A silver-catalyzed intramolecular amidation of saturated C-H bonds

Article abstract of DOI:10.1002/anie.200454243

Opportunities with silver: A dinuclear silver(I) compound was found to efficiently catalyze the intramolecular amidation of saturated C-H bonds of carbamates and sulfamates (see scheme). This highly regioselective, stereospecific reaction offers a practical method for the construction of cyclic nitrogen-containing organic molecules.

Full text of DOI:10.1002/anie.200454243

Communications
À
C H Activation
complex gave clean aziridine products in the silver-mediated
reactions tested with a variety of different ligands. This led us
to propose that the disilver structure may be important for the
unique activity of this complex, and that the same compound
may catalyze other reactions, such as the activation of
A Silver-Catalyzed Intramolecular Amidation of
Saturated C H Bonds**
À
À
saturated C H bonds. After several experiments we found
that 1 generated in situ catalyzed the efficient intramolecular
Yong Cui and Chuan He*
À
amidation of saturated C H bonds, as shown in Scheme 1b.
À
Oxidative insertion reactions into saturated C H bonds to
form amines and amine derivatives under transition-metal
catalysis are attractive synthetic methodologies.[1–14] Besides
providing facile access to various organic structures, these
reactions also offer opportunities to study the fundamental
With the indanol-derived carbamate 2 as the substrate,
AgNO3 (4 mol%) with 4,4’,4’’-tri-tert-butyl-2,2’:6’,2’’terpyri-
dine (tBu3tpy, 4 mol%) as the catalyst, and PhI(OAc)2
(2.0 equiv) as the oxidant, the cyclized product indanooxazo-
lidin-2-one (3) was isolated from the reaction in acetonitrile in
83% yield (Table 1 and entry 2 of Table 2). A color change
À
mechanism of C H activation. It has been demonstrated that
À
Rh- and Ru-based compounds mediate C H amidation
reactions particularly well.[2,4,14–19] A dirhodium(ii) tetracar-
Table 1: Conditions for the oxidative cyclization of a carbamate.[a]
boxylate system was shown to catalyze the intramolecular
À
amidation of C H bonds of carbamates and sulfamates with
high regioselectivity and good to excellent diastereoselectiv-
ity.[14–16] It is thought that a metal-bound nitrene species might
À
be formed which then inserts oxidatively into saturated C H
bonds in these reactions. Despite extensive studies of metal-
Ligand
Yield [%]
[b]
pyridine
4-tert-butylpyridine
bipyridine
4,4’-di-tert-butylbipyridine
2,2’:6’,2’’-terpyridine (tpy)
4,4’,4’’-tBu3tpy
4-tert-butylbis(oxazoline)
4-phenylbis(oxazoline)pyridine
–
–
À
catalyzed amidations of C H bonds, efficient and practical
examples of such processes are limited. We report herein an
efficient amidation reaction of saturated C H bonds cata-
[b]
22
35
41
89
À
lyzed by silver(i).
Silver complexes exhibit interesting activity, and silver-
catalyzed organic transformations have attracted attention
recently.[20–23] In particular, several nonradical oxidative
group-transfer reactions catalyzed by silver were discov-
ered.[24–27] We reported a disilver(i) compound (1, Scheme 1a)
that efficiently catalyzes the aziridination of olefins.[26]
Surprisingly, subsequent studies showed that only this disilver
[b]
–
–
[b]
[a] All reactions were conducted at 828C in CH3CN with 2.0 equivalents
ofPhI(OAc) 2. The reported yields were determined by NMR spectroscopy
by using mesitylene as an internal standard. [b] Product 3 was not
observed.
from pale yellow to deep brown was observed during
the course of the reaction, which may suggest the
involvement of high-valent silver species.
Although different types of ligands were tested
(Table 1), a clean reaction was observed and the
product was obtained in high yield only when tBu3tpy
was used, as had also been observed for the olefin
aziridination reaction.[26] Combinations of other
ligands with silver(i) salts were either completely
inactive or led to a low yield of the product. Sulfamate
substrates were also tested, and very similar results
were obtained. Although both silver and copper can
catalyze olefin aziridination in the presence of tBu3tpy,[26]
copper, unlike silver, is not able to catalyze insertion into
Scheme 1. a) A unique disilver(i) catalyst [Ag2(tBu3tpy)2(NO3)](NO3) (1). b) Oxidative
cyclization ofcarbamates [Eq. (1)] and sulaf mates [Eq. (2)].
[*] Dr. Y. Cui, Prof. C. He
À
C H bonds. It was reported that copper ions can mediate the
Department ofChemistry
University ofChicago
5735 S. Ellis Avenue, Chicago, IL 60637 (USA)
Fax: (+1)773-702-0805
E-mail: chuanhe@uchicago.edu
À
amidation of saturated C H bonds when an electron-defi-
cient ligand is used.[6] In this case a reactive electrophilic
copper nitrene species may form. In the system described
herein, it appears that the silver nitrene species are more
oxidative than analogous copper species. The high oxidation
potential often observed for silver complexes supports this
notion.[28]
A range of carbamates and sulfamates were tested, as
shown in Table 2. Five-membered-ring and six-membered-
ring insertion products were generated preferentially from
[**] This research was supported by the University ofChicago. We
acknowledge the donors ofthe American Chemical Society
Petroleum Research Fund for support of this research (PRF 38848-
G3). C.H. is also a recipient ofa Research Innovation Award
(RI1179) from Research Corporation.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200454243
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Angewandte
Chemie
Table 2: Disilver(i)-catalyzed oxidative cyclization.[a]
entries 5, 6, 10, and 12). However, excess amounts of the
monodentate ligand 4-tert-butylpyridine (10 equiv with
respect to the silver catalyst) had a deleterious effect on the
reaction. The exact role played by 4-tert-butylpyridine is not
clear. Perhaps the binding of a monodentate ligand to one
terminal side of disilver influences the activity on the other
end. We also employed several amides as potential substrates.
However, no product was observed under the various reaction
conditions tested so far. Efforts to use catalyst 1 or other silver
compounds to mediate insertion reactions of amides and
other substrates are in progress.
Entry
Substrate
Product
Yield [%]
81
1
2
3
83[b]
85[b]
A substrate derived from (S)-2-methyl-1-butanol was
employed to probe the mechanism of this reaction and
determine its stereoselectivity (Table 2, entry 13).[14] A mix-
ture of enantiomers would be expected as the product if a
discrete radical species is the reactive intermediate. Retention
of configuration should occur if a nitrene intermediate is
4
5
85
76 (68)[c]
À
produced and inserts directly into the C H bond. The product
of this reaction has the same optical rotation as an enantio-
merically pure sample of the oxazolidinone[14] shown in
entry 13 of Table 2.[29] The product obtained from the
corresponding racemic starting material in a control experi-
ment showed no optical rotation (Table 2, entry 5). This result
indicates that the silver-catalyzed reaction is stereospecific. It
also provides support for the involvement of a nitrene-type
oxidant in the reaction, although the involvement of a short-
lived radical as the reactive intermediate can not be excluded
completely.
6
7
74 (70)[c]
58
8
87
The disilver(i) catalyst 1 was probed by electrospray mass
spectrometry (MS(ESI)) in acetonitrile. The molecular peak
for the disilver(i) cation core [Ag2(tBu3tpy)2NO3]+ is clearly
visible in the spectrum (Figure 1). Mononuclear silver(i)
9
78
10
65 (57)[b,c]
11
12
90
53[c,d]
13
73
[a] All reactions were conducted at 828C in CH3CN in the presence of
AgNO3, tBu3tpy (4 mol%), and PhI(OAc)2 (1.4 equiv for sulfamates and
2.0 equiv for carbamates). The yield of the isolated product is reported.
[b] The cis product was obtained. [c] The reaction was carried out with 4-
tert-butylpyridine (2 mol%; the yield determined by NMR spectroscopy
for the reaction in the absence of 4-tert-butylpyridine is reported in
parenthesis). [d] syn/anti=2.2:1.0.
Figure 1. The MS(ESI) spectrum ofcomplex 1 in CH3CN. MS(ESI+):
calcd for [Ag2(tBu3tpy)2NO3]+: 1080, [Ag(tBu3tpy)2]+: 911,
[Ag(tBu3tpy)]+: 508.
species were also observed. These species perhaps result
from the decomposition of 1 under the conditions of mass
spectrometry; our previous NMR spectroscopic studies
suggest that 1 remains a dimer in acetonitrile.[26] The peak
for the disilver cation core remained in the spectrum after
carbamates and sulfamates, respectively (in particular, see
entries 7 and 12 of Table 2). Good to excellent yields
comparable to or better than those found with other catalytic
systems[14–16] were observed with our silver catalyst. The
addition of a stoichiometric amount of a base was not
necessary. The addition of 2 mol% of 4-tert-butylpyridine led
to better yields and cleaner products in several cases (Table 2,
=
compound 1 was treated with PhI NTs (Ts = p-toluenesul-
fonyl) for 2 h at room temperature. A color change from pale
yellow to brown occurred during the course of the reaction. A
weak peak corresponding to a molecular weight of 1189 was
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ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
[18] J. L. Liang, J. S. Huang, X. Q. Yu, N. Y. Zhu, C. M. Che, Chem.
Eur. J. 2002, 8, 1563.
[19] J. L. Liang, S. X. Yuan, J. S. Huang, W. Y. Yu, C. M. Che, Angew.
Chem. 2002, 114, 3615; Angew. Chem. Int. Ed. 2002, 41, 3465.
[20] J. Cirakovic, T. G. Driver, K. A. Woerpel, J. Am. Chem. Soc.
2002, 124, 9370.
also observed, and assigned as the molecular peak of the
=
disilver complex associated with the nitrenoid moiety NTs.
(The molecular weight of Ag2(tBu3tpy)2 with the nitrenoid
=
=
NTs is 1188.) When PhI NSO2Ph was used, this peak shifted
to 1175 as expected, which corresponds to the removal of a
methyl group.[30] There is no direct evidence to indicate that
this species is a silver–nitrene intermediate. However, the
results seem to suggest that the disilver structure can remain
intact when in contact with the nitrenoid group. Evidently
further mechanistic studies are required to fully elucidate the
mechanism. We did not observe similar species upon the
treatment of complex 1 with PhI(OAc)2 and carbamates. No
color change occurred at room temperature after complex 1
was mixed with PhI(OAc)2 and a carbamate. In this case, the
reaction intermediates may not be stable to analysis by mass
spectrometry.
[21] N. S. Josephsohn, M. L. Snapper, A. H. Hoveyda, J. Am. Chem.
Soc. 2003, 125, 4018.
[22] N. Momiyama, H. Yamamoto, J. Am. Chem. Soc. 2003, 125, 6038.
[23] C. M. Wei, Z. G. Li, C. J. Li, Org. Lett. 2003, 5, 4473.
[24] K. Burgess, H. J. Lim, A. M. Porte, G. A. Sulikowski, Angew.
Chem. 1996, 108, 192; Angew. Chem. Int. Ed. Engl. 1996, 35, 220.
[25] H. V. R. Dias, R. G. Browning, S. A. Polach, H. V. K. Diyaba-
lanage, C. J. Lovely, J. Am. Chem. Soc. 2003, 125, 9270.
[26] Y. Cui, C. He, J. Am. Chem. Soc. 2003, 125, 16202. A copper-
catalyzed olefin aziridination is described in the Supporting
Information.
[27] H. V. R. Dias, R. G. Browning, S. A. Richey, C. J. Lovely,
Organometallics 2004, 23, 1200.
In summary, we have reported an efficient amidation
[28] H. N. Po, Coord. Chem. Rev. 1976, 20, 171.
À
reaction of saturated C H bonds catalyzed by a unique
[29] The value of the optical rotation of the product ([a]2D9 = + 5.2; c =
0.5, CHCl3) compares well with that of a reference sample
([a]2D9 = + 5.0; c = 0.5, CHCl3).
disilver(i) complex. The results suggest the involvement of a
silver–nitrene intermediate. The reaction is stereospecific and
practical for constructing amine-containing organic mole-
cules. Both the ligand tBu3tpy and the silver salt are
commercially available. With their inherent high oxidation
potential, we believe silver ions are good oxidation catalysts
that could exhibit other unique activities.
[30] The mass spectra are shown in the Supporting Information.
Received: March 11, 2004
Revised: June 7, 2004 [Z54243]
À
Keywords: amidation · C H activation · homogeneous
catalysis · silver · synthetic methods
.
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4212
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www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 4210 –4212