Selenium-catalyzed oxidative c(sp2)-h amination of alkenes exemplified in the expedient synthesis of (Aza-)indoles

Article abstract of DOI:10.1021/acs.orglett.5b01156

A new selenium-catalyzed protocol for the direct, intramolecular amination of C(sp²)-H bonds using N-fluorobenzenesulfonimide as the terminal oxidant is reported. This method enables the facile formation of a broad range of diversely functionalized indoles and azaindoles derived from easily accessible ortho-vinyl anilines and vinylated aminopyridines, respectively. The procedure exploits the pronounced carbophilicity of selenium electrophiles for the catalytic activation of alkenes and leads to the formation of C(sp²)-N bonds in high yields and with excellent functional group tolerance.

Full text of DOI:10.1021/acs.orglett.5b01156

Letter
pubs.acs.org/OrgLett
Selenium-Catalyzed Oxidative C(sp2)−H Amination of Alkenes
Exemplified in the Expedient Synthesis of (Aza-)Indoles
Stefan Ortgies and Alexander Breder*
Institut fur Organische und Biomolekulare Chemie, Georg-August-Universitat, Tammannstrasse 2, 37077 Gottingen, Germany
̈
̈
̈
S
* Supporting Information
ABSTRACT: A new selenium-catalyzed protocol for the direct,
intramolecular amination of C(sp2)−H bonds using N-fluorobenzene-
sulfonimide as the terminal oxidant is reported. This method enables
the facile formation of a broad range of diversely functionalized
indoles and azaindoles derived from easily accessible ortho-vinyl
anilines and vinylated aminopyridines, respectively. The procedure exploits the pronounced carbophilicity of selenium
electrophiles for the catalytic activation of alkenes and leads to the formation of C(sp2)−N bonds in high yields and with
excellent functional group tolerance.
xidative functionalization of C−H bonds constitutes a
there is still a dearth of potent metal-free catalysis concepts for
direct manipulation of olefinic carbon−hydrogen bonds. Seminal
work by Engman11 on the Se(IV)-mediated C−H chlorination of
alkenes indicated that the carbon−halogen bond formation
presumably occurs through the transient formation of a
seleniranium species that subsequently undergoes ring opening
by a chloride anion followed by dehydrodeselenation. A similar
scenario for the initial electrophilic activation of the alkene
moiety was recently suggested by Denmark and co-workers in
the context of a syn-specific dichlorination of olefins by a
selenium(IV) catalyst.12 Other chalcogen-based electrophiles,
such as sulfenium ions, were also shown to function as competent
promoters for stoichiometric C−H amination reactions of
alkenes (Scheme 1).13
O
privileged and step-economic1 strategy for the incorpo-
ration of heteroatoms into given hydrocarbon frameworks. In
this direction, the oxidative C−H amination of nonactivated
alkenes, arenes, and alkyl groups has evolved into a highly active
area in contemporary chemical sciences.2 For almost four
decades, many research groups have devised elegant methods to
address the challenging task of nitrogenating C(sp2)−H bonds in
a regio- and chemoselective manner. Large portions of these
protocols use transition metal catalysts such as Pd,3,4 Cu,5 Rh,6
and Ru complexes (Scheme 1).2,7 Conversely, oxidative
Scheme 1. Indole Syntheses through C(sp2)−H Amination
Our group has recently reported the first Se-catalyzed C−H
imidation of (hetero)cyclic alkenes and styrenes using N-
fluorobenzenesulfonimide (NFSI) as both the terminal oxidant
and nitrogen source.14,15 On the basis of these initial
investigations, we became interested in the design of a novel
metal-free concept for the electrophilic activation of simple
alkenes that would allow for the facile construction of C−N
bonds from C(sp2)−H entities. Since indoles and azainoldes
represent structural motifs with important pharmaceutical,
agrochemical, and material scientific applications,16,17 we
reasoned that such target structures would provide an ideal
platform for the development of a Se-catalyzed C−H amination
protocol. As a result, we present herein a useful and wide-ranging
metal-free method for the synthesis of diversely functionalized
indoles and azaindoles from ortho-vinylated anilines and
aminopyridines, respectively, using (PhSe)2 as a highly potent
and cost-efficient catalyst.
amination of nonactivated olefinic C−H bonds enabled by
catalysts that possess nonmetallic main group elements as
catalytically active species still constitutes a great challenge in
current methodological research. Although significant and
indicative progress has been made in the realm of iodine-
mediated and, to some extent, -catalyzed C−H aminations of
alkenes,8 (hetero)arenes,9 and dinitrogenations of olefins,10
At the outset of our studies, we used anilide 1a as the test
substrate to determine optimal conditions for the title reaction.
Treatment of compound 1a with 1.05 equiv of NFSI in the
Received: April 21, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01156
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Organic Letters
Letter
Scheme 2. Scope of the Se-Catalyzed C(sp2)−H Amination
presence of 10 mol % of (PhSe)2 at ambient temperature in
various solvents (Table 1, entries 1−12) revealed that the best
a
with Varying R1 and R2 Substituents
a
Table 1. Optimization of Reaction Conditions
b
entry
catalyst (mol %)
(PhSe)2 (10)
solvent
THF
temp (°C) yield (%)
1
2
22
22
14
12
11
9
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (10)
(PhSe)2 (2.5)
(BnSe)2 (10)
(4-MeO-C6H4Se)2 (10)
DBDS (10)
Et2O
3
1,4-dioxane
MeCN
MeNO2
DMF
22
4
22
5
22
10
0
6
22
7
DMSO
hexane
benzene
toluene
DCM
22
0
8
22
17
46
47
43
0
9
22
10
11
12
13
14
15
16
17
18
19
20
22
22
MeOH
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
22
100
100
100
100
100
100
100
100
71
37
13
20
65
0
a
Reactions were carried out on a 0.3 mmol scale in 5 mL of toluene
with 1.05 equiv of NFSI, 2.5 mol % of (PhSe)2, and 4 Å molecular
sieves for 16 h. Yields refer to isolated compounds. Numbers in
parentheses refer to yields determined by 1H NMR using 4-
PhSeBr (2.5)
PdCl2 (10)
b
bromoanisole as an internal standard. 5 mol % of (PhSe)2 was used.
FeCl3 (10)
0
none
4
a
With regard to the substitution pattern on the alkene, both
aromatic and aliphatic groups were tolerated under the reaction
conditions. For instance, anilide 1f was readily converted into
corresponding indole 2f in an isolated yield of 79%. It is
noteworthy to point out that anilides homologous to structures
1a and 1f were shown to be unsuitable for cognate cyclizations
under single-electron oxidation conditions.19 Thus, our results
underscore the high degree of chemoselectivity and functional
group tolerance exerted by selenium catalysis. Furthermore,
other aliphatic and alicyclic substituents, such as cyclohexyl (2g),
methoxymethyl (2h), cyclopentylmethyl (2i), and cyclopropyl
groups (2j), were tolerated in the C−H amination (65−87%).
The conversion of substrate 1n to tetrahydrocarbazole 2n
required an increased catalyst loading of 5 mol % and resulted in
an isolated yield of 66%. Particularly good results were recorded
for substrates with aromatic residues in the β-position of the
alkene. Anilides 1l and 1m underwent oxidative cyclization with
isolated yields of 97 and 98%, respectively. In the case of
substrate 1m, a 1:1.4 mixture of isomeric indoles 2m (41%) and
2m′ (57%) was isolated. The formation of 3-arylated indole 2m′
is believed to involve a phenonium migration from the 2- into the
3-position.7,19,20
To further demonstrate the generality and utility of the Se-
catalyzed C−H amination protocol, our efforts continued with
the investigation of steric and electronic effects exerted by
substituents within the anilide ring (Scheme 3). For this purpose,
a series of anilide derivatives 1o−1x, which possess electron-
donating and electron-withdrawing groups in various positions,
were synthesized. In general, the title transformation exhibited a
broad functional group tolerance since halides, ethers, a nitrile,
and an ester were compatible with the reaction conditions.
Moreover, it was found that the electronic nature of the R4 group
did not have a significant impact on the yield. For example,
trifluoromethylated compound 2p and dioxoloindole 2q were
Reactions were carried out on a 0.17 mmol scale in 1.5 mL of solvent
b
with 1.05 equiv of NFSI and 4 Å molecular sieves for 16 h. Yields
were determined by H NMR using 4-bromoanisole as an internal
standard. DBDS = dibenzo[c,e][1,2]diselenine.
1
result was obtained in toluene (0.1 M) with a yield of 47% for
indole 2a. At 100 °C, a catalyst loading of only 2.5 mol % led to a
yield of 71% (entry 13). Next, we tested a number of other
selenium catalysts, such as (4-anisylSe)2, (BnSe)2, dibenzo[c,e]-
[1,2]diselenine, and PhSeBr (entries 14−17), for their catalytic
activity, but none of these species provided results superior to
those of (PhSe)2. At no instance during this study was the
formation of fluorinated products detected. To exclude the
possibility that the title reaction was actually catalyzed by traces
of palladium or iron salts, which may have arisen from the
substrate synthesis or the employed equipment, we also tested
PdCl2 and FeCl3 (10 mol % each, entries 18 and 19) as potential
catalysts. However, neither experiment resulted in detectable
quantities of product 2a. Treatment of anilide 1a with NFSI at
100 °C in the absence of any catalyst led to product 2a only in
trace amounts (entry 20).
With optimized conditions in hand, investigations continued
with the exploration of the scope and limitations of our
amination protocol (Schemes 2 and 3). Initially, various
substituents at the nitrogen atom (R1) and the alkene (R2 and
R3) were tested (Scheme 2). While tosyl, nosyl, and mesyl groups
were compatible with the reaction conditions (2a−c, 42−47%),
the corresponding carboxamide derivatives (1d and 1e) did not
participate in the reaction. These observations suggest that the
pKa of the N−H units plays an important role in the reaction
turnover. Presumably, neither of the carboxamide groups is acidic
enough to be deprotonated under the reaction conditions. This
finding is congruent with reports by other research groups on
related cyclization reactions.13,18
B
DOI: 10.1021/acs.orglett.5b01156
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Scope of the Se-Catalyzed C(sp2)−H Amination
transiently formed indoline intermediate. Thus, we synthesized
N-tosylindoline and exposed it to 1.05 equiv of NFSI in toluene
at 100 °C in both the absence and presence of (PhSe)2. However,
in neither case was the formation of the respective indole product
detected, indicating that a hydroamination step is very unlikely.
Next, we wanted to determine whether the C−H amination is
catalyzed by (PhSe)2 itself or by oxidized derivatives of it, such as
PhSeF or PhSeN(SO2Ph)2. Hence, we initially examined the
crude mixture of a completed reaction of substrate 1h and NFSI
under standard conditions. From this mixture, (PhSe)2 was
reisolated in 81% yield. To elucidate whether the diselane
remains intact throughout the entire catalytic cycle or whether it
undergoes a fragmentation/recombination sequence, we con-
ducted a series of crossover experiments, in which substrate 1l
was initially converted with a 1:1 mixture of (PhSe)2 and (4-
tolylSe)2 (10 mol % each) in the presence of 1.05 equiv of NFSI.
Analysis of the reisolated selenium catalysts by gas chromatog-
raphy revealed no indication of the presence of a mixed
diselane.23 Ostensibly, this result was indicative of a mechanism
in which the Se−Se bond may not undergo oxidative scission. To
obtain a consistent picture of the catalytically active species,
investigations continued with the analysis of arylselenyl halides as
catalysts. Therefore, the cyclization of substrate 1p was
conducted with 10 mol % of PhSeBr to elucidate the fate of
this catalyst upon completion of the reaction. Upon full
conversion of the starting material, (PhSe)2 was the only
selenium species isolable (20%) from the reaction mixture. This
finding proved that the recombination of the SeII species to
diselenides is possible under the title conditions.24 Next, we
performed another crossover experiment using PhSeBr and 4-
tolylSeBr (10 mol % each) as catalysts under otherwise unaltered
conditions. In contrast to our expectations, the gas chromato-
gram of the isolated selenium species displayed signals
corresponding to only (PhSe)2 and (4-tolylSe)2. This result
implied that a recombination of diselenides via Se−Se bond
homolysis under thermal conditions might be possible during the
GC experiment, which potentially obviates the detection of a
mixed diselane.25 Eventually, we subjected the diselane samples
from both crossover experiments to 77Se NMR analysis (CDCl3,
cf. Supporting Information). The spectra exhibited resonances
corresponding to (PhSe)2 (462.2 ppm) and (4-tolylSe)2 (473.6
ppm). In addition, two resonances at 459.1 and 476.1 ppm were
recorded, which could be assigned to the mixed diselane via 2D
77Se,77Se COSY experiments. From these data, it became evident
that a mixed diselane was formed in both crossover experiments
together with (PhSe)2 and (4-tolylSe)2 in a statistical 2:1:1 ratio.
Consequently, we postulate the following catalytic cycle
(Scheme 4). Following oxidative scission of the Se−Se bond
through reaction of (PhSe)2 with NFSI, the resulting selenium
electrophiles presumably attack the alkene moiety to form
seleniranium ion I. This, in turn, is then attacked by the adjacent
sulfonamide group to furnish adduct II. Nucleofugic activation of
the PhSe group within adduct II by another selenium
electrophile initiates an elimination step, upon which the indole
product and (PhSe)2 are released.
a
with Varying R4 Substituents and Aminopyridines
a
Reactions were carried out on a 0.3 mmol scale in 5 mL of toluene
with 1.05 equiv of NFSI, 2.5 mol % of (PhSe)2, and 4 Å molecular
sieves for 16 h. Yields refer to isolated compounds. Numbers in
parentheses refer to yields determined by 1H NMR using 4-
b
bromoanisole as an internal standard. 5 mol % of (PhSe)2 was used.
isolated in 87% yield. Even better results were recorded for 5-
methylated indole 2r and 5-chlorinated indole 2s with isolated
yields of 99 and 96%, respectively. On the other hand, steric
factors were found to have a more substantial impact on the yield.
More precisely, 2-bromoanilide 1o was converted into
corresponding indole 2o in a moderate yield of 42%. This result
was rationalized by considering steric repulsions between the Br
atom and the adjacent tosylamide group.
Given that the electronic nature of the R4 group was virtually
inconsequential with regard to the yields of products 2, we
surmised that other electron-deficient entities, such as pyridines,
might also be tolerated under the title conditions. The use of
vinylated aminopyridines as aza analogues of anilides 1a−1x
would provide a facile and expedient route toward azaindoles.21
Thus, vinylated aminopyridines 1y−1aa were tested in the Se-
catalyzed C−H amination. From initial experiments, in which
aminopyridine 1y was used as the test substrate, we learned that
an increased catalyst loading of 5 mol % under otherwise
unchanged reaction conditions led to 7-azaindole 2y in a
satisfying yield of 81%. Application of the modified reaction
parameters to aminopyridines 1z and 1aa provided access to
azaindoles 2z and 2aa in isolated yields of 57 and 78%,
respectively. The slightly diminished yield of azaindole 2z was
rationalized with the electronically deactivating effect of the
endocyclic nitrogen atom in the 4-position on the adjacent
alkene. Attempts to synthesize the 6-azaindole analogue failed,
presumably due to insufficient solubility of the respective 3-
aminopyridine substrate in toluene.
In summary, we have delineated a novel Se-catalyzed C(sp2)−
H amination protocol via electrophilic activation of simple
alkenes. Both ortho-aminostyrenes and vinylated aminopyridines
have been successfully employed as substrates to access a broad
range of indoles and azaindoles, respectively, in good to excellent
yields. In general, the title reaction exhibits an excellent
functional group tolerance and allows for a facile route to
diverse substitution patterns around the (aza)indole core.
In addition to the substrate scope, we also wanted to gain some
insight into the mechanism of the title transformation. At the
outset of this study, we wanted to exclude the possibility that the
C−N bond formation may actually proceed by Brønsted acid
catalyzed hydroamination22 followed by oxidation of a
C
DOI: 10.1021/acs.orglett.5b01156
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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■
S
Experimental procedures and compound characterization data.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01156.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: abreder@gwdg.de.
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Fonds der
Chemischen Industrie (FCI, Liebig-Fellowship to A.B., and
Chemiefonds Fellowship to S.O.) and the Deutsche For-
schungsgemeinschaft (DFG, Emmy Noether Fellowship to
A.B.). We thank Prof. Dr. Lutz Ackermann (University of
Gottingen, Germany) for generous support of our work. Dr.
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