Nitrene transfer reactions catalyzed by gold complexes

Article abstract of DOI:10.1021/jo060016t

We report here the first gold-catalyzed nitrene transfer reaction. A gold(I) compound, supported by 4,4′,4″-tri-tert-butyl-2,2′: 6′,2″-terpyridine (tBu₃tpy) as the ligand, efficiently catalyzes olefin aziridination with the use of the commercia

Full text of DOI:10.1021/jo060016t

Nitrene Transfer Reactions Catalyzed by Gold Complexes
Zigang Li, Xiangyu Ding,‡ and Chuan He*
Department of Chemistry, The UniVersity of Chicago, 5735 South Ellis AVenue, Chicago, Illinois 60637
chuanhe@uchicago.edu
ReceiVed January 4, 2006
We report here the first gold-catalyzed nitrene transfer reaction. A gold(I) compound, supported by 4,4′,4′′-
tri-tert-butyl-2,2′:6′,2′′-terpyridine (tBu3tpy) as the ligand, efficiently catalyzes olefin aziridination with
the use of the commercially available oxidant PhI(OAc)2 and sulfonamides. This system also mediates
carbene insertion into benzene.
Introduction
explore nitrene transfer reactions that could be catalyzed by
similar complexes of the other two coinage metals, gold and
copper. Reported herein is the first homogeneous gold-catalyzed
olefin aziridination that proceeds with the use of commercially
available oxidant PhI(OAc)2 and different sulfonamides. This
process avoids the use of the sulfonyliminoiodinane-type
nitrenoids such as PhIdNTs,4 which are commonly employed
in olefin aziridination reactions and have to be synthesized
beforehand in unsatisfying yields.1c,5 Further study of the new
activity reported here may lead to a better understanding of the
redox chemistry of gold and development of other gold-mediated
oxidation reactions.
Metal-catalyzed nitrene transfers to unsaturated and saturated
organic substrates are important tools in synthetic chemistry.1
Various transition metal complexes such as those of rhodium,
copper, ruthenium, cobalt, iron, manganese, and nickel have
been shown to catalyze these transformations.2 Our laboratory
also discovered a unique disilver(I)-based catalyst, with 4,4′,4′′-
tri-tert-butyl-2,2′:6′,2′’terpyridine (tBu3tpy) as the ligand, which
mediates efficient olefin aziridination and intramolecular ami-
dation of saturated C-H groups.3 This success prompted us to
‡ Current address: Illinois Mathematics and Science Academy.
Gold-based catalysis has recently attracted intensive interest
from the chemistry community.6 Especially in the past few years,
numerous reactions catalyzed by gold(I) and gold(III) complexes
have been reported. In most cases, the metal ions act as Lewis
acids that activate alkyne or alkene substrates.7 Gold(III) is also
(1) (a) Halfen, J. A. Curr. Org. Chem. 2005, 9, 657. (b) Dauban, P.;
Dodd, R. H. Synlett 2003, 1571. (c) Mueller, P.; Fruit, C. Chem. ReV. 2003,
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V. K. Synlett 1998, 329.
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Moran, M.; Naegeli, I. J. Phys. Org. Chem. 1996, 9, 341. (b) Liang, J.-L.;
Yuan, S.-X.; Chan, P. W. H.; Che, C.-M. Org. Lett. 2002, 4, 4507. (c)
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Soc. 2004, 126, 15378. (e) Fiori, K. W.; Fleming, J. J.; Du Bois, J. Angew.
Chem., Int. Ed. 2004, 43, 4349. (f) Espino, C. G.; Wehn, P. M.; Chow, J.;
Du Bois, J. J. Am. Chem. Soc. 2001, 123, 6935. (g) Espino, C. G.; Du
Bois, J. Angew. Chem., Int. Ed. 2001, 40, 598. For Cu, see: (h) Li, Z.;
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Am. Chem. Soc. 1999, 121, 9120. (m) Man, W.-l.; Lam, W. W. Y.; Yiu,
S.-M.; Lau, T.-C.; Peng, S.-M. J. Am. Chem. Soc. 2004, 126, 15336. For
Co, see: (n) Gao, G.-Y.; Harden, J. D.; Zhang, X. P. Org. Lett. 2005, 7,
3191. For Fe, see: (o) Mahy, J. P.; Battioni, P.; Mansuy, D. J. Am. Chem.
Soc. 1986, 108, 1079. (p) Vyas, R.; Gao, G.-Y.; Harden, J. D.; Zhang, X.
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Yu, X.-Q.; Zhu, N.-Y.; Che, C.-M. Chem. Eur. J. 2002, 8, 1563. (r) Mansuy,
D.; Mahy, J. P.; Dureault, A.; Bedi, G.; Battioni, P. J. Chem. Soc., Chem.
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He, C. Angew. Chem., Int. Ed. 2004, 43, 4210.
(4) A few examples directly using sulfonamide have been reported with
Rh and Cu recently, For Rh, see: (a) Guthikonda, K.; Du Bois, J. J. Am.
Chem. Soc. 2002, 124, 13672. For Cu, see: (b) Dauban, P.; Saniere, L.;
Tarrade, A.; Dodd, R. H. J. Am. Chem. Soc. 2001, 123, 7707. (c) Han, H.;
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Kwong, H.-L.; Liu, D.; Chan, K.-Y.; Lee, C.-S.; Huang, K.-H.; Che, C.-M.
Tetrahedron Lett. 2004, 45, 3965.
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Chim. Acta 2003, 342, 301. (b) Taylor, S.; Gullick, J.; McMorn, P.; Bethell,
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(6) For recent reviews, see: (a) Hashmi, A. S. K. Gold Bull. 2004, 37,
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10.1021/jo060016t CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/01/2006
5876
J. Org. Chem. 2006, 71, 5876-5880

Nitrene Transfer Reactions Catalyzed by Gold Complexes
TABLE 1. Effect of Selected Gold-based Precatalysts on an Olefin Aziridination Reactiona
entry
precatalyst
conversion (%)b
1
2
3
AuCl
AuCl + AgOTf
Au(PPh3)Cl
0
0
0
4
5
6
7
8
9
10
11
12
13
14
Au(PPh3)Cl + AgOTf
7
Au(Pt-Bu3)Cl
0
AuCl + 4,4′-di-tert-butyl-2,2′-bipyridine
AuCl + 2,2′:6′,2′′-terpyridine
AuCl + 4,4′,4′′-tri-tert-butyl-2,2′: 6′,2′′-terpyridine
[Au(4,4′-di-tert-butyl-2,2′-bipyridine)]OTf
[Au(2,2′:6′,2′′-terpyridine)]OTf
[Au(4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine)]OTf
[Au(4,4′,4′′-tri-tert-butyl-2,2′:6′,2′′-terpyridine)]OTf
[Au(bathophenanthroline)]OTf
18c
50c
56c
60d
70d
83 (75)d,e,f
>95 (88)d,e,g
55h
[Au(bathophenanthroline)]OTf
80 (70)d,e,g
a All reactions were carried out under nitrogen in predried solvent with 3% gold catalyst and 0.5 mmol amide (styrene:amide:PhI(OAc)2 ) 2:1:1.2).
b NMR yield with 1,3,5-trimethoxybenzene as an internal standard. c AuCl and the ligand were mixed first and the formed precipitate was filtered off before
the reaction was conducted. d Homogeneous gold complex was used (see the Supporting Information for the preparation procedure). e Isolated yield in
parentheses. f Cl-, NO3-, and ClO4- have been tested as counteranions which gave 35-65% conversion. g p-Nitrosulfonamide was used. h Other phenanthroline-
type ligands gave lower conversions.
known to readily metalate arenes8 and this activity has been
utilized in hydroarylation reactions.9 Despite this progress,
homogeneous gold-catalyzed redox transformations are still rare.
Very recently, Corma and co-workers reported a homo-coupling
of boronic acid with nano-CeO2-supported gold(I);10 Ito, Sawa-
mura, and co-workers reported an interesting dehydrogenative
coupling of silane and alcohol with Au(xantphos)Cl;11 Nolan
and Pere´z reported a gold(I)-NHC complex that mediates
carbene insertions to benzene, O-H, and N-H bonds;12 and
Shi and co-workers reported an interesting alcohol oxidation
chemistry catalyzed by gold(I).13 These examples showed that
the redox potential gap between gold(I)/gold(III)14 could pos-
sibly be tuned by employing ligands or in crystalline states,
which could result in unique redox properties.
Results and Discussion
In our initial investigation, we employed o-nitrosulfonamide
(0.5 mmol) as the nitrene source, PhI(OAc)2 (0.6 mmol) as the
oxidant, and styrene (1 mmol) as the olefin substrate. The
reaction was run in acetonitrile at 50 °C with 3 mol % of gold
catalyst. As indicated in Table 1, simple gold(I) complexes
did not exhibit any significant activity (Table 1, entries 1-5).
Mixing AuCl with ligands such as pyridine, dipyridine, and
terpyridine typically led to precipitations in common organic
solvents, which complicates the characterization of the cata-
lyst and the reaction (for selected examples see Table 1, entries
6-8). However, with ligands bearing tert-butyl or phenyl
substitutions, we could prepare gold complexes15 that have
good solubilities in either CH2Cl2 or CH3CN. Some of these
compounds serve as excellent catalysts in mediating styrene
aziridination (Table 1). Most noticeable is the gold(I) com-
plex of 4,4′,4′′-tri-tert-butyl-2,2′:6′,2′’terpyridine, denoted as
[Au(tBu3tpy)](OTf), which gave 83% conversion and 75%
isolated yield of the aziridine product in an overnight reaction
at 50 °C (Table 1, entry 11). The reaction also proceeded at
room temperature; however, only about 40% conversion was
observed overnight. Thus, our standard reaction conditions are
set at 50 °C overnight. Counteranions did affect the product
yield to a certain extent as OTf- is superior to Cl-, NO3-, and
(7) For selected examples, see: (a) Hashmi, A. S. K.; Rudolph, M.;
Weyrauch, J. P.; Woelfle, M.; Frey, W.; Bats, J. W. Angew. Chem., Int.
Ed. 2005, 44, 2798. (b) Yang, C.-G.; He, C. J. Am. Chem. Soc. 2005, 127,
6966. (c) Staben, S. T.; Kennedy-Smith, J. J.; Toste, F. D. Angew. Chem.,
Int. Ed. 2004, 43, 5350. (d) Zhang, L. M.; Kozmin, S. A. J. Am. Chem.
Soc. 2004, 126, 11806. (e) Wei, C.; Li, C.-J. J. Am. Chem. Soc. 2003, 125,
9584. (f) Mizushima, E. S.; Sato, K.; Hayashi, T.; Tanaka, M. Angew. Chem.,
Int. Ed. 2002, 41, 4563. (g) Hashmi, A. S. K.; Frost, T. M.; Bats, J. W. J.
Am. Chem. Soc. 2000, 122, 11553. (h) Yao, X.; Li, C.-J. J. Am. Chem.
Soc. 2004, 126, 6884. (i) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph, M.;
Kurpejovic, E. Angew. Chem., Int. Ed. 2004, 43, 6545. (j) Sherry, B. D.;
Toste, F. D. J. Am. Chem. Soc. 2004, 126, 15978. (k) Nieto-Oberhuber, C.;
Munoz, M. P.; Bunuel, E.; Nevado, C.; Cardenas, D. J.; Echavarren, A. M.
Angew. Chem., Int. Ed. 2004, 43, 2402. (l) Sromek, A. W.; Gevorgyan, M.
R. V. J. Am. Chem. Soc. 2005, 127, 10500.
(8) Kharasch, M. S.; Isbell, H. S. J. Am. Chem. Soc. 1931, 53, 3053.
(9) (a) Hashmi, A. S. K.; Schwarz, L.; Choi, J. H.; Frost, T. M. Angew.
Chem., Int. Ed. 2000, 39, 2285. (b) Reetz, M. T.; Sommer, K. Eur. J. Org.
Chem. 2003, 18, 3485. (c) Shi, Z.; He, C. J. Org. Chem. 2004, 69, 3669.
(d) Shi, Z.; He, C. J. Am. Chem. Soc. 2004, 126, 5964. (e) Shi, Z.; He, C.
J. Am. Chem. Soc. 2004, 126, 13596. (f) Li, Z.; Shi, Z.; He, C. J. Organomet.
Chem. 2005, 690, 5049. (g) Luo, Y.; Li, C.-J. Chem. Commun. 2004, 1930.
(10) Carretin, S.; Guzman, J.; Corma, A. Angew. Chem. 2005, 117, 2282;
Angew. Chem., Int. Ed. 2005, 44, 2242.
-
ClO4 (Table 1). Furthermore, a more reactive p-nitrosulfona-
mide was tested as the nitrenoid, which led to an almost
quantitative conversion of styrene to the corresponding aziridine
(Table 1, entry 12).16
Then p-nitrosulfonamide (NsNH2) was employed to react with
different olefins. The scope of the reaction was studied as shown
in Table 2. The use of styrenes bearing different functional
groups led to excellent yields of the corresponding aziridines.
Norbornene and cyclooctene, as well as a substrate for an
(11) Ito, H.; Takagi, K.; Miyahara, T.; Sawamura, M. Org. Lett. 2005,
7, 3001.
(12) Fructos, M. R.; Belderrain, T. R.; Fre´mont, P.; Scott, N.; Nolan, S.
P.; D´ıaz-Requejo, M. M.; Pe´rez, P. J. Angew. Chem., Int. Ed. 2005, 44,
5284.
(13) Guan, B.; Xing, D.; Cai, G.; Wan, X.; Yu, N.; Fang, Z.; Yang, L.;
Shi, Z. J. Am. Chem. Soc. 2005, 127, 18004.
(14) Rao, B. V. Anal. Chim. Acta 1992, 258, 167.
(15) Please see the Supporting Information for procedures.
(16) For the reactivity comparison of different sulfonamides, see: Leung,
S. K.-Y.; Tsui, W.-M.; Huang, J.-S.; Che, C.-M.; Liang, J.-L.; Zhu, N. J.
Am. Chem. Soc. 2005, 127, 16629.
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Li et al.
TABLE 2. Aziridination of Olefins by [Au(tBu3Tpy)](OTf)a
TABLE 3. Effect of Different Nitrene Sourcesa
a All reactions were carried out overnight at 50 °C. b Isolated yield.
in Table 1, all commonly used cationic gold(I) Lewis acids did
not show catalytic activity, which seems to argue strongly
against the Lewis acid mechanism.
Other types of sulfonamides can be employed in this system
(Table 3), albeit with lower product yields in some cases. It
should be noted that the reactions with 5-methylpyridine-2-
sulfonamide and trichloroethylsulfamate ester as the nitrene
sources did not work for the disilver(I)-based catalytic system
we reported previously,3 which indicates reactivity difference
between the two systems (Scheme 1). This reaction has a
limitation that the aziridination of unactivated olefins could not
be catalyzed so far. More research is required to obtain a more
active gold-based catalyst in the future.
Electron-spray mass spectrometry (ESI-MS) was used to
characterize the precatalyst [Au(tBu3tpy)](OTf). The molecular
peak at ∼598.2 bearing a 2+ charge was observed (Figure 1),
indicating a molecular formula of [Au2(tBu3tPy)2]2+ (calculated
m/z 598), which is in agreement with the elemental analysis of
the complex.17 This result suggests that the gold(I) precatalyst
is in a dimeric [Au2(tBu3tPy)2](OTf)2 form in solution, which
may resemble the disilver(I) structure we reported previously.3
The [Au(tBu3tpy)](OTf) system exhibits other interesting
reactivities as well. It can mediate a carbene insertion to neat
benzene with ethyl diazoacetate (EDA). A 57% conversion was
obtained overnight at 80 °C with diethyl fumarate and diethyl
maleate produced as side products in an almost identical ratio
(Scheme 2).18 When the reaction temperature was lowered to
50 °C, only 28% of the benzene insertion product was generated
with the remaining EDA untouched. The [Au(bathophenanthro-
line)](OTf) complex exhibits similar activity, as shown in
Scheme 2. Recently, Nolan and Pe´rez reported a NHC-gold
complex that can mediate the carbene insertion.12 They detected
the carbene insertion into the aromatic C-H bond, instead of
the aromatic ring. In our case we did not detect any C-H
insertion product; the result may suggest that gold can mediate
a All reactions were carried out overnight at 50 °C. b Isolated yield. c Only
the trans isomer was detected. d Exo product. e NMR yield.
intramolecular example, also worked for this reaction. The
aziridination of simple aliphatic olefins could not be mediated
by our gold catalyst, except for activated ones and intramolecular
reactions (Table 2, entries 11-13).
In entry 9 of Table 2, we did observe total conversion of the
cis-olefin substrate to the trans aziridine product, which may
suggest a stepwise mechanism instead of a concerted one. Che
and co-workers used isolated Ru-nitrene complexes to react
with cis-olefins.2l In their experiments mixed products were
detected and several different mechanisms were proposed to
explain the result. Interestingly, some metal-porphyrin com-
plexes also gave trans-aziridine product from cis-olefins in
aziridination reactions,2j and in most cases, the stereoselectivity
largely depended on reaction temperatures (for example -20
vs 0 °C).2 In our case, we cannot completely rule out the
possibility that gold(I) acts as a Lewis acid; however, as shown
(17) Main peaks at 598.7 and 598.2 indicate a 2+ charge, which sug-
gests a dimmer structure. Elemental analysis for [AutBu3tpy]2(OTf)2,
[C28H35AuF3N3O3S]2 found (%): C 45.19, H 4.66, N 5.42. Calcd: C 44.98,
H 4.72, N 5.62.
(18) Please see spectra in the Supporting Information.
5878 J. Org. Chem., Vol. 71, No. 16, 2006

Nitrene Transfer Reactions Catalyzed by Gold Complexes
SCHEME 1. Difference between Gold- and Silver-Mediated Nitrene Transfer Reactions
a All are isolated yields. bThe silver-catalyzed reactions were also repeated under different conditions; no improvement was found with the change of
temperature, solvent, and reaction time.
reactions mediated by gold with pyridine-based ligands are rare.
We hope to further explore pyridine-based ligands and study
the scope, mechanism, and applications of the chemistry
described here. This discovery provides opportunities for more
extensive exploration of gold-mediated redox transformations.
Potential intermediates may be stabilized and characterized in
the future, which would significantly enhance our understanding
of the gold oxidation chemistry and the metal-nitrene interac-
tion in general.
Experimental Section
General Procedure To Prepare Gold Complexes. In general,
gold(I) complexes were made according to a previously reported
procedure,19 except that 0.95 equiv of silver was used to eliminate
potential silver contamination. The yields were around 50% to 60%
based on gold. In a typical process, a portion of AuSMe2Cl (100
mg, 0.337 mmol) was dissolved in 30 mL of CH2Cl2 at room
temperature. Then 77 mg of AgOTf (0.320 mmol, 0.95 equiv) was
added and the mixture was stirred vigorously in the dark for 2 h.
The precipitate was filtered off, 136 mg of tBu3tpy (0.337 mmol,
1 equiv) was added, and the solution was stirred in the dark for an
additional 2 h. The reaction solution was filtered again to remove
any potential residual precipitate and evaporated to dryness. Then
the residue was dissolved in CH2Cl2 and hexanes were added to
precipitate the gold complex, which was dried under vacuum. This
process has been repeated numerous times in different scales, and
the resulting products showed the same catalytic reactivity.
General Procedure for the Olefin Aziridination Reaction. The
gold(I) catalyst (3 mol %) was dissolved in 4 mL of CH3CN
with104 mg of styrene (1 mmol), 100 mg of NsNH2 (0.5 mmol),
193 mg of PhI(OAc)2 (0.6 mmol), and 500 mg of 4 Å molecular
sieves under nitrogen. The reaction was then stirred at 50 °C
overnight. The reaction mixture was filtered through a short Celite
pad, washed with CH2Cl2, and concentrated and the residue was
applied to column chromatography to afford the purified product.
Proton and Carbon NMR Data for 4-[1-(4-Nitrobenzene-
sulfonyl)aziridin-2-yl]benzonitrile. IR (% T): 3105 (w), 3056 (w),
2916 (w), 2231 (m), 1652 (w), 1533 (s), 1349 (s), 1265 (s), 1167
FIGURE 1. ESI-MS spectrum for [Au(tBu3tPy)]2(OTf)2
SCHEME 2. Gold-Mediated Carbene Insertion to Benzene
b
a NMR conversions. Reaction at 50 °C with 3% catalyst.
carbene insertion via different reaction pathways depending on
the ligand systems employed.
1
(s), 1092 (m), 737 (s). H NMR (δ, ppm): 8.41-8.39 (dd, 2H, J
) 6.9,1.9 Hz), 8.20-8.18 (dd, 2H, J ) 6.9, 1.9 Hz), 7.63-7.61
(dd, 2H, J ) 6.7, 1.7 Hz), 7.37-7.35 (dd, 2H, J ) 6.7, 1.7 Hz),
3.96-3.93 (dd, 1H, J ) 7.2, 4.4 Hz), 3,15-3.13 (d, 1H, J ) 7.2
Hz), 2.47-2.46 (d, 1H, J ) 4.4 Hz). 13C NMR (δ, ppm): 150.8,
Conclusions
In summary, we have described the first example of the gold-
catalyzed nitrene transfer reaction. Styrenes bearing various
functional groups could react with different sulfonamides
directly to give good yields of the aziridine products. Organic
(19) Uson, R.; Laguna, A.; Navarro, A.; Parish, R. V.; Moore, L. S.
Inorg. Chim. Acta 1986, 112, 205.
J. Org. Chem, Vol. 71, No. 16, 2006 5879

Li et al.
143.4, 139.6, 132.6, 129.3, 127.2, 124.5, 118.2, 112.6, 40.5, 37.1.
37.1. HRMS: MW calcd 329.3305, MW found (M + 1) 330.3430.
(RI1179) from Research Corporation and an Alfred P. Sloan
Research Fellowship.
Supporting Information Available: The characterization of all
products. This material is available free of charge via the Internet
at http://pubs.acs.org.
Acknowledgment. This research is supported by the Uni-
versity of Chicago. Acknowledgment is made to the donors of
the Petroleum Research Fund, administered by the American
Chemical Society, for support of this research (PRF 38848-
G3). C.H. is also a recipient of a Research Innovation Award
JO060016T
5880 J. Org. Chem., Vol. 71, No. 16, 2006