Formamidines - Versatile ligands for zinc-catalyzed hydrosilylation and iron-catalyzed epoxidation reactions

Article abstract of DOI:10.1002/ejoc.201000648

In the present study the abilities of catalysts modified by formamidine ligands have been examined in the zinc-catalyzed hydrosilylation of ketones and the iron-catalyzed epoxidation of stilbene, In case of hydrosilylation diethylzinc combined with easily accessible formamidine ligands allow for the efficient reduction of various aryl and alkyl ketones. By using a convenient in situ catalyst system high turnover frequencies up to more than 1.000 h ^-1 and a broad functional group tolerance were achieved. Moreover, the formamidine ligands were successfully applied, in the iron-catalyzed epoxidation of stilbene with hydrogen peroxide in good yield and chemoselectivity.

Full text of DOI:10.1002/ejoc.201000648

FULL PAPER
DOI: 10.1002/ejoc.201000648
Formamidines – Versatile Ligands for Zinc-Catalyzed Hydrosilylation and
Iron-Catalyzed Epoxidation Reactions
Stephan Enthaler,*[a] Kristin Schröder,[b] Shigeyoshi Inoue,[a] Björn Eckhardt,[a]
Kathrin Junge,[b] Matthias Beller,[b] and Matthias Drieß[a]
Keywords: Homogeneous catalysis / Zinc / Iron / Amidine ligands / Hydrosilylation / Epoxidation
In the present study the abilities of catalysts modified by form-
amidine ligands have been examined in the zinc-catalyzed
hydrosilylation of ketones and the iron-catalyzed epoxidation
of stilbene. In case of hydrosilylation diethylzinc combined
with easily accessible formamidine ligands allow for the ef-
ficient reduction of various aryl and alkyl ketones. By using
a convenient in situ catalyst system high turnover fre-
quencies up to more than 1.000 h–1 and a broad functional
group tolerance were achieved. Moreover, the formamidine
ligands were successfully applied in the iron-catalyzed epox-
idation of stilbene with hydrogen peroxide in good yield and
chemoselectivity.
variations can be only done directly at α-position to the
nitrogen atoms due to the fixed position of the heterocyclic
ring carbons. By removal of the C2-bridge a higher degree
of structural variations is possible. The resulting formamid-
ine structure allows for an easy variation of substituents on
the imine nitrogen and an additional group bonded to the
amine part. In consequence a convenient tuning is possible
compared to imidazole ligands.[7]
Introduction
The quest for sustainable and more efficient synthetic
methods continues to be a primary research target in
modern chemistry. In this regard, catalysis is a key technol-
ogy, since high atom efficiency, reduced amounts of waste
as well as energy, and in consequence advantageous eco-
nomics are feasible.[1] Especially, the use of organometallic
compounds as catalysts has become a well-established tool
for both academia and industry.[2] The reactivity and selec-
tivity of the reaction are significantly influenced by the se-
lection of the central metal and by the abilities of sur-
rounded ligands. Hence, a vast number of ligands were de-
veloped during the last decades. In recent times a special
focus for new ligand approaches is devoted to the design of
artificial catalysts/ligands inspired by active sites of “natu-
ral” biocatalysts.[3] Moreover, a second aspect, the substitu-
tion of expensive metals (e.g., Rh, Ru, Pd) by low-toxic and
cheap biorelevant metals, e.g., iron or zinc, is currently
highly requested, since the advantages are obvious.[4]
Recently, some of us became interested in this field of
chemistry and showed the successful application of imid-
azole ligands in iron-catalyzed oxidation reactions.[5] This
ligand concept is based on the occurrence of histidine,
which participates in manifold biological processes.[6] In gen-
eral, in metalloproteins the imine nitrogen of histidine co-
ordinates to the corresponding metal. Hence, imidazole-
based complexes should allow for similar catalytic applica-
tions (Figure 1).[5] However, applying imidazole ligands
Figure 1. Bio-inspired catalysts related to histidine.
Herein, we report on our initial studies on the applica-
tion of formamidine ligands in the zinc-catalyzed hydro-
silylation of carbonyl compounds and the iron-catalyzed
epoxidation of olefins.
Results and Discussion
Formamidines were synthesized following reported pro-
cedures. Thus, equimolar amounts of dimethylformamide
were treated with phosphorus oxychloride in diethyl ether
under inert conditions to form the corresponding Vilsmeier
reagent. Then, the reagent was quenched with primary
[a] Technische Universität Berlin, Department of Chemistry,
Cluster of Excellence “Unifying Concepts in Catalysis”,
Straße des 17. Juni 135, 10623 Berlin, Germany
Fax: +49-3031429732
E-mail: stephan.enthaler@tu-berlin.de
[b] Leibniz-Institut für Katalyse an der Universität Rostock e.V.,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
amines to obtain formamidines
2 after basification
(Table 1).[8] The crude products were purified by vacuum
distillation. Moderate yields were achieved for arylated ami-
dines, while alkyl derivatives containing cyclohexyl, tert-
butyl and adamantyl groups are derived only in low yields.
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The formamidines based on methylphenylformamide and Table 2. Characterization of the formamidines.
diphenylformamide were not accessible under these stan-
dard conditions. However, refluxing formamide and phos-
phorus oxychloride in toluene and subsequently heating to
reflux after amine addition gave moderate yields of for-
L
1H NMR
13C NMR IR
λ
∆E[d]
δ [ppm][a]
δ [ppm][b]
[cm–1][c]
[cm–1][e] [kJ/mol]
2a
2b
7.52
7.51
7.53
7.40
7.54
7.47
7.46
7.18
7.15
7.82
8.18
153.2
153.1
153.2
153.0
153.8
153.7
153.8
153.2
152.5
151.0
150.6
1634
1635
1639
1638
1603
1696
1647
1644
1644
1634
–
269.4
270.5
273.0
274.1
275.5
269.3
269.0
263.8
270.7
264.8
276.5
–30.6
–30.8
–29.3
–22.0
–29.6
–30.0
–31.5
–30.8
–21.2
–10.7
–25.3
mamidines 3 and 4. Noteworthy, in all cases we observed 2c
2d
the selective formation of the E-isomer, which is in agree-
2e
ment with the literature.[9]
2f
2g
Table 1. Synthesis of a formamidine library.
2h
2i
3
4
[a] The chemical shift (1H-NMR) of the formamidine proton was
measured in [D1]chloroform at 25 °C. [b] The chemical shift (13C-
NMR) of R–N=C(H)NR2 was measured in [D1]chloroform at
25 °C. [c] KBr. [d] ∆E = (Energy E-isomer)-(Energy Z-isomer), en-
ergies obtained by calculations: B3LYP/6-31G(d). [e] All measure-
ments were carried out in EtOH at 25 °C.
Ligand
R1
R2
R3
Yield [%]
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
C6H5
C6H5
57
35
50
37
54
62
49
69
45
28
44
4-MeC6H4
4-tBuC6H4
4-MeOC6H4
4-CF3C6H4
3,5-F2C6H3
3,4,5-F3C6H2
2,4,6-Me3C6H2
2,6-iPr2C6H3
2,4,6-Me3C6H2
C6H5
distances were measured (129.0 vs. 133.4 pm), which is in
agreement with literature values.[11] In case of 3 the amine
bond is similar to the imidazole bond (136.2 pm), while the
imine bond is shorter (126.8 pm). The open structure in
formamidines results in an elongation of the angle N1–C1–
N2 124.1° (2g), 123.5° (3) compared to 111.8° in N-2,4,6-
trimethylphenylimidazole.
3
4
In addition, we studied the influence of the various sub-
stituents in comparison to parent structure 2a. In Table 2
typical analytical parameters are given. First, the for-
mamidine proton was applied as sensor to detect geometri-
1
cal and electronic changes. By measuring the H chemical
shift no significant influence was found when alkyl groups,
electron-withdrawing groups as well as electron-donating
groups are placed in the para-position, while increasing the
bulkiness in the 2,6-position by methyl or isopropyl a high
field shift was observed. Substitution of the Me2N moiety
by MePhN or Ph2N led to a dramatic downfield shift. In
general, only a small influence was seen by using the form-
amidine C-atom as sensor, since all values are in the range
of 150–154 ppm.
In order to get insights into the geometrical structures
of the formamidines, DFT-calculations at the RB3LYP/6-
31G(d) level were performed.[9] In agreement with experi-
mental observations (vide supra) the E-configuration corre-
sponds to the thermodynamically most stable motif
(Table 2). The E-isomer is favoured by approximately 30 kJ/
mol. An exception is observed for compound 3 with a dif-
Figure 2. Molecular structure of 2g (a), and 3 (b). Hydrogen atoms
have been omitted for clarity except the formamidine protons. Se-
lected bond lengths [pm] and angles [°]: 2g: N1–C1 133.4(2), N2–
C1 129.0(2), N1–C1–N2 124.1(16); 3: N1–C1 136.3(3), N2–C1
126.8(3), N1–C1–N2 123.5(3).
After preparation and characterization of a small library
ference of only 10 kJ/mol. The calculated geometries for 2g of formamidine ligands, we were interested in the catalytic
and 3 are in agreement with those recorded by X-ray crys- performance of this ligand class in homogeneous catalysis.
tallography (Figure 2). In all cases the E-isomer was found. First we focused on the catalytic hydrosilylation of carbonyl
In addition, we compared the obtained structures with compounds. Up to now manifold catalysts for the hydro-
structural information available for imidazoles, e.g., N- silylation of ketones rely on precious metals such as rho-
2,4,6-trimethylphenylimidazole.[10] Due to the aromatic dium, ruthenium, and iridium.[12] Due to the high price and
character of imidazole the bond lengths of the embedded sometimes toxicity less expensive “biological” metal-based
carbon to the “imine” and “amine” nitrogen are 132.6 pm catalysts are highly requested.[4] At this point the use of iron
vs. 136.8 pm, whereas in case of formamidine 2g shorter and zinc complexes is probably of most interest.[13] Several
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Formamidines as Versatile Ligands for Catalysts
research groups have demonstrated the abilities of zinc cata- based on chelating diamines containing NH-functionalities
lysts in hydrosilylation reactions.[14] For example, it was to create a covalent N–Zn bond. We asked ourselves if it is
shown by Mimoun that unmodified ZnEt2 is not able to possible to obtain active catalysts containing monodentate
catalyze the hydrosilylation of carbonyl compounds, but the nitrogen ligands without NH group, since a plethora of sim-
addition of ligands can improve the outcome of the reaction ple amines is available. Based on this idea initial studies on
significantly.[14] Most ligands applied for this reaction are the influence of the reaction conditions were carried out
with acetophenone 5 as benchmark substrate by using 1
mol-% of an in-situ-generated zinc catalyst composed of di-
Table 3. Influence of the hydride source on the hydrosilylation of
ethylzinc and formamidine 2c in a ratio of 1:1 (Table 3).
The generated silyl ether was hydrolyzed under basic condi-
tions to obtain the corresponding alcohol 5a. First, the abil-
ities of some hydride sources were studied at 60 °C for one
hour. High conversion (98%) was found in case of trieth-
oxysilane as hydrogen source (Table 1, entry 1). Good yields
of 1-phenylethanol 5a were obtained with diphenylsilane
and phenylsilane (Table 3, entries 4 and 5), while poly-
(methylhydrosiloxane) (PMHS) furnished only moderate
amounts of 1-phenylethanol. In contrast, with triphenyl-
silane, dimethylphenylsilane, and triethylsilane no hydride
transfer was detected. Replacement of one ethoxy by a
methyl group led to a dramatic decrease of product forma-
tion. For further understanding of the abilities of applied
hydride sources, the hydridic character of the Si–H bond
was determined by NMR spectroscopy. Unfortunately, no
correlation was observed.
To clarify the influence of the ligand unmodified dieth-
ylzinc was also applied as catalyst (Table 4, entry 1). Here,
lower conversion was observed, while in the absence of any
catalyst no reaction took place. Apparently, the addition of
the formamidine ligand is of importance for acceleration of
the reaction. Next, the zinc to ligand ratio was examined in
the range of 1:1 to 1:5 (Table 4, entries 4–7). No significant
effects were observed. In addition, the influence of the reac-
tion temperature was investigated. At ambient condition a
turnover number of 58 h–1 was obtained. At 90 °C full con-
acetophenone 5.
Entry[a] H source
δ [ppm] H-
δ [ppm] 29Si-
Yield
[%]
1
NMR[c]
NMR[d]
1
2
3
4
5
6
7
8
(EtO)3SiH
Me(EtO)2SiH
Ph3SiH
Ph2SiH2
PhSiH3
4.22
4.66
5.49
5.02
4.29
4.61
3.66
4.74
–58.2
–34.4
–17.8
–33.1
–58.7
–16.7
–0.5
98
17
Ͻ1
95
92
Me2PhSiH
Et3SiH
Ͻ1
Ͻ1
53
PMHS[b]
–34.4[e]
[a] Reactions were carried out with 0.024 mmol ZnEt2 (1.0  in n-
hexane), 0.024 mmol 2c, 2.4 mmol acetophenone, 3.6 mmol silane,
1.0 mL of THF for 1 h at 60 °C, The conversion was determined by
GC (30 m Rxi-5ms column, 40–300 °C; 1-phenylethanol: 6.20 min,
acetophenone: 6.28 min) with dodecane: 7.84 min as the internal
standard. [b] PMHS = poly(methylhydrosiloxane). [c] Chemical
shift for the hydride. [d] With TMS (tetramethylsilane) as internal
standard. [e] Besides the major signal at –34.4 ppm [according to
literature assigned as (RO)2MeSi–H subunit] the following signals
were observed: –35.4, –34.9, –34.5, 10.3 ppm.
Table 4. Catalytic hydrosilylation of acetophenone 5: influence of
various reaction parameters.
Table 5. Catalytic hydrosilylation of acetophenone 5: variation of
ligands.
Entry[a] Cat. loading [mol-%]
M/L
T [°C]
Yield [%]
Entry[a]
Ligand
R1/R2/R3
Yield [%]
1[b]
2[c]
3[d]
4
5
6
7
8
9
1.0
0
0
1.0
1.0
1.0
1.0
1.0
0.1
1:0
0:0
0:1
1:1
1:2
1:3
1:5
1:1
1:1
60
60
60
25
25
25
25
90
60
40
Ͻ 1
Ͻ 1
51
46
48
48
Ͼ 99
Ͼ 99
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5/CH3/CH3
37
38
51
39
37
82
33
87
18
53
33
4-MeC6H4/CH3/CH3
4-tBuC6H4/CH3/CH3
4-MeOC6H4/CH3/CH3
4-CF3C6H4/CH3/CH3
3,5-F2C6H3/CH3/CH3
3,4,5-F3C6H2/CH3/CH3
2,4,6-Me3C6H2/CH3/CH3
2,6-iPr2C6H3/CH3/CH3
2,4,6-Me3C6H2/CH3/C6H5
C6H5/C6H5/C6H5
9
10
11
[a] Reactions were carried out with 0.024 mmol ZnEt2 and
0.024 mmol 2c, 2.4 mmol acetophenone, 3.6 mmol triethoxysilane,
1.0 mL of THF for 1 h, The conversion was determined by GC (30
m Rxi-5ms column, 40–300 °C; 1-phenylethanol: 6.20 min, aceto-
phenone: 6.28 min) with dodecane: 7.84 min as the internal stan-
dard. [b] 0.024 mmol diethylzinc (1.0  in n-hexane) without ad-
dition of ligand were applied as catalyst precursor. [c] Without cata-
lyst, 96% of acetophenone was recovered. [d] Addition of 1.0 mol-
% ligand, without diethylzinc.
3
4
[a] Reactions were carried out with 0.024 mmol ZnEt2 (1.0  in
n-hexane), 0.024 mmol ligand, 2.4 mmol acetophenone, 3.6 mmol
triethoxysilane, 1.0 mL of THF for 1 h at 25 °C. The conversion
was determined by GC (30 m Rxi-5ms column, 40–300 °C; 1-phen-
ylethanol: 6.20 min, acetophenone: 6.28 min) with dodecane:
7.84 min as the internal standard.
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S. Enthaler et al.
FULL PAPER
version was reached within one hour, even if a black pre- and 7). Best performance was demonstrated with the in-
cipitate was formed after several minutes. Reducing the situ-catalyst containing ligand 2h.
catalyst loading to 0.1 mol-% demonstrated excellent ac-
tivity at 60 °C with a catalyst turnover frequency of at least % catalyst, 60 °C, 1 h) for the hydrosilylation of the bench-
999 h–1.
mark substrate, the scope and limitations of the catalyst
After establishing suitable reaction conditions (1.0 mol-
The influence of the ligand structure was explored by composed of diethylzinc and formamidine 2c in a ratio of
combination of various formamidines with diethylzinc 1:1 were studied. The obtained silyl ethers were hydrolyzed
(Table 5). Those in-situ-generated pre-catalyst were sub- under basic conditions to yield the corresponding alcohols.
jected to the hydrosilylation of acetophenone at room tem- As shown in Table 6 an influence of substitution on the
perature. The parent ligand 2a gave a conversion of 37% phenyl-moiety of acetophenone was observed. Excellent
and substitution in 4-position showed no significant differ- yields were attained with electron-withdrawing as well as
ence. Only with ligand 2c slightly higher conversion was with electron-donating substituents (Table 6, entries 2, 4–9).
observed. However, fluoro-substitution in 3,5-position pro- However, ortho-substituted substrates showed no conver-
vided high conversion (82%), whereas an additional fluoro- sion at all under these conditions (Table 6, entry 3). In con-
substituent diminished the yield again (Table 5, entries 6 trast to this, an increase of bulkiness on the alkyl chain
Table 6. Scope and limitations of the in situ system composed of ZnEt2 and ligand 2c.
[a] Reactions were carried out with 0.024 mmol ZnEt2 and 0.024 mmol 2c, 2.4 mmol ketone, 3.6 mmol triethoxysilane, 1.0 mL of THF
1
for 1 h at 60 °C; the conversion was determined by GC (30 m Rxi-5ms column, 40–300 °C) and the yield by H NMR after isolation.
[b] In brackets the isolated yield is stated. [c] Mixture of products: 4-phenylbut-3-en-2-ol (78%), 4-phenylbutan-2-one (22%).
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Formamidines as Versatile Ligands for Catalysts
showed no influence on the conversion (Table 6, entries 10– ited moderate to good reactivity and high selectivity. No-
13). Hydrosilylation of ketones in the presence of an alkynyl tably, ligands 2e, 2h and 2i gave the highest yields (Ͼ75%).
group gave selectively the corresponding alcohol, whereas However, ligands with electron donating- or electron with-
23 yielded a mixture of the allylic alcohol and the saturated drawing-substituents at the aromatic ring which are owing
ketone (Table 6, entries 17 and 18). In addition, several di- to inductive effects (-alkyl, -CF3) displayed significantly
alkyl ketones were reduced with excellent conversions by higher yields than those which showed a complete or partly
the zinc catalyst (Table 6, entries 19–23).
donation or withdrawal by conjugation effects (-OMe, -F).
In addition to the zinc-catalyzed hydrosilylation, the ge- Besides, ligands 4, 2g and 2f demonstrate an interesting be-
neral catalytic potential of the formamidine ligands was haviour. After the first hour of hydrogen peroxide addition
demonstrated by applying them also in iron-catalyzed epox- the ligands showed only low conversions. However, the
idations of C–C double bonds. In general, oxiranes pro- product yield increased to 22% for 4, to 54% for 2g, and
vided by catalytic epoxidation are of fundamental impor- to 56% for 2f after stirring for 24h. These results guided us
tance in various fields of chemistry ranging from pharma- to explore the stability of the ligands under slightly acidic,
ceutical intermediates to monomers for bulk polymers. airy and hydrous conditions. When applying our reaction
Hence, there is an ongoing interest in active, simple and conditions to a mixture of FeCl3·6H2O, pyridine-2,6-dicar-
selective catalyst systems. Environmentally benign epoxid- boxylic acid, tert-amyl alcohol and ligand 2a no change in
ation catalysts should work with inexpensive ligands with- structure was observed by GC-MS. Besides also the use of
out tedious complex preparation and with sustainable ter- hydrogen peroxide (30%) and trans-stilbene gave no decom-
minal oxidants. For our purposes hydrogen peroxide is re- position of the ligand.
garded as oxidant of choice due to ecological and economic
Apart from trans-stilbene this protocol was also applied
considerations with only water as by-product.[15] Based on to cis-stilbene, styrene, and cyclooctene. For cyclooctene
the earlier developed approaches of iron-catalyzed epoxid- and cis-stilbene the reactivity was diminished compared to
ation reactions with pyridine-2,6-dicarboxylic acid 29 and the benchmark reaction. Nevertheless, cyclooctene oxide
nitrogen ligands like pyrrolidine or benzylamines[16] as well was obtained in 20% yield with iron trichloride hexahydrate
as with imidazole ligands,[5] we adopted these biomimetic modified by ligands 2a and 2b. To our delight styrene was
methods on the formamidine ligands. Fortunately, starting oxidized in excellent yield, e.g., ligand 2h gave up to 99%
with the benchmark substrate trans-stilbene (30) we ob- conversion with a yield of 92%, whereas ligand 2a and 2b
tained good yields up to 78% in the presence of ligand 2i gave good yields towards the corresponding oxiranes.[17]
(Table 7). Generally, all formamidine ligands with two
methyl groups attached to the amine nitrogen (2a–2i) exhib-
Conclusions
Table 7. Epoxidation of trans-stilbene.
In the present study we demonstrated the catalytic poten-
tial of formamidine ligands both in zinc-catalyzed hydro-
silylations as well as iron-catalyzed epoxidations. In case of
hydrosilylation diethylzinc combined with easily accessible
formamidine ligands allowed the efficient reduction of ace-
tophenone with high catalyst turnover frequencies up to
1.000 h–1. Furthermore, the hydrosilylation of various aryl
Entry[a]
Ligand R1/R2/R3
Conv.
[%][b]
Yield
Sel.
and alkyl ketones can be performed with broad functional
group tolerance. Moreover, the combination of formamid-
ine ligands with iron(III) chloride and pyridine-2,6-dicarb-
oxylic acid yielded a practical in situ catalyst for the epox-
idation of C–C double bonds. Using hydrogen peroxide as
terminal oxidant the stilbene epoxide was formed in selec-
tivities up to 99% at mild reaction conditions. Further work
in the direction of enantioselective hydrosilylation and
epoxidation, based on Zn and Fe catalysts modified by chi-
ral formamidines, are under way in our laboratories.
[%][b] [%][c]
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2h
2i
C6H5/CH3/CH3
77
77
80
24
87
36
21
77
79
61
6
69
69
63
22
76
30
17
77
78
56
6
90
90
79
98
87
85
83
99
99
92
97
4-MeC6H4/CH3/CH3
4-tBuC6H4/CH3/CH3
4-MeOC6H4/CH3/CH3
4-CF3C6H4/CH3/CH3
3,5-F2C6H3/CH3/CH3
3,4,5-F3C6H2/CH3/CH3
Mes/CH3/CH3
9
10
11
2,6-iPr2C6H3/CH3/CH3
Mes/CH3/C6H5
3
4
C6H5/C6H5/C6H5
[a] Reaction conditions: 0.5 mmol trans-stilbene,
5
mol-%
Experimental Section
FeCl3·6H2O and 12 mol-% formamidine ligand, 5 mol-% H2-pydic,
tert-amyl alcohol, 0.44 mmol dodecane, (100 µL, internal standard)
were added in sequence at room temp. in air. To this mixture, a
solution of 30% H2O2 (170 µL, 1.5 mmol) in tert-amyl alcohol
(830 µL) was added over a period of 1h at room temp. by a syringe
pump. [b] Conversion and yield were determined by GC analysis,
for reproducibility the reactions were at least repeated twice. [c]
Selectivity refers to the chemoselectivity of epoxide from olefin.
General: All manipulations with oxygen- and moisture-sensitive
compounds were performed under dinitrogen using standard
Schlenk techniques. THF was distilled from sodium/benzophenone
ketyl under dinitrogen. Diethylzinc solution (1.0  in n-hexane pur-
1
chased from Aldrich) was used without further manipulations. H
and 13C NMR spectra were recorded on a Bruker AFM 200 spec-
Eur. J. Org. Chem. 2010, 4893–4901
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4897

S. Enthaler et al.
145.0 (C ), 149.3 (C ), 153.2 [N=C(H)N] ppm. IR (KBr): ν = 2958
(m), 1639 (s), 1600 (m), 1515 (m), 1416 (m), 1364 (m), 1258 (m),
1182 (m), 1098 (m), 974 (w), 834 (m), 569 (w), 519 (w) cm–1. MS
(ESI): m/z (%) = 205 (18) [M+], 190 (56), 160 (100). HRMS calcd.
for C13H20N2+H: 205.16993; found 205.16930. UV/Vis: (EtOH,
25 °C): λ = 273.0 nm.
FULL PAPER
trometer (1H: 200.13 MHz; 13C: 75.5 MHz) using the proton sig-
nals of the deuterated solvents as reference. Single-crystal X-ray
diffraction measurements were recorded on an Oxford Diffraction
Xcalibur S Saphire spectrometer. IR spectra were recorded either
on a Nicolet Series II Magna-IR-System 750 FTR-IR or on a Per-
kin–Elmer Spectrum 100 FT-IR. Electron-impact mass spectra (EI-
MS) were recorded on a Finnigan MAT95S. Melting points (m.p.)
were determined on a BSGT Apotec II capillary-tube apparatus
and are uncorrected. UV/Vis spectra were recorded on a Perkin–
Elmer Lambda 20 spectrometer. GC–MS measurements were car-
ried out on a Shimadzu GC-2010 gas chromatograph (30 m Rxi-
5ms column) linked with a Shimadzu GCMA-QP 2010 Plus mass
spectrometer.
˜
q
q
N,N-Dimethyl-NЈ-(4-methoxyphenyl)formamidine (2d): Yield 37%;
colourless liquid; b.p. 108–110 °C (1 mbar). 1H NMR (200 MHz,
CDCl3, 25 °C): δ = 2.913 (s, 3 H,CH3), 2.914 (s, 3 H,CH3), 3.69 (s,
3 H, OCH3), 6.70–6.86 (m, 4 H, C6H4), 7.40 [s, 1 H, N=C(H)N]
ppm. 13C NMR (50 MHz, CDCl3, 25 °C): δ = 34.1 (NCH3), 39.6
(NCH3), 55.2 (OCH3), 114.3 (CH), 121.5 (CH), 145.3 (Cq), 152.8
(C ), 155.2 [N=C(H)N] ppm. IR (KBr): ν = 1638 (s), 1509 (s), 1368
˜
q
Safety Consideration: During our studies we used triethoxysilane
as reducing reagent without incident. However, Buchwald et al. re-
ported on difficulties when working with triethoxysilane.[18]
(m), 1239 (s), 1179 (m), 1103 (m), 1036 (m), 831 (m), 765 (m), 591
(w), 552 (w) cm–1. MS (ESI): m/z (%) = 179 (61) [M+], 134 (100).
HRMS calcd. for C10H14ON2+H: 179.11789; found 179.11797.
UV/Vis: (EtOH, 25 °C): λ = 274.1 nm.
Synthesis of N,N-Dimethylformamidine (2): Dimethylformamide
(25.3 mmol) was dissolved in dry diethyl ether (50 mL) and cooled
to 10 °C. Under an atmosphere of nitrogen phosphorus oxychlor-
ide (25.3 mmol) was added carefully maintaining the reaction tem-
N,N-Dimethyl-NЈ-(4-trifluoromethylphenyl)formamidine (2e): Yield
54%; white solid crystallized from ethanol; m.p. 47–49 °C; b.p. 92–
1
94 °C (1 mbar). H NMR (200 MHz, CDCl3, 25 °C): δ = 3.06 [s, 6
perature, while a white precipitate was formed. The mixture was H, N(CH3)2]; 6.99–7.03 (m, 2 H, C2H4); 7.48–7.51 (m, 2 H, C2H4);
stirred for two hours at room temperature yielding an insoluble (in
diethyl ether) oily residue. A white precipitate was obtained after
cooling to 10 °C and slow addition of the corresponding primary
amine (25.3 mmol). The stirring was continued for 12 h at room
temperature. The mixture was transferred to a separation funnel
and carefully treated with water (50 mL) and aqueous sodium hy-
droxide solution until pH Ͼ 8 for the aqueous layer. The aqueous
layer was extracted with diethyl ether (2ϫ50 mL). The combined
organic layers were washed with water (50 mL) and brine (50 mL).
Drying with Na2SO4 and removal of the solvent yield the crude
product, this was purified by distillation.
7.54 [s, 1 H, N=C(H)N] ppm. 13C NMR (50 MHz, CDCl3, 25 °C):
δ = 34.4 (NCH3), 40.1 (NCH3), 114.0 (CH), 121.1 (CH), 126.1
(CH), 127.4 (C ), 153.7 [N=C(H)N], 155.2 (C ) ppm. IR (KBr): ν
˜
q
q
= 2926 (m), 1915 (w), 1603 (s), 1516 (m), 1443 (m), 1373 (m), 1331
(s), 1263 (m), 1161 (m), 1107 (s), 1066 (s), 975 (m), 846 (m), 663
(m), 599 (m), 516 (w), 484 (w) cm–1. MS (ESI): m/z (%) = 217
(100) [M+], 172 (46). HRMS calcd. for C10H11F3N2+H: 217.09471;
found 217.09473. UV/Vis: (EtOH, 25 °C): λ = 275.5 nm.
N,N-Dimethyl-NЈ-(3,5-difluorophenyl)formamidine (2f): Yield 62%;
colourless solid (–25 °C), colourless liquid (room temp.); b.p. 92–
1
93 °C (1 mbar). H NMR (200 MHz, CDCl3, 25 °C): δ = 2.98 [s, 6
N,N-Dimethyl-NЈ-phenylformamidine (2a): Yield 57%; colourless li-
H, N(CH3)2], 6.31–6.51 (m, 3 H, C6H4), 7.47 [s, 1 H, N=C(H)N]
1
quid; b.p. 84–85 °C (1 mbar). H NMR (200 MHz, CDCl3, 25 °C): ppm. 13C NMR (50 MHz, CDCl3, 25 °C): δ = 34.3 (s, NCH3), 40.0
1
2
δ = 3.02 (s, 6 H, CH3), 6.92–7.06 (m, 3 H, C6H5), 7.20–7.32 (m, (s, NCH3), 97.1 (t, J = 26.07 Hz, C6H3), 103.8 (dd, J = 9.34, J =
3H C6H5), 7.52 [s, 1 H, N=C(H)N] ppm. 13C NMR (50 MHz, 15.69 Hz, CH), 153.7 [N=C(H)N], 154.9 (t, J = 11.67 Hz, Cq),
CDCl3, 25 °C): δ = 34.3 (NCH3), 39.8 (NCH3), 120.9 (C6H5), 122.1
160.9 (d, J = 15.38 Hz, CF), 165.7 (d, J = 15.36 Hz, CF) ppm. IR
(KBr): ν = 3431 (m), 2921 (m), 1696 (m), 1617 (m), 1384 (s), 1310
(C6H5), 128.6 (C6H5), 151.9 (Cq), 153.1 [N=C(H)N] ppm. IR
˜
(KBr): ν = 3440 (m), 3055 (m), 3027 (m), 2987 (m), 2922 (s), 2853 (w), 1257 (w), 1102 (m), 987 (m), 840 (w), 680 (w), 509 (w) cm–1.
˜
(m), 2801 (m), 2588 (w), 2470 (w), 2004 (w), 1933 (m), 1634 (s), MS (ESI): m/z
= 185.1. HRMS calcd. for C9H10F2N2+H:
1589 (s), 1482 (m), 1384 (s), 1258 (m), 1214 (m), 1170 (m), 1099 185.08848; found 185.08835. UV/Vis: (EtOH, 25 °C): λ = 269.3 nm.
(s), 970 (m), 834 (m), 761 (m), 696 (m), 637 (m), 549 (m), 503 (m)
N,N-Dimethyl-NЈ-(3,4,5-trifluorophenyl)formamidine (2g): Yield
cm–1. MS (ESI): m/z (%) = 149 (100) [M+], 104 (21). HRMS calcd.
49%; white solid; m.p. 38 °C; b.p. 84–85 °C (1 mbar). 1H NMR
for C9H12N2+H: 149.10732; found 149.10721. UV/Vis: (EtOH,
25 °C): λ = 269.4 nm.
(200 MHz, CDCl3, 25 °C): δ = 3.03 [s, 6 H, N(CH3)2], 6.46–6.64
(m, 2 H, C6H2), 7.46 (s, 1 H), 7.54 [s, 1 H, N=C(H)N] ppm. 13C
N,N-Dimethyl-NЈ-(4-methylphenyl)formamidine (2b): Yield 35%; NMR (50 MHz, CDCl3, 25 °C): δ = 34.3 (s, NCH3), 40.1 (s,
colourless liquid; b.p. 95 °C (1 mbar). 1H NMR (200 MHz,
CDCl3): δ = 2.32 (s, 3 H, C6H4CH3), 3.00 [s, 6 H, N(CH3)2], 6.84– 6.65, J = 1.03 Hz, C6H2), 133.2 (t, J = 15.8 Hz, Cq), 138.0 (t, J =
NCH3), 98.4 (d, J = 23.25 Hz, C6H2), 104.6 (td, 1J = 7.68, 2J =
3
6.93 (m, 2 H, C6H4), 7.04–7.14 (m, 2 H, C6H4), 7.51 [s, 1 H,
N=C(H)N] ppm. 13C NMR (50 MHz, CDCl3): 20.6 3.35 Hz, Cq), 148.7 (dd, 1J = 10.35, 2J = 5.72 Hz, Cq), 153.5
(C6H4CH3), 34.6 (NCH3), 39.7 (NCH3), 120.8 (CH), 129.4 (CH), [N=C(H)N] ppm. IR (KBr): ν = 3491 (w), 2928 (m), 2808 (w), 1647
15.86 Hz, Cq), 148.1 (qd, 1J = 13.35, 2J = 9.83, 3J = 9.48, 4J =
δ
=
˜
131.5 (C ), 149.4 (C ), 153.1 [N=C(H)N] ppm. IR (KBr): ν = 3441
(s), 1622 (s), 1522 (s), 1430 (m), 1381 (m), 1332 (m), 1229 (m), 1099
˜
q
q
(s), 2921 (m), 1635 (m), 1509 (w), 1384 (m), 1258 (w), 1173 (w), (m), 1038 (m), 856 (m), 788 (m), 681 (m), 640 (m), 532 (m) cm–1.
1100 (m), 1064 (w), 970 (w), 816 (w), 588 (w), 502 (w) cm–1. MS MS (ESI): m/z (%) = 203 (74) [M+], 158 (100). HRMS calcd. for
(ESI): m/z (%) = 163 (100) [M+], 118 (27). HRMS calcd. for
C10H14N2+H: 163.12298; found 163.12286. UV/Vis: (EtOH,
25 °C): λ = 270.5 nm.
C9H9F3N2+H: 203.07906; found 203.07922. UV/Vis: (EtOH,
25 °C): λ = 269.0 nm.
N,N-Dimethyl-NЈ-(2,4,6-trimethylphenyl)formamidine (2h): Yield
69%; colourless liquid; b.p. 100–102 °C (1 mbar). 1H NMR
N,N-Dimethyl-NЈ-(4-tert-butylphenyl)formamidine (2c): Yield 50%;
b.p. 120 °C (1 mbar). 1H NMR (200 MHz, CDCl3, 25 °C): δ = 1.32 (200 MHz, CDCl3, 25 °C): δ = 2.13 (s, 6 H), 2.26 (s, 3 H, p-Me),
[s, 9 H, C(CH3)3], 3.01 [s, 6 H, N(CH3)2], 6.86–6.95 (m, 2 H, C6H4),
7.24–7.33 (m, 2 H, C6H4), 7.53 [s, 1 H, N=C(H)N] ppm. 13C NMR 13C NMR (50 MHz, CDCl3, 25 °C): δ = 18.4 [C6H2(CH3)2], 20.5
(50 MHz, CDCl3, 25 °C): δ = 31.4, 34.0, 120.5 (CH), 125.7 (CH), (C6H2CH3), 37.0 (NCH3), 128.3 (C6H2), 129.2 (Cq), 130.7 (Cq),
3.01 (s, 6 H), 6.84 (m, 2 H, Ar-H), 7.18 [s, 1 H, N=C(H)N] ppm.
4898
www.eurjoc.org
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 4893–4901

Formamidines as Versatile Ligands for Catalysts
147.4 (C ), 153.2 [N=C(H)N] ppm. IR (KBr): ν = 3435 (m), 3282
(m), 2921 (s), 2854 (m), 2805 (m), 2729 (m), 1950 (w), 1644 (s),
brine (50 mL). Drying with Na2SO4 and removal of the solvent
˜
q
yields a yellow powder; yield 44%; yellow solid; m.p. 85 °C. 1H
1607 (m), 1479 (m), 1436 (m), 1384 (s), 1260 (m), 1213 (m), 1094 NMR (200 MHz, CDCl3, 25 °C):
δ = 7.01–7.46 [m, 1 H,
(m), 852 (m), 744 (m), 588 (m) cm–1. MS (ESI): m/z (%) = 191 (C6H5)2N, C6H5N=], 8.18 [s, 1 H, N=C(H)N] ppm. 13C NMR
(100) [M+], 146 (65). HRMS calcd. for C12H18N2+H: 191.15428;
(50 MHz, CDCl3, 25 °C): δ = 121.2 (CH), 123.5 (CH), 124.9 (CH),
found 191.15437. UV/Vis: (EtOH, 25 °C): λ = 263.8 nm.
125.5 (CH), 128.9 (CH), 129.2 (CH), 143.3 (Cq), 150.6 [N=C(H)-
N], 151.1 (C ) ppm. IR (KBr): ν = 3053 (m), 2923 (m), 1686 (w),
˜
q
N,N-Dimethyl-NЈ-(2,6-diisopropylphenyl)formamidine (2i): Yield
45%; colourless solid (–25 °C), colourless liquid (room temp.); b.p.
90–93 °C (1 mbar). H NMR (200 MHz, CDCl3, 25 °C): δ = 1.18
[d, J = 6.90 Hz, 12 H, CH(CH3)2], 2.82 [s, 6 H, N(CH3)2], 3.15
[sept, J = 6.90 Hz, 2 H, CH(CH3)2], 6.98–7.12 (m, 3 H, Ar), 7.15
[s, 1 H, N=C(H)N] ppm. 13C NMR (50 MHz, CDCl3, 25 °C): δ =
22.4 [CH(CH3)2], 23.7 [CH(CH3)2], 27.6 (NCH3), 27.8 (NCH3),
118.4 (CH), 122.6 (CH), 132.3 (Cq), 140.3 (Cq), 147.8 (Cq), 152.5
1633 (s), 1586 (s), 1494 (s), 1449 (m), 1348 (s), 1284 (m), 1207 (s),
1138 (m), 966 (m), 759 (s), 693 (s), 555 (m) cm–1. MS (ESI): m/z
(%) = 273 (47) [M+], 170 (100). HRMS calcd. for C19H16N2+H:
273.13863; found 273.13889. UV/Vis: (EtOH, 25 °C): λ = 276.5 nm.
1
General Procedure for the Hydrosilylation Reaction: A pressure tube
was charged with an appropriate amount of the corresponding
formamidine (0.024 mmol, 1.0 mol-%) and purged with nitrogen
(nitrogen/vacuum, three cycles). The formamidine was dissolved
in anhydrous tetrahydrofuran (760 µL). Diethylzinc [240 µL
(0.024 mmol) of a stock solution (1.0 mL of diethylzinc, 1.0  in n-
hexane, diluted with 9.0 mL of THF)] was added and the mixture
was stirred for 10 min at room temperature. To the clear solution
acetophenone (2.4 mmol) and triethoxysilane (3.6 mmol,
1.5 equiv.) were added via syringe. The reaction mixture was stirred
in a preheated oil bath at 60 °C for 60 min. The mixture was cooled
on an ice bath and was treated with dodecane (10 µL) as GC stan-
dard (for GC analysis), methanol (1.0 mL), and aqueous sodium
hydroxide solution (1.0 mL) with vigorous stirring (Caution: The
reaction mixture bubbled vigorously upon addition of the base).
The reaction mixture was stirred for 60 min at 0 °C and was then
extracted with diethyl ether (2ϫ10.0 mL). The combined organic
layers were washed with brine, dried with anhydrous Na2SO4, fil-
tered, and an aliquot was removed for GC-analysis. The yield was
monitored by GC (30 m Rxi-5ms column, 40–300 °C; 1-phenyl-
ethanol: 6.20 min, acetophenone: 6.28 min). The analytical proper-
ties of the corresponding alcohols are in agreement with literature.
[N=C(H)N] ppm. IR (KBr): ν = 3408 (m), 3061 (w), 2959 (s), 2926
˜
(m), 2869 (m), 1647 (s), 1588 (m), 1459 (m), 1441 (m), 1363 (m),
1322 (w), 1262 (m), 1089 (s), 934 (w), 833 (w), 800 (w), 757 (m)
cm–1. MS (ESI): m/z (%) = 203 (74) [M+], 158 (100). HRMS calcd.
for C17H20N2+H: 233.20123; found 253.16904. UV/Vis: (EtOH,
25 °C): λ = 270.7 nm.
N-Methyl-N-phenyl-NЈ-(2,4,6-trimethylphenyl)formamidine (3): The
corresponding formamide (25.3 mmol) was dissolved in dry toluene
(50 mL) and cooled to 10 °C. Under an atmosphere of nitrogen
phosphorus oxychloride (25.3 mmol) was added carefully while
maintaining the reaction temperature. The mixture was refluxed
for three hours. After cooling to 10 °C the corresponding aniline
(25.3 mmol) was added carefully. The stirring was continued for 12
h at room temperature. The mixture was refluxed for three hours
and afterwards transferred to a separation funnel and carefully
treated with water (50 mL) and an aqueous sodium hydroxide solu-
tion until the aqueous layer was alkaline (pH Ͼ 8). The aqueous
layer was extracted with diethyl ether (2ϫ50 mL). The combined
organic layers were washed with water (50 mL) and brine (50 mL).
Drying with Na2SO4 and removal of the solvent yields the crude
product. yield 28%; colourless crystals (crystallized from ethanol/
General Procedure for the Epoxidation of Olefins: In a test tube,
FeCl3·6H2O (0.025 mmol), tert-amyl alcohol (9 mL), formamidine
ligand (0.060 mmol), pyridine-2,6-dicarboxylic acid (0.025 mmol),
olefin (0.50 mmol) and dodecane (GC internal standard, 100 µL)
were added in sequence at room temperature in air. To this mixture
a solution of 30% hydrogen peroxide (aqueous, 170 µL, 1.5 mmol)
in tert-amyl alcohol (830 µL) was added over a period of 1 h at
room temperature by a syringe pump. Conversion and yield were
determined by GC analysis without further manipulations. Reac-
tions were carried out twice due to reproducibility.
1
n-hexane 90:10); m.p. 81 °C. H NMR (200 MHz, CDCl3, 25 °C):
δ = 2.18 [s, 6 H, C6H2(CH3)2], 2.27 (s, 3 H, C6H2CH3), 3.58 (s, 3
H, NCH3), 6.87 (s, 2 H, C6H2), 7.18–7.05 (m, 3 H, C6H5), 7.42–
7.30 (m, 2 H, C6H5), 7.28 [s, 1 H, N=C(H)N] ppm. 13C NMR
(50 MHz, CDCl3, 25 °C): δ = 18.6 (CH3), 20.6 (CH3), 33.4 (NCH3),
116.1 (CH), 123.4 (CH), 128.5 (CH), 128.9 (Cq), 129.3 (CH), 131.6
(C ), 145.0 (C ), 146.8 (C ), 151.0 [N=C(H)N] ppm. IR (KBr): ν
˜
q
q
q
= 2915 (m), 1634 (s), 1595 (m), 1501 (m), 1483 (m), 1374 (w), 1346
(m), 1264 (m), 1210 (m), 1148 (m), 1126 (m), 1035 (w), 974 (w),
856 (w), 762 (m), 693 (m), 520 (w) cm–1. MS (ESI): m/z (%) = 253
(43) [M+], 146 (55), 131 (24), 120 (26), 108 (100), 93 (26). HRMS
calcd. for C17H20N2+H: 253.16993; found 253.16904. UV/Vis:
(EtOH, 25 °C): λ = 264.8 nm.
Single-Crystal X-ray Structure Determination: Crystals were each
mounted on a glass capillary in perfluorinated oil and measured
under a flow of nitrogen. The data of 2g, and 3 were collected
on an Oxford Diffraction Xcalibur S Sapphire at 123 K (Mo-Kα
radiation, λ = 0.71073 Å). The structures were solved by direct
methods and refined on F2 with the SHELX-97[19] software pack-
age. The positions of the H atoms were calculated and considered
isotropically according to a riding model.
N,N,NЈ-Triphenylformamidine
(4):
Diphenylformamide
(25.3 mmol) was dissolved in dry toluene (50 mL) and cooled to
10 °C. Under an atmosphere of nitrogen phosphorus oxychloride
(25.3 mmol) was added carefully while maintaining the reaction
temperature. The clear mixture was refluxed for three hours. A
white precipitate was obtained after cooling to 10 °C and slow ad-
dition of aniline (25.3 mmol). The stirring was continued for 12 h
at room temperature. The mixture was refluxed for three hours.
After cooling to room temperature the solid was filtered off and
washed with diethyl ether. The solid was transferred to a separation
funnel and treated with water (50 mL) and diethyl ether (50 mL).
A solution of sodium hydroxide was carefully added until pH Ͼ 8.
The aqueous layer was extracted with diethyl ether (2ϫ50 mL).
The combined organic layers were washed with water (50 mL) and
2g: Monoclinic, space group P21/n; a = 7.3941(3), b = 16.0027(6),
c = 7.7584(3) Å; b = 100.526(4)°; V = 902.57(6) Å3; Z = 4; ρcalc
=
1.488 mg m–3; µ(Mo-Kα) = 0.092 mm–1; 4462 collected reflections;
1585 crystallographically independent reflections (Rint = 0.0259);
1585 reflections with I Ͼ 2σ(I); θmax = 25.00°; R(Fo) = 0.0380
[IϾ2σ(I)]; wR(F2o) = 0.0903 (all data); 129 refined parameters.
3: Monoclinic, space group P21/n; a = 6.8218(5), b = 7.0009(4), c
= 15.1640(10) Å; b = 93.431(6)°; V = 722.92(8) Å3; Z = 2; ρcalc
=
1.159 mg m–3; µ(Mo-Kα) = 0.092 mm–1; 3155 collected reflections;
2136 crystallographically independent reflections (Rint = 0.0251);
Eur. J. Org. Chem. 2010, 4893–4901
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
4899

S. Enthaler et al.
FULL PAPER
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Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P.
Hratchian, J. B. Cross, C. Adamo, J. Jaramillo, R. Gomperts,
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elli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P.
Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich,
A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui,
A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu,
A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J.
Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara,
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Wong, C. Gonzalez, J. A. Pople, Gaussian 03, rev. B.02,
Gaussian, Inc., Pittsburgh PA, 2003.
2136 reflections with I Ͼ 2σ(I); θmax = 25.00°; R(Fo) = 0.0476
[I Ͼ 2σ(I)]; wR(F2o) = 0.0858 (all data); 176 refined parameters.
CCDC-765371 (for 2g) and -765372 (for 3) contain the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[8]
[9]
Acknowledgments
This work was supported by the Cluster of Excellence “Unifying
Concepts in Catalysis”, sponsored by the Deutsche Forschungsge-
meinschaft (DFG), and administered by the Technische Universität
Berlin. K. S. appreciates financial support provided by the Gradu-
iertenkolleg 1213 (“Neue Methoden für Nachhaltigkeit in Katalyse
und Technik”) and the Max-Buchner-Forschungsstiftung
(DECHEMA). We thank Katja Pavel for excellent technical
assistance.
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Received: May 7, 2010
Published Online: July 16, 2010
Eur. J. Org. Chem. 2010, 4893–4901
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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