Chiral aziridination of olefins using a chiral sulfanamide as the nitrogen source

Article abstract of DOI:10.1055/s-0029-1218545

Chiral aziridination of cyclic a-bromoenones is achieved by the use of the lithium salt of (S_s)-(+)-p-toluenesulfinamide, which leads to products with diastereomeric excesses in the range of 30-65% using a simple protocol. A key factor associated with chiral induction is the incorporation of the reacting olefin in a cycle, indicating the importance of conformational restriction in the reacting double bond. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1218545

LETTER
145
Chiral Aziridination of Olefins Using a Chiral Sulfinamide as the Nitrogen
Source
Aziridinationof
O
a
lefinswith
s
a
Chiral
S
c
ulfinamideo D. B. Bonifácio,a Concepción González-Bello,b Henry S. Rzepa,c Sundaresan Prabhakar,*a Ana M. Lobo*a
a
Chemistry Department, REQUIMTE/CQFB, and SINTOR–UNINOVA, Faculty of Sciences and Technology,
Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
Fax +351(21)2948550; E-mail: aml@fct.unl.pt
Departamento de Química Orgánica, Facultad de Química, Universidad de Santiago de Compostela,
15782 Santiago de Compostela, Spain
b
c
Chemistry Department, Imperial College London, South Kensington Campus, Exhibition Road, London SW7 2AY, UK
Received 28 July 2009
room temperature, as shown in Scheme 2, a diastereomer-
ic mixture of the corresponding aziridines 3Aa/3Ba was
obtained. Optimization of the conditions for this reaction
is summarized in Table 1.
Abstract: Chiral aziridination of cyclic a-bromoenones is achieved
by the use of the lithium salt of (SS)-(+)-p-toluenesulfinamide,
which leads to products with diastereomeric excesses in the range
of 30–65% using a simple protocol. A key factor associated with
chiral induction is the incorporation of the reacting olefin in a cycle,
indicating the importance of conformational restriction in the react-
ing double bond.
X
H
N
N
R
R
Hal
Y
Key words: aminations, chirality, cyclizations, enones, Michael
additions
Y
b
a
R
H
N
R
X
N
Hal
Aziridines are important intermediates for the synthesis1
of bioactive compounds2 including antiviral3 natural prod-
ucts.4,5 In fact the three-membered ring of aziridines is
considered one of the most versatile structural units in the
synthesis of nitrogen-containing compounds.6 Amongst
the various methods for obtaining aziridines, the transfer
of the nitrogen atom directly (such as in Scheme 1, a) or
stepwise (such as in Scheme 1, b) to an olefin is one of the
most actively investigated.7 In the absence of a chiral ele-
ment in the olefin, chirality in the final aziridine will have
to rely on the other partner(s) of the reaction, that is, it will
have to be part of the nitrogen moiety or part of a catalyst.8
Our own work in this field has followed the approach of
‘a’ in Scheme 1, and the leaving group used has been an
acyloxy, as in O-acyl hydroxylamines,9 or a halogen, as in
Bromamine-T.10 The use of chiral phase-transfer catalysts
associated with hydroxamic acids, in situ precursors of O-
acyl hydroxylamines, led to enantiomeric excesses rang-
ing from good to moderate,11 a method which has recently
been improved.12 The present Letter reports our work in
the aziridination of olefins, using a chiral sulfinamide and
strategy ‘b’ in Scheme 1. The advantage of sulfinamide
aziridines is the facile removal of the sulfinyl atom from
the three-membered-ring nitrogen,13 and the fact that there
are several commercially available chiral sulfinamides in
both enantiomeric forms.
Y
Y
R
N
R
N
Hal
Y
Y
aziridine
Scheme 1 Strategies for aziridine formation
When the lithium salt of (SS)-(+)-p-toluenesulfinamide
(1),14,15 was allowed to react in THF with electrodeficient
a-bromoolefins such as 2a16 from –78 °C and warming to
SYNLETT 2010, No. 1, pp 0145–0149
x
x.
x
x.
2
0
0
9
Advanced online publication: 02.12.2009
DOI: 10.1055/s-0029-1218545; Art ID: D20109ST
© Georg Thieme Verlag Stuttgart · New York
Scheme 2 Aziridination of 2a and X-ray crystal structure of 3Aa
with one molecule of H2O

146
V. D. B. Bonifácio et al.
LETTER
Table 1 Reaction of Aziridination of Compound 2a Using (SS)-(+)-p-Toluenesulfinamide and n-Butyllithium as in Scheme 2
Entry
Olefin 2a
(equiv)
Sulfinamide
(equiv)
BuLi
(equiv)
Yield of aziridines Ratio of
Recovered materials
Yield of sulfinamide (%)
3Aa/3Ba (%)a
3Aa/3Bab
Yield of 2a (%)
1
2
3
4
5
1
1
1
1
1
1
2
3
3
2
1
1
3
1
2
12
19
62c
22
31
70:30
50
60
50
60
62
30
20
0
76:24
77:23
71:29
35
0
73:27
a Isolated combined yields.
b Ratio of aziridines calculated from 1H NMR of crude reaction mixtures (ring protons H2).
c Use of NaH as base led to a lower yield of 48.5%.
As can be easily seen, the best results (entry 3) were ob-
tained by using a threefold molar excess of both the chiral
sulfinamide 1 and of n-BuLi, relative to the bromoenone.
The diastereomeric aziridines 3Aa/3Ba, formed in 62%
yield, had a de of 53% (Table 2, entry 1). In this particular
case it was possible to separate chromatographically both
O
O
O
O
MCPA
3Aa/3Ba
+
S
N
S
N
p-Tol
p-Tol
O
O
4Aa/4 Ba
53% ee
Scheme 3 Oxidation of N-sulfinamide aziridines 3a to N-sulfon-
amide aziridines 4a
aziridines, and the structure of the major compound was
analyzed by single-crystal X-ray diffraction analysis17 and
found to be the (SS,2S,3S)-aziridine 3Aa (Scheme 2) indi-
cating that the preferential attack of the sulfinamide anion
proceeds on the Si face of 2a. Figure 1 shows the calculat-
ed B3LYP/6-31G(d,p) transition state where an additional
stabilization is provided by the chelation of lithium by the
sulfur oxygen and (in this example) two aditional THF
molecules acting as coordinated solvent.18 The calculated
preference in the free energy DGπ for 3Aa over 3Ba is 1.1
kcal/mol (for one coordinated THF molecule) or 0.5 kcal/
mol (for two coordinated THF molecules), which is com-
mensurate with the observed de. The de obtained in this
reaction was confirmed when MCPA oxidation of 3Aa/
3Ba led to the corresponding enantiomeric N-sulfonyl
Other results, using the described method, are summa-
rized in Table 2.22 When the olefin is embedded in a five-
membered ring, though the chemical yields were lower
relative to the six-membered ring, the chiral induction re-
mained of the same order of magnitude (entries 4 and 6 vs.
entries 1 and 2), the biggest reduction in both yield and de
occurring when the bromine of 2 was replaced by iodine
(entries 4 and 5). Comparison of the aziridination of 2d
with 2h showed yields of the same order of magnitude but
a remarkable drop in the chiral induction for the olefin not
part of a ring (entries 4 and 8). Replacing the activating
carbonyl of 2h by a sulfonyl group as in 2i led to a similar
result but at much slower rate (entries 8 and 9), and the
open-chain bromo product 4i was also identified.
1
aziridines 4Aa/4Ba19 (Scheme 3). Analysis of their H
NMR spectra for the aziridine protons (H2), after addition
of the chiral shift reagent Eu(hfc)3,20 confirmed an ee of
53%.
The presence of substituents in the olefin proved determi-
nant for the absence of the aziridination, in line with ear-
lier findings from our laboratory.9 In the case of 4-
bromocholestenone no aziridine was formed, possibly in-
dicating the hindrance of the olefinic carbon C-5 to the
initial Michael addition.
In conclusion, the addition of the lithium salt of a chiral
sulfinamide to an electron-deficient cyclic olefin leads to
chiral aziridines, with de in the order of 30–65% in a sim-
ple and easy to use protocol. Further studies to enhance
the diastereoselection are in progress.
Acknowledgment
We thank Fundação para a Ciência e Tecnologia (Lisbon, Portugal),
PRAXIS program, for partial financial support. One of us (V.D.B.
Bonifácio) also thanks PRAXIS program for the award of a doctoral
fellowship.
Figure 1 Calculated transition state for the formation of (SS,2S,3S)-
3Aa. Two coordinated THF solvent molecules are shown as oxygen
atoms only for clarity. The forming N–C bond is 2.22 Å.21
Synlett 2010, No. 1, 145–149 © Thieme Stuttgart · New York

LETTER
Aziridination of Olefins with a Chiral Sulfinamide
147
Table 2 Aziridination of Compounds 2 via (SS)-(+)-p-Toluenesulfinamide and Base
O
R2
R2
R2
R1
R1
R1
S
O
O
Ar
NHLi
+
(CH2)n
(CH2)n
S
N
S
N
(CH2)n
THF
Ar
Ar
X
O
O
O
3A/3B
2
Entry
Compound 2
Time
Yield (%)a of aziridine 3
de (%)b
53
Other products
1
2
3
4
5
6
7
2a X = Br; R1 = R2 = H; n = 2
2b X = Br; R1 = R2 = Me; n = 2
2c X = Br; R1 = R2 = Ph; n = 2
2d X = Br; R1 = R2 = H; n = 1
2e X = I; R1 = R2 = H; n = 1
2f X = Br; R1 = R2 = Me; n = 1
2g X = Br; R1 = R2 = H; n = 3
30 min
3 h
62
96
56
22
11
41
25
–
–
–
–
–
–
–
44
3 h
56
30 min
30 min
11 h
65
12
53
3 h
30
8
9
1.5 h
30
50
<1
4
–
Br
O
2h
O
S
O
O
Br
S
21 h
N
p-Tol
S
Ph
H
O
O
Br
4i (17)c
2i
a Isolated yields.
b Determined on the crude reaction mixture from 1H NMR (ring protons H2 or H3).
c Isolated as a 1:1 mixture (by 1H NMR) of diastereomers.
J. S.; Richardson, T. E. J. Am. Chem. Soc. 1999, 121, 9088.
(c) Coleman, R. S.; Li, J.; Navarro, A. Angew. Chem. Int. Ed.
2001, 40, 1736.
References and Notes
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Synlett 2010, No. 1, 145–149 © Thieme Stuttgart · New York

148
V. D. B. Bonifácio et al.
LETTER
entries: 10042/to-2308, 10042/to-23010, 10042/to-2311,
10042/to-2312, 10042/to-2314 and 10042/to-2315, resolved
as e.g. http://dx.doi.org/10042/to-2308. An interactive
version of this figure can be viewed via the HTML version
of this article.
Celik, A.; Kazaz, C. Tetrahedron Lett. 2005, 46, 197.
(f) Chenna, P. H. D.; Peillard, F. R.; Dauban, P.; Dodd, R. H.
Org. Lett. 2004, 6, 4503. (g) Cavallo, A. S.; Roje, M.;
Welter, R.; Sinjic, V. J. Org. Chem. 2004, 69, 1409.
(h) Redlich, M.; Hossain, M. M. Tetrahedron Lett. 2004, 45,
8987. (i) Calhorda, M. J.; Vaz, P. D. Chemtracts: Inorg.
Chem. 2004, 17, 396. (j) Barros, M. T.; Maycock, C. D.;
Ventura, M. R. Tetrahedron Lett. 2002, 43, 4329.
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Westmoreland, I.; Thickitt, C. P.; Scott, P. J. Org. Chem.
2002, 67, 3450. (l) Nishimura, M.; Minakata, S.; Takahashi,
T.; Oderaotoshi, Y.; Komatsu, M. J. Org. Chem. 2002, 67,
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S. E. J. Org. Chem. 2002, 67, 2335. (n) Wei, H.-X.; Kim,
S. H.; Li, G. Tetrahedron 2001, 57, 8401.
(22) Typical Experimental Procedure for Aziridination
To a dry THF solution of 2-bromocyclohex-2-en-1-one (2a,
0.286 mmol), under nitrogen atmosphere and protected from
light at –78 °C, was added the lithium salt of chiral
sulfinamide 1 (0.858 mmol) and the reaction allowed to
reach r.t. After 30 min the solvent was removed under
reduced pressure and the residue dissolved in EtOAc,
washed with aq NH4Cl (10%), and after drying and solvent
removal, the remaining residue was purified via silica gel
PTLC (EtOAc–n-hexane, 1:1) to afford a mixture of the title
aziridines 3Aa (major)/3Ba (minor); yield 62%. Separation
of both diastereomers was achieved by 2 × PTLC of the
mixture.
(9) Pereira, M. M.; Santos, P. P. O.; Reis, L. V.; Lobo, A. M.;
Prabhakar, S. J. Chem. Soc., Chem. Commun. 1993, 38.
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Lobo, A. M.; Branco, P. S.; Prabhakar, S. Tetrahedron 2007,
63, 7009. (b) Antunes, A. M. M.; Marto, S. J.; Branco, P. S.;
Prabhakar, S.; Lobo, A. M. Chem. Commun. 2001, 405.
(11) (a) Aires-de-Sousa, J.; Prabhakar, S.; Lobo, A. M.; Rosa, A.
M.; Gomes, M. J. S.; Corvo, M. C.; Williams, D. J.; White,
A. J. P. Tetrahedron: Asymmetry 2002, 12, 3349. (b) Aires-
de-Sousa, J.; Lobo, A. M.; Prabhakar, S. Tetrahedron Lett.
1996, 37, 3183.
(12) Murugan, E.; Siva, A. Synthesis 2005, 2022.
(13) (a) Davis, F. A.; Wu, Y.; Yan, H.; McCoull, W.; Prasad,
K. R. J. Org. Chem. 2003, 68, 2410. (b) Davis, F. A.; Zhou,
P.; Reddy, G. V. J. Org. Chem. 1994, 59, 3243.
(14) The lithium salt of (SS)-(+)-1 was prepared as in: Wenschuh,
V.; Fritzsche, B. J. Prakt. Chem. 1970, 312, 129; its
configuration stability in the chiral sulfur was ascertained by
recovering (SS)-(+)-1 with the same specific rotation of
+85.6 (c 0.95, CH3Cl) from the salt.
Compound 3Aa(major): mp 70–71 °C (EtOAc–n-hexane);
[a]D23 +64.6 (c 0.99, CHCl3).
Compound 3Ba(minor): mp 90–91 °C (EtOAc–n-hexane);
[a]D23 +89.4 (c 1.44, CHCl3). 1H NMR (400 MHz, CDCl3;
3Aa/3Ba = 77:23): d = 7.60 (2 H, d, J = 8.2 Hz, minor), 7.55
(2 H, d, J = 8.2 Hz, major), 3.23 (1 H, d, J = 6.3 Hz, minor),
3.13 (1 H, d, J = 6.3 Hz, major), 3.06 (1 H, d, J = 6.3 Hz,
major), 2.94 (1 H, d, J = 6.3 Hz, minor). MS (CI): m/z (%) =
249 (100) [M+]. Anal. Calcd for C13H15NO2S: C, 62.62; H,
6.06; N, 5.62. Found: C, 62.31; H, 6.34; N, 5.74.
Compound 3b: oil. 1H NMR (400 MHz, CDCl3): d = 7.59 (2
H, d, J = 8.1 Hz, minor), 7.55 (2 H, d, J = 8.1 Hz, major),
3.08 (1 H, d, J = 6.3 Hz, major), 2.94 (1 H, d, J = 6.4 Hz,
minor), 2.85 (1 H, d, J = 6.4 Hz, minor), 2.77 (1 H, d, J = 6.3
Hz, major). HRMS: m/z calcd for C15H19NO2S: 277.11365;
found: 277.11344. Anal. Calcd for C15H19NO2S: C, 64.95;
H, 6.90; N, 5.05. Found: C, 65.27; H, 7.16; N, 4.95.
Compound 3c: mp 64–65 °C (EtOAc–n-hexane). 1H NMR
(400 MHz, CDCl3): d = 3.79 (2 H, d, J = 5.4 Hz, minor), 3.64
(1 H, d, J = 5.6 Hz, major), 3.41 (1 H, d, J = 6.2 Hz, major),
3.30 (1 H, d, J = 6.2 Hz, minor). MS (EI): m/z (%) = (1.1)
402 [M + 1]+. Anal. Calcd for C25H23NO2S: C, 74.78; H,
5.77; N, 3.49. Found: C, 75.16; H, 5.74; N, 3.37.
(15) This compound is commercially available from Sigma-
Aldrich Co (http://www.sigmaaldrich.com).
(16) All a-bromo(iodo)olefins 2 were obtained by dihalogenation
of the double bond followed by base-catalyzed elimination
of the b-halogen while reforming the olefin: (a) Compound
2a: Kowalski, C. J.; Weber, A. E.; Fields, K. W. J. Org.
Chem. 1982, 47, 5088. (b) Compounds 2b,c: Bordwell,
F. G.; Wellman, K. M. J. Org. Chem. 1963, 28, 2544.
(c) Compound 2d: Smith, A. B.; Branca, S. J.; Guaciaro,
M. A.; Wovkulich, P. M.; Korn, A. Org. Synth., Coll. Vol.
VII 1990, 271. (d) Compound 2e: Johnson, C. R.; Adams,
J. P.; Braun, M. P.; Senanayake, C. B. W.; Wovkulich,
P. M.; Uskokovic, M. R. Tetrahedron Lett. 1992, 7, 917.
(e) Compound 2f: Wakui, T.; Otsuji, Y.; Imoto, E. Bull.
Chem. Soc. Jpn. 1974, 2267. (f) Compound 2g: Amice, P.;
Blanco, L.; Conia, J. M. Synthesis 1976, 196.
Compound 3d (major): mp 97–98 °C (Et2O–n-hexane);
[a]D23 +22.0 (c 0.90, CHCl3).
Compound 3d (minor): mp 105–106 °C (Et2O–n-hexane);
[a]D23 +12.4 (c 2.03, CHCl3). 1H NMR (400 MHz, CDCl3):
d = 7.62 (2 H, d, J = 8.2 Hz, minor), 7.54 (2 H, d, J = 8.2 Hz,
major), 3.56 (1 H, m, minor), 3.53 (1 H, m, major), 3.11 (1
H, d, J = 4.3 Hz, major), 3.08 (1 H, d, J = 4.3 Hz, minor).
HRMS–FAB: m/z calcd for C12H14NO2S: 236.074526;
found: 236.075172.
Compound 3f: oil. 1H NMR (400 MHz, CDCl3): d = 7.60 (2
H, d, J = 7.8 Hz, minor), 7.54 (2 H, d, J = 7.8 Hz, major),
3.23 (1 H, d, J = 4.4 Hz, minor), 3.21 (1 H, d, J = 4.4 Hz,
major), 3.13 (1 H, d, J = 4.4 Hz, major), 3.10 (1 H, d, J = 4.4
Hz, minor). MS (FI): m/z (%) =(100) 263 [M+]. Anal. Calcd
for C14H17NO2S: C, 63.85; H, 6.51; N, 5.32. Found: C,
62.95; H, 6.81; N, 5.17.
(g) Compound 2h: Crossland, I.; Bock, K.; Norrestam, R.
Acta Chem. Scand., Ser. B 1985, 39, 7. (h) Nield, C. H.
J. Am. Chem. Soc. 1945, 67, 1145. (i) Compound 2i:
Carlier, P.; Gelas-Mialhe, Y.; Vessiere, R. Can. J. Chem.
1977, 55, 3190.
(17) CCDC 741870 contains the crystallographic data which can
be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, by emailing data_request@ccdc.cam.
ac.uk, or by contacting CCDC, UK; fax: +44 (1223)336033.
(18) For stabilization involving lithium chelation in an eight-
membered ring, see: Davis, F. A.; Reddy, R. T.; Reddy,
R. E. J. Org. Chem. 1992, 57, 6387.
Compound 3g: mp 92–93 °C (Et2O–n-pentane). 1H NMR
(400 MHz, CDCl3): d = 7.61 (2 H, d, J = 8.1 Hz, minor), 7.56
(2 H, d, J = 8.1 Hz, major), 3.13 (1 H, J = 7.7, 1.5 Hz,
major), 3.06 (1 H, m, minor), 2.97 (1 H, dd, J = 7.7, 4.6 Hz,
major), 2.80 (1 H, m, minor). MS (EI): m/z (%) =(18) 263
[M+]. Anal. Calcd for C14H17NO2S: C, 63.85; H, 6.51; N,
5.32. Found: C, 62.62; H, 6.72; N, 5.29.
(19) Molander, G. A.; Stengel, P. J. Tetrahedron 1997, 53, 8887.
(20) Parker, D. Chem. Rev. 1991, 91, 1441.
(21) Full coordinates and other information about the
calculations can be found via the following digital repository
Compound 3h: diasteriomeric mixture (1:1), mp 69–71 °C,
77–80 °C (EtOAc). 1H NMR (400 MHz, CDCl3): d = 2.81
(1 H, d, J = 4.1 Hz), 2.79 (1 H, d, J = 7.3 Hz), 2.71 (1 H, d,
Synlett 2010, No. 1, 145–149 © Thieme Stuttgart · New York

LETTER
J = 6.9 Hz), 2.24 (1 H, d, J = 3.7 Hz). HRMS (EI): m/z calcd
Aziridination of Olefins with a Chiral Sulfinamide
149
MHz, CDCl3): d = 3.70 (1 H, dd, J = 6.3, 3.2 Hz), 2.75 (1 H,
d, J = 6.3 Hz), 2.43 (1 H, br d); the attribution to 3Ai and 3Bi
may be reversed. HRMS (EI): m/z calcd for C15H15NO3S2:
321.04933; found: 321.04958.
for C16H15NO2S: 285.08235; found: 285.083598. Anal.
Calcd for C16H15NO2S: C, 67.34; H, 5.30; N, 4.91. Found: C,
67.34; H, 5.30; N, 4.91.
Compounds 3i: 3Ai: mp 103–105 °C (EtOAc); [a]D23 –15.1
(c 3.30, CHCl3). 1H NMR (400 MHz, CDCl3): d = 3.91 (1 H,
dd, J = 6.5, 3.3 Hz), 2.89 (1 H, d, J = 3.3 Hz), 2.66 (1 H, d,
J = 6.5 Hz). Anal. Calcd for C15H15NO3S2: C, 56.05; H, 4.70;
N, 4.36. Found: C, 55.87; H, 4.76; N, 4.38. 3Bi: mp 120–
121 °C (EtOAc); [a]D23 +7.4 (c 2.04, CHCl3). 1H NMR (400
Oxidation of 3Aa/3Ba
To a solution of 3Aa/3Ba (0.2 mmol) was added MCPBA
(0.4 mmol), and after 18 h at r.t. the solvent was removed and
the enantiomeric mixture of 4Aa/4Ba isolated (yield 99%);
ee 53% from 1H NMR (400 MHz, CDCl3): d = 3.45 (1 H, H2)
split by addition of Eu(hfc)3.
Synlett 2010, No. 1, 145–149 © Thieme Stuttgart · New York

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