Thieme chemistry journal awardees-where are they now? bifunctional organocatalysis with N-Formyl-l-proline: A novel approach to epoxide ring opening and sulfide oxidation

Article abstract of DOI:10.1055/s-0029-1219363

A conceptually distinct approach to the aminolysis of 1,2-epoxides, which involves Lewis base-Br?nsted acid catalysis employing N-formyl-l-proline as an easily accessible bifunctional organocatalyst and water as a solvent is presented. The potential of N-formyl-l-proline as organocatalyst for the sulfide oxidation reaction using aqueous hydrogen peroxide as environmentally benign and readily available oxidant is also demonstrated. Good to high yields are achieved for both reactions. Georg Thieme Verlag Stuttgart ? New York.

Full text of DOI:10.1055/s-0029-1219363

LETTER
707
Thieme Chemistry Journal Awardees – Where Are They Now?
Bifunctional Organocatalysis with N-Formyl-L-Proline: A Novel Approach to
Epoxide Ring Opening and Sulfide Oxidation
Bifunctional
O
h
rganocatalys
e
is with
N
-F
n
ormyl-
L
-Pro
g
line wei Wei, Kerstin A. Stingl, Katharina M. Weiß, Svetlana B. Tsogoeva*
Department of Chemistry and Pharmacy, Chair of Organic Chemistry I, University of Erlangen-Nuremberg,
Henkestr. 42, 91054 Erlangen, Germany
Fax +49(9131)8526865; E-mail: tsogoeva@chemie.uni-erlangen.de
Received 9 October 2009
Abstract: A conceptually distinct approach to the aminolysis of
1,2-epoxides, which involves Lewis base–Brønsted acid catalysis
employing N-formyl-L-proline as an easily accessible bifunctional
organocatalyst and water as a solvent is presented. The potential of
N-formyl-L-proline as organocatalyst for the sulfide oxidation reac-
tion using aqueous hydrogen peroxide as environmentally benign
and readily available oxidant is also demonstrated. Good to high
yields are achieved for both reactions.
Key words: bifunctional organocatalysis, Lewis base–Brønsted
acid, epoxides, ring opening, amino alcohols, sulfide oxidation,
sulfoxides
Svetlana B. Tsogoeva, born in 1973, studied chemistry at St. Peters-
burg State University, Russian Federation, where she completed her
doctoral thesis supported by Procter&Gamble on the ‘Synthesis of
Modified Analogues of Steroid Estrogens’ in 1998. Then, she moved
to the research group of Prof. Göbel at the Johann Wolfgang Goethe
University Frankfurt am Main, Germany, for a postdoctoral project
under the sponsorship of the DFG. In July 2000 she joined the Degus-
sa AG Fine Chemicals Division in Hanau-Wolfgang, Germany. In Ja-
nuary 2002 she was appointed Junior Professor of Chemistry at the
Georg-August University of Göttingen, Germany. Since February
2007 she holds the position of Professor of Organic Chemistry at the
Friedrich-Alexander University Erlangen-Nuremberg, Germany.
Svetlana B. Tsogoeva is one of the 2007 Thieme Chemistry Journal
Award recipients. Her research is currently focused on asymmetric
organocatalysis, organo-autocatalysis, asymmetric oxidations with
nonhem iron complexes, as well as synthesis of natural product hy-
brids for medicinal chemistry.
One of the important features of organocatalysts1a is that
the same catalyst may be able to catalyze various reac-
tions, which follow entirely different mechanisms.1b,c
Recently, we reported the first example of an organocata-
lytic enantioselective allylation of simple aldimines using
chiral mono- and bisformamides synthesized in our labo-
ratory.2a,b We further demonstrated the potential of these
Lewis basic compounds as organocatalysts for the reduc-
tion of ketimines.2b In the context of these studies on form-
amides derived from L-proline, we also found that even
simple N-formyl-L-proline,2a,b which initially served as a
building block for chiral formamide-based catalysts, was
able to catalyze Strecker synthesis and reduction of
ketimines with trichlorosilane.2c These results prompted
us to expand the scope of N-formyl-L-proline-mediated
catalysis.
proline as organocatalyst for the sulfide oxidation reaction
using aqueous hydrogen peroxide as environmentally
friendly and readily available oxidant.
As part of ongoing studies in our laboratory, aimed at the
development of new catalytic systems for epoxide ring
opening by amine as nucleophile and the sulfide oxidation
reaction, we explored N-formyl-L-proline and other Lewis
basic compounds as catalysts for the target reactions un-
der various conditions.
Aminolysis of 1,2-Epoxides
The ring-opening reaction of epoxides with amines repre-
sents one of the most important and straightforward meth-
ods for preparing b-amino alcohols, which are versatile
synthons in the synthesis of biologically active natural
products, unnatural amino acids, and b-blockers, as well
as insecticidal agents and chiral auxiliaries.3
Herein, we wish to report our findings and to disclose a
novel approach to the aminolysis of 1,2-epoxides, which
involves Lewis base–Brønsted acid bifunctional catalysis
using easily accessible N-formyl-L-proline as an organo-
catalyst and water as a cheap and safe solvent. We also
demonstrate for the first time the potential of N-formyl-L-
Among the catalysts reported for this type of reaction,
Lewis acid catalysis is the most widely used method for
epoxide ring openings with amines.4 The Lewis acids
used include metal halides and triflates, besides others.5
However, the use of strong Lewis acids could be problem-
atic, because they may trigger undesired isomerization of
the epoxides with formation of carbonyl compounds.6
SYNLETT 2010, No. 5, pp 0707–0711
Advanced online publication: 08.02.2010
DOI: 10.1055/s-0029-1219363; Art ID: S11909ST
1
6.
3
.2
0
1
0
© Georg Thieme Verlag Stuttgart · New York

708
S. Wei et al.
LETTER
The reactions reported sometimes also require at least
stoichiometric amounts of Lewis acid or reflux condi-
tions. Accordingly, the development of a more efficient
and milder catalytic methodology for the epoxide opening
is desired.
Table 1 Aminolysis of Cyclohexene Oxide (5) by Aniline (6) in the
Presence of Catalysts 1–4
catalyst
OH
(10 mol%)
O
+
H2N
solvent, r.t., 48 h
NH
Ph
5
7
6
Recently, a few examples of aminolysis of epoxides by
employing organocatalysts were presented.7 The organo-
catalysts reported include tributylphosphine,7a tertiary
amines,7b b-cyclodextrin,7c thioureas,7d–f and poly(ami-
doamine) dendrimers.7g
Entry
Catalyst
Solvent
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
H2O
Yield (%)a
1
2
–
1
2
3
4
–
1
2
3
4
n.r.
9
While aminolysis of 1,2-epoxides with arylamines has
been recently performed even in the presence of bases
such as 1,4-diazabicyclo[2.2.2]octane (DABCO) or tri-
ethylamine,7b to the best of our knowledge, no report is
known on Lewis basic formamides or N-oxides as mild
organocatalysts for these reactions.
3
>99
n.r.
5
4
5
6
34
44
>99
58
40
This motivated us to test readily available compounds 1–
4 (Figure 1) as catalysts for the synthesis of 2-amino alco-
hols. As a model transformation we studied the addition of
aniline (6) to cyclohexene oxide (5) in the presence of 10
mol% of the appropriate catalyst candidate, with the reac-
tion proceeding at room temperature and in CH2Cl2 as a
solvent (Table 1).
7
H2O
8
H2O
9
H2O
10
H2O
a Yields of isolated products after chromatography.
O
OH
O
OH
shown in Table 2: all reactions afforded products in good
yields (entries 1–10, Table 2). Racemic or nearly racemic
products were obtained in all cases.
N
N
N+
O–
N
H
O
O
H
H
2
1
3
4
The catalyst 2 was shown to be less efficient for the
epoxide ring opening of cis-stilbene oxide (entries 11 and
12). When styrene oxide was used, two regioisomers A
and B were formed with the ratio of 85:15 (by NMR) and
in good yield (73%, entry 13).
Figure 1 Screened organocatalysts
Notably, no reaction was observed in the absence of cata-
lyst under the reaction conditions. Interestingly, while
pyridine-N-oxide (1), L-proline (3), and N-formylpyrroli-
dine (4) afforded no to low yields, over 99% yield was ob-
served under the same reaction conditions with the N-
formyl-L-proline (2) as a catalyst (entry 3 vs. entries 2, 4,
and 5, Table 1).
Table 2 Scope of Substrates: Aminolysis in the Presence of
N-Formyl-L-proline (1) Using Water as a Solvent12
R1
OH
catalyst 2
R1
R1
(10 mol%)
H2NR2
R2
+
O
R1
N
H
H2O, r.t.
These experiments show that the Lewis basic formamide
group in 4 alone is not able to facilitate the reaction with
a good yield, and thus the prerequisite for high yield is that
the catalyst possesses both a formamide group and a car-
boxylic moiety (entries 3 vs. entries 4, 5).
Entry
Product
Time (h)
Yield (%)a
70
OH
OMe
1
2
3
4
48
48
48
48
N
H
Similar trends, albeit with better yields, were observed
with catalysts 1–4 when water was used as a solvent:8
whereas the yields ranged only between 34–58% without
or with catalysts 1, 3, and 4, nearly quantitative yield was
achieved again with catalyst 2 (entry 8 vs. entries 6, 7, 9,
and 10, Table 1). This is interesting since potential advan-
tages of using water as a solvent are its safety, low cost,
and ease of use. In addition, unique reactivity and selec-
tivity are often observed when water is used as a solvent.8
OH
72
>99
78
N
H
OMe
OH
N
H
OH
We next examined the scope of the reaction for different
amines (aromatic, aliphatic, and cyclic) and cyclohexene
oxide (5) with selected bifunctional Lewis basic–Brønsted
acidic catalyst 2, using water as a solvent. The results are
N
H
F
Synlett 2010, No. 5, 707–711 © Thieme Stuttgart · New York

LETTER
Bifunctional Organocatalysis with N-Formyl-L-Proline
709
Table 2 Scope of Substrates: Aminolysis in the Presence of
N-Formyl-L-proline (1) Using Water as a Solvent12 (continued)
voted to developing metal-free approaches to this oxida-
tion process.11
R1
OH
Therefore, the search for new methodologies for sulfide
oxidation under mild reaction conditions with favorable
environmental impact still remains a challenging task.
catalyst 2
R1
R1
(10 mol%)
H2NR2
R2
+
O
R1
N
H
H2O, r.t.
Given our interest in the development of new organocata-
lytic method for sulfide oxidation, we subsequently tested
compounds 1–4 (Figure 1) as catalysts in the oxidation of
methyl phenyl sulfide (entries 2–5, Table 3).14 Our initial
studies were performed at room temperature (for 24 h) in
CH2Cl2 as a solvent and using readily available aqueous
hydrogen peroxide as an oxidant (entries 2–5 of Table 3).
Interestingly, again N-formyl-L-proline (2) performed
with better conversion with respect to compounds 1, 3,
and 4 (entry 3 vs. 2, 4, and 5). No overoxidation of sulfox-
ide was observed under these reaction conditions (entry 3,
Table 3). Only 9% conversion of 8 was afforded after 24
hours in the absence of catalyst. These results proved that
the bifunctional compound 2 is able to promote also the
sulfide oxidation reaction.
Entry
Product
Time (h)
Yield (%)a
44
OH
Cl
5
6
24
48
N
H
OH
66
80
N
H
N
OH
7
8
48
48
N
H
OH
N
75
We next screened a range of solvents (CHCl3, hexane, tol-
uene, MeOH, and H2O) at room temperature for the reac-
tion catalyzed by the selected organocatalyst 2 (entries 6–
13, Table 3). Interestingly, the results in CHCl3 resemble
those in CH2Cl2 (entries 3 and 6). In hexane and toluene,
N-formyl-L-proline (2) displayed much lower activity
than in CH2Cl2 as a solvent (entry 3 vs. 7, 8). Furthermore,
while 65% conversion and no overoxidation product was
observed in CH2Cl2, the conversion of 36% and the prod-
uct of overoxidation (94:6 sulfoxide/sulfone ratio) was
observed when the reaction was carried out in toluene (en-
try 3 vs. 8, Table 3). In polar solvents – MeOH and H2O –
the adduct was obtained after 24 hours with 64% and 45%
yields, respectively (entries 9 and 10). The yield in H2O
was further improved to 70% when the reaction was run
for 120 hours (entry 11). Yields were also improved in
CH2Cl2 or CHCl3 when the reaction time was extended to
120 hours (82% and 88%, respectively, entries 12 and 13).
OH
9
10
11
12
24
24
48
48
72
71
33
5
N
H
OH
N
H
Cl
Ph
OH
Ph
Ph
N
H
OH
Ph
N
H
OH
Acceleration of this oxidation process was observed in
CH2Cl2 on mild isothermal heating to only 40 °C with re-
spect to the results obtained at room temperature (entry 14
vs. 3 and 12). Thus, we observed 97% conversion with
high chemoselectivity (98:2 sulfoxide/sulfone ratio),
when carrying out the reaction in CH2Cl2 at 40 °C for 24
hours (entry 14). Notably, using CHCl3 or H2O as a sol-
vent at the same slightly elevated reaction temperature
gave no improvement in the reaction outcome (entries 15,
16 vs. 11, 13).
Ph
N
H
A
B
73
13
24
85:15 (A/B)
H
N
Ph
OH
a Yields of isolated products after chromatography.
Sulfide Oxidation with H2O2
While good to high yields were obtained for both investi-
gated reactions, the products were isolated with almost no
enantioselectivity.
The oxidation of sulfides to sulfoxides has attracted much
attention over the years due to the importance of sulfox-
ides as versatile building blocks in the synthesis of biolog-
ically active compounds or as chiral auxiliaries.9
Although further studies are needed to elucidate the reac-
tion mechanisms of these N-formyl-L-proline-catalyzed
epoxide ring opening and sulfide oxidation reactions, we
propose two corresponding working hypotheses shown in
Figure 2 and which could also explain the nearly racemic
outcome in both reactions.
Whereas some progress has been made in recent years in
developing transition-metal-based methodologies for sul-
fide oxidation reaction employing different sources of
oxygen donors,10 comparably less work has yet been de-
Synlett 2010, No. 5, 707–711 © Thieme Stuttgart · New York

710
S. Wei et al.
LETTER
base–Brønsted acid bifunctional organocatalyst, can cata-
lyze the aminolysis of 1,2-epoxides in water and a sulfide
oxidation with hydrogen peroxide. These results provide
novel insights in the development of enantioselective
Lewis basic–Brønsted acidic bifunctional organocatalyts
showing high enantioselectivity and general superiority
for these and other reaction types. This work is in progress
in our laboratory.
Table 3 Sulfide Oxidation in the Presence of Catalysts 1–4
O
S
O
O
catalyst
(20 mol%)
S
S
Me
Me
Me
+
H2O2 (1.2 equiv)
solvent
8
9a
9b
Entry
Catalyst Solvent
Temp
(°C)
Time
(h)
Conv. Ratioof
(%)a a/ba
1
2
–
1
2
3
4
2
2
2
2
2
2
2
2
2
2
2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
hexane
toluene
MeOH
H2O
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
40
40
24
24
9
15
65
7
n.d.c
100:0
100:0
n.d.c
100:0
100:0
n.d.c
94:6
n.d.c
n.d.c
n.d.c
n.d.c
n.d.c
98:2
100:0
n.d.c
Acknowledgment
The authors gratefully acknowledge generous financial support
from the Deutsche Forschungsgemeinschaft (SPP 1179 ‘Organo-
catalysis’) and BMBF.
3
24
4
24
References and Notes
5
24
15
64
7
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Synlett 2010, No. 5, 707–711 © Thieme Stuttgart · New York

LETTER
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Seraglia, R. Synthesis 1984, 325. (b) Pitchen, P.; Duñach,
E.; Desmukh, M. N.; Kagan, H. B. J. Am. Chem. Soc. 1984,
106, 8188. (c) Palucki, M.; Hanson, P.; Jacobsen, E. N.
Tetrahedron Lett. 1992, 33, 7111. (d) Komatsu, N.;
Hashizume, M.; Sugita, T.; Uemura, S. J. Org. Chem. 1993,
58, 4529. (e) Bolm, C.; Bienewald, F. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2640; Angew. Chem. 1995, 107, 2883.
(f) Kokubo, C.; Katsuki, T. Tetrahedron 1996, 52, 13895.
(g) Lattanzi, A.; Bonadies, F.; Senatore, A.; Soriente, A.;
Scettri, A. Tetrahedron: Asymmetry 1997, 8, 2473.
(h) Gunaratne, H. Q. N.; McKervey, M. A.; Feutren, S.;
Finlay, J.; Boyd, J. Tetrahedron Lett. 1998, 39, 5655.
(i) Legros, J.; Bolm, C. Angew. Chem. Int. Ed. 2004, 43,
4225; Angew. Chem. 2004, 116, 4321. (j) Drago, C.;
Caggiano, L.; Jackson, R. F. W. Angew. Chem. Int. Ed. 2005,
44, 7221; Angew. Chem. 2005, 117, 7387. (k) Legros, J.;
Bolm, C. Chem. Eur. J. 2005, 11, 1086. (l) Mba, M.; Prins,
L. J.; Licini, G. Org. Lett. 2007, 9, 21. (m) Egami, H.;
Katsuki, T. J. Am. Chem. Soc. 2007, 129, 8940. (n) Zeng,
Q.-L.; Tang, H.-Y.; Zhang, S.; Liu, J.-C. Chin. J. Chem.
2008, 26, 1435. (o) Sahoo, S.; Kumar, P.; Lefebvre, F.;
Halligudi, S. B. J. Catal. 2009, 262, 111.
(12) General Procedure for the Aminolysis of 1,2-Epoxides
A mixture of epoxide (0.31 mmol, 1 equiv), amine (2 equiv),
and N-formyl-L-proline (0.1 equiv) in H2O (150 mL) was
stirred at r.t. for 24 or 48 h. The reaction mixture was diluted
with H2O (1.5 mL) and extracted with CH2Cl2 (3 × 2 mL).
The combined organic layers were washed with brine (2
mL), dried (Na2SO4), and the solvent was removed under
reduced pressure. The residue was purified by chromatog-
raphy (SiO2, PE–EtOAc) to provide the desired b-amino
alcohols in yields given in Table 2. Products from the entries
1–5 and 7–13 are known compounds. Their constitution was
ascertained by comparison of spectroscopic data with
reported literature data.7h, 13 b-Amino alcohol from entry 6 is
a new compound, and its characterization data are reported
below.
1H NMR (300 MHz, DMSO-d6): d = 7.26 (t, 1 H, J = 7.5
Hz), 6.29–6.35 (m, 2 H), 5.65 (br s., 1 H), 3.24–3.45 (m, 2
H), 2.23 (s, 3 H), 1.85–1.96 (m, 2 H), 1.60–1.63 (m, 2 H),
1.08–1.22 (m, 4 H) ppm. 13C NMR (75 MHz, DMSO-d6):
d = 158.33, 154.56, 137.41, 110.26, 105.50, 73.21, 56.18,
34.20, 30.97, 24.13, 23.71 ppm. MS (MALDI): m/z = 207
[M + H]+; the exact molecular mass m/z = 206.1419 1.7
ppm [M+] was confirmed by HRMS (EI, 70 eV).
(13) (a) Overman, L. E.; Flippin, L. A. Tetrahedron Lett. 1981,
22, 195. (b) Woodgate, P. D.; Singh, Y.; Rickard, C. E. F.
J. Organomet. Chem. 1998, 560, 197. (c) Periasamy, M.;
Kumar, N. S.; Sivakumar, S.; Rao, V. D.; Ramanathan,
C. R.; Venkatraman, L. J. Org. Chem. 2001, 66, 3828.
(d) Carrée, F.; Gil, R.; Collin, J. Org. Lett. 2005, 7, 1023.
(e) Kureshy, R. I.; Singh, S.; Khan, N. H.; Abdi, S. H. R.;
Agrawal, S.; Mayani, V. J.; Jasra, R. V. Tetrahedron Lett.
2006, 47, 5277. (f) Bonollo, S.; Fringuelli, F.; Pizzo, F.;
Vaccaro, L. Synlett 2008, 1574. (g) Kureshy, R. I.; Prathap,
K. J.; Agrawal, S.; Khan, N. H.; Abdi, S. H. R.; Jasra, R. V.
Eur. J. Org. Chem. 2008, 18, 3118. (h) Hosseini-Sarvari,
M. Can. J. Chem. 2008, 86, 65.
(14) A Typical Procedure for the Oxidation of Methyl Phenyl
Sulfide
To a solution of catalyst (0.048 mmol) in a solvent (500 mL)
methyl phenyl sulfide (0.242 mmol) was added at r.t. To this
mixture 30% H2O2 (29.6 mL, 1.2 equiv) was added in one
portion and stirred for 24 h at r.t. After quenching the
reaction with Na2SO3, the conversion was determined by GC
analysis using hexadecane 99% (30 mL) as internal standard.
The yield of isolated product was obtained directly by
column chromatography (PE–EtOAc, 1:1). The isolated
methyl phenyl sulfoxide was identified through comparison
of 1H NMR spectra with literature data. See ref. 10g,k,m.
(11) For metal-free procedures of sulfoxidation, see: (a) Tohma,
H.; Takizawa, S.; Watanabe, H.; Fukuoka, Y.; Maegawa, T.;
Kita, Y. J. Org. Chem. 1999, 64, 3519. (b) Bethell, D.;
Synlett 2010, No. 5, 707–711 © Thieme Stuttgart · New York

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