Silver-catalyzed intermolecular amination of C-H groups

Article abstract of DOI:10.1002/anie.200700760

(Chemical Equation Presented) Teamwork: The dinuclear silver complex [Ag₂(OTf)₂(bp)₂] (OTf = trifluoromethane- sulfonate, bp = bathophenanthroline) catalyzes the intermolecular amination of saturated C-H bonds by using Phl=NNs (Ns = p-nitrosulfonyl) as the nitrene precursor (see scheme). Preliminary investigations suggest that the dinuclear structural motif is crucial to catalytic performance.

Full text of DOI:10.1002/anie.200700760

Communications
DOI: 10.1002/anie.200700760
Transition-Metal Catalysis
À
Silver-Catalyzed Intermolecular Amination of C H Groups**
Zigang Li, D. A. Capretto, R. Rahaman, and Chuan He*
Despite advances made in the past, the direct transformation
(4)]. Furthermore, 2 is easily prepared from cost-efficient and
of saturated C H bonds into different functional groups
commercially available chemicals.[10a]
À
remains a challenging task.[1] Nitrogen-containing functional
groups are prevalent in various natural products and phar-
maceutical reagents. As a result, newmethods to introduce
We then tested the more challenging intermolecular C H
À
amination reaction. As shown in Table 1, silver complex 2 was
À
able to catalyze intermolecular C H aminations using the
À
À
C N functionality by direct C H activation are both impor-
tant and highly desirable. In recent years, iminoiodanes
=
(PhI NSO2R) have been broadly utilized as nitrene precur-
sors to access nitrogen-containing molecules. However,
success has been mostly restricted to aziridination of olefins,
[2]
À
sulfoxide imination, and intramolecular C H amidation.
À
C H amination chemistry has long been an active
research area. In the past two decades, various catalysts
utilizing iron, manganese, rhodium, ruthenium, and copper
have been developed to mediate nitrene insertion reac-
tions.[3–7] Advances have also been made in the amination of
unactivated hydrocarbons, including a copper trispyrazolyl-
borate system[7b] as well as rhodium-based systems.[5] We have
reported a disilver(I) compound [Ag2(tBu3tpy)2(NO3)](NO3)
(1; tBu3tpy = 4,4’,4’’-tri-tert-butylterpyridine) that efficiently
catalyzes olefin aziridination[8] and intramolecular C H
À
amidation reactions.[9] However, the intramolecular C H
À
amidation reactions with 1 must be carried out at 828C. Also,
À
1 fails to induce intermolecular C H amination even at high
temperatures. These shortcomings prompted us to seek ways
to improve the efficiency of the silver-based system.
We hypothesized that with a less electron-donating ligand
system, we might enhance the electrophilicity of the putative
À
silver intermediate responsible for oxidation of C H groups.
We tested a range of different synthetic and commercially
available bidentate ligands. To our delight, a complex of
silver(I) trifluoromethanesulfonate (AgOTf) with 4,7-
diphenyl-1,10-phenanthroline (bathophenanthroline, bp)
À
worked as an excellent catalyst for intramolecular C H
amidations. This newcatalyst 2 efficiently converts a secon-
À
=
dary C H bond (3a) into the cyclized product at 508C in
oxidant PhI NNs (Ns = p-nitrosulfonyl) as the nitrene
dichloromethane with PhI(OAc)2 as the oxidant [Eq. (1)].
The 75% yield of isolated product for this reaction is
significantly higher than that obtained with catalyst 1 at
828C (58%).[9] Other substrates could be readily oxidized to
give the expected products in good yields, as well [Eqs. (2)–
source; the resulting NsNH group is easily deprotected to
afford amines. Good yields were achieved with exclusive
regioselectivity for benzylic C H bonds (entries 1–5, Table 1).
While exploring related reactions, Pelletier and Powell
reported a copper phenanthroline system that is capable of
À
[7c]
À
catalyzing amination of benzylic C H bonds. Previously,
PØrez and co-workers reported intermolecular aminations of
À
inert C H bonds catalyzed by a copper-based system;
[*] Z. Li, D. A. Capretto, R. Rahaman, Prof. C. He
Department ofChemistry, University ofChicago
5735 South Ellis Avenue Chicago, IL 60637 (USA)
Fax: (+1)773-702-0805
however, a special electron-deficient ligand had to be
employed in that case.[7a] Our catalyst seems to be more
reactive than the copper phenanthroline system in its ability
À
to functionalize inert C H bonds of alkanes. We observed
amination of cyclohexane and cyclopentane, albeit with lower
yields than for benzylic C H groups (entries 6–9, Table 1).
With mesitylene as the hydrocarbon substrate, primary
E-mail: chuanhe@uchicago.edu
[**] This research was supported by the University ofChicago and a
Research Fellowship from the Alfred P. Sloan Foundation (C.H.)
[10]
À
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
À
benzylic C H activation could also be observed (entry 10,
5184
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Angew. Chem. Int. Ed. 2007, 46, 5184 –5186

Angewandte
Chemie
À
Table 1: Intermolecular C H amination catalyzed by 2.
stacked over each other (Figure 1). The stacking pattern of
this complex is similar to that of dipalladium(II) phenanthro-
line complexes.[12] Ag2 is coordinated in a distorted square-
Entry
1
Substrate
Product
Yield[a]
70[b]
2
3
68[b]
57[b]
4
5
65[b]
Figure 1. Crystal structure ofcomplex 2. Thermal ellipsoids are set at
50% probability. H atoms are not shown. Selected bond lengths [Š]
and angles [8]: Ag1-Ag2 3.386(1), N1-Ag1 2.248(3), N2-Ag1 2.319(3),
O1-Ag1 2.194(2), N3-Ag2 2.318(3), N4-Ag2 2.352(3), O2-Ag2 2.349(2),
O3-Ag2 2.547(3); N1-Ag1-N2 72.78(10), N1-Ag1-O1 152.24(10), N2-
Ag1-O1 131.93(10), O2-Ag2-N3 164.41(9), O3-Ag2-N4 154.41(9), N3-
Ag2-N4 70.95(11), O2-Ag2-O3 81.79(9).
71[b,c]
6
7
40[d]
35[d]
planar geometry by two nitrogen atoms from bp and two
OTfÀ ions. Ag1 is bound in a distorted three-coordinate
geometry with a terminal water ligand, which is within
hydrogen-bonding distance (O1···O4 and O1···O5 ca. 2.8 Š)
of the OTfÀ ions bound to Ag2. The silver–silver separation of
3.386 Š observed in the structure indicates a weak interac-
tion.[11]
8
9
33[d]
39[b,e]
10
25
A common feature shared by precatalysts 1 and 2 is a
disilver core in their crystal structures. It is hard to believe
that this similarity is just a coincidence. Many other silver
[a] Yield ofisolated product. [b] Carried out on a 0.25-mmol scale with
=
5:1 hydrocarbon/PhI NNs in 4 mL CH2Cl2. [c] The product ratio was
determined by 1H NMR spectroscopy. [d] Carried out on 0.5-mmol scale
À
=
compounds have been tested and failed to mediate C H
with 10:1 hydrocarbon/PhI NNs in 4 mL CH2Cl2. [e] Other two isomers
1
were also detected by H NMR spectroscopy as minor products. Major
product/minor products=88:12.
amination. To probe whether the dinuclear feature is
important for the catalytic oxidation activity, we investigated
the reactivity of various substituted phenanthroline deriva-
tives. Studies of substitution effects on palladium phenanthro-
line complexes have shown that 2,9-substituted phenanthro-
lines prevent formation of dinuclear palladium species in the
crystalline form, while dinuclear structures are favored with
complexes of palladium with unsubstituted phenanthro-
line.[12,13] Using the intermolecular amination reaction
shown in Scheme 1, we probed the activity of silver complexes
with different phenanthroline derivatives. None of the com-
Table 1). Although modest yields were obtained for inert
C H substrates with the current catalytic system, this discov-
À
À
ery of intermolecular C H amination is the first for silver-
based catalysis and opens opportunities for further develop-
ments.
=
Instead of PhI NNs, we also attempted to use NsNH2 as
the nitrene source in combination with commercially avail-
able oxidants. The use of PhI(OAc)2 gave the expected
amination product with a lower yield [ca. 30%, Eq. (5)] than
with PhI = NNs, while persulfate and peroxides showed no
reactivity.
plexes with derivatives substituted in the 2- and 9-positions
catalyze the C H amination reaction. The silver(I) com-
[10]
À
plex with unsubstituted phenanthroline showed some con-
version, but its lowsolubility in organic solvents limited its
activity and thus its utility. We then tested a phenanthroline
ligand with monosubstitution in the 2-position. Again, no
activity was observed when its silver complex was employed
in the test reaction. Substitution in the 2- or 9-position (or
both) of phenanthroline prevents formation of dinuclear
metal complexes.[12,13] These results, together with our pre-
Why is 2 an active catalyst? The crystal structure of this
compound shows a disilver(I) core with two bp ligands
Angew. Chem. Int. Ed. 2007, 46, 5184 –5186
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Scheme 1. The effect of ligand substitution on silver amination chemistry. All
=
reactions were carried out with 0.25 mmol PhI NNs and five equivalents hydro-
carbon; yields ofisolated product are given.
À
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catalyst 1, strongly indicate the requirement of a dinuclear
core for catalyzing nitrene transfer oxidation chemistry in
silver complexes. Considering the high oxidation potentials of
silver ions, a two-electron oxidation of the disilver(I) catalyst
to form a disilver(II) nitrene intermediate with the nitrene
species bridging two silver ions is an attractive hypothesis.
Moreover, with two silver(I) ions involved in catalysis,
potential radical pathways with mononuclear silver com-
pounds may be avoided.
We report herein a silver-catalyzed intermolecular ami-
À
À
nation of saturated C H bonds to install C N groups. This
À
catalytic system is not only efficient at activating benzylic C
H bonds but also capable of functionalizing inert cyclo-
alkanes.[14] Structural characterization of the catalyst revealed
a disilver core, a feature resembling the only other known
silver-based nitrene transfer catalyst 1, discovered by us
previously. Based on this common feature and on studies of
substitution effects of phenanthroline ligands, we believe a
dinuclear core is key to silver-based nitrene transfer. Future
mechanistic studies to understand the system, together with
catalyst design for achieving better reactivity, will focus on
exploiting this unique property. These investigations are
currently underway.
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Received: February 19, 2007
Published online: May 30, 2007
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[14] For preliminary results with aliphatic linear hydrocarbons,
please see the Supporting Information.
À
Keywords: amination · C H activation · homogeneous catalysis ·
nitrene transfer · silver
.
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