Microencapsulated Cu(acac)2: A recoverable and reusable polymer-supported copper catalyst for aziridination of olefins

Article abstract of DOI:10.1016/j.tetlet.2003.09.213

Microencapsulated copper(II) acetylacetonate was prepared and used in the aziridination of alkenes employing [N-(p-tolylsulfonyl)imino]phenyliodinane (PhI=NTs) as the nitrogen source. Microencapsulated copper(II) acetylacetonate [MC-Cu(acac)₂] catalyst was reused for several cycles with consistent activity.

Full text of DOI:10.1016/j.tetlet.2003.09.213

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 9029–9032
Microencapsulated Cu(acac)2: a recoverable and reusable
polymer-supported copper catalyst for aziridination of olefins
M. Lakshmi Kantam,a,* B. Kavita,a V. Neeraja,a Y. Haritha,a M. K. Chaudhurib,* and
S. K. Dehuryb
aIndian Institute of Chemical Technology, Hyderabad 500 007, India
bDepartment of Chemistry, Indian Institute of Technology, Guwahati 81039, India
Received 20 August 2003; revised 28 September 2003; accepted 29 September 2003
Abstract—Microencapsulated copper(II) acetylacetonate was prepared and used in the aziridination of alkenes employing
[N-(p-tolylsulfonyl)imino]phenyliodinane (PhIꢀNTs) as the nitrogen source. Microencapsulated copper(II) acetylacetonate [MC-
Cu(acac)2] catalyst was reused for several cycles with consistent activity.
© 2003 Elsevier Ltd. All rights reserved.
Aziridines are an important class of compounds in
organic chemistry1 and especially useful as intermedi-
ates for functional group modifications, efficient chiral
auxiliaries2 and ligands in asymmetric catalysis.3 The
synthesis of aziridines has therefore been a subject of
considerable research interest over the last few years.
some limitations such as tedious catalyst preparation,
low yields of aziridines and long reaction times.
Immobilized catalysts have been of great interest due to
several advantages, such as ease of product separation,
isolation and reuse of the catalyst.14 Recently,
Kobayashi et al. have reported the use of microencap-
sulation as a technique for immobilizing metal com-
plexes.15–18 Microencapsulation is a rather new method
for immobilizing catalysts onto polymers accomplished
by physical envelopment by the polymers. This allows
interactions between the p electrons of benzene rings of
the polystyrene-based polymers and vacant orbitals of
the catalyst (metal compounds) rendering the catalyst
system more effective. This technique has been used in
a variety of reactions such as [MC-Sc(OTf)3] in aza
Diels–Alder, Strecker, cyanation and alkylation,19
Mannich-type and aldol reactions,15 silylation of
alcohols20 and microencapsulated osmium tetroxide
[MC-OsO4] was used for dihydroxylation of olefins16,21
and [MC-Pd(PPh3)]18 for allylic substitution, Suzuki
coupling and Mizoroki Heck reactions.
A single atom transfer to olefins is the shortest route to
three-membered cyclic compounds. Catalytic addition
of a carbene moiety to an imine results in the formation
of an aziridine, but the yields are often low.4 The
transfer of a nitrogen atom onto an olefin offers an
attractive synthesis of aziridines.
Evans et al. reported Cu(I) and Cu(II) salts5 as the
most effective and extensively used homogeneous cata-
lysts for aziridination of olefins using [N-(p-tolylsul-
fonyl)imino] phenyl iodinane (PhIꢀNTs) as the nitrene
donor. Taylor and co-workers6,7 and Ando et al.8 have
reported copper-catalyzed aziridination of alkenes using
chloramine-T as the source of the nitrene. Hutchings et
al.9,10 have, however, for the first time used heteroge-
neous copper-exchanged zeolite Y (CuHY) catalyst,
using PhIꢀNTs as a nitrene donor for aziridination
reactions. Later, a polymer-supported Ru-porphyrin
catalyst,11 a silica-supported polypyrazolylborate cop-
per complex12 and a polymer-supported manganese(II)
complex13 were also used for the aziridination of olefins
by several other workers. However, these catalysts have
Considering the advantages of the microencapsulation
technique, we have developed a MC-Cu(acac)2 catalyst
* Corresponding authors. Tel./fax: 91-40-27160921; 91-361-2690762;
e-mail: mlakshmi@iict.res.in; mkc@iitg.ernet.in
Scheme 1.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.09.213

9030
M. Lakshmi Kantam et al. / Tetrahedron Letters 44 (2003) 9029–9032
Table 2. MC-Cu(acac)2-catalyzed aziridination of alkenesa
by immobilizing Cu(acac)2 on polystyrene, and used
this in the aziridination of alkenes. Incidentally, we
have at our disposal a very clean and easy-to-operate
22
method for the preparation of highly pure Cu(acac)2
which facilitated the catalyst preparation. The prepara-
tion of the MC-Cu(acac)2 is very simple and was
accomplished following the Kobayashi protocol15 using
1 g of polystyrene and 120 mg of Cu(acac)2. Measure-
ment of the mass increase of the resultant MC-Cu(a-
cac)2 indicates that 100 mg of Cu(acac)2 is
encapsulated, and the copper content is 0.38 mmol/g.
MC-Cu(acac)2 catalyses the aziridination of a wide
range of alkenes employing [N-(p-tolylsulfonyl)imino]-
phenyl iodinane (PhIꢀNTs) as the nitrogen source
(Scheme 1).
We have carried out aziridination of styrene using
different nitrene donors such as chloramine-T, bro-
mamine-T and PhIꢀNTs and the leaching of the metal
for these nitrene donors was determined using an
Atomic Absorption Spectrometer. The results are sum-
marized in Table 1. The results confirmed that
PhIꢀNTs is the preferred nitrene donor for our catalytic
system.
A variety of alkenes were examined for this MC-Cu(a-
cac)2 catalyzed aziridination using PhIꢀNTs. Under the
standardized conditions (MeCN, 5.6 mol% catalyst, 1
equiv. of PhIꢀNTs, 5 equiv. of olefin, 25°C), good
yields of aziridines were obtained with both aromatic
and aliphatic olefins. The results are summarized in
Table 2. PhIꢀNTs, like its oxygen analogue PhIꢀO, is
insoluble in a variety of solvents, including MeCN,
dissolution of this reagent in the reaction indicates the
completion of the reaction. It is clear from Table 2 that
the catalyst gives the best results with phenyl substi-
tuted alkenes and moderate to good yields with simple
olefins like trans-2-octene and 1-octene (entries 10 and
11, Table 2). All aliphatic olefins afforded good yields
of aziridines without allylic insertion. The reaction of
PhIꢀNTs with norbornene (entry 7, Table 2) occurs
from the less hindered exo face of the bicyclic nucleus
to provide the exo adduct in high yield and sterically
hindered olefins like trans-stilbene (entry 6, Table 2)
afforded the corresponding aziridines in good yields.
tion of PhIꢀNTs. No product formation was observed.
Further, the recovered catalyst could be reused for
several cycles with consistent activity (Table 3).
The present microencapsulated catalyst is more active
than its homogeneous analogue, Cu(acac)2 or a hetero-
geneous copper-exchanged zeolite Y (CuHY) catalyst
(Table 2, entry 1). It is noteworthy that in earlier
studies in the homogeneously catalyzed reaction,4 the
yield of aziridine decreased to 37% when the molar
ratio of styrene: PhIꢀNTs was 1:1, due to the compet-
In order to confirm that this process is truly heteroge-
neous, the catalyst was removed after the reaction by
filtration, fresh aliquots of reactants were added to the
filtrate and the reaction was monitored by the dissolu-
Table 3. Recovery and reuse of the MC-Cu(acac)2-catalyst
for aziridination of alkenesa
Table 1. Aziridination of styrene using various nitrene
donors
Entry
Cycle no.
Isolated yield (%)
Nitrene donor
Time (h)
Yield (%)a
Cu-Leaching
(Wt%)
1
2
3
4
5
1
2
3
4
5
92
92
92
90
90
PhIꢀNTs
Chloramine-T
Bromamine-T
1
6
6
92
36
45
0.025
16.56
10.28
a Isolated yields.
a MeCN, 25°C, alkene: PhIꢀNTs=5:1.

M. Lakshmi Kantam et al. / Tetrahedron Letters 44 (2003) 9029–9032
9031
Acknowledgements
ing breakdown of the PhIꢀNTs reagent to yield tolu-
ene-p-sulfonamide, whereas with our catalyst the yield
is 85%. This is a particularly important observation
because this procedure can be applied to the aziridina-
tion of expensive alkenes.
B.K., V.N. and Y.H. thank the Council of Scientific
and Industrial Research, India, for fellowships, S.K.D.
is a IITG research fellow.
In conclusion, MC-Cu(acac)2 is used as an effective
catalyst for the aziridination of alkenes using PhIꢀNTs
as the nitrene source with low catalyst loading. MC-
Cu(acac)2 catalyst can be reused for several cycles with
consistent activity. The advantages of MC-Cu(acac)2
are the ease of preparation and high activity.
References
1. (a) Padwa, A.; Woolhouse, A. D. In Comprehensive Het-
erocyclic Chemistry; Lwowski, W., Ed.; Pergamon:
Oxford, 1983; Vol. 7, p. 47; (b) Kemp, J. E. G. In
Comprehensive Organic Synthesis; Trost, B. M.; Fleming,
I., Eds.; Pergamon: Oxford, 1991; Vol. 7, p. 469.
2. Tanner, D. Angew. Chem., Int. Ed. Engl. 1994, 33, 599–
619.
3. (a) Tanner, D.; Andersson, P. G.; Harden, A.; Somfai, P.
Tetrahedron Lett. 1994, 35, 4631–4634; (b) Andersson, P.
G.; Guijarro, D.; Tanner, D. Synlett 1996, 727–728; (c)
Tanner, D.; Korno, H. T.; Guijarro, D.; Andersson, P.
G. Tetrahedron 1998, 54, 14213–14232.
4. (a) Hansen, K. B.; Finney, N. S.; Jacobsen, E. N. Angew.
Chem., Int. Ed. Engl. 1995, 34, 676–678; (b) Rasmussen,
K. G.; Jorgensen, K. A. J. Chem. Soc., Chem. Commun.
1995, 1401–1402.
5. (a) Evans, D. A.; Faul, M. M.; Bilodean, M. T. J. Am.
Chem. Soc. 1994, 116, 2742–2753; (b) Evans, D. A.; Faul,
M. M.; Bilodean, M. T. J. Org. Chem. 1991, 56, 6744–
6746.
Preparation of copper(II) acetylacetonate: Copper(II)
acetate monohydrate (10 g, 50.09 mmol) was dissolved
in 300 mL of water in a 500 mL beaker by warming
at 60°C for 15 min. To the cooled solution, 20% aq.
KOH was added slowly till the pH was raised to ca. 8
with constant stirring to precipitate the metal as its
hydrated oxide. The metal hydroxide was washed with
water till it was free of alkali by decantation, filtered
and washed twice with cold water. Distilled acetylace-
tone (11.06 mL, 110 mmol) was added to the precipi-
tate and mixed thoroughly with a glass rod. An
exothermic reaction set in leading to the formation of
blue shiny crystals of Cu(acac)2. The mixture was
allowed to stand at room temperature for 30 min and
then placed in an ice-water bath for 15 min. The solid
product was filtered through Whatmann No. 42 filter
paper and dried in vacuo over fused CaCl2 (yield:
12.49 g, 95%). The chemical analyses, IR and mass
spectra of the compound match very well with those
reported in literature.23
6. Aujla, P. S.; Baird, C. P.; Taylor, P. C.; Manger, H.;
Valle´e, Y. Tetrahedron Lett. 1997, 38, 7453–7456.
7. Aujla, P. S.; Albone, D. P.; Taylor, P. C. J. Org. Chem.
1998, 63, 9569–9571.
Preparation of the catalyst: Polystyrene (1.00 g, pur-
chased from Aldrich) was dissolved at 40°C in cyclo-
hexane (20 mL), and to this solution was added
Cu(acac)2 (0.12 g). This mixture was stirred for 1 h at
this temperature and then slowly cooled to 0°C with
vigorous stirring. The polystyrene solidified around the
metal catalyst dispersed in the solution. Hexane (30
mL) was added to harden the capsule walls. The mix-
ture was stirred at room temperature for 1 h, and the
capsules were washed with acetonitrile several times to
remove unencapsulated Cu(acac)2 and then dried
under vacuum.
8. Ando, T.; Minakata, S.; Ryu, I.; Komatsu, M. Tetra-
hedron Lett. 1998, 39, 309–312.
9. Langham, C.; Piaggo, P.; Bethell, D.; Lee, D. F.;
McMorn, P.; Bulman Page, P. C.; Willock, D. J.; Sly, C.;
Hancock, F. E.; King, F.; Hutchings, G. J. J. Chem. Soc.,
Chem. Commun. 1998, 1601–1602.
10. Langham, C.; Taylor, S.; Bethell, D.; McMorn, P.; Bul-
man Page, P. C.; Willock, D. J.; Sly, C.; Hancock, F. E.;
King, F.; Hutchings, G. J. J. Chem. Soc., Perkin Trans. 2
1999, 1043–1049.
11. Zhang, J. L.; Che, C. M. Org. Lett. 2002, 4, 1911–1914.
12. Mar Diaz-Requejo, M.; Perez, P. J. J. Organomet. Chem.
2001, 617–618, 110–118.
General procedure: To acetonitrile (3 mL) were added
MC-Cu(acac)2 (0.15 g, 5.6 mol%), styrene (0.56 ml, 5
mmol) and PhIꢀNTs (0.372 g, 1 mmol) and the reac-
tion mixture was stirred at room temperature. The
reaction was monitored by disappearance of the
PhIꢀNTs from the reaction mixture. After completion
of the reaction, the catalyst was filtered and the filtrate
concentrated and purified by column chromatography
(hexane/ethyl acetate, 95/5, v/v) to afford pure product
13. Vyas, R.; Chanda, B. M.; Belhekar, A. A.; Patel, D. R.;
Rain, R. N.; Bedekar, A. V. J. Mol. Catal. A 2000, 160,
237–241.
14. For reviews, see: (a) Bailey, D. C.; Langer, S. H. Chem.
Rev. 1981, 81, 109–148; (b) Akelah, A.; Sherrington, D.
C. Chem. Rev. 1981, 81, 557–587; (c) Frechet, J. M. J.
Tetrahedron 1981, 37, 663–683.
15. Kobayashi, S.; Nagayama, S. J. Am. Chem. Soc. 1998,
120, 2985–2986.
16. Nagayama, S.; Endo, M.; Kobayashi, S. J. Org. Chem.
1
as white solid. Yield: 0.251 g, 92%. H NMR (CDCl3,
400 MHz) l 7.86 (d, 2H, J=8.3 Hz, Ar-H), 7.27 (m,
7H, Ar-H), 3.77 (dd, 1H, Jcis=7.2 Hz, Jtrans=4.5 Hz,
CHPh), 2.98 (d, 1H, J=7.8 Hz, cis-CH-aziridine) 2.43
(s, 3H, Ar-Me), 2.38 (d, 1H, J=4.4 Hz, trans-CH-
aziridine).
1998, 63, 6094–6095.
17. Kobayashi, S.; Ishida, T.; Akiyama, R. Org. Lett. 2001,
3, 2649–2652.
18. Akiyama, R.; Kobayashi, S. Angew. Chem., Int. Ed. 2001,
40, 3469–3471.

9032
M. Lakshmi Kantam et al. / Tetrahedron Letters 44 (2003) 9029–9032
22. Chaudhuri, M. K.; Dehury, S.; Dhar, S. S.; Bora, U.;
Choudary, B. M.; Lakshmi Kantam, M. US Patent
appln. no. 10/335, 103, December 31, 2002.
23. Bhattacharjee, M. N.; Chaudhuri, M. K.; Devi, M.;
Dutta Purkayastha, R. N.; Hiese, Z.; Khathig, D. T. Int.
J. Mass Spectrom. Ion Processes 1986, 71, 109–117.
19. Kobayashi, S.; Akiyama, R. J. Chem. Soc., Chem. Com-
mun. 2003, 449–460.
20. Suzuki, T.; Watahiki, T.; Oriyama, T. Tetrahedron Lett.
2000, 41, 8903–8906.
21. Kobayashi, S.; Endo, M.; Nagayama, S. J. Am. Chem.
Soc. 1999, 121, 11229–11230.