Promoting reductive tandem reactions of nitrostyrenes with Mo(CO)6 and a palladium catalyst to produce 3 h -indoles

Article abstract of DOI:10.1021/jacs.5b02946

The combination of Mo(CO)₆ and 10 mol % of palladium acetate catalyzes the transformation of 2-nitroarenes to 3H-indoles through a tandem cyclization-[1,2] shift reaction of in situ generated nitrosoarenes. Mo(CO)₆ appears to have dual roles in this transformation: generate CO and promote C-N bond formation to increase the yield of the N-heterocycle product.

Full text of DOI:10.1021/jacs.5b02946

Communication
pubs.acs.org/JACS
Promoting Reductive Tandem Reactions of Nitrostyrenes with
Mo(CO)6 and a Palladium Catalyst To Produce 3H‑Indoles
Navendu Jana, Fei Zhou, and Tom G. Driver*
Department of Chemistry, University of Illinois at Chicago, 845 West Taylor Street, Chicago, Illinois 60607-7061, United States
S
* Supporting Information
Table 1. Development of Optimal Conditions
ABSTRACT: The combination of Mo(CO)6 and 10 mol
% of palladium acetate catalyzes the transformation of 2-
nitroarenes to 3H-indoles through a tandem cyclization-
[1,2] shift reaction of in situ generated nitrosoarenes.
Mo(CO)6 appears to have dual roles in this trans-
formation: generate CO and promote C−N bond
formation to increase the yield of the N-heterocycle
product.
a
yield, %
entry
catalyst
ligand
phen
reductant
solv
9:10:11
b
1
Pd(OAc)2
Pd(OAc)2
Pd(TFA)2
Pd(TFA)2
Pd(TFA)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
CO (1.5 atm) DMF
20:40:0
12:21:0
44:8:0
b
2
tmphen CO (1.5 atm) DMF
b
3
phen
phen
phen
phen
phen
phen
phen
CO (1.5 atm) DMF
CO (1.5 atm) DMF
bc
,
4
5
6
7
8
9
62:0:0
he potential of triggering carbon−nitrogen bond formation
bc
,
CO (3 atm)
Mo(CO)6
Mo(CO)6
Mo(CO)6
Mo(CO)6
Mo(CO)6
DMF
DMF
THF
DCE
THF
DCE
37:0:0
T
using a nitro-group as the source of the nitrogen atom
inspires significant research because of the widespread availability
of nitroarenes.1 These research efforts have eased the trans-
formation of nitroarenes into indoles and carbazoles by forming
sp2-C−N bonds from sp2-C−H bonds.2 This C−H bond
amination is traditionally achieved using superstoichiometric
quantities of a reductant such as phosphite,3 zinc dust,4 Grignard
reagent,5 or high pressures of carbon monoxide,6 which convert
the nitro-group into an electrophilic nitrogen species. In contrast,
creating partially saturated, nonplanar N-heterocycles from
nitroarenes has been slow to emerge,7 and these processes are
traditionally typified by low yields and significant byproduct
formation.8 Our group has developed transition-metal-catalyzed
methods to convert aryl azides into complex, functionalized N-
heterocycles through an electrocylization−migration tandem
reaction of rhodium N-aryl nitrene 2 (Scheme 1).9 During the
d
d
d
e
30:0:35
48:0:50
80:0:0
16:15:38
68:0:0
f
10
phen
1
a
As determined using H NMR spectroscopy using CH2Br2 as the
internal standard. 20 mol % of Pd(OAc)2 and 40 mol % ligand used.
0.4 equiv of trifluoroacetic acid added. 1.0 equiv of Mo(CO)6 used.
0.5 equiv of Mo(CO)6 used. 5 mol % of Pd(OAc)2 and 10 mol %
b
c
d
e
f
phen used.
To determine if a cyclization−migration cascade could be
triggered, nitroarene 8a was examined toward transition-metal
catalysts and reductants (Table 1). The substrate for our study
was easily constructed by cross-coupling the commercially
available 2-nitrophenylboronic acid with the vinyl triflate derived
from 2-phenylcyclohexanone. At the outset of our study,
nitroarene 8a was exposed to a range of different transition-
metal complexes using 1.5 atm of carbon monoxide as the
reductant. To our dismay, examination of common Rh-,10
Ru-,7a,10a and Pt-catalysts11 for the reduction of nitroarenes
produced only aniline 11a. In contrast, in situ generated
phenanthroline palladium(II) complexes triggered the desired
cyclization (entry 1).6 Unfortunately, the migration step was
short-circuited by deprotonation to result in both spirocycle 9a
and indoline 10a. Changing the identity of the phenanthroline
ligand did not improve the ratio of spirocycle 9a to 10a (entry
2).10b,12b Deprotonation could be inhibited by using Pd(TFA)2
as the precatalyst (entry 3), and the addition of trifluoroacetic
acid further improved both the yield and selectivity of the
reaction (entry 4). Increasing the pressure of CO proved
detrimental to this result (entry 5). In an attempt to improve the
Scheme 1. Rh2(II)-Promoted Tandem Reactions
course of our studies, we were curious if spirocyclic hetero-
cyclesinaccessible from aryl azides using our methodscould
be formed from nitroarenes. Herein, we report that the reactivity
embedded in trisubstituted nitrostyrenes can be unlocked using
Mo(CO)6 and a palladium catalyst to enable access to
functionalized 3H-indoles.
Received: March 20, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b02946
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 2. Examination of the Electronic Nature of the
Nitroarene
Table 3. Scope and Limitations of 3H-Indole Formation
ab
,
entry
no.
R1
R2
R3
9 yield, %
1
2
3
4
5
6
7
8
9
a
b
c
d
e
f
H
MeO
Me
F
H
H
H
H
H
H
H
H
H
H
H
77
60 (82)
64
c
H
H
72
c
F3C
H
H
54
MeO
Me
F
68
77
88
75
g
h
i
H
H
H
CO2Me
a
Conditions: 10 mol % Pd(OAc)2 20 mol % phenanthroline,
b
Mo(CO)6 (1 equiv), DCE, 120 °C, 16 h. Isolated after silica gel
chromatography. 20 mol % Pd(OAc)2 used.
c
yield of our transformation, alternative sources of CO were
screened. Metal carbonyl complexes are well established to
release CO upon heating,13 and Mo(CO)6 is a proven CO
equivalent in palladium-catalyzed carbonylation reactions.14 To
our delight, when CO source was changed to Mo(CO)6, neither
the deprotonation product 10a nor oligomerization was
observed (entries 6−8). Dichloroethane was found to be the
optimal solvent enabling clean formation of spirocycle 9a in 80%
without any observed aniline byproduct (entry 8). Both aniline
11a and 10a byproducts were observed, however, when the
amount of Mo(CO)6 was reduced from 1 to 0.5 equiv (entry 9).
In contrast, reducing the catalyst loading of Pd(OAc)2 and
phenanthroline proved less detrimental, while the yield of
spirocycle 9a was attenuated to 68%, no byproducts were formed
(entry 10).
Using these optimal conditions, we examined the scope and
limitations of palladium-catalyzed reductive cyclization−migra-
tion reaction (Table 2). This investigation was significantly eased
by the modular nature of our substrate synthesis, which enables
rapid construction of nitroarenes through a Suzuki cross-
coupling reaction between 2-nitroarylboronic acid 12 and vinyl
triflate 13a. First, the effect of changing the nitroarene
substituent was examined. To our delight, we found that our
reaction tolerated a range of electron-releasing or electron-
withdrawing aryl R1-substituents without significant attenuation
the yield (entries 1−5). Next to demonstrate that our reaction
enables access to 3H-indoles, which cannot be formed as single
isomers using Fischer-indole-type reactions,15 the identity of the
R2-substituent was changed. Spirocycle 9 was formed smoothly
irrespective of the electronic identity of the substituent (entries
6−9).
a
b
Isolated after silica gel chromatography. 20 mol % Pd(OAc)2 used.
indole 16 (entry 5). Carboxylate migration was not dependent
on the ring size of the ortho-substituent: six-, seven- and even
eight-membered cycloalkenyl substrates could be smoothly
converted to 3H-indole 16 (entries 5−7). Next, changing the
composition of the tether was examined (entries 8−9): 3H-
indoles could be accessed from nitroarenes 14h and 14i bearing
ortho-heterocycle substituents if a higher catalyst loading was
used. The diastereo-selectivity of our reaction was next probed.
Scheme 2. Possible Mechanism for 3H-Indole Formation
Next, the scope of the reaction was surveyed by varying the
identity of the o-alkenyl substituent of the nitroarene (Table 3).
First, higher yields were obtained with electron-rich Rβ-aryl
substituent (entries 1 and 2). In contrast to the expected alkyl
shift, untethering the α- and β-alkyl substituents in 14c triggered
a [1,2] phenyl shift to form 3H-indole 15c (entry 3). Next, the
effect of changing the identity of the Rβ-substituent was
investigated (entries 4−12). While ring contraction was observed
with an Rβ-methyl to afford spirocycle 15d (entry 4), switching to
a carboxylate group changed the identity of the product to 3H-
B
DOI: 10.1021/jacs.5b02946
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Journal of the American Chemical Society
Communication
The allylic methyl group in 14j directed the stereoselectivity of
the transformation to afford 3H-indole 16j as a 91:9 mixture of
diastereomers (entry 10). Moving the stereocenter to the
homoallylic position in 14k attenuated the diastereoselectivity
slightly to 80:20 (entry 11). This ratio could be improved by
increasing size of the migrating group to a tert-butyl carboxylate
(entry 12).
either nitroarene 8a or 2,5-di-tert-butylnitrobenzene using
Mo(CO)6. It could only be intercepted from 30 using CO as
the reductant to produce oxazine 31.
To further investigate the effect of Mo(CO)6, the reactivity of
2-tert-butylnitrosobenzene 32 was examined (Eq 1). As expected,
exposure to 2,3-dimethylbutadiene resulted in formation of
oxazine 33. The addition of Mo(CO)6, however, completely
inhibited cycloadditionirrespective of the equivalents of
butadiene or 1,5-cyclooctadiene presentleading to aniline.
Together these experiments indicate that Mo(CO)6 is not simply
functioning as a source of CO, but that it coordinates the
nitrosoarene to induce cyclization and migration.20 Because
Pd(OAc)2 is required for high yields,25 our data suggests that role
of the palladium phenanthroline complex is to catalyze the N−O
bond reduction in nitroarene 8a and spirocycle 21.26
Our initial mechanistic hypothesis for 3H-indole formation is
based on the previous investigations into the catalytic cycle of
palladium-mediated nitroarene reduction (Scheme 2).12,16
Reduction of (phen)Pd(OAc)2 complex by Mo(CO)6 would
produce the palladium CO complex 17, which could exist either
as a monomer or a cluster.17 Oxidative addition of nitroarene
produces palladacycle 18.18 Reductive elimination of CO2
produces palladium-nitrosoarene 19. Cyclization of 19 could
occur through electrocyclization or attack by the adjacent π-
system.19 The resulting benzylic cation in 20 triggers a ring
contraction to produce spirocycle N-oxide 21. Reduction
produces 3H-indole 9a and regenerates the palladium−carbonyl
catalyst. Alternatively, Mo(CO)6 could have multiple functions:
in addition to serving as the CO-source for Pd-catalyzed N−O
bond reduction, it could coordinate to nitrosoarene 23 to
facilitate attack by the pendant olefin to afford 20.20 A third
possible mechanism is that spirocycle formation occurs via a
metal nitrene intermediate. These intermediates have been
posited as potential catalytic intermediates in related pro-
cesses:7,8 a ruthenium nitrene was implicated by Cenini and co-
workers in their intermolecular allylic C−H bond amination of
cyclohexene with nitroarenes.7a Palladium nitrosoarene complex
could be reduced to produce palladacycle 24.21 Reductive
elimination of CO2 then produces palladium nitrene 25, which
could undergo a 4π-electron-5-atom-electrocyclization-ring
contraction to produce the 3H-indole product.
Further insight into the mechanism was serendipitously
provided by α-pinene-derived nitroarene 14m (Scheme 4).
Exposure of this nitroarene to reaction conditions produced only
indoline 35 as a single diastereomer. The expected 1,2-
carboxylate migration was short-circuited by deprotonation of
one of the bridgehead methyl groups in 34 inducing a
fragmentation to produce 35.27 We found that the migration of
the methyl carboxylate did not require palladium: exposure of
indoline 35 to Mo(CO)6 formed 3H-indole 16m as a single
diastereomer.28 The chemoselectivity of this migration step
appears to result from the transition state or intermediate
containing the more stable iminium ion (e.g., TS-36).
Several experiments were performed to distinguish between
Scheme 4. Separation of Cyclization and Migration Steps
these possible mechanisms (Scheme 3).22 First, we submitted
Scheme 3. Attempted Interception of Catalytic Intermediates
In conclusion, we have shown that the combination of
palladium acetate and Mo(CO)6 can unlock the reactivity
embedded in 2-alkenyl-substituted nitroarenes and trigger
cyclization−migration reactions to produce spirocyclic 3H-
indoles. Our future studies are aimed at further exploring the
reactivity of palladium nitrosoarene complexes to produce
complex, functionalized N-heterocycles from readily accessible
2-substituted nitroarenes.
nitroarene 26 to reaction conditions to examine if palladium
nitrene 25 was a catalytic intermediate. We expected that if this
intermediate was formed that some 2-phenylindoline would be
observed.23 In contrast, only aniline was produced to suggest that
metal nitrenes are not catalytic intermediates. Next, interception
of the nitrosoarene intermediate was attempted through the
addition of 2,3-dimethylbutadiene to the reaction mixture.24 To
our surprise, the putative nitrosoarene could not be trapped from
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectroscopic and analytical data. The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/jacs.5b02946.
C
DOI: 10.1021/jacs.5b02946
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Gasperini, M.; Cenini, S. Adv. Synth. Catal. 2004, 346, 63. (f) Gasperini,
M.; Ragaini, F.; Remondini, C.; Caselli, A.; Cenini, S. J. Organomet.
Chem. 2005, 690, 4517.
AUTHOR INFORMATION
Corresponding Author
*tgd@uic.edu
Notes
■
(13) (a) Phillips, J.; Dumestic, J. A. Appl. Catal. 1984, 9, 1.
(b) Armarego, W. L. F.; Perrin, D. D. Purifications of Laboratory
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(b) Kaiser, N.-F. K.; Hallberg, A.; Larhed, M. J. Comb. Chem. 2002, 4,
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Synlett 2007, 2557. (d) Spencer, J.; Rathnam, R. P.; Patel, H.; Anjum, N.
Tetrahedron 2008, 64, 10195. (e) He, L.; Sharif, M.; Neumann, H.;
Beller, M.; Wu, X.-F. Green Chem. 2014, 16, 3763.
(15) cf. (a) Phillips, R. R. Org. React. 1959, 10, 1143. (b) Robinson, B.
Chem. Rev. 1963, 63, 373. (c) Schammel, A. W.; Boal, B. W.; Zu, L.;
Mesganaw, T.; Garg, N. K. Tetrahedron 2010, 66, 4687.
(16) (a) Ragaini, F.; Larici, H.; Rimoldi, M.; Caselli, A.; Ferretti, F.;
Macchi, P.; Casati, N. Organometallics 2011, 30, 2385. (b) Mooibroek,
T. J.; Bouwman, E.; Drent, E. Organometallics 2012, 31, 4142.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the University of Illinois at Chicago, Office of
the Vice-Chancellor for Research for their generous financial
support. We thank Mr. Furong Sun (UIUC) for high resolution
mass spectrometry data.
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D
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Journal of the American Chemical Society
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(28) The combination Pd(OAc)2, phenanthroline and Mo(CO)6 also
promoted the formation of spirocycle 9a from 10a. No 1,2 migration
was observed when Mo(CO)6 was omitted from the reaction mixture.
E
DOI: 10.1021/jacs.5b02946
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX