Ruthenium(II)-salen complexes-catalyzed olefination of aldehydes with ethyl diazoacetate

Article abstract of DOI:10.1016/j.tetlet.2006.01.055

Several salen-ruthenium(II) complexes, which are derived from commercial ligands or simply ethylenediamine, can be successfully applied as catalysts for the olefination of a broad variety of aldehydes. Depending on the electron richness of the applied aldehydes, good to very good olefin yields and high E:Z selectivities are reached at 60 or 80°C reaction temperature with ethyl diazo acetate being the reaction partner. The reaction rate depends on the electron donor capabilities of the aldehydes. Electron poor aldehydes undergo faster reactions than electron rich aldehydes, but both electron rich and bulky aldehydes can be transformed to corresponding olefins in very good yields and high E-selectivity.

Full text of DOI:10.1016/j.tetlet.2006.01.055

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Tetrahedron Letters 47 (2006) 1993–1996
Ruthenium(II)–salen complexes-catalyzed olefination of
aldehydes with ethyl diazoacetate
Wei Sun, Bingsheng Yu and Fritz E. Kuhn*
¨
Lehrstuhl fu¨r Anorganische Chemie der Technischen Universita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85747 Garching bei Mu¨nchen, Germany
Received 8 December 2005; revised 10 January 2006; accepted 13 January 2006
Available online 3 February 2006
Abstract—Several salen–ruthenium(II) complexes, which are derived from commercial ligands or simply ethylenediamine, can be
successfully applied as catalysts for the olefination of a broad variety of aldehydes. Depending on the electron richness of the applied
aldehydes, good to very good olefin yields and high E:Z selectivities are reached at 60 or 80 °C reaction temperature with ethyl diazo
acetate being the reaction partner. The reaction rate depends on the electron donor capabilities of the aldehydes. Electron poor
aldehydes undergo faster reactions than electron rich aldehydes, but both electron rich and bulky aldehydes can be transformed
to corresponding olefins in very good yields and high E-selectivity.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
applied. Thus, we were interested in alternative, more
practical catalyst systems. In this present work, we
report on several simple Ru(II)–salen complexes 2–4,
which are derived from ethylenediamine.
The Wittig reaction occupies an important position in
the assembly of organic molecules as it generates a dou-
ble bond with usually high levels of geometrical con-
trol,1,2 however, it still has several drawbacks.3 To
avoid the basic conditions required for the generation
of phosphorane precursors, the catalytic alternative to
the classic Wittig reaction for stereoselective synthesis
of alkenes has proven to be quite promising.4,5 Up to
now, several efficient catalytic aldehyde-olefination reac-
tions have been reported based on Re,6–12 Ru,13–17
Rh,16,18 Fe,15,19–22 and Co23 complexes. In some cases
they operate under very mild conditions, with short
reaction times, and quantitative olefin yields with high
E-selectivities. Recently, we demonstrated that a ruthe-
nium(II)–salen complex of formula 1, derived from
cyclohexanediamine, is capable of catalyzing the alde-
hyde olefination reaction with both high yield and
selectivity.24
2. Results and discussion
Salen ligands with more flexible structures, which can be
easily introduced by substitution, have been attached to
a broad variety of transition metals and the resulting
compounds have been successfully applied for a pleth-
ora of catalytic reactions.25–36 Ru(II)(salen)(PPh3)2
complex 2 can be easily synthesized from commercially
available salen ligand and Ru(II)(PPh3)3Cl2 according
to the literature procedures.37 Complexes 3 and 4 were
prepared according to the literature procedures by react-
ing Ru(PPh3)2Cl2 with H2 salen in ethanol.25
As a control experiment, complexes 2–4 were applied as
catalysts in the olefination reaction of 4-methylbenzalde-
hyde, utilizing aldehyde (see Table 1), ethyl diazo acetate
(EDA), and triphenylphosphine (PPh3) as substrates in a
1:1.2:1.2 relationship. All reactions were performed in
toluene at 60 or 80 °C. 1 mol % of catalyst was applied.
The general catalytic reaction is given as Eq. 1. All the
catalysts exhibit high catalytic activity, leading to the
desired product in excellent yields. The reaction also
can be performed at lower temperatures with high excess
of E-olefin, however, the reactions need more time.
Although the catalytic activity of this ruthenium(II)
complex is often good, in order to synthesize this com-
plex an expensive trans-cyclohexanediamine has to be
Keywords: Aldehyde olefination; Homogeneous catalysis; Ruthenium;
Salen.
*
Corresponding author. E-mail: fritz.kuehn@ch.tum.de
0040-4039/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.01.055

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1994
W. Sun et al. / Tetrahedron Letters 47 (2006) 1993–1996
PPh3
PPh3
N
O
N
N
O
N
Ru
Ru
Cl
O
Cl
O
PPh3
2
PPh3
Cl
Cl
1
PPh3
PPh3
N
N
N
N
Ru
Ru
Cl
O
O
Cl I
O
O
I
PPh3
PPh3
Cl
I
Cl
I
3
4
Table 1. Catalytic results obtained with complexes 2–4 in toluene as the solventa
Entry
Substrate
Catalysta
Temperature
Time (h)
Yieldb (%)
E/Z-Ratio
1
2
3
4
2
2
3
4
80
60
80
80
1
3
1
1
98
96
97
97
17.6
18.9
11.3
9.45
CHO
a Reactions were carried out at 60 or 80 °C in 2.5 ml toluene for 1–3 h under N2 with 0.5 mmol of aldehyde, 0.6 equiv of EDA, 0.6 equiv of Ph3P, and
1 mol % of salen–Ru(II) catalysts.
b Isolated yield.
R
is predominant while the Z,Z-product is minor
(19.9:1). Z,E-Product is only found in traces (see Eq.
2). The salen–Ru(II) complexes, derived from ethylene-
diamine, also exhibit much higher E-selectivity than
complex 1.23
Cat. 2-4
Toluene
RCHO + N2=CHCOOEt + PPh3
COOEt
ð1Þ
The scope of the olefination reactions by the Ru(II)(sa-
len)(PPh3)2 complex of formula 2 was further explored
for a variety of aldehydes, the results are summarized
in Table 2. With all applied aldehydes, regardless
whether they are electron neutral (Table 2, entries 1–3)
or electron poor (Table 2, entry 7) isolated yields of benz-
aldehydes >90% (quantitative GC-yields) can be
reached at 60 °C within 3 h. Furthermore, also sterically
demanding aldehydes lead to quantitative product yields
under the same conditions (Table 2, entry 2). The less-
reactive electron rich 4-methoxybenzaldehydes can be
transformed to the desired olefin in 97% yield and
10.8:1 E/Z-selectivity with EDA (Table 2, entry 4).
The electron rich 4-dimethylaminobenzaldehyde (Table
2, entries 8–9), which can be regarded as an nonactiva-
ted aldehyde, was selectively converted to the corre-
sponding olefin in high yield and E-selectivity with
longer reaction time (24 h) and higher temperature
(100 °C). Non-aromatic aldehydes, such as cyclic alde-
hyde (Table 2, entry 10), were also suitable as substrates
for olefination with excellent yields.
COOEt
COOEt
COOEt
CHO
EDA, PPh3
Cat. 2-4
+
+
CHO
COOEt
COOEt
COOEt
E,E
E,Z
Z,Z
ð2Þ
3. Conclusions
Several ruthenium–salen complexes derived from simple
ethylenediamine can be successfully applied as catalysts
with low catalyst loadings for the olefination of a broad
variety of aldehydes, reaching good E/Z selectivities and
quantitative isolated olefin yields. Even sterically
demanding and electron rich olefins, presenting difficul-
ties for several other catalysts can be quantitatively
transformed to the desired olefins. Work to further
In the case of a dialdehyde (Table 2, entries 13–15) both
yield and selectivity are comparable to a mono-aldehyde
(entry 1), all the catalysts show quantitative yields with
high E-selectivity within 2 h at 80 °C. The E,E-product

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W. Sun et al. / Tetrahedron Letters 47 (2006) 1993–1996
Table 2. Catalytic results obtained with complexes 2 in toluene as the solventa
1995
Entry
1
Substrate
Temperature
60
Time (h)
3
Yieldb (%)
97
E/Z-Ratio
11.4
10.1
11.1
10.8
20.2
9.2
CHO
2
3
4
5
6
7
60
60
80
60
60
60
3
3
2
3
3
3
96
98
97
95
99
99
CHO
CHO
MeO
F3C
Cl
CHO
CHO
CHO
16.2
O2N
CHO
CHO
8
9
80
100
30
24
91
95
20.0
47.7
N
10
11
80
80
2
2
92
90
13.9
8.7
CHO
CHO
N
12
80
2
91
4.3
CHO
13
14
15
80
2
1.5
1.5
95
94
94
19.9
19.1
17.7
80c
80d
OHC
CHO
a Reactions were carried out at 60 or 80 °C in 2.5 ml toluene for 1–3 h under N2 with 0.5 mmol of aldehyde, 0.6 equiv of EDA, 0.6 equiv of Ph3P, and
1 mol % of salen–Ru(II) catalyst 2.
b Isolated yield.
c 1 mol % complex 3.
d 1 mol % complex 4.
expand the scope of these metal–salen complexes to
apply them to carbonyl substrates and tandem catalytic
reactions is currently underway in our laboratory.
tored by GC–MS on a Hewlett–Packard instrument
HP 6890/5973 GC–MS equipped with an Optima-1 col-
umn (Column No. 2965/12; SCT-Seitz GmbH). The E/
Z-ratios of products were determined by GC/MS prior
to purification of the products. All organic and inor-
ganic starting materials were purchased from Aldrich
or Acros and used without further purification. Com-
pounds 2–4 were prepared as described in Refs. 25 and
37.
4. Experimental
4.1. General remarks
Only freshly distilled, dry, and oxygen-free solvents were
1
used. H and 13C NMR spectra were recorded with a
4.2. General procedure for aldehyde olefination in toluene
400 MHz Bruker Avance DPX-400 spectrometer. IR
spectra were recorded on a Perkin–Elmer 1650 spec-
trometer (KBr). Elemental analyses were performed in
the Mikroanalytisches Labor of the Technische Univer-
In an oven dried Schlenk tube 1 mol % of catalyst, PPh3
(157.4 mg, 0.6 mmol) and 1 ml of toluene were placed.
Aldehyde (0.5 mmol), and EDA (0.6 mmol) dissolved
in 1.5 ml of toluene was added and heated to 60 or
sita¨t Munchen (M. Barth). Catalytic runs were moni-
¨

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W. Sun et al. / Tetrahedron Letters 47 (2006) 1993–1996
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mixture was kept for the times given in Table 1. The
solution was then cooled to room temperature and con-
centrated. The residue was chromatographed over a sil-
ica gel column with n-hexane/ethyl acetate (20:1–8:1)
affording the olefins.
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2002, 124, 176.
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