Rh2(II)-catalyzed ester migration to afford 3 H-indoles from trisubstituted styryl azides

Article abstract of DOI:10.1021/ol503541z

Rh₂(II)-Complexes trigger the formation of 3H-indoles from ortho-alkenyl substituted aryl azides. This reaction occurs through a 4π-electron-5-atom electrocyclization of the rhodium N-aryl nitrene followed by a [1,2]-migration to afford only 3H-indoles. The selectivity of the migration is dependent on the identity of the β-styryl substituent.

Full text of DOI:10.1021/ol503541z

Letter
pubs.acs.org/OrgLett
Rh2(II)-Catalyzed Ester Migration to Afford 3H‑Indoles from
Trisubstituted Styryl Azides
Chen Kong and Tom G. Driver*
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607, United States
S
* Supporting Information
ABSTRACT: Rh2(II)-Complexes trigger the formation of 3H-
indoles from ortho-alkenyl substituted aryl azides. This reaction
occurs through a 4π-electron-5-atom electrocyclization of the
rhodium N-aryl nitrene followed by a [1,2]-migration to afford only
3H-indoles. The selectivity of the migration is dependent on the
identity of the β-styryl substituent.
he wide-ranging potency of bioactive N-heterocycles
continues to inspire the development of new synthetic
To determine if our tandem process could yield 3H-indole
products, the reactivity of aryl azide 9a toward transition metal
catalysts was investigated (Table 1). Aryl azide 9a was chosen
T
transformations that simplify access to their complex and
diverse structural motifs.1,2 In comparison to other N-
heterocycles, the antiproliferation activity of 3H-indoles has
only recently been recognized.3,4 As a consequence, general
methods for the construction of 3H-indolesparticularly
nonoxygenated oneshas lagged despite their biological
activity and potential value as synthetic intermediates.5−7 This
structural motif can be formed using an interrupted Fischer-
indole reaction;8 however, this reaction is neither regio- nor
stereoselective.8d Densely functionalized carbocycles can be
created using cyclization reactions to trigger structural
rearrangements,9 and the use of the Nazarov reaction, in
particular, has proved to be a successful strategy to access these
carbocycles.10 Our previous investigations established that
related tandem reactions can be initiated from ortho-substituted
aryl azides:11 the electrocyclization of 2 triggered [1,2]
migration to form a 1,2,3-trisubstituted indole (4) (Scheme
1).11d We anticipated that we might be able to transform
Table 1. Determination of Optimal Conditions for 3H-
Indole Formation
a
entry
catalyst
mol %
solvent
t (°C)
%, yield
b
1
none
−
10
10
10
5
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
DME
DCE
140
140
140
140
140
140
140
140
140
140
100
trace
2
FeBr2
49
40
33
33
25
64
78
72
53
26
3
CoTPP
4
RuCl3·nH2O
[(cod)Ir(OMe)]2
Rh2(O2CC3F7)4
Rh2(O2CC7H15)4
Rh2(esp)2
5
6
5
7
5
8
5
Scheme 1. Rh2(II)-Promoted Tandem Reactions
9
Rh2(esp)2
5
10
Rh2(esp)2
5
11
Rh2(esp)2
5
PhMe
a
As determined using 1H NMR spectroscopy using CH2Br2 as an
b
internal standard. Decomposition of 9a observed.
to start our investigation because of its facile construction from
2-azidophenylboronate 7a12,13 and β-ketoester-derived vinyl
triflate 8.14,15 This azide proved to be remarkably robust: very
little reaction occurred below 140 °C in the absence of
transition metal complexes. At this temperature submission of
9a to a series of Fe,16 Co,17 Ru,18 or Ir complexes19all
established N-atom transfer metal catalystsdid induce
decomposition but only poor yields of N-heterocyclic products
were observed (entries 2−5). In contrast, clean conversion was
trisubstituted styryl azides 5 into 3H-indoles 6 by varying the
identity of the β-substituent to change the regioselectivity of the
[1,2] shift. Herein, we report that readily accessible β-
carboxylate- and β-methoxy-susbtituted stryryl azides are
efficiently converted to 3H-indoles and oxindoles using a
rhodium(II) carboxylate catalyst.
Received: December 8, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol503541z
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
observed when 9a was exposed to rhodium(II)-carboxylate
complexes (entries 6−8). To our surprise, the 3H-indole
product resulted from ester migration, a phenomenon not
predicted by our migratorial aptitude scale.11b While 3H-indole
was observed with both perflourinated and alkyl carboxylate
complexes, DuBois’s tetradentate Rh2(esp)2 produced 10a in
the highest, most reproducible yield (entry 8),20 which we
attribute to the thermal robustness of this catalyst.21 A screen of
different solvents revealed toluene to be the ideal reaction
media: lower conversions were obtained in ethereal or
chlorinated solvents (entries 8−10).22 Lowering the temper-
ature also led to incomplete reactions (entry 11).
Table 3. Effect of Changing the o-Alkenyl Substituent on 3H-
Indole Formation
Using these optimized conditions,23 the scope of our
transformation was investigated by varying the identity of the
aryl azide moiety (Table 2). Aryl azides bearing a range of
Table 2. Effect of Changing the Aryl Azide Moiety on 3H-
Indole Formation
a
entry
#
R1
R2
%, yield
1
a
b
c
d
e
f
H
H
H
H
H
H
80
77
62
86
71
74
70
77
68
65
72
2
Me
F
3
4
Cl
OCF3
H
5
6
NHBoc
OMe
Me
7
g
h
i
H
8
H
9
H
F
10
11
j
H
CF3
Ac
k
H
a
Isolated yield of 10 after neutral alumina chromatography; only
product obtained.
a
Isolated yield of 12 after neutral alumina chromatography; only
functional groups were efficiently transformed into 3H-indoles
irrespective of the electronic nature of the R1- or R2-substituent.
The success of 5-substituted aryl azides (entries 6−11)
demonstrates that our transformation can generate 3H-indoles,
which cannot be formed as single isomers using Fischer-indole
or Fischer-indole-type reactions (entries 6−11).24
product obtained.
While only modest diastereoselectivity was observed with an
allylic substituent (entry 10), an 82:18 ratio of diastereomers
was observed with a homoallylic tert-butyl group (entry 11).
This diastereoselective ratio was increased only slightly to 86:14
if the methyl ester was replaced with a tert-butyl ester albeit
with a reduced yield (entry 12).
The scope was further investigated by systematically varying
the identity of the ortho-substituent of the aryl azide (Table 3).
First, the o-cyclohexenyl group could be enlarged to cyclo-
heptene or cyclooctene without attenuating the yield of 3H-
indole (entries 1 and 2). Further, aryl azides containing O- or
N-atoms in the o-heterocycle were efficiently converted to
product (entries 3 and 4). The latter β-carboline is a ubiquitous
structural motif in bioactive alkaloids,1 and our method
represents a novel approach to this important substructure. A
cycloalkenyl o-substituent is not required in our tandem
reaction: aryl azide 11e was smoothly converted into 3H-
indole 12e (entry 5). Next, the identity of the migrating ester
was modified (entries 6−9). We found that 3H-indoles could
be produced using isopropyl, tert-butyl, or allyl esters.
Unfortunately, the menthol ester 11i exhibited poor conversion
and only modest diastereoselectivity (entry 9). The diaster-
eoselectivity of our process was further probed by introducing
substitution on the o-cyclohexenyl substituent (entries 10−14).
While several mechanisms are possible,25 3H-indole
formation is attributed to the tandem electrocyclization−[1,2]
migration outlined in Scheme 2. Rhodium-catalyzed decom-
position of aryl azide 9a produces rhodium nitrene 13,17d,26
which undergoes a 4π-electron-5-atom electrocyclization. The
resulting N-heterocycle 14 contains a benzylic carbocation,
which can undergo ring contraction to produce spirocycle 15 or
a [1,2] ester shift to produce 16.10d−f We believe that ester
migration is favored because the buildup of positive charge in
the transition state leading to 15 will be destabilized by the
ester group.10f Double crossover experiments revealed that the
ester does not escape the solvent sheath during the shift.22
Release of the rhodium carboxylate affords 3H-indole 10a.
If this catalytic cycle were operating, we anticipated that the
3H-indole product might be controlled by the identity of the β-
B
DOI: 10.1021/ol503541z
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Possible Mechanism for Rh2(II)-Catalyzed 3H-
Indole Formation
ASSOCIATED CONTENT
* Supporting Information
■
S
Complete experimental procedures, spectroscopic and ana-
lytical data for the products. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: tgd@uic.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Science Foundation (CHE-
126563) and the Office of the Vice Chancellor of Research at
the University of Illinois at Chicago for their generous support.
We also thank Mr. Furong Sun for mass spectrometry data.
substituent (Scheme 2). Our analysis of the two possible [1,2]
migration reactive intermediates (or transition states leading to
them) indicated that ring contraction might be favored if the
ester substituent was replaced with one that would stabilize 15
in comparison to 16. We anticipated that incorporation of an
electron-donating group (EDG) at the β-position of styryl azide
17 would trigger a [1,2] alkyl shift in 18 to enable formation of
3H-indoles 19.
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