Chiral derivatives of 1,2-benzenedisulfonimide as efficient Bronsted acid catalysts in the Strecker reaction

Article abstract of DOI:10.1039/c4ob00552j

Two chiral derivatives of 1,2-benzenedisulfonimide, namely 4-methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide and 4,5-dimethyl-3,6-bis(o- tolyl)-1,2-benzenedisulfonimide, have been easily synthesized in good overall yields (respectively 34% and 41%) by me

Full text of DOI:10.1039/c4ob00552j

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Chiral derivatives of 1,2-benzenedisulfonimide as
efficient Brønsted acid catalysts in the Strecker
reaction†
Cite this: Org. Biomol. Chem., 2014,
12, 3902
Margherita Barbero, Silvano Cadamuro, Stefano Dughera* and Roberta Torregrossa
Two chiral derivatives of 1,2-benzenedisulfonimide, namely 4-methyl-3,6-bis(o-tolyl)-1,2-benzenedi-
sulfonimide and 4,5-dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide, have been easily synthesized in
good overall yields (respectively 34% and 41%) by means of an eleven-step synthetic protocol from com-
mercially available 2-methyl-6-nitroaniline or 2,3-dimethyl-6-nitroaniline. 4,5-Dimethyl-3,6-bis(1-
naphthyl)-1,2-benzenedisulfonimide was also synthesized, but the overall yield from 2,3-dimethyl-6-
nitroaniline was lower (9%). The atropisomers of these compounds have been resolved and (−)atrop-
isomers have been demonstrated to be efficient chiral catalysts in the Strecker reaction.
Received 12th March 2014,
Accepted 7th April 2014
DOI: 10.1039/c4ob00552j
www.rsc.org/obc
Two different research groups have very recently reported
that chiral binaphthyl sulfonimides are very effective chiral
Introduction
Asymmetric catalysis is still one of the major challenges in Brønsted acid catalysts. A number of binaphthyl sulfonimides
modern organic chemistry,1,2 and chiral Brønsted organic acid have been synthesized by Lee et al. and tested in the asym-
catalysis, in particular, is an emerging area.3 In fact, asym- metric Friedel–Crafts alkylation of indoles with imines.7
metric Brønsted acid catalysis has proved to be a very efficient List et al. developed chiral binaphthyl sulfonimide-catalyzed
tool for the synthesis of chiral molecules as it can boast of a Mukaiyama aldol reactions, vinylogous and bisvinylogous
number of impressive capabilities that contribute to obtaining Mukaiyama aldol reactions, hetero Diels–Alder reactions as
the target products with very good enantioselectivity.
well as Hosomi–Sakurai reactions.8 Both groups obtained
Currently, BINOL-derived phosphoric acids3c,d (first pro- excellent enantioselectivity results, especially when sulfon-
posed by Akiyama et al.4 and Terada et al.5) and the corres- imides bore bulky aryl groups with electron-withdrawing
ponding N-triflyl phosphoramides (first proposed by substituents.
Yamamoto et al.6) are probably the most widely used chiral
We have recently reported on the use of o-benzenedisulfon-
Brønsted acid catalysts, and this well established class of imide9a (OBS; 1; Fig. 1), in catalytic amounts, as a safe, non-
chiral Brønsted acid catalysts has been applied to an ever- volatile and noncorrosive Brønsted acid in several acid-catalyzed
increasing number of useful transformations.
organic reactions under very mild and selective conditions.9b–m
The catalyst was easily recovered and purified, and was ready
to be used in further reactions, giving economic and ecological
advantages.
The results and advantages of using 1 are very promising
considering its applications in the field of asymmetric cata-
lysis. We have, therefore, proposed the synthesis of a 1,2-benzene-
disulfonimide chiral derivative, namely 4-methyl-3-(o-tolyl)-
Dipartimento di Chimica, Università di Torino, C.so Massimo d’Azeglio 48,
10125 Torino, Italy. E-mail: stefano.dughera@unito.it
†Electronic supplementary information (ESI) available: Synthesis of anilines 5
and their physical and spectral data; p. 2. Synthesis of diiodonitro derivatives 7
and their physical and spectral data; pp. 2–3. Synthesis of 4-nitro-3,6-bis(o-tolyl)-
o-xylene (15) and its physical and spectral data; p. 3. Synthesis of diiodoanilines
8 and their physical and spectral data; p. 4. Synthesis of 2,5-bis(o-tolyl)-3,4-
dimethylaniline (16) and its physical and spectral data; pp. 4–5. Synthesis of
diiodoisatins 10 and their physical and spectral data; p. 5. Synthesis of 5,6-
dimethyl-4,7-bis(o-tolyl)isatin (11b) and its physical and spectral data; pp. 5–6.
Synthesis of diarylisatins 11 and their physical and spectral data; pp. 6–7. Syn-
thesis of 2-aminobenzoic acids 12 and their physical and spectral data; pp. 7–8.
Synthesis of 1,3-benzodithioles 13 and their physical and spectral data; pp. 8–9.
1H NMR and 13C NMR spectra of unknown products; pp. 10–63. HPLC spectra of
sulfonylchlorides 14; pp. 64–71. Spectral and physical data of nitriles 21;
pp. 72–74. 1H NMR and 13C NMR spectra of nitriles 21; pp. 75–90. Chiral GC
spectra of nitriles 21; pp. 91–110. See DOI: 10.1039/c4ob00552j
Fig. 1 OBS 1 and its chiral derivative 2.
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1,2-benzenedisulfonimide (2; Fig. 1).10a Its chirality is due to
the hindered rotation of the aryl group (atropisomerism). We
used it as a chiral catalyst in the Strecker reaction and
obtained unsatisfactory enantioselectivity results.10b
Results and discussion
Synthesis of chiral derivatives of 1
Our goal was now to synthesize a chiral derivative of 1 which
can be more efficient than 2 in asymmetric catalysis.
Fig. 2 Chiral adducts 3a, 3b, 3c.
The key to achieving good enantioselectivity results in the
presence of strong chiral Brønsted acids as catalysts is the
hydrogen bonding established between the protonated sub-
strate and the conjugate chiral anion. Furthermore, a Brønsted
acid should have the following attributes if it is to be con-
sidered a good chiral catalyst: adequate acidity11a in order to
catch up the substrate through hydrogen bonding without loose
ion-pair formation,5b type C2 symmetry,11b,c and the presence of
bulky substituents closer to the acidic function that increase
the stereochemical communication between the catalyst and the
substrate.8a Actually, 2 does not present type C2 symmetry and
has only one bulky group in the ortho position. In order to
improve the chiral structure of 2, we herein report the syn-
thesis of 4-methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide 3a
(Fig. 2) which has two bulky groups in the ortho positions
but does not show C2 symmetry, 4,5-dimethyl-3,6-bis(o-tolyl)-
1,2-benzenedisulfonimide 3b and 4,5-dimethyl-3,6-bis-
(1-naphthyl)-1,2-benzenedisulfonimide 3c (Fig. 2) which have
two bulky groups in ortho positions and C2 symmetry. These
new catalysts were tested in the Strecker reaction.
The precursor to 3a is the corresponding anthranilic acid
12a which was obtained from commercially available 2-methyl-
6-nitroaniline (4a) as reported in Scheme 1.
12a was then converted into 3a, using the same synthetic
protocol10a as that employed in the preparation of
2
(Scheme 2). The overall yield of 3a, obtained with this
synthetic sequence from 4a, was 34%.
Conversely, the overall yield of imide 3b, starting from 2,3-
dimethyl-6-nitroaniline (4b), was very low (only 3%). In fact,
the acid cyclization of 9b mainly afforded a monodeiodinated
adduct. Moreover, the Suzuki coupling of 10b afforded the
adduct 11b in a low yield (28%). It was possible to increase the
Suzuki coupling yield (89%) by reacting 10b with o-tolylboronic
Scheme 1 Synthetic protocol for anthranilic acids 12.
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Scheme 2 Synthetic protocol for sulfonimides 3.
in the presence of Pd2(dba)3, Sphos and K3PO4 and toluene as into 3b (Scheme 2). The overall yield thus increased consider-
a solvent12 and the overall yield increased by up to 10%. Fur- ably (41%).
thermore, we modified the synthetic protocol to further
increase the overall yield as described below (Scheme 3).
3c was also synthesized using the procedures reported in
Schemes 1 and 2. The overall yield from 4b was 9% when the
First of all, we carried out the Suzuki coupling on adduct Suzuki coupling was carried out in the presence of Pd2(dba)3,
7b and in the presence of Pd2(dba)3, Sphos and K3PO4.12 In Sphos and K3PO4.
this way, we obtained the diarylated adduct 15 in a very good
Unfortunately, it was impossible to obtain the adduct
yield (95%).
derived from the Suzuki coupling of 7b with adequate purity
The reduction of the nitro group allowed us to obtain in this case (Scheme 3).
amine 16 which was transformed into the corresponding
isatine 11b in good yields (89%).
Separation of atropisomers
It is worth noting that it was necessary to use MeSO3H After preparing chiral sulfonimides 3, the next step was the
instead of H2SO4 to transform intermediate 17 into 11b. It was resolution of their atropisomers. However, 3 are highly acidic
then converted into anthranilic acid 12b, which was converted (their pKa value is most likely similar to that of parent OBS 1
Scheme 3 Synthetic protocol for isatine 11b.
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Fig. 3 Atropisomers of 14.
which, measured, was 6.09 in butanone13) and polar com- Use of (−)atropisomers of 3 as catalysts
pounds, so their chromatographic separation can be con-
We reported that 2 catalyzed the Strecker reaction between
acetophenone (18a), aniline (19a) and TMSCN (20) although it
gave poor enantioselectivity (Table 1, entries 2 and 3).10b
In the light of this, we decided to test the (−)atropisomer of
3 in this model reaction (Scheme 4), under the same con-
ditions as those previously employed.10b
The results are encouraging and are collected in Table 1.
The use of (−)3a led to an increase in ee in 2-phenylamino-
propanenitrile (21a; Table 1, entries 4 and 5). But the best
results were obtained using (−)3b. The ee increased up to 84%
when the reaction was carried out at 0 °C (Table 1, entry 6)
and further cooling to −20° C increased it up to 94% (Table 1,
entry 7). The yield, however, diminished as a small amount of
the unreacted starting products was still present; 3c was also a
good chiral catalyst (Table 1, entries 8 and 9). From these data,
sidered extremely difficult. We preferred trying to separate the
atropisomers of disulfonyl chlorides 14 that have the same
chiral structure as 3. Their separation was possible by semi-
preparative chiral HPLC.
In particular, 14a has only one structural motif that leads to
atropisomerism and this is due to the hindered rotation of the
aryl group bonded to the 3 position carbon. It is worth noting
that no hindered rotation of the aryl group bonded to the 6
position carbon takes place due to the lack of a methyl group
in the 5 position.10a
The racemic mixture of 14a (40 mg) was chromatographed
on
a semipreparative Chiralpak IC column. The two
atropisomers (−)14a and (+)14a, eluted as single peaks,
were collected (respectively 19.2 mg and 20.8 mg; ratio
1 : 1.08), after the solvents were removed under nitrogen flow
(Fig. 3).
The situation is different for 14b and 14c for which there
are two identical structural motifs that lead to the atropisomer-
ism. For this reason, there is also a meso form in addition to
the two atropisomers. Regarding 14b, the mixture of atrop-
isomers and meso form (40 mg) was chromatographed on a
semipreparative Chiralpak IC column using an isocratic
elution. We thus separated the meso form (the first eluted com-
pound, 19.8 mg) from a racemic mixture of two atropisomers
(the second eluted compound, 20.2 mg); ratio 1 : 1.02. Then,
another run (20 mg) allowed us to separate the atropisomer (−)
14b (Fig. 3; the first eluted compound, 9.5 mg), from the atrop-
isomer (+)14b (Fig. 3; the second eluted compound, 10.5 mg);
ratio 1 : 1.1. The same procedure was employed to separate the
atropisomers of 14c (40 mg). First, we separated the meso form
(the first eluted compound, 20.4 mg) from a racemic mixture
of two atropisomers (the second eluted compound, 19.6 mg);
ratio 1.04 : 1. Then, another run (19 mg) allowed us to separate
the atropisomer (−)14c (Fig. 3; the first eluted compound,
9.4 mg), from the atropisomer (+)14c (Fig. 3; the second eluted
compound, 9.6 mg); ratio 1 : 1.02.
Table 1 Three-component Strecker reaction between 18a, 19a and 20
catalyzed by chiral catalysts (−)2 and (−)3
Temp.
(°C)
Time
(min)
Yielda,b
(%) of 21a
er in
21a
ee (%)
in 21a
Entry
Catalyst
110b
210b
310b
4
5
6
7
8
9
1
2
2
3a
3a
3b
3b
3c
3c
rt
rt
0
0
−20
0
−20
0
−20
5
5
95
92
88
85
65
86
70
84
72
51 : 49
66 : 34
78 : 22
82 : 18
84 : 16
92 : 8
97 : 3
90 : 10
96 : 4
—
32
56
64
68
84
94
80
92
180
120
360
120
360
120
360
a All the reactions were performed with 5 mol% of 3 (10 mg); reactants
18a and 19a were in equimolar amounts (see the Experimental
section). TMSCN (20) was in slight excess (1.1 eq.). b Yields refer to the
pure and isolated products.
Finally,
the
separated
atropisomers
(−)14a–c
were easily transformed into the corresponding (−)3a–c
(Scheme 2).
Scheme 4 Strecker reaction with chiral catalysts 3.
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in order to fully explain the role of 3b as the chiral catalyst,
depth mechanistic studies are planned.
Table 2 Consecutive runs with recovered (−)3b
Time
(min)
Yield (%)
Recovery (%)
of 3b
ee (%)
in 21
According to these data, this chiral Brønsted acid catalyzed
reaction could be classified as ACDC.2a In fact, List2a has
recently introduced the concept of asymmetric counteranion-
directed catalysis (ACDC) in which the induction of enantio-
selectivity in a reaction proceeds through a cationic intermediate
by means of ion pairing with a chiral and enantiomerically pure
anion provided by the catalyst.
Entry
of 21a,b
1
2
3
120
120
180
86
85
89
95, 9.5 mgc
86, 8.2 mgd
82, 6.7 mg
84
82
81
a Yields refer to the pure products. b The reaction was performed at
0 °C with 0.0468 mmol of 18a and 19a, 0.05148 mmol of 20 and 5 mol%
of 3b (10 mg, 0.0234 mmol). c Recovered 3b was used as a catalyst in
entry 2. d Recovered 3b was used as a catalyst in entry 3.
Conclusions
it is evident that C2 symmetry is essential for obtaining a good
enantioselectivity.
A protocol for the synthesis of three chiral derivatives of
Catalyst 3b was easily recovered and used in another two o-benzenedisulfonimide, namely 3a, 3b and 3c, was developed
consecutive catalytic cycles. The results are listed in Table 2. under easy and moderate reaction conditions and with good
The enantiomeric excess was good in all cases.
overall yields.
The high enantiomeric excesses achieved encouraged us to
In particular, the atropisomers (−)3b showed a good
further exploit catalyst 3b in Strecker reactions. A selection (15 enantioselectivity in the Strecker reaction (16 examples,
examples) of good results is presented in Table 3. The presence average ee 91%) and proved to be interesting chiral catalysts.
of electron-donating or electron-withdrawing groups on the In fact, C2 symmetry11a,b and the presence of aryl substituents
aromatic ring of 18 or 19 did not affect the enantioselectivity on the sulfonimidic ring (even without electron-withdrawing
of the reactions which generally provided excellent enantio- groups) created a chiral fashion allowing to have a high
meric excess. Even with aliphatic 18l and aldehydes 18m–p enantioselectivity. In order to confirm these good results, (−)
very good results were obtained.
3b will, of course, be tested in further reactions. Clearly, a
Theoretical calculations have allowed us to explain the shortcoming of this synthetic protocol is its lack of enantio-
mechanism of the Strecker reaction catalyzed by 1.10b The selectivity. It is well known that chiral biaryl compounds have
same mechanism should be used in the reactions catalyzed by been easily synthesized in very good ee via catalytic asymmetric
3b (Scheme 5).
Suzuki coupling.14 In order to obtain enantiomerically pure 15
In particular, the intermediate 26a should be crucial for (one of the precursors to 3b; Scheme 3), we could carry out
carrying out this reaction in a highly enantioselective fashion. this coupling in the presence of a chiral ligand. However, in
However, it must be stressed that this is only a hypothesis, and the step in which 12 was converted into 13, the formation of a
Table 3 Three-component Strecker reaction catalyzed by (−)3b
Entry
R in 18, 21
R′ in 18, 21
R″ in 19, 21
Products 21
Yielda,b (%)
Time (h)
ee in 21 (%)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
H
18b
18c
18d
18e
18f
18g
18h
18i
18j
18k
18l
18m
18n
18o
18p
4-MeOC6H4
4-NO2C6H4
4-BrC6H4
4-FC6H4
2-MeOC6H4
3-MeOC6H4
Ph
4-NO2C6H4
4-MeOC6H4
4-MeOC6H4
Ph
19b
19c
19d
19e
19f
19g
19h
19j
19k
19l
19m
19n
19o
19p
19q
21b
21c
21d
21e
21f
21g
21h
21i
69
65
61
64
68
70
68
72
68
74
71
80
85
88
84
6
6
7
5
6
6
7
6
5
8
8
2
2
2
2
95
93
92
93
82
89
88
93
97
95
87
88
95
89
89
4-MeC6H4
4-MeC6H4
4-MeC6H4
4-NO2C6H4
nPr
9
21j
10
11
12
13
14
15
21k
211
21m
21n
21o
21p
Ph
Ph
Ph
Ph
Ph
4-NO2C6H4
4-MeC6H4
2-Thienyl
H
H
H
a All the reactions were performed at −20 °C with 5 mol% of 3b (10 mg); the reactants 18 and 19 were in equimolar amounts (see the
Experimental section). TMSCN (20) was in slight excess (1.1 eq.). b Yields refer to the pure and isolated products.
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Scheme 5 Mechanism of the three-component acid-catalysed Strecker reaction catalyzed by 3b.
benzyne intermediate occurred (Scheme 2). Most likely, this of compounds 5, 7, 8, 10, 11, 12, 13, 15 and 16 precursors of
benzyne is not enough hindered to prevent rotation of the aryl 14a, 14b and 14c are reported in ESI.†
group so that the previously achieved enantiomeric purity is
lost.
Synthesis of 1,2-benzenedisulfonyl chlorides 14. 1,3-Benzo-
dithiole (13; 2 mmol) was dissolved in t-butyl alcohol (20 mL),
Since (−)3b and (−)3c have proved to be (at least in the CH2Cl2 (16 mL) and H2O (3 mL). The resulting mixture was
Strecker reaction) good chiral catalysts, we will dedicate our cooled to 0 °C. Chlorine was bubbled through while the temp-
future research to developing an effective enantioselective syn- erature was maintained at 0 °C and the reaction mixture vigor-
thetic protocol that allows strictly chiral derivatives of 1 to be ously stirred. The reaction was monitored on TLC (PE–EtOAc
obtained, without having to separate the various atropisomers. 7 : 3). After 1 h, when the spot of 13 disappeared and there was
In addition, we will also evaluate the possibility of synthesizing only one other spot, the reaction was complete. The reaction
derivatives with more bulky aromatic groups which may also mixture was poured into CH2Cl2–H2O (100 mL, 1 : 1). The
bear electron-withdrawing substituents.
aqueous layer was separated and extracted with CH2Cl2
(100 mL). The combined organic extracts were washed with a
5% NaOH solution (100 mL), dried over Na2SO4 and evapor-
ated under reduced pressure. The crude residue, when purified
Experimental
General
in
pure 14.
4-Methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonyl
a chromatography column (PE–EtOAc 7 : 3), afforded
chloride
Analytical grade reagents and solvents were used and the reac-
tions were monitored by GC, GC-MS and TLC. Column
chromatography and TLC were performed on Merck silica gel
60 (70–230 mesh ASTM) and GF 254, respectively. Petroleum
ether (PE) refers to the fraction boiling in the range 40–70 °C.
Room temperature (rt) is 20–25 °C. Mass spectra were recorded
on an HP5989B mass selective detector connected to an HP
5890 GC with a cross-linked methyl silicone capillary column.
ESI-MS spectra were obtained using a Waters micromass ZQ
spectrometer equipped with an ESI ion source. Chiral analyses
were performed on a Perkin-Elmer Autosystem GC connected
to a J&W Scientific Cyclosil-B column; stationary phase: 30%
(14a). Couple of atropisomers. White waxy solid (0.86 g, 91%
yield). Found: C 53.76; H 3.92; Cl 15.07; S 13.71. C21H18Cl2O4S2
requires: C 53.74; H 3.87; Cl 15.11; S 13.66%. 1H NMR
(200 MHz, CDCl3): δ = 7.51 (s, 1H), 7.32–7.15 (m, 8H), 2.24 (s,
3H), 2.09 (s, 3H) 2.01 (s, 3H). 13C NMR (50 MHz, CDCl3): δ =
147.9, 147.5, 145.8, 145.2, 143.5, 141.2, 140.5, 138.2, 137.0,
136.0, 135.8, 130.9, 130.5, 129.7, 129.5, 128.7, 126.2, 125.6,
21.2, 21.1, 20.5. MS (ESI+) m/z: 469.12 (M + H)+. IR (neat)
ν (cm−1): 1383, 1175 (SO2).
Compound 14a (40 mg) was chromatographed on a semi-
preparative Chiralpak IC column (F 10 × 250 mm, Daicel,
Osaka, Japan) using an isocratic elution with heptane–CH2Cl2
(4 : 1) at a flow rate of 3.0 mL min−1. The compounds eluted
from the column were monitored using a photodiode array
detector. Two compounds (respectively 19.2 mg and 20.8 mg),
eluted as single peaks at 7.90 and 11.97 min, were collected,
after removing the solvents under nitrogen flow.
heptakis
(2,3-di-O-methyl-6-O-t-butyldimethylsilyl)-β-cyclo-
dextrin in DB-1701. 1H NMR and 13C NMR spectra were recorded
on a Bruker Avance 200 spectrometer at 200 and 50 MHz
respectively. IR spectra were recorded on an IR Perkin-Elmer
UATR-two spectrometer. HPLC separations were performed on
HPLC Waters 1525 connected to a column Chiralpak IA-SFC.
For the determination of optical rotations, a Jasco P-2000
polarimeter was used. All reagents were purchased from
Sigma-Aldrich. Satisfactory microanalyses were performed for
all new compounds. Syntheses and physical and spectral data
The first one was (−)4-methyl-3,6-bis(o-tolyl)-1,2-benzenedi-
sulfonyl chloride (14a; waxy white solid). [α]2D0 −12.3 (c 0.18,
CH2Cl2). Spectral data were identical to what had been
reported for the couple of two atropisomers.
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The second one was (+)4-methyl-3,6-bis(o-tolyl)-1,2-benzene- (200 MHz, CDCl3): δ = 7.68–7.44 (m, 10H), 1.98 (s, 6H). 13C
disulfonyl chloride (14a; waxy white solid). [α]2D0 +12.9 (c 0.18, NMR (50 MHz, CDCl3): δ = 137.8, 137.6, 134.6, 134.2, 133.9,
CH2Cl2). Spectral data were identical to what had been 133.8, 133.6, 132.5, 132.0, 131.8, 131.7, 128.4, 128.1, 127.5,
reported for the couple of two atropisomers.
127.1, 126.9, 126.6, 126.2, 126.1, 125.2, 125.1, 19.2, 19.0. MS
Circular dichroism studies are in progress to determine the (ESI+) m/z: 555.31 (M + H)+. IR (neat) ν (cm−1): 1384, 1171
absolute configuration of the atropisomers (−)14a.
4,5-Dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonyl chloride
(SO2).
Compound 14c (40 mg) was chromatographed on a semi-
(14b). Mixture of diastereomers (meso isomer and a couple of preparative Chiralpak IC column (F 10 × 250 mm, Daicel,
atropisomers). Waxy white solid (0.84 g; 87% yield). Found: C Osaka, Japan) using an isocratic elution with heptane–CH2Cl2
54.61; H 4.21; Cl 14.78; S 13.19. C22H20Cl2S2O4 requires: C (3 : 2) at a flow rate of 5.0 mL min−1. The compounds eluted
1
54.66; H 4.17; Cl 14.67; S 13.26%. H NMR (200 MHz, CDCl3): from the column were monitored with a photodiode array
δ = 7.39–7.10 (m, 8H), 2.09, 2.03, 1.98 and 1.96 (4 s, 12H). 13C detector. Two compounds (respectively 20.4 mg and 19.6 mg),
NMR (50 MHz, CDCl3): δ = 146.8, 144.6, 144.3, 137.8, 136.7, eluted as single peaks at 12.11 and 17.23 min, were collected,
136.3, 135.7, 130.7, 130.4, 130.2, 129.5, 129.4, 125.9, 125.8, after removing the solvents under nitrogen flow.
20.6, 20.4, 19.0, 18.6. MS (ESI+) m/z: 483.24 (M + H)+. IR (neat)
ν (cm−1): 1380, 1173 (SO2).
The first one was the meso isomer. 1H NMR (200 MHz,
CDCl3): δ = 7.68–7.44 (m, 10H), 1.98 (s, 3H). 13C NMR
Compound 14b (40 mg) was chromatographed on a semi- (50 MHz, CDCl3): δ = 137.8, 134.6, 134.2, 133.9, 133.6, 132.5,
preparative Chiralpak IC column (F 10 × 250 mm, Daicel, 131.7, 128.4, 127.5, 127.1, 126.6, 126.2, 125.2, 19.2. [α]2D7.5 −0.3
Osaka, Japan) using an isocratic elution with heptane–CH2Cl2 (c 0.15, CH2Cl2). The second one was a couple of atropisomers.
(3 : 2) at a flow rate of 5.0 mL min−1. The compounds eluted 1H NMR (200 MHz, CDCl3): δ = 7.68–7.40 (m, 10H), 1.98 (s,
from the column were monitored using a photodiode array 6H). 13C NMR (50 MHz, CDCl3): δ = 137.6, 134.6, 134.2, 133.8,
detector. Two compounds (respectively 19.8 mg and 20.2 mg), 133.6, 132.0, 131.7, 128.1, 127.5, 126.9, 126.6, 126.1, 125.1,
eluted as single peaks at 5.12 and 6.57 min, were collected 19.0. [α]2D7.5 −0.1 (c 0.15, CH2Cl2).
after removing the solvents under nitrogen flow.
The second eluted compound (19 mg) was again chromato-
The first one was the meso isomer. 1H NMR (200 MHz, graphed on a semipreparative Chiralpak IC column (F 10 ×
CDCl3): δ 7.39–7.10 (m, 8H), 2.09 (s, 3H), 1.98 (s, 3H). 13C NMR 250 mm, Daicel, Osaka, Japan) using an isocratic elution with
(50 MHz, CDCl3): δ = 146.8, 144.6, 137.8, 136.7, 135.7, 130.7, heptane–MeOH (9 : 1) at a flow rate of 3.0 mL min−1. The com-
130.4, 129.5, 125.9, 20.6, 19.0. [α]2D7.5 0.0 (c 0.10, CH2Cl2). The pounds eluted from the column were monitored with a photo-
1
second one was a couple of atropisomers. H NMR (200 MHz, diode array detector. Two compounds (respectively 9.4 mg and
CDCl3): δ = 7.35–7.10 (m, 8H), 2.03 (s, 3H), 1.96 (s, 3H). 9.6 mg), eluted as single peaks at 9.33 and 14.27 min, were col-
13C NMR (50 MHz, CDCl3): δ 146.6, 144.3, 137.8, 136.3, 135.7, lected, after removing the solvents under nitrogen flow.
130.7, 130.2, 129.4, 125.8, 20.4, 18.6). [α]2D7.5 +0.1 (c 0.10,
CH2Cl2).
The first one was (−)4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-
benzenedisulfonyl chloride (14c; waxy white solid). [α]D21.5
The second eluted compound (20 mg) was again chromato- −38.7 (c 0.10, CH2Cl2). Spectral data were identical to what had
graphed on a semipreparative Chiralpak IC column (F 10 × been reported for the couple of atropisomers.
250 mm, Daicel, Osaka, Japan) using an isocratic elution with
The second one was (+)4,5-dimethyl-3,6-bis(1-naphthyl)-1,2-
heptane–CH2Cl2 (4 : 1) at a flow rate of 3.0 mL min−1. The com- benzenedisulfonyl chloride (14c; waxy white solid). [α]2D1.5
pounds eluted from the column were monitored with a photo- +38.3 (c 0.10, CH2Cl2). Spectral data were identical to what had
diode array detector. Two compounds (respectively 9.5 mg and been reported for the couple of atropisomers.
10.5 mg), eluted as single peaks at 10.44 and 14.42 min, were
collected, after removing the solvents under nitrogen flow.
The first one was (−)4,5-dimethyl-3,6-bis(o-tolyl)-1,2-benzene-
Circular dichroism studies are in progress to determine the
absolute configuration of the atropisomers (−)14c.
Synthesis of 1,2-benzenedisulfonimides 3. 1,2-Benzenedi-
disulfonyl chloride (14b; waxy white solid). [α]2D7.5 −22.9 sulfonyl chloride (14; 2 mmol) was dissolved in toluene (8 mL)
(c 0.15, CH2Cl2). Spectral data were identical to what had been and EtOH (12 mL). The resulting mixture was cooled to 0 °C.
reported for the couple of atropisomers.
Ammonia was bubbled through while the temperature was
The second one was (+) 4,5-dimethyl-3,6-bis(o-tolyl)-1,2- maintained at 0 °C and the reaction mixture was vigorously
benzenedisulfonyl chloride (14b; waxy white solid). [α]D27.5 +23.5 stirred. The reaction was monitored by TLC (PE–EtOAc 7 : 3).
(c 0.15, CH2Cl2). Spectral data were identical to what had been After 30 min, the reaction was complete. The mixture was first
reported for the couple of atropisomers.
Circular dichroism studies are in progress to determine the orated under reduced pressure. The crude residue, when dis-
absolute configuration of the atropisomers (−)14b. solved in H2O and passed through a Dowex (HCR-W2) column
4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonyl (H2O), afforded pure 3.
chloride (14c). Mixture of diastereomers (meso isomer and a 4-Methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide
filtered in order to eliminate NH4Cl and then solvent was evap-
(3a).
couple of atropisomers). Waxy white solid (0.89 g; 80% yield). Grey waxy solid (0.75 g, 90%). Found: C 61.02; H 4.68; N 3.33;
Found: C 60.51; H 3.67; Cl 12.80; S 11.52. C28H20Cl2S2O4 S 15.54. C21H19NO4S2 requires: C 61.00; H 4.63; N 3.39; S
1
requires: C 60.54; H 3.63; Cl 12.76; S 11.54%. 1H NMR 15.51%. H NMR (200 MHz, CDCl3): δ = 7.49 (s, 1H), 7.32–7.18
3908 | Org. Biomol. Chem., 2014, 12, 3902–3911
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(m, 8H), 2.15 (s, 3H), 2.12 (s, 3H), 2.07 (s, 3H). 13C NMR that had been cooled to 0 °C. The mixture was stirred at 0 °C
(50 MHz, CDCl3): δ = 145.9, 145.7, 138.4, 138.0, 137.4, 136.7, for 3 hours until GC and GC-MS analyses showed complete
136.5, 136.3, 135.4, 134.2, 132.7, 130.5, 129.8, 129.6, 126.4, disappearance of the starting compounds and complete for-
126.3, 125.9, 125.8, 20.5, 20.4, 19.9. MS (ESI+) m/z: 414.63 mation of 2-phenyl-2-phenylaminopropanenitrile (21a). Cold
(M + H)+. IR (neat) ν (cm−1): 3004 (NH), 1362, 1151(SO2).
H2O (2 ml) was added to the reaction mixture under vigorous
(−)4-Methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide (3a). stirring. The resulting solid was filtered on a Hirsch funnel
The title compound (grey waxy solid; 32 mg, 77% yield) and washed with additional cold H2O (2 × 1 ml) and a small
was obtained from (−)4-methyl-3,6-bis(2-tolyl)-1,2-benzenedi- amount of PE (1 ml). It was virtually pure (GC, GC-MS,
sulfonyl chloride (14a; 0.1 mmol, 47 mg) as reported above.
1H NMR, and 13C NMR), 2-phenyl-2-phenylaminopropane-
[α]2D1 −14.8 (c 0.15, CH2Cl2). Spectral data were identical to nitrile (21a; white solid; 89 mg, 83% yield). Spectral and physical
what had been reported for the mixture of two atropisomers.
data were identical to what had been reported in the litera-
4,5-Dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide (3b). ture.15 After analyzing 21a on a GC with a chiral column, the
Mixture of diastereomers (meso isomer and a couple of atrop- presence of two enantiomers was found; the enantiomeric
isomers). Grey waxy solid (0.77 g; 90% yield). Found: C 61.77; ratio (er) was 82 : 18; the enantiomeric excess (ee) was 64%.
H 4.92; N 3.33; S 15.04. C22H21NO4S2 requires: C 61.81; H 4.95;
2. The reaction was carried out at −20 °C and small
N 3.28; S 15.00%. 1H NMR (200 MHz, CDCl3): δ = 7.39–7.14 (m, amounts of 18a and 19a were detected by GC and GC-MS ana-
8H), 2.05 (s, 12H). 13C NMR (50 MHz, CDCl3): δ = 144.9, 137.1, lyses after 6 hours. However, the reaction had stopped. The
136.9, 136.7, 135.4, 133.6, 130.5, 129.9, 129.8, 126.3, 126.2, yield of 21a was 65% (70 mg).
20.1, 19.9, 17.6. MS (ESI+) m/z: 428.01 (M + H)+. IR (neat)
ν (cm−1): 3001 (NH), 1360, 1156 (SO2).
After analyzing 21a on a GC with a chiral column, the pres-
ence of two enantiomers was found; er was 87 : 12; ee was
(–)4,5-Dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide 75%.
(3b). The title compound (pale grey waxy solid; 35 mg, 82%
(–)4,5-Dimethyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide (3b)
yield) was obtained from (−)4,5-dimethyl-3,6-(bis-2-tolyl)-1,2- as a catalyst. 1. (−)4,5-Dimethyl-3,6-bis(o-tolyl)-1,2-benzenedi-
benzenedisulfonyl chloride (14b, 0.1 mmol, 48 mg) as reported sulfonimide (3b; 5 mol%; 10 mg, 0.0234 mmol). The amounts
above.
of 18a, 19a and 20 were, respectively, 56 mg (0.0468 mmol),
1
[α]2D7 −18.4 (c 0.12, CH2Cl2); H NMR (200 MHz, CDCl3): δ = 43 mg (0.0468 mmol), and 51 mg (0.0515 mmol). At 0 °C the
7.39–7.18 (m, 8H), 2.05 (s, 12H). 13C NMR (50 MHz, CDCl3): yield of 21a was 86% (88 mg). After analyzing 21a on a GC with
δ = 144.9, 137.1, 136.9, 136.7, 135.4, 133.6, 130.5, 129.8, 126.3, a chiral column, the presence of two enantiomers was found;
19.9, 17.6.
er was 92 : 8; ee was 84%. The aqueous washings were collected
4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide and evaporated under reduced pressure. After the removal of
(3c). Mixture of diastereomers (meso isomer and a couple of H2O, a virtually pure (1H NMR) 3b was recovered (9.5 mg, 95%
atropisomers). Grey waxy solid (0.98 g; 88% yield). Found: C yield). The recovered 3b was employed in another two catalytic
67.40; H 4.21; N 2.74; S 12.84. C28H21NO4S2 requires: C 67.32; cycles under the conditions described above. Table 2 reports
H 4.24; N 2.80; S 12.83%. 1H NMR (200 MHz, CDCl3): δ = the yields of 21a and the yields of recovered 3b.
8.43–8.29 (m, 2H), 7.92–7.29 (m, 8H), 2.05 (s, 6H). 13C NMR
2. The reaction was carried out at −20 °C and small
(50 MHz, CDCl3): δ = 140.1, 140.0, 137.5, 137.4, 135.7, 135.5, amounts of 18a and 19a were detected by GC and GC-MS ana-
135.4, 135.1, 133.1, 130.2, 128.6, 128.2, 128.1, 128.0, 127.0, lyses after 6 hours. However, the reaction had stopped. The
126.3, 120.3, 120.2, 19.3, 19.2. MS (ESI+) m/z: 500.11 (M + H)+. yield of 21a was 70% (73 mg). After analyzing 21a on a GC with
IR (neat) ν (cm−1): 2998 (NH), 1357, 1159 (SO2).
a chiral column, the presence of two enantiomers was found;
(–)4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide er was 97 : 3; ee was 94%.
(3c). The title compound (pale grey waxy solid; 36 mg, 72%
Under the same conditions, 2-(4-methoxyphenyl)-2-phenyl-
yield) was obtained from (−)4,5-dimethyl-3,6-(bis-1-naphthyl)- aminopropanenitrile (21b),16 2-(4-nitrophenyl)-2-phenylamino-
1,2-benzenedisulfonyl chloride (0.1 mmol, 55 mg) as reported propanenitrile (21c),10b 2-(4-bromophenylamino)-2-phenyl-
above.
propanenitrile (21d),10b 2-(4-fluorophenylamino)-2-phenyl-
1
[α]2D1 −45.4 (c 0.15, CH2Cl2); H NMR (200 MHz, CDCl3): δ = propanenitrile (21e),10b 2-(2-methoxyphenyl)-2-phenylamino-
8.36–8.29 (m, 2H), 8.36–8.29 (m, 2H), 7.92–7.29 (m, 8H), 2.05 propanenitrile
(21f),10b
2-(3-methoxyphenylamino)-2-
(s, 6H). 13C NMR (50 MHz, CDCl3): δ = 140.0, 137.5, 135.7, phenylpropanenitrile (21g),16 2-phenylamino-2-(4-tolyl)propa-
135.4, 135.1, 133.1, 130.2, 128.6, 128.2, 128.0, 127.0, 126.3, nenitrile (21h),17 2-(4-nitrophenylamino)-2-(4-tolyl)propane-
120.2, 19.3.
nitrile
(21i),10b
2-(4-methoxyphenylamino)-2-(4-tolyl)-
propanenitrile (21j),10b 2-(4-methoxyphenylamino)-2-(4-nitro-
phenyl)propanenitrile (21k),16 2-methyl-2-phenylaminopentane-
Chiral sulfonimides 3 as catalysts in the Strecker reaction
(–)4-Methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfonimide (3a) as nitrile (21l), 2-phenyl-2-phenylaminoacetonitrile (21m),18
a catalyst. 1. TMSCN (20, 53 mg, 0.53 mmol) was added to a 2-(4-nitrophenyl)-2-phenylaminoacetonitrile (21n),19 2-(4-tolyl)-
mixture of (−)4-methyl-3,6-bis(o-tolyl)-1,2-benzenedisulfon- 2-phenylaminoacetonitrile (21o)20 and 2-(2-thienyl)-2-phenyl-
imide (3a; 5 mol%; 10 mg, 0.0242 mmol), acetophenone (18a, aminoacetonitrile (21e)21 were obtained. The yields and the ee
58 mg, 0.484 mmol) and aniline (19a, 45 mg, 0.484 mmol) of each of these compounds are reported in Table 3. Their
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spectral and physical data (reported in ESI†) are identical to
those reported in the literature, with the only exception of 21l,
unknown in the literature.
K. Fuchibe and T. Akiyama, J. Am. Chem. Soc., 2007, 129,
6756; (c) T. Akiyama, Chem. Rev., 2007, 107, 5744.
5 (a) D. Uraguchi and M. Terada, J. Am. Chem. Soc., 2004,
126, 5356; (b) M. Terada, Synthesis, 2010, 1929;
(c) M. Terada, Curr. Org. Chem., 2011, 15, 2227.
6 D. Nakashima and H. Yamamoto, J. Am. Chem. Soc., 2006,
128, 9626.
7 (a) H. He, L.-Y. Chen, W.-Y. Wong, W.-H. Chan and
A. W. M. Lee, Eur. J. Org. Chem., 2010, 4181; (b) L.-Y. Chen,
H. He, W.-H. Chan and A. W. M. Lee, J. Org. Chem., 2011,
76, 7141.
8 (a) P. Garcia-Garcia, F. Lay, P. Garcia-Garcia, C. Rabalakos
and B. List, Angew. Chem., Int. Ed., 2009, 48, 4363;
(b) L. Ratjen, P. Garcia-Garcia, F. Lay, M. E. Beck and
B. List, Angew. Chem., Int. Ed., 2011, 50, 754; (c) J. Guin,
C. Rabalakos and B. List, Angew. Chem., Int. Ed., 2012, 51,
754; (d) M. Mahlau, P. Garcia-Garcia and B. List, Chem. –
Eur. J., 2012, 18, 16283; (e) S. Gandhi and B. List, Angew.
Chem., Int. Ed., 2013, 52, 2573.
2-Methyl-2-phenylaminopentanenitrile (21l). Viscous oil
(65 mg, 71%). Found: C 76.51; H 8.55; N 14.92. C12H16N2
requires: C 76.56; H 8.57; N 14.88%. 1H NMR (200 MHz,
CDCl3): δ = 7.24–7.10 (m, 2H), 6.88–6.82 (m, 3H), 3.58 (br s,
1H), 1.97–1.75 (m, 2H), 1.64–1.44 (m, 2H), 1.58 (s, 3H), 0.95 (t,
J = 7.2 Hz, 3H). 13C NMR (50 MHz, CDCl3): δ = 144.1, 129.5,
121.9, 120.6, 117.4, 53.0, 42.7, 25.8, 17.7, 14.1. MS (EI) m/z: (%)
188 [M+] (5), 161 (35), 146 (45), 118 (100), 77 (55). IR (CHCl3) ν
(cm−1): 3431 (NH), 2249 (CN).
(–)4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-benzenedisulfonimide
(3c) as
a catalyst. 1. 4,5-Dimethyl-3,6-bis(1-naphthyl)-1,2-
benzenedisulfonimide (3c; 5 mol%; 10 mg, 0.02 mmol). The
amounts of 18a, 19a and 20 were, respectively, 48 mg
(0.04 mmol), 37 mg (0.04 mmol), and 44 mg (0.044 mmol). At
0 °C the yield of 21a was 84% (75 mg). After analyzing 21a on a
GC with a chiral column, the presence of two enantiomers was
found; er was 90 : 10; ee was 80%.
9 (a) M. Barbero, S. Bazzi, S. Cadamuro and S. Dughera,
Curr. Org. Chem., 2011, 15, 576; (b) M. Barbero,
S. Cadamuro and S. Dughera, Synth. Commun., 2013, 43,
758; (c) M. Barbero, S. Cadamuro and S. Dughera,
ARKIVOC, 2012, ix, 262; (d) M. Barbero, S. Cadamuro,
S. Dughera, C. Magistris and P. Venturello, Org. Biomol.
Chem., 2011, 9, 8393; (e) M. Barbero, S. Bazzi, S. Cadamuro,
S. Dughera, C. Magistris, A. Smarra and P. Venturello, Org.
Biomol. Chem., 2011, 9, 2192; (f) M. Barbero, S. Bazzi,
S. Cadamuro and S. Dughera, Tetrahedron Lett., 2010, 51,
6356; (g) M. Barbero, S. Bazzi, S. Cadamuro and
S. Dughera, Tetrahedron Lett., 2010, 51, 2342;
(h) M. Barbero, S. Bazzi, S. Cadamuro, C. Piccinini and
S. Dughera, Synthesis, 2010, 315; (i) M. Barbero, S. Bazzi,
S. Cadamuro, S. Dughera and G. Ghigo, Eur. J. Org. Chem.,
2009, 4346; ( j) M. Barbero, S. Cadamuro, A. Deagostino,
S. Dughera, P. Larini, E. G. Occhiato, C. Prandi, S. Tabasso,
R. Vulcano and P. Venturello, Synthesis, 2009, 2260;
(k) M. Barbero, S. Bazzi, S. Cadamuro and S. Dughera,
Eur. J. Org. Chem., 2009, 430; (l) M. Barbero, S. Cadamuro,
S. Dughera and P. Venturello, Synthesis, 2008, 3625;
(m) M. Barbero, S. Cadamuro, S. Dughera and
P. Venturello, Synthesis, 2008, 1379; (n) M. Barbero,
S. Cadamuro, S. Dughera and P. Venturello, Synlett, 2007,
2209.
2. The reaction was carried out at −20 °C and small
amounts of 18a and 19a were detected by GC and GC-MS ana-
lyses after 6 hours. However, the reaction had stopped. The
yield of 21a was 72% (64 mg). After analyzing 21a on a GC with
a chiral column, the presence of two enantiomers was found;
er was 96 : 4; ee was 92%.
Acknowledgements
This work was supported by the Ministero dell’Università e
della Ricerca and by the University of Torino.
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