Use of the chiral pool - Practical asymmetric organocatalytic strecker reaction with quinine

Article abstract of DOI:10.1002/adsc.200800798

An efficient, organocatalytic enantiose-lective synthesis of N-arylsulfonyl α-amino nitriles from the corresponding α-amido sulfones has been developed. This quinine-catalyzed Strecker reaction provides the corresponding cyanated products in good yields a

Full text of DOI:10.1002/adsc.200800798

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DOI: 10.1002/adsc.200800798
Use of the Chiral Pool – Practical Asymmetric Organocatalytic
Strecker Reaction with Quinine
Rꢀdiger Reingruber,a,b Thomas Baumann,c Stefan Dahmen,c,d,*
and Stefan Brꢁsea,b,*
a
Institute of Organic Chemistry, University of Karlsruhe (TH), Fritz-Haber-Weg 6, 76131 Karlsruhe, Germany
Fax : (+49)-721-608-8581; phone: (+49)-721-608-2902; e-mail: stefan.braese@kit.edu
Institute of Organic Chemistry, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany
cynora GmbH, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
New address: BASF SE, ED/DN, 67056 Ludwigshafen, Germany
b
c
d
Received: December 21, 2008; Revised: March 23, 2009; Published online: May 5, 2009
Dedicated to Armin de Meijere on the occasion of his 70th birthday.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800798.
Abstract: An efficient, organocatalytic enantiose-
lective synthesis of N-arylsulfonyl a-amino nitriles
from the corresponding a-amido sulfones has been
developed. This quinine-catalyzed Strecker reaction
provides the corresponding cyanated products in
good yields and enantioselectivities.
naturally occurring alkaloid quinine (2)[4] using potas-
sium cyanide (KCN) as a safe and convenient cyanat-
ing agent (Scheme 1).
The required N-carbamoyl a-amido sulfone 1, a
synthetic precursor of the corresponding aldimines,
can be readily prepared from aldehydes and the cor-
responding carbamate following known protocols in
the literature, and easily purified by recrystalliza-
tion.[7]
As a model reaction, we investigated the reactivity
of the representative substrate 1a, which can easily be
synthesized from benzaldehyde, tert-butyl carbamate
and NaSO2Ph. To avoid the highly toxic and volatile
HCN, we used potassium cyanide (KCN), a more con-
Keywords: asymmetric cyanation; chiral pool; orga-
nocatalysis; potassium cyanide; quinines; Strecker
reaction
The catalytic asymmetric cyanation of imines, the so- venient cyanation reagent, which is easier to handle.
called Strecker reaction,[1] is one of the most powerful In this process, KCN plays two roles. It first acts as a
and efficient strategies for the synthesis of chiral base, liberating the free N-carbamoyl imine by depro-
a-amino acids and their derivatives. In the last tonation of precursor 1 and subsequent elimination of
decade, both metal-catalyzed[2] processes and organo-
catalytic approaches[3,4] have been developed for this
purpose and a large number of highly successful and
efficient protocols, providing a reliable access to a
wide range of optically active a-amino nitriles, have
been reported.[5] However, more efforts are still
needed in this area, because the highly efficient cyan-
ation of imines to afford the precursors of pharma-
ceutically important a-amino acids is always prevail-
ing and the quest for a universal and powerful orga-
nocatalytic system which avoids the use of hazardous
and expensive cyanation agents like TMSCN or HCN
still remains a challenge.
Herein, we present an enantioselective cyanation of
several N-carbamoyl aldimines, in situ generated from
a-amido sulfones 1,[6] promoted by the powerful and Scheme 1. Organocatalytic Strecker reaction with quinine.
Adv. Synth. Catal. 2009, 351, 1019 – 1024
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Rꢀdiger Reingruber et al.
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Scheme 2. In situ formation of the N-carbamoyl imine.
Scheme 3. Phase-transfer-catalyzed enantioselective Strecker reactions of a-amido sulfones with cyanohydrins.[4]
the sulfinate. The intermediately formed HCN then
Trying to find the ideal temperature (Table 1, en-
acts as the cyanating agent giving enantioenriched tries 5–7), it is important to mention that the selectivi-
product 3 in the presence of suitable chiral catalysts ty dropped significantly when we performed the reac-
(Scheme 2).
tion at room temperature and that there was no reac-
A screening of various catalysts revealed readily
available commercial alkaloids as the best choice for
further optimization of the selectivity of the catalytic
system (Table 1 and Table 2).
Table 1. Screening of reaction conditions for the organocata-
lytic Strecker reaction.[a]
In a publication from 2006, Ricci et al. have de-
scribed a phase-transfer-catalyzed Strecker reaction
that is related to the described process (Scheme 3).[4]
They applied a quaternary ammonium salt of quinine
as catalyst in a biphasic system containing potassium
carbonate as a base and cyanohydrin as a cyanide
source. Using an electron-deficient benzylic substitu-
ent on the quinine catalyst, they successfully achieved
the Strecker reaction with aliphatic substrates.
In contrast to the work of Ricci, however, it was
found that chiral phase-transfer catalysts did not show
any activity on our system. The use of other cyanating
agents like NaCN, TMSCN or acetone cyanohydrin
(even in presence of a base) did not show any signifi-
cant improvement and led to depletion of yield and/
or selectivity.
From screening various solvents, it was found that
the polarity of the solvent had a significant effect on
the outcome of the reaction (Table 1, entries 1–4).
Thus, CH2Cl2 and toluene were proven to be supe-
rior to any other solvents. While polar, non coordinat-
ing solvents like CH2Cl2 improve reaction yield by en-
abling sufficient solubility of substrate as well as KCN
salt, polar, coordination solvents like diethyl ether
drastically diminish enantioselectivity.
Entry
Solvent
T [8C]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
CH2Cl2
CHCl3
toluene
Et2O
CH2Cl2
CH2Cl2
CH2Cl2
0.5 mL CH2Cl2
10 mL CH2Cl2
0
0
0
0
+25
À10
À40
0
67
88
30
40
80
82
NR
88
NR
50 (R)[d]
37 (R)
43 (R)
5 (S)
19 (R)
57 (R)
–
33 (R)
–
0
[a]
Reactions were performed on 250 mmol scale using 10
mol% of quinine (2) and 2.0 equiv. of KCN in 1.0 mL of
the specified solvent at the stated temperature for 24 h.
Isolated yields after purification.
Determined by HPLC analysis on chiral stationary
phase.
[b]
[c]
[d]
Determined by comparison with literature values of the
optical rotation.[8]
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Use of the Chiral Pool – Practical Asymmetric Organocatalytic Strecker Reaction
Table 2. Catalyst and additive screening for the organocatalytic Strecker reaction. Selected examples.[a]
COMMUNICATIONS
Entry
Catalyst
Additive
Yield [%][b]
ee [%][c]
1
2
10 mol% cinchonidine
30 mol% N-methylephedrine
5 mol% O-benzoylquinine
5 mol% quinidinium sulfate
5 mol% quinine N-oxide
10 mol% quinine
10 mol% quinine
5 mol% quinine
5 mol% quinine
–
–
–
–
46
48
58
92
51
76
95
57
62
10 (R)[e]
16 (S)
37 (S)
32 (S)
25 (R)
49 (R)
27 (R)
40 (R)
46 (R)
3
4[d]
5
–
6
7
8
9
1 mL heptane
1 mL H2O
10 mL AcOH
6 mL H3PO4
[a]
Reactions were performed on 250 mmol scale using 2.0 equiv. KCN in 1.0 mL CH2Cl2 at À108C for 24 h.
Isolated yields after purification.
[b]
[c]
[d]
[e]
Determined by HPLC analysis on chiral stationary phase.
2.0 equiv. NaCN as cyanation reagent.
Determined by comparison with literature values of the optical rotation.[8]
tion at all (even after 3 days), when we went down to turnover, blocking the free OH group of quinine dras-
À408C. According to this observation the optimal tically reduces activity as well as selectivity, even re-
temperature to achieve high yield and selectivity moval of the aromatic methoxy group (e.g., with free
turned out to be À108C (Table 1, entry 6). We assume cinchonidine) leads to drastically diminished results.
that the main influence of the temperature comes At this point we began to assume that the deprotona-
about by carefully balancing the amounts of free dis- tion of the substrate as well as the addition of cyanide
solved reactive intermediates N-carbamoyl imine and proceed via a proton shuttle process facilitated by the
KCN/HCN in the reaction mixture.
catalyst. Further evidence are the facts that water is
The appropriate amount of catalyst used for the de- tolerated to a high amount (Table 2, entry 7, biphasic
scribed reaction was found to be 5–10 mol% as there system) and even the addition of acids does not nega-
was no further improvement when using more than tively impact the enantioselectivity (Table 2, entries 8
10 mol%. Looking at the ideal concentration while and 9). Especially the latter in combination with the
performing the reaction, we found that the selectivity fact that quinidinium sulfate as catalyst (Table 2,
dropped when the mixture was too concentrated and entry 4) still resulted in reasonable enantioselectivity
no reaction took place when it was too highly diluted and that the addition of Li salts dramatically dimin-
(Table 1, entries 8–9). The use of other catalysts such ishes the ee gives strong evidence that the enantio-dis-
as cinchonidine, N-methylephedrine or quinine deriv- criminating step involves an N-protonated catalyst in-
atives (Table 2, entries 1–5) showed only minor cata- termediate.
lytic activity. Thus, the naturally occurring quinine
Aliphatic imine substrates bearing active alpha hy-
itself was proven to be the most effective catalyst for drogens like the ones applied by Ricci et al. are usual-
the reaction, both in terms of efficiency and enantio- ly less reactive than aromatic substrates. This also
selectivity (Table 1, entry 6).
manifests in the finding that aliphatic sustrates are
In order to further improve the reaction outcome, prone to undergo enamine side reactions when ap-
various additives were tested but no overall improve- plied in (metal-) catalyzed addition reactions.[6d] In
ment could be observed (Table 2, entries 6–9). How- our experience, aromatic substrates tend to be more
ever, there are several striking features of this reac- reactive but also more sensitive to hydrolysis. Al-
tion that shed some light on a possible reaction mech- though mechanistic details for both processes are still
anism. While trying to optimize the catalyst structure, elusive, we assume that the inherent differences in the
we screened various N,O ligands in this reaction. It reaction mechanism explain why Ricci only reported
turned out that all structural features of quinine are the use of aliphatic substrates while our system gives
necessary to obtain optimal reactivity and selectivity. rise to aromatic products. While the enantioselective
Primary and secondary amino alcohols give poor step in the Ricci reaction would have to include a
Adv. Synth. Catal. 2009, 351, 1019 – 1024
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Rꢀdiger Reingruber et al.
Table 3. Generality and scope of the asymmetric organocatalytic Strecker reaction with quinine (2).[a]
COMMUNICATIONS
Entry
R1
R2
T [8C]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
Ph
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
t-Bu
À10
À10
À10
À15
À10
À10
82
99
99
95
35
95
57 (R)[d]
61 (R)
74 (R)
62 (R)
62 (S)
79 (R)
4-MeC6H4
3-MeC6H4
2-MeC6H4
3-MeOC6H4
1-naphthyl
7
Ph
À10
34
62 (R)
8
Ph
À10
68
60 (S)
9
Ph
À10
0
95
53
98
25
56 (R)
64 (S)
80 (S)
54 (S)
10
11
12
Ph
1-naphthyl
Ph
À10
À10
13
14
15
3-ClC6H4
2,6-di-ClC6H3
1-furyl
t-Bu
t-Bu
t-Bu
À10
À10
À10
80
91
94
34 (S)
48 (R)
43 (R)
[a]
Reactions were performed on 500 mmol scale using 5 mol% of quinine (2) and 2.0 equiv. of KCN in 1.0 mL of CH2Cl2 at
the stated temperature for 24 h.
Isolated yields after purification.
[b]
[c]
[d]
Determined by HPLC analysis on chiral stationary phase.
Determined by comparison with literature values of the optical rotation.[8]
ꢃhardꢄ anionic PTC-bound cyanide giving rise to a fast in good yield and selectivity. Although the selectivity
addition step, our system likely includes a ꢃsoftꢄ hy- dropped down significantly when using electron-poor
drogen cyanide bound to a proton-shuttle catalyst. imine precursors, the corresponding nitriles could still
Additionally, we would expect the phase-transfer de- be obtained in synthetically useful yields (Table 3, en-
protonation step in the Ricci variant to be the rate- tries 13 and 14). As far as aliphatic substrates con-
determining step, while in our case deprotonation and cerned, we tested a pivalaldehyde derivative which,
addition are both strongly influenced by the catalyst however, could not be converted into the correspond-
and are estimated to be both equally fast under the ing nitrile. This finding also strengthens our hypothe-
chosen reaction conditions.
sis about the reaction mechanism as discussed above.
Having established the optimum conditions for the
The resulting N-carbamoyl-a-aminonitriles 3 can be
reaction (Scheme 1), we proceeded to investigate its converted to the corresponding amino acids by reflux-
generality and scope. As demonstrated in Table 3, a ing with aqueous HCl as described in the litera-
series of optically active a-amino nitriles bearing
ture.[6a,9] The amino acid hydrochlorides 4 were ob-
ACHTUNGTRENNUNG
bulky substituents at the aromatic core as well as at tained by evaporation of the solvent and recrystalliz-
the carbamoyl group result from the Strecker reaction ing the residue with water (Scheme 4).
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Use of the Chiral Pool – Practical Asymmetric Organocatalytic Strecker Reaction
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General Procedure for the Strecker Reaction of N-
Carbamoyl-a-(phenylsulfonyl)amines 1
A 0.5M solution of the N-carbamoyl-a-(phenylsulfonyl)-
amine (500 mmol, 1.00 equiv.) and quinine (2) (5 mol%) in
CH2Cl2 (1.0 mL) was cooled to À108C under argon. At this
temperature KCN (1.00 mmol, 2.00 equiv.) was added and
the reaction mixture was stirred vigorously for 24 h at
À108C. After completion, the mixture was quenched with
saturated aqueous NaHCO3 (1.00 mL), extracted with
CH2Cl2 (2ꢅ5 mL), dried over MgSO4 and concentrated
under vacuum. The resulting residue was further purified by
column chromatography to give the pure title compounds.
The enantiomeric excess (ee) of each product was deter-
mined by HPLC analysis on chiral stationary phase.
Scheme 4. Hydrolysis of the N-carbamoyl-a-aminonitriles 3.
In summary, we have demonstrated an organocata-
lytic enantioselective Strecker reaction making use of
the chiral pool and a-amido sulfones.[10] The overall
process is highly efficient, encompasses a broad sub-
strate scope and was accomplished by making use of
the powerful and naturally occurring catalyst quinine
(2) and KCN as a less hazardous cyanide source. In
the presence of 5–10 mol% catalyst, excellent yields
up to 99% and reasonable enantioselectivities up to
80% ee were achieved for many substrates. Thus the
reported method provides a convenient route to un-
natural amino acids that may find an application in
medicinal chemistry.
Acknowledgements
We thank the BMBF (Wing – QuatCat) for financial support.
R. R. is a fellow of the Landesgraduiertenfçrderung Baden-
Wꢀrttemberg.
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Carbamoyl-a-(phenylsulfonyl)amines 1 (Method A)
A mixture of the carbamate (1.00 equiv.) and benzenesulfin-
ic acid salt (2.00 equiv.) was suspended in a solution of
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(1.50 equiv.) was added in one portion, followed by formic
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General Procedure for the Preparation of N-
Carbamoyl-a-(phenylsulfonyl)amines 1 (Method B)
A mixture of the carbamate (2.00 equiv.) and benzenesulfin-
ic acid salt (1.50 equiv.) was suspended in a mixture of tolu-
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(1.00 equiv.) was added in one portion, followed by formic
acid (98%, 1.50 equiv.) and TMSCl (1.10 equiv.). The result-
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ether, washed with brine, dried over MgSO4 and concentrat-
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and washed with water and diethyl ether. The precipitate
was then recrystallized from CH2Cl2/hexane to obtain the
title compound as a crystalline material.
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