Iron(II) bromide-catalyzed intramolecular c-h bond amination [1,2]-shift tandem reactions of aryl azides

Article abstract of DOI:10.1021/ja3113565

Iron(II) bromide catalyzes the transformation of ortho-substituted aryl azides into 2,3-disubstituted indoles through a tandem ethereal C-H bond amination [1,2]-shift reaction. The preference for the 1,2-shift component of the tandem reaction was established to be Me < 1 < 2 < Ph.

Full text of DOI:10.1021/ja3113565

Communication
pubs.acs.org/JACS
Iron(II) Bromide-Catalyzed Intramolecular C−H Bond Amination
[1,2]-Shift Tandem Reactions of Aryl Azides
Quyen Nguyen, Tuyen Nguyen, and Tom G. Driver*
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, IL, 60607-7061 United States
S
* Supporting Information
Scheme 1. Observation of a Fe(II)-Promoted Tandem
Reaction
ABSTRACT: Iron(II) bromide catalyzes the transforma-
tion of ortho-substituted aryl azides into 2,3-disubstituted
indoles through a tandem ethereal C−H bond amination
[1,2]-shift reaction. The preference for the 1,2-shift
component of the tandem reaction was established to be
Me < 1° < 2° < Ph.
he ability of tandem reactions to rapidly increase the
Tmolecular complexity of simple substrates continues to
inspire the efforts of synthetic groups to incorporate new reactions
into these cascades.1,2 While transition-metal-catalyzed C−H bond
amination is emerging as a useful synthetic process,3−5 this reaction
has never been harnessed to initiate a cascade reaction. Further, the
incorporation of migratorial processes into these cascade sequences
remains rare despite the potential of these processes to transform
simple substrates into complex, functionalized products.6 We have
demonstrated that metal nitrenes originating from ortho-alkenyl-
substituted aryl azides can engage in cascade reactions where
electrocyclization of the rhodium nitrene triggers a subsequent,
selective 1,2-shift.7 Initiating these tandem reactions with a C−H
bond amination reactionideally using an inexpensive, nontoxic
first-row transition-metal catalystwould be highly appealing, as it
would minimize the amount of functionality required in the
starting material. Towards this goal, we report our development
of an iron(II) bromide-catalyzed ethereal C−H bond amination
1,2-migration tandem reaction that efficiently and selectively
transforms ortho-substituted aryl azides into 2,3-disubstituted indoles.
An unexpected observation during our optimization study
into the formation of indoline 2 from aryl azide 1 prompted our
interest in using an amination reaction to initiate a tandem
reaction sequence (Scheme 1). A screen of transition-metal
complexes identified Rh2(esp)2 to be the most efficient catalyst
for the intramolecular C−H bond amination of 1, which
provided indoline 2.8 This screen also revealed that FeBr2
decomposed the aryl azide. The expected indoline, however,
was not observed. Instead, a mixture of 2,3-dimethylindole 3
and aniline were formed. We attributed the formation of these
products to an Fe-mediated oxidation of indoline 2,9 which
would produce iminium ion 4 and aniline if the aryl azide was
the oxidant.10 A 1,2-methyl shift from 4 would then produce
the observed indole. We anticipated that this tandem
amination−migration process might be rendered a viable
synthetic method if the mechanism for iminium ion formation
were changed from an oxidative process (requiring a
stoichiometric oxidant, azide) to an elimination step. We
envisioned that this modification could be achieved if one of
the β-hydrogen atoms in 1 was replaced with a leaving group.
Transition-metal-catalyzed C−H bond amination of 5 would
form indoline 6, which could undergo Lewis acid-catalyzed
elimination of the leaving group to form iminium ion 7 and
trigger the desired 1,2-migratorial process.11
Our pursuit of triggering a tandem C−H bond amination−
elimination−migration sequence started by investigating the
reactivity of aryl azides 8 toward transition-metal complexes
(Table 1). We began by substituting the β-hydrogen atom in 1
with an alkoxy group and examining the reactivity of the resulting
azides toward iron(II) bromide.11−13 While the use of an acetate
lead only to aniline formation (entry 1), changing R to Et led to
nearly complete 2,3-dimethylindole formation (entry 2). The
reaction conversion was dependent on both the temperature as
well as catalyst loading with severe attenuation of indole formation
observed when either was reduced (entries 2−4).
Upon completion of our initial optimization studies using
iron(II) bromide, other transition-metal complexes were
examined to determine if they could catalyze this tandem
reaction (Table 1). Despite their proven ability to catalyze N-
atom transfer reactions from azides, our survey of Rh2(II),14
Ir(I),15 Co(I),16 Ru(III),17 or Cu(I) complexes18 did not
identify any competent catalyst for our process (entries 5−8).
The reaction also proved sensitive to the Lewis acidity of the
iron salt:19 no reaction was observed if the counterion or the
oxidation state was changed (entries 9−11). From our studies,
iron(II) bromide appears unique in its ability to catalyze the C−H
bond amination reaction, the elimination, and the 1,2-methyl shift
Received: November 25, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja3113565 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Table 1. Development of Optimal Conditions
Table 3. Effect of Changing the Migrating Group Identity on
the Tandem Reaction
b
a
entry
catalyst
mol %
R
temp
yield, %
c
1
2
FeBr2
FeBr2
20
20
10
20
20
20
20
20
20
20
20
Ac
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
120
120
120
140
120
120
120
120
120
120
120
10
75
16
85
0
3
FeBr2
4
FeBr2
5
Rh2(esp)2
CoTTP
RuCl3·nH2O
CuI
6
0
7
0
8
0
9
ZnI2
trace
0
10
11
FeCl2
FeBr3
20
a
As determined using 1H NMR spectroscopy and CH2Br2 as the
b
internal standard. No desired product was observed in transition-
metal catalyst absence; only azide decomposition was obtained.
c
Aniline formed.
with the optimal conditions to be 20 mol % catalyst loading in
toluene at 140 °C.
With optimal conditions, the scope of our iron(II) bromide-
catalyzed tandem C−H bond amination−elimination 1,2-
methyl migration reaction was investigated (Table 2). We
Table 2. Scope of Fe(II)-Catalyzed Tandem Reaction
a
entry
no.
R1
R2
yield, %
1
2
3
4
5
6
7
a
b
c
d
e
f
H
H
H
H
H
H
H
Br
85
70
98
85
50
81
79
OMe
Me
Ph
PhCHCH
Br
H
g
a
Isolated after silica gel chromatography.
found that the reaction yield was not affected by the electronic
nature of the aryl azide with consistent yields of the 2,3-
dimethylindole obtained for both electron-releasing and
-withdrawing groups. Despite the established reactivity of
olefins with iron nitrenes,9c we found that aryl azide 8e bearing
a styryl group was transformed into the indole product, albeit
with a diminished yield (entry 5). Our reaction enables the
synthesis of 6-substituted indoles (e.g., 10g), which cannot be
made regioselectively using the Fischer indole reaction.20 These
results indicate that changing the electronic nature of the aryl
azide is not detrimental to the outcome of our tandem reaction.
Next, the effect of changing the identity of the migrating
group on the Fe(II)-catalyzed C−H bond amination 1,2-
migration reaction was investigated (Table 3). We found that
our reaction was not limited to 1,2-methyl shifts, but that ethyl
group migrations as well as ring expansions could be triggered
(entries 1−4). For the latter, the reaction was not constrained
by the alleviation of ring strain: The highest reaction yield was
a
b
Isolated after silica gel chromatography. Aniline obtained as a
c
byproduct. Determined using 1H NMR spectroscopy with CH2Br2 as
internal standard.
obtained from the expansion the cyclohexyl substituted aryl
azide 11d in comparison to cyclobutyl and cyclopentyl
substrates (entries 2−4).
Next, we determined if any selectivity could be observed
during the migration component of the tandem reaction. We
began by examining aryl azides that contained both methyl and
aryl groups (Table 3, entries 5−7). To our delight, we found
that submission of these substrates to reaction conditions
resulted in exclusive aryl group migration to afford 2-aryl-3-
methylindoles, presumably via a phenonium ion. This reaction,
however, was dependent on the electronic nature of the aryl
group with only decomposition observed for azide 11g bearing
B
dx.doi.org/10.1021/ja3113565 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
an electron-deficient arene. With successful differentiation
between sp2 and sp3 carbons, we were curious if our reaction
could distinguish between different sp3-substituted migrating
carbons (entries 8 and 9). To examine this, we submitted aryl
azide 11h to reaction conditions. To our surprise, we found
that only ethyl group migration occurred to provide 2-ethyl-3-
methylindole as the solitary product. To determine if this high
selectivity was general, azide 11i bearing both isopropyl and
ethyl groups was submitted to reaction conditions, affording 2-
isopropyl-3-ethylindole as the only product (entry 9). Finally,
to test for alkyl or aryl group migration in the presence of an α-
hydrogen azides, 11j and 11k were examined (entries 10 and
11). While diminished yields were obtained, only 3 substituted
indoles 11j and 11k were observed from these azides, revealing
that these groups do not migrate when a hydrogen is present.21
From these results, a preliminary migratorial aptitude scale of
our reaction can be established to be: Me < 1° < 2° < Ph.
While a number of mechanisms can explain our trans-
formation,21 we interpret our results to indicate that iron(II)
bromide functions as both an N-atom transfer catalyst and a
Lewis acid (Scheme 2). Coordination of the iron catalyst to the
Scheme 3. Isolation of Reactive Intermediates
occurs after elimination of the ethoxide group. In contrast to
13n, thermolysis of 24b forms the 2,3-dimethylindole product
in the absence of the Lewis acid. Together these results suggest
that iron(II) bromide is required for both C−H bond
amination as well as iminium ion formation but not for 1,2-
alkyl migration.
Scheme 2. Possible Mechanisms for Fe-Catalyzed Tandem
Reaction
To probe the 1,2-shift reaction mechanism, a double crossover
experiment was performed (eq 1). Exposure of a 1:1 mixture of
8a and 11a to reaction conditions resulted in the formation of
only two indoles. The lack of crossover products suggests that
the 1,2-shift component of our tandem reaction is a concerted
process, or if stepwise, the shift occurs faster than diffusion of
the migrating group.
In conclusion, we have discovered that iron(II) bromide
promotes tandem C−H bond amination 1,2 migration reactions of
ortho-substituted aryl azides to enable the formation of 2,3-
disubstituted indoles. The 1,2-shift component of our tandem
reaction is remarkably selective, and our results enable prediction
of the migration aptitude to be Me < 1° < 2° < Ph. Our future
studies are aimed at achieving a better understanding of the
mechanism of our tandem reaction as well as further exploring
iron-catalyzed C−H bond amination reactions.
aryl azide (to form 14)23 triggers the extrusion of N2 to form
iron nitrene 15.24 While the ethereal C−H bond amination
reaction could be concerted (via TS-16),11a a stepwise process
is also possible: hydride transfer from 15 forms oxocarbenium
ion 17 that is attacked by the proximal amine to form indoline
18.25 Coordination of the Lewis acidic iron salt to the ethyl
ether promotes the generation of iminium ion 21, which
triggers the 1,2-shift.26 Subsequent deprotonation of 22 by iron
ethoxide completes the catalytic cycle.
In our optimization studies, we isolated two potential
heterocyclic intermediates, whose reactivity toward the reaction
conditions support our proposed mechanism (Scheme 3).
When cyclopropyl-substituted aryl azide 11n was exposed to
iron(II) bromide, a mixture of indoline 13n and indole 23n was
isolated. Isolation of indoline 13n provides support that C−N
bond formation occurs through an ethereal C−H bond
amination reaction. The lack of fragmentation of the cyclo-
propane suggests that this amination reaction does not proceed
through an H-atom abstraction−radical recombination reac-
tion.22e,h,27 Resubmission of 13n to reaction conditions
produced indole 23n; in the absence of iron(II) bromide no
reaction was observed. The reactivity of methoxy-substituted 8b
was also consistent with our mechanistic hypothesis. The
isolation of 3H-indole 24b indicates that the 1,2-methyl shift
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures and analytical data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
tgd@uic.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Institutes of Health NIGMS
(R01GM084945) and the University of Illinois at Chicago for
their generous financial support. We thank Mr. Furong Sun
(UIUC) for high-resolution mass spectrometry data.
C
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Journal of the American Chemical Society
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(b) Lop
Org. Lett. 2006, 8, 4303.
́ ́
ez-Alvarado, P.; Caballero, E.; Avendano, C.; Menendez, J. C.
̃
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D
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