
49]
edge position,
but the reso-
nance shift at about 7.13 keV is
in accordance with the forma-
tion of a negatively charged car-
>99
>99
18.9
>
99
88
80
100
100
6
6
22
>99
88
94
15.4
4.2
4.4
[
49]
[
[
e]
e]
bonyl cluster.
The so-called
9
94
pre-edge signal at the low-
energy side of the edge at
around 7.12 keV shows two sig-
nals for [Fe (CO) ], which is
10
>99
80
6
>99
2.3
>
99
99
99
80
100
100
6
6
22
>99
99
99
3.6
3.8
3.9
[e]
e]
3
12
11
12
[
a result of the nonequivalent
iron centers. In solution, only
one shoulder can be found, and
equivalent iron centers are as-
sumed. The FeÀFe coordination
number is proof of the intact Fe3
cluster core. In the course of iso-
merization of 1-octene, the FeÀC
coordination number of five is in
good agreement with the most
98
80
20
98
2.0
66
100
100
6
22
66
>99
3.8
3.4
[
f]
1
3
>99
[
3
a] Reactions were carried out at 808C with [Fe (CO)12] (1 mol%), diglyme (2 mL), aq. KOH (0.16 mL, 3n), and
substrate (5 mmol). [b] Determined by performing GC. [c] Reaction temperature was 1008C and 2.5 mmol of
substrate was used. [d] THF was used as solvent. [e] KCl used instead of KOH. [f] Yield and selectivity deter-
mined by performing NMR spectroscopy.
ChemSusChem 2012, 5, 734 – 739
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
737

M. Beller et al.
Most notably, functional groups such as halides, hydroxyl, and
amines are tolerated by the iron catalyst (Table 4, entries 3, 4,
involving volatile compounds were carried out in an autoclave.
1
Product characterization was performed by comparing their H and
13
C NMR and HR–MS spectra to those of authentic samples,.
10, and 12). In a number of cases, unprecedented isomeriza-
tion reactions take place. For example, the generation of 2-
butene-1-ol from 3-butene-1-ol is remarkable (Table 4, entry 4).
To the best of our knowledge, all known isomerization meth-
ods for this substrate led to butyraldehyde, which is the ther-
modynamically stable product of further isomerization and
subsequent tautomerization reactions. Similarly, N-benzyl-N-3-
butenylamine (Table 4, entry 6) leads to the corresponding en-
amine, which undergoes uncatalyzed tautomerization towards
the imine (Table 4, entry 5). 2-Vinylcyclohexane also gave the
less stable exocyclic olefin in excellent yield (97%; Table 4,
entry 7). Typically, using this type of substrates isomerization to
endocyclic olefins is achieved.
As an example, 1-hexadecene (2.5 mmol, 730 ml) was used follow-
1
ing the general procedure. H NMR (300 MHz, CDCl ): d=5.42–5.38
3
(2H, m), 2.03–1.91 (2H, m), 1.64–1.57 (3H, m), 1.25 (22H, br. s),
13
0
.89–0.85 ppm (3H, m); C NMR (75.5 MHz, CDCl ): d=131.7
3
(0.75CH), 130.9 (0.25CH), 124.5 (0.75CH), 123.6 (0.25CH), 32.6
(
CH ), 32 (CH ), 29.7–29.3 (9CH ), 22.7 (CH ), 18 (CH ), 14.1 ppm
2
2
2
2
3
+
(CH ); MS (70 eV, EI): m/z (%): 224 ([M] , 41.10), 111 (45.07), 97
3
(
82.35), 83 (80.15), 69 (81.61), 55 (100.00), 43 (55.00), 41 (57.40), 29
+
(18.29); HR-MS (EI): m/z calcd for C H : 224.24985 [M] ; found:
16
32
224.249942.
Acknowledgements
From a practical point of view, the licorice-fragrance anet-
hole was easily available from the isomerization reaction of es-
tragole in >99% yield (Table 4, entry 9). In this case, the reac-
M.B. wishes to thank the ꢂngstrømquelle Karlsruhe ANKA for pro-
vision of beamtime at the XAS-beamline (Dr. Stefan Mangold)
within the framework of proposal MR-128.
tion exclusively with [Fe(CO) ] gave an E/Z ratio of 6.7 at 1408C
5
[
50]
after 8 h.
Applying our system under milder conditions
(808C), we obtained full conversion after 1 h and observed
Keywords: alkenes
·
homogeneous catalysis
·
iron
·
a significantly improved E/Z ratio of 15.4 after 6 h. Likewise,
the isomerization of allyl benzene gave b-methylstyrene in ex-
cellent yield (>99%) with high E/Z selectivity (Table 4, entry 8).
Using renewable products, such as citronellene, demonstrated
that it was possible to selectively shift a terminal double bond
in the presence of internal double bonds (Table 4, entry 12).
Even more complex natural compounds such as quinine were
transformed into the corresponding b-olefins (Table 4,
entry 13).
isomerization · selectivity
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