Cobalt-catalyzed efficient aziridination of alkenes

Article abstract of DOI:10.1021/ol050896i

(Chemical Equation Presented) Cobalt porphyrins are capable of catalyzing the aziridination of alkenes with bromamine-T as the nitrene source. Among cobalt complexes of different porphyrins, Co(TDCIPP) is an effective catalyst that can aziridinate a wide

Full text of DOI:10.1021/ol050896i

ORGANIC
LETTERS
2005
Vol. 7, No. 15
3191-3193
Cobalt-Catalyzed Efficient Aziridination
of Alkenes
Guang-Yao Gao, Jeremiah D. Harden, and X. Peter Zhang*
Department of Chemistry, UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600
pzhang@utk.edu
Received April 22, 2005
ABSTRACT
Cobalt porphyrins are capable of catalyzing the aziridination of alkenes with bromamine-T as the nitrene source. Among cobalt complexes of
different porphyrins, Co(TDClPP) is an effective catalyst that can aziridinate a wide variety of alkenes. The catalytic system can operate at
room temperature in a one-pot fashion with alkenes as limiting reagents, forming the desired N-sulfonylated aziridine derivatives in high to
excellent yields with NaBr as the byproduct.
Aziridines are a class of synthetically and biologically
important three-membered heterocyclic compounds that have
found many applications.1 Among synthetic methodologies,
transition metal complex mediated aziridination of alkenes
with a nitrene source represents a direct and powerful
approach for the construction of the aziridine rings.2 The
most widely used nitrene sources for aziridination are the
reagent [N-(p-toluenesulfonyl)imino]phenyliodinane (PhId
NTs) and related iminoiodane derivatives.3 To overcome
several limitations associated with the use of PhIdNTs,4
alternative nitrene sources such as chloramine-T,5 brom-
amine-T,6 and tosyl azide7 have also been actively pursued.
With these nitrene sources, complexes of Mn, Fe, Ru, Rh,
and Cu that are supported by different ligands have been
identified to catalyze aziridination.1-9
The unique ligand environment and metal coordination
mode of metalloporphyrins render them a class of attractive
catalysts for aziridination and related atom/group transfer
reactions.10 Indeed, the first transition metal complexes that
were discovered for aziridination catalytic activity are
metalloporphyrins.11 To date, porphyrin complexes of Fe,
(5) (a) Simkhovich, L.; Gross, Z. Tetrahedron Lett. 2001, 42, 8089. (b)
Albone, D. P.; Aujla, P. S.; Taylor, P. C.; Challenger, S.; Derrick, A. M. J.
Org. Chem. 1998, 63, 9569. (c) Mairena, M. A.; Diaz-Requejo, M. M.;
Belderrain, T. R.; Nicasio, M. C.; Trofimenko, S.; Perez, P. J. Organome-
tallics 2004, 23, 253.
(6) (a) Chanda, B. M.; Vyas, R.; Bedekar, A. V. J. Org. Chem. 2001,
66, 30. (b) Antunes, A. M. M.; Marto, S. J. M.; Branco, P. S.; Prabhakar,
S.; Lobo, A. M. Chem. Commun. 2001, 405.
(1) (a) Hu, X. E. Tetrahedron 2004, 60, 2701. (b) Sweeney, J. B. Chem.
Soc. ReV. 2002, 31, 247. (c) Zwanenburg, B.; ten Holte, P. Top. Curr. Chem.
2001, 216, 93. (d) McCoull, W.; Davis, F. A. Synthesis 2000, 1347. (e)
Tanner, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 599.
(7) (a) Omura, K.; Uchida, T.; Irie, R.; Katsuki, T. Chem. Commun. 2004,
2060. (b) Li, Z.; Quan, R. W.; Jacobsen, E. N. J. Am. Chem. Soc. 1995,
117, 5889.
(2) (a) Muller, P.; Fruit, C. Chem. ReV. 2003, 103, 2905. (b) Jacobsen,
E. N. In ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: Berlin, 1999; Vol. 2, p 607. (c) Osborn, H.
M. I.; Sweeney, J. Tetrahedron: Asymmetry 1997, 8, 1693.
(3) (a) Dauban, P.; Dodd, R. H. Synlett 2003, 1571. (b) Koser, G. F.
Top. Curr. Chem. 2003, 224, 137. (c) Yamada, Y.; Yamamoto, T.; Okawara,
M. Chem. Lett. 1975, 361.
(4) For the use of in situ formed iminoiodane, see: (a) Yu, X.-Q.; Huang,
J.-S.; Zhou, X.-G.; Che, C.-M. Org. Lett. 2000, 2, 2233. (b) Dauban, P.;
Saniere, L.; Tarrade, A.; Dodd, R. H. J. Am. Chem. Soc. 2001, 123, 7707.
(c) Guthikonda, K.; Du Bois, J. J. Am. Chem. Soc. 2002, 124, 13672.
(8) For selected examples, see: (a) Li, Z.; Conser, K. R.; Jacobsen, E.
N. J. Am. Chem. Soc. 1993, 115, 5326. (b) Evans, D. A.; Faul, M. M.;
Bilodeau, M. T. J. Am. Chem. Soc. 1994, 116, 2742.
(9) A recent addition to the catalyst series is a Ag complex of a substi-
tuted terpyridine ligand: Cui, Y.; He, C. J. Am. Chem. Soc. 2003, 125,
16202.
(10) The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: San Diego, 2000-2003; Vols. 1-20.
(11) (a) Groves, J. T.; Takahashi, T. J. Am. Chem. Soc. 1983, 105, 2073.
(b) Mansuy, D.; Mahy, J.-P.; Dureault, A.; Bedi, G.; Battioni, P. J. Chem.
Soc., Chem. Commun. 1984, 1161.
10.1021/ol050896i CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/23/2005

Mn, and Ru have been known to catalyze aziridination.12
As part of our ongoing efforts in developing metallopor-
phyrin-based practical atom/group transfer catalytic sys-
tems,13 we reveal herein the first cobalt-based catalytic
system that is efficient for the aziridination of different
alkenes.14-16 Cobalt(II) porphyrins (Figure 1) were shown
Table 1. Aziridination of Styrene by Cobalt Porphyrinsa
mol
temp time yield
entry S:BTb
[Co(Por)]c
Co(TPP)
Co(TMeOPP)
Co(TTMeOPP)
Co(TPFPP)
(%) solvent (°C) (h) (%)d
1
2
3
4
5
6
7
8
1:2
1:2
1:2
1:2
1:2
5
5
5
5
5
5
5
5
5
5
5
5
2
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
THF
23
23
23
23
23
23
23
23
23
18
16
20
17
18
18
17
20
17
19
17
18
17
7
18
<5
0
53
83
75
67
0
Co(TDClPP)
1:1.2 Co(TDClPP)
5:1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
Co(TDClPP)
9
CH2Cl2
<5
0
10
11
12
13
14
15
CH3C6H5 23
CH3CN
CH3CN
CH3CN
CH3CN
40
82
23
23
23
84
66
80
71
82
10 CH3CN
7
a Carried out under N2 in the presence of 5 Å molecular sieves with a
concentration of 0.1 mmol styrene/2 mL solvent. b The mole ratio of styrene
substrate to bromamine-T. c See Figure 1. d Isolated yields.
Figure 1. Structures of cobalt(II) porphyrin complexes.
to be effective catalysts for the aziridination of a wide variety
of alkenes with bromamine-T (Scheme 1).
could aziridinate styrene in a low yield, the Co complexes
of electron-rich porphyrins such as Co(TTMeOPP) and Co-
(TMeOPP) furnished no or only a trace amount of the desired
product (Table 1, entries 1-3). The production of aziridine,
however, was tripled when the reaction was catalyzed by
the Co complex of an electron-deficient porphyrin Co-
(TPFPP) (Table 1, entry 4). Significant further improvement
was achieved with the Co complex of an electron-deficient
and sterically hindered porphyrin Co(TDClPP) as the catalyst,
producing the desired aziridine in 83% isolated yield (Table
1, entry 5). Whereas change in the ratio of styrene to
bromamine-T from 1:2 to 1:1.2 had no significant influence
on the catalytic reaction, an excess of styrene resulted in a
relatively lower yield (Table 1, entries 6 and 7). Acetonitrile
appeared the solvent of choice for the catalytic reaction, as
the uses of other solvents such as tetrahydrofuran, methylene
chloride, or toluene gave no or only a trace amount of the
desired product (Table 1, entries 8-10). Although a slightly
better yield was obtained at 40 °C, further increase in reaction
temperature caused a lower yield (Table 1, entries 11 and
12). The room temperature reaction could be effectively
carried out at a lower catalyst loading without affecting the
yield (Table 1, entry 13). A relatively lower yield was
observed when the reaction time was shortened (Table 1,
entry 14). The employment of higher catalyst loading,
however, could allow the reaction to be finished in a short
time without decrease of the yield (Table 1, entry 15).
Using the above optimized reaction conditions, the new
Co(TDClPP)-based catalytic system was found to be suitable
for many different types of alkene substrates (Table 2). In
addition to styrene, derivatives of styrene with alkyl sub-
stituents could be equally aziridinated to afford the desired
products in high yields (Table 2, entries 1-4). Functional
groups in styrene derivatives could be well tolerated to
cleanly generate the corresponding aziridines (Table 2, entries
5-7). Sterically hindered derivatives such as 2,4,6-trimeth-
Scheme 1. Aziridination of Alkenes with Bromamine-T
Catalyzed by Cobalt(II) Porphyrin Complexes
Using styrene as a model substrate, we first evaluated the
catalytic aziridination activities of Co complexes supported
by various porphyrins under practical conditions (room
temperature, one-pot protocol, and styrene as the limiting
reagent). The results are summarized in Table 1. Although
the Co complex of the most common porphyrin Co(TPP)
(12) (a) Mahy, J.-P.; Bedi, G.; Battioni, P.; Mansuy, D. J. Chem. Soc.,
Perkin Trans. 2 1988, 1517. (b) Lai, T.-S.; Kwong, H.-L.; Che, C.-M.;
Peng, S.-M. Chem. Commun. 1997, 2373. (c) Simonato, J.-P.; Pecaut, J.;
Scheidt, W. R.; Marchon, J.-C. Chem. Commun. 1997, 989. (d) Au, S.-M.;
Huang, J.-S.; Yu, W.-Y.; Fung, W.-H.; Che, C.-M. J. Am. Chem. Soc. 1999,
121, 9120. (e) Liang, J.-L.; Huang, J.-S.; Yu, X.-Q.; Zhu, N.; Che, C.-M.
Chem. Eur. J. 2002, 8, 1563.
(13) (a) Chen, Y.; Huang, L.; Ranade, M. A.; Zhang, X. P. J. Org. Chem.
2003, 68, 3714. (b) Chen, Y.; Huang, L.; Zhang, X. P. J. Org. Chem. 2003,
68, 5925. (c) Chen, Y.; Huang, L.; Zhang, X. P. Org. Lett. 2003, 5, 2493.
(d) Huang, L.; Chen, Y.; Gao, G.-Y.; Zhang, X. P. J. Org. Chem. 2003,
68, 8179. (e) Lee, M.-Y.; Chen, Y.; Zhang, X. P. Organometallics 2003,
22, 4905. (f) Chen, Y.; Zhang, X. P. J. Org. Chem. 2004, 69, 2431 (g)
Vyas, R.; Gao, G.-Y.; Harden, J. D.; Zhang, X. P. Org. Lett. 2004, 6, 1907.
(h) Chen, Y.; Fields, K. B.; Zhang, X. P. J. Am. Chem. Soc. 2004, 126,
14178.
(14) For cobalt porphyrin-mediated amination with aryl azides, see:
Ragaini, F.; Penoni, A.; Gallo, E.; Tollari, S.; Gotti, C. L.; Lapadula, M.;
Mangioni, E.; Cenini, S. Chem. Eur. J. 2003, 9, 249.
(15) Under microwave conditions, CoCl2 was shown to catalyze aziri-
dination of styrene with bromine-T to form the desired aziridine in 56%
yield when excess styrene was used (styrene/bromamine-T ) 5: 1). See
ref 6a.
(16) Co(TPP) was previously found to catalyze aziridination of styrene
with bromine-T to form the desired aziridine in 27% yield when excess
styrene was used (styrene/bromamine-T ) 5: 1). See ref 13a.
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Org. Lett., Vol. 7, No. 15, 2005

electron-deficient pentafluorostyrene could be successfully
converted to the desired aziridines in good to high yields
(Table 2, entries 10-13). To the best of our knowledge, this
represents the first example of aziridination of pentafluoro-
styrene. In addition, both R-substituted and â-substituted
(cyclic and acyclic) styrenes were suitable substrates for the
catalytic process (Table 2, entries 14-21). Whereas a
competitive amidation was presumably responsible for the
low aziridination yield of 1,2-dihydronaphthalene (Table 2,
entry 16), excellent yields were obtained for acyclic â-sub-
stituted styrenes in both cis and trans forms (Table 2, entries
18-21). For the latter substrates, the catalytic reactions by
Co(TDClPP) appeared to lack stereospecificity, although high
trans-stereoselectivity was observed for both cis- and trans-
â-methylstyrenes. In addition to aromatic and conjugated
alkenes, acyclic and cyclic aliphatic alkenes with different
ring sizes are suitable substrates for the catalytic system
(Table 2, entries 22-25). Under similar conditions, exo-
methylene carbocycles such as methylenecyclohexane could
also be successfully aziridinated to afford the desired
spirocyclic aziridine in 67% isolated yield (Table 2, entry
26).
Table 2. Aziridination of Different Alkenes by Co(TDClPP)a
In summary, we have developed the first Co-based
catalytic system that is efficient for aziridination of various
alkenes.14-16 We demonstrated that the new Co(TDClPP)/
bromamine-T catalytic system can be effectively operated
under mild and practical conditions with alkenes as limiting
reagents and is generally suitable for a wide variety of alkene
substrates. Further improvement of the catalytic system
including stereospecificity and the development of its asym-
metric variant are in progress. The successful demonstration
of the catalytic capability of Co(II) porphyrins for aziridi-
nation will also likely stimulate future studies to address
some interesting mechanistic issues.
Acknowledgment. We are grateful for financial support
of this work from the UT Department of Chemistry, the UT
Center of Excellence for Structural Biology (CESB), Oak
Ridge Associated Universities (ORAU) for a Ralph E. Powe
Junior Faculty Enhancement Award, Hereditary Disease
Foundation (HDF), and High Q Foundation (HQF).
a Carried out at room temperature in CH3CN overnight under N2 with
alkenes as limiting reagent (alkene/bromamine-T ) 1:2) using 5 mol %
Co(TDClPP) in the presence of 5 Å molecular sieves at concentration of
0.2 mmol alkene/4-5 mL CH3CN. b Isolated yields. c Performed with
alkene/bromamine-T ) 5:1. d Performed with alkene/bromamine-T ) 1:3
using 10 mol % Co(TDClPP). e cis:trans ) 9:91. f cis:trans ) 47:53. g cis:
trans ) 8:92. h cis:trans ) 58:42.
Supporting Information Available: Analytical data for
all products. This material is available free of charge via
the Internet at http://pubs.acs.org.
ylstyrene as well as 2-vinylnaphthalene could also be
catalytically aziridinated, albeit in lower yields (Table 2,
entries 8 and 9). Halogenated styrenes including highly
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Org. Lett., Vol. 7, No. 15, 2005
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