Copper-catalyzed intermolecular aminoazidation of alkenes

Article abstract of DOI:10.1021/ol500513b

Copper-catalyzed intermolecular aminoazidation of alkenes is described. This novel methodology provides an efficient approach to vicinal amino azides which can easily be transformed into other valuable amine derivatives. The commercially available N-fluorobenzenesulfonimide (NFSI) is used as a nitrogen-radical precursor and TMSN₃ as the N₃ source. Yields are moderate to excellent, and for internal alkenes aminoazidation occurs with excellent diastereoselectivity.

Full text of DOI:10.1021/ol500513b

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Intermolecular Aminoazidation of Alkenes
Bo Zhang and Armido Studer*
Institute of Organic Chemistry, Westfalische Wilhems-University, Corrensstraße 40, 48149 Munster, Germany
̈
̈
S
* Supporting Information
ABSTRACT: Copper-catalyzed intermolecular aminoazida-
tion of alkenes is described. This novel methodology provides
an efficient approach to vicinal amino azides which can easily
be transformed into other valuable amine derivatives. The
commercially available N-fluorobenzenesulfonimide (NFSI) is
used as a nitrogen-radical precursor and TMSN3 as the N3 source. Yields are moderate to excellent, and for internal alkenes
aminoazidation occurs with excellent diastereoselectivity.
Scheme 1. Copper-Catalyzed Intermolecular
Aminoazidation of Alkenes
rganic azides are highly important and valuable
Ocompounds which have gained great attention not only
because of their widespread application as versatile inter-
mediates and building blocks in organic synthesis but also
because of their remarkable biological activity.1 Moreover,
azides have been intensively used as reactive functionalities in
materials science,2 supramolecular chemistry,3 medicinal
chemistry4 and biotechnology.5 Along these lines, the Cu-
catalyzed [3 + 2] cycloaddition of an azide with an alkyne as the
bona fide click reaction has recently emerged as a powerful tool
in drug discovery.6 In light of the importance of this compound
class, there is continuing interest in the development of novel
synthetic methods for C−N3 bond formation.
As part of our ongoing interest in intermolecular vicinal
radical difunctionalization of alkenes, we recently reported
radical azidooxygenation,7a oxyarylation7b trifluoromethylami-
noxylation,7c and hydroxyarylation.7d Encouraged by these
results, we became interested in the development of other
radical alkene difunctionalization processes. Considering the
importance of the azide and the amino group, we planned to
introduce these two functionalities sequentially in a cascade
process allowing the straightforward preparation of vicinal
amino azides. To the best of our knowledge, intermolecular
vicinal aminoazidation has not been reported to date.7e
We envisioned to use a radical approach where a nitrogen-
centered radical adds to an alkene providing a C-radical which
then undergoes azidation.8 First results on a copper-catalyzed
intermolecular aminoazidation of alkenes using commercially
available N-fluorobenzenesulfonimide (NFSI) as nitrogen-
radical precursor and TMSN3 as N3 source leading to vicinal
amino azides are disclosed herein (Scheme 1).
Recently, much attention has been paid to transition-metal-
catalyzed difunctionalization of alkenes.9 Such reactions allow
direct vicinal installation of two functional groups across a C−C
double bond. Among these reactions, difunctionalization of
alkenes, comprising a C−N bond formation, is particularly
interesting allowing the synthesis of important nitrogen-
containing compounds. Examples are diamination,10 amino-
oxygenation,11 aminohalogenation,12 and carboamination.13
The intramolecular aminative difunctionalization of alkenes
has been intensively investigated. In our eyes, intermolecular
variants are more attractive but certainly far more challenging.
Therefore, development of new methods for intermolecular
aminative difunctionalization of alkenes is important.
We decided to use NFSI as the N-radical precursor because it
has been already demonstrated that the bisphenylsulfonyla-
midyl radical can be generated by reaction of NFSI with Cu(I)
salts.13a,b TMSN3 was chosen as the commercially available N3
source. Aminoazidation of styrene 1a in the presence of CuCl
as catalyst in DCE at 70 °C under argon atmosphere was
investigated first. Unfortunately, only traces of the targeted
product 2a were identified (Table 1, entry 1). Studies were
continued by screening various additives L1−L5. Pleasingly, the
yield was significantly improved (57−78%), and 1,10-
phenanthroline L1 turned out to be well-suited, affording 2a
in 78% yield (Table 1, entries 2−6). Solvent effects were
investigated, and we found that DCE is the best solvent for this
transformation. Other solvents, such as CH3CN, dioxane, and
DMF provided worse results (Table 1, entries 7−9). Next, a
series of Cu(I) salts were tested. CuBr, CuI, CuOAc,
[(MeCN)4Cu]PF6, and CuTc provided 2a in 55−74% yield
showing that CuCl performs slightly better (Table 1, entries
10−14). As compared to TMSN3, alternative N3 reagents, such
as NaN3, TsN3, and DPPA afforded lower yields of 2a (Table 1,
entries 15−17). The reaction did not work at room
Received: February 18, 2014
Published: March 3, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/ol500513b | Org. Lett. 2014, 16, 1790−1793

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Letter
a
Table 1. Reaction Optimization Using Styrene as Substrate
Scheme 2. Copper-Catalyzed Intermolecular
a,b
Aminoazidation of Alkenes: Scope
b
entry
catalyst
ligand N3 source
solvent
yield (%)
1
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl
CuBr
CuI
none
L1
L2
L3
L4
L5
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
L1
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
TMSN3
NaN3
DCE
DCE
DCE
DCE
DCE
DCE
CH3CN
dioxane
DMF
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
trace
78
75
58
76
57
56
52
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
74
70
55
68
74
trace
trace
20
0
CuOAc
[(MeCN)4Cu]PF6
CuTc
CuCl
CuCl
TsN3
CuCl
DPPA
c
18
CuCl
TMSN3
TMSN3
TMSN3
TMSN3
d
19
CuCl
78
0
e
20
CuCl
21
none
0
a
Reaction conditions: 1a (0.25 mmol), catalyst (10 mol %), ligand (10
mol %), N3 source (0.375 mmol), and NFSI (0.375 mmol) in solvent
b
c
(1.0 mL) at 70 °C under argon for 1 h. Isolated yields. The reaction
d
was conducted at rt. Using 5 mol % of CuCl and 5 mol % of L1.
Using 2 mol % of CuCl and 2 mol % of L1. DPPA =
e
diphenylphosphoryl azide.
a
Reaction conditions: 1b−u (0.25 mmol), CuCl (5 mol %), L1 (5 mol
temperature (Table 1, entry 18). Catalyst and ligand loading
could be lowered to 5 mol % without affecting the yield (Table
1, entry 19). However, the reaction did not work by using 2
mol % of catalyst and ligand (Table 1, entry 20). In addition,
the reaction did not proceed in the absence of copper catalyst
(Table 1, entry 21). These results imply that both the copper
catalyst and ligand are essential for alkene aminoazidation
(Table 1, entries 1 and 21).
With optimized reaction conditions in hand, we next
explored the scope of the aminoazidation reaction (Scheme
2). Styrene derivatives bearing electron-donating substituents at
the p-position of the arene ring provided the vicinal amino
azides 2b (4-Me), 2e (4-t-Bu), and 2f (4-MeO) in moderate to
excellent yields (47−75%). Ortho- and meta-substituted
congeners of 1b gave similar results (2c,d). Styrene derivatives
carrying halogen substituents such as F, Cl, and Br were
successfully converted to the corresponding protected amino
azides 2g−l in good to excellent yields (60−75%). Substrates
bearing other electron-withdrawing substituents, such as CF3,
OAc, and CN underwent vicinal difunctionalization smoothly
to provide the corresponding products in moderate to good
yields (2m: 66%, 2n: 60%, 2q: 74%, 2r: 55%). p-Vinylbiphenyl
afforded 65% of 2o, and a slightly lower yield was obtained with
2-vinylnaphthalene (2p: 45%). In addition, we could show that
%), TMSN3 (0.375 mmol), and NFSI (0.375 mmol) in DCE (1.0 mL)
at 70 °C under argon for 1 h. Isolated yields.
b
formation of quaternary C centers is possible using this novel
method (see 2s: 78%).
To address the diastereoselectivity, we investigated the
aminoazidation of internal alkenes (Scheme 2). The selectivity
1
was readily determined by H NMR analysis of the crude
product. β-Methylstyrene reacted under optimized conditions
with complete regioselectivity to 2t, which was isolated in 70%
yield. Pleasingly, 2t was formed with excellent diastereoselec-
tivity (19:1). For assignment of the relative configuration we
transformed 2t to the known diamine 5 (see below). The
selectivty can be explained by considering the allylic A[1,3]
strain model.14 Dihydronaphthalene reacted with excellent
trans-selectivity (>98:2) and complete regioselectivity to
provide the corresponding products 2u in moderate yield.
Unfortunately, aliphatic alkenes such as 1-heptene did not react
under the optimized conditions.
We further applied the novel protocol to an estrone
derivative 1v, which was smoothly converted to corresponding
product 2v in 57% yield with 1:1 diastereoselectivity (Scheme
3).
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dx.doi.org/10.1021/ol500513b | Org. Lett. 2014, 16, 1790−1793

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Letter
Scheme 3. Aminoazidation of an Estrone Derivative
Scheme 5. Proposed Reaction Mechanism
To illustrate the synthetic utility of the aminoazidation
products, we next investigated the follow-up chemistry
(Scheme 4). Click reaction of 2a with phenylacetylene using
Scheme 4. Transformations of 2a and 2t
amine derivatives. Further studies on the synthetic applications
and the mechanism are ongoing in our laboratory.
CuSO4 as a catalyst provided triazole 3 in 65% yield. Azide
reduction was easily achieved by treating 2a with CuSO4/
NaBH4 in MeOH at 0 °C to afford the monoprotected 1,2-
diamine 4 in good yield (85%). It is important to note that both
sulfonyl protecting groups in the bissulfonamides can be readily
removed under acidic conditions.10a,15 Amino azide 2t was
converted to 1,2-diamine 5 by azide reduction and subsequent
desulfonylation (56% overall yield). The trans-configuration
was assigned by comparing NMR data of 5 with known
literature values.16 In analogy, 2u was transformed to the
corresponing trans-1,2-diamine (38%, not shown). 1,2-
Diamines are important functional groups present in various
natural products, in biologically active compounds, and as
ligands in metal catalysts.10,17 Hence, our novel protocol
provides an alternative and complementary approach to 1,2-
diamines.
ASSOCIATED CONTENT
* Supporting Information
Experimental details and characterization data for the products.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: studer@uni-muenster.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the Deutsche Forschungs-gemeinschaft (DFG) for
supporting our work.
■
A plausible reaction mechanism is illustrated in Scheme 5. As
previously suggested,13a,b CuCl first reduces NFSI to provide a
Cu(III) species A18 which could exist in equilibrium with a
Cu(II)-stabilized N-centered radical B.13a,b,19 Both are sources
of the bis-sulfonylamidyl radical, which adds to the alkene to
generate radical C along with the Cu(II) species D. In addition,
B could react as complexed N-radical with the alkene. Two
probable routes have to be considered. In path a, trapping of C
with D provides Cu(III) species E, which undergoes ligand
exchange with TMSN3 to give Cu(III) complex F and TMSF.
Reductive elimination of F eventually affords 2, thereby
regenerating the Cu(I) catalyst. Direct amidocupration of the
alkene with A to E can be excluded since only the trans-product
was obtained for aminoazidation of 1u. Alternatively, in path b,
C gets oxidized by D to a cationic intermediate G which is
trapped by TMSN3 to form 2.20
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