Rhodium(I)-catalyzed cycloisomerization of nitrogen-tethered indoles and alkylidenecyclopropanes: Convenient access to polycyclic indole derivatives

Article abstract of DOI:10.1002/chem.201302331

At the end of its tether: A new synthetic protocol for the preparation of polycyclic indole derivatives has been developed from a rhodium(I)-catalyzed cycloisomerization of a nitrogen-tethered indole and alkylidenecyclopropane, affording the corresponding

Full text of DOI:10.1002/chem.201302331

DOI: 10.1002/chem.201302331
Rhodium(I)-Catalyzed Cycloisomerization of Nitrogen-Tethered Indoles and
Alkylidenecyclopropanes: Convenient Access to Polycyclic Indole
Derivatives
Di-Han Zhang, Xiang-Ying Tang,* Yin Wei,* and Min Shi*[a]
The indole moiety is a privileged structural motif in many
biologically active and medicinally valuable molecules.[1]
Polycyclic frameworks lead to relatively rigid structures that
might be expected to show substantial selectivity in their in-
teractions with enzymes or receptors.[2] Construction of poly-
cyclic indoles usually requires multistep approaches.[3] The
preparation of polyfunctional indoles is therefore an impor-
tant research field.
The tetrahydro-b-carboline ring system, which belongs to
the polycyclic indole family, is widely found in natural prod-
ucts and pharmaceuticals.[4] Representative examples, such
Scheme 1. Construction of tetrahydro-b-carboline derivatives.
as jafrine, mitragynine, and ajmalicine, all have tetrahydro-
b-carboline structures (Figure 1). The Pictet–Spengler reac-
tion has been recognized as one of the most direct and effi-
cient methods for the construction of tetrahydro-b-carboline
frameworks.[5] However, to date, most of the reported meth-
ods are accomplished by the treatment of tryptamine with a
carbonyl functionality in the presence of an acidic catalyst
(Scheme 1a).
Alkylidenecyclopropanes (ACPs), containing a coordinat-
ing double bond and a strained carbocycle, can undergo a
number of interesting metal-assisted transformations,[6–8] and
our group has developed a series of transformations of these
exo-methylene three-membered carbocycles.[9] Herein, we
report a new method of construction of polycyclic indoles
by a rhodium(I)-catalyzed cycloisomerization reaction from
alkylidenecyclopropanes toward tetrahydro-b-carboline de-
rivatives (Scheme 1b).
Initial studies by using indole–alkylidenecyclopropane 1a
(0.1 mmol) as the substrate in the presence of [RhClACHTUNGTRENNUNG(PPh3)3]
were aimed at determining the
reaction outcome and subse-
quently optimizing the reaction
conditions. To our delight, we
found that an interesting tetra-
hydro-b-carboline derivative,
2a, was formed by using the
rhodium catalyst. Further ex-
amination of the reaction with
[Rh
ACHTUGTNREN(UNG cod)ACHUTNTGREN(NUGN IPr)Cl] (cod=1,5-cy-
Figure 1. Selected naturally occurring compounds containing tetrahydro-b-carboline.
clooctadiene, IPr=1,3-bis(2,6-
diisopropylphenyl)imidazole-2-ylidene) revealed that the
conjugated diene 3a could be obtained in 76% yield
(Scheme 2). After the catalyst and ligand screening and the
investigation of solvent effects, reaction temperature, and
concentration on the reaction outcome, we determined that
the optimal reaction conditions involve carrying out the re-
[a] Dr. D.-H. Zhang, Prof. X.-Y. Tang, Prof. Y. Wei, Prof. Dr. M. Shi
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai 200032 (P.R. China)
Fax: (+86)21-64166128
E-mail: siocxiangying@sioc.ac.cn
action in toluene (0.0125m) at 1108C by using [RhClACHTUNGTRENNUNG(PPh3)3]
weiyin@sioc.ac.cn
Mshi@mail.sioc.ac.cn
(5 mol%)/PPh3 (15 mol%) as the catalyst (Scheme 2, see
Table SI-2 in the Supporting Information for details). The
structure of compound 2a was confirmed by use of NMR
spectroscopic data and X-ray diffraction analysis (see the
Supporting Information).[10]
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201302331. It contains experi-
mental procedures and spectral data for all new compounds, as well
as X-ray crystal structures and data for 2a and 5b.
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Chem. Eur. J. 2013, 19, 13668 – 13673

COMMUNICATION
Table 1. Substrate scope of the rhodium(I)-catalyzed cycloisomerization
reaction.[a]
Entry Substrate 1
Product 2, t [h]
Yield
[%][b]
Scheme 2. Optimization of reaction conditions for rhodium(I)-catalyzed
cyclization.
1
2
3
4
5
6
7
8
9
1b, R1 =5-MeO
2b, 8
2c, 7
2d, 9
2e, 9
2 f, 7
2g, 7
2h, 8
2i, 7
2j, 7
95
82
76
75
80
52
56
91
88
1c, R1 =5-Me
1d, R1 =5-Br
1e, R1 =5-Cl
1 f, R1 =6-Me
1g, R1 =6-F
We next examined the generality of the reaction with re-
spect to the substrate under the optimized conditions and
the results are shown in Table 1. A variety of indole–alkyli-
denecyclopropanes 1b–1j, having either electron-donating
or -withdrawing groups as substituents at the 4-, 5-, 6-, or 7-
position of the benzene ring of the indole, underwent the re-
actions smoothly, to give the corresponding products 2b–2j
in 52–95% yields (Table 1, entries 1–9). In the case of other
N-sulfonated amines (X=Ns or Ms), the cycloisomerized
compounds 2k and 2l were obtained in 85 and 73% yields,
respectively (Table 1, entries 10 and 11). Examination of the
reaction with substrate 1m (R2 =Me) revealed that the de-
sired product 2m could be formed in 73% yield with good
diastereoselectivity (4.3:1; Table 1, entry 12). As for longer-
chain substrate 1n, the seven-membered heterocyclic com-
pound 2n was obtained in 68% yield (Table 1, entry 13).
Only in the case of carbon-tethered alkylidenecyclopropane
(1o), the corresponding conjugated diene 3o was formed in
51% yield rather than the cyclized product (Table 1,
entry 14). As for substrate 1p, only 10% of the desired
product 2p was obtained and 64% of 1p was recovered
(Table 1, entry 15). The product structures of 2b–2p were
determined by use of NMR spectroscopic data, MS, and
HRMS (see the Supporting Information).
1h, R1 =4-Me
1i, R1 =7-Me
1j, R1 =7-BnO
10
11
1k, X=Ns
1l, X=Ms
2k, 12
2l, 11
85
73
12
13
1m
1n
2m, 13
2n, 24
73 (4.3:1)[c]
68
14
15
1o
1p
3o, 24
2p, 24
51
Further transformations of product 2a are shown in
Scheme 3. The 1,7-diene 4a could be obtained in 84% yield
by treatment with sodium hydride and allyl bromide, and
subsequently gave the polycyclic indole 5a in 67% yield in
the presence of Grubbs first-generation catalyst (10 mol%).
Meanwhile, we synthesized allene 4b in excellent yield. By
10 (64)[d]
[a] All reactions were carried out by using 1 (0.1 mmol) in the presence
of [RhCl(PPh3)3] (5 mol%)/PPh3 (15 mol%) in toluene (8.0 mL) at 110–
1208C. The reactant concentration was 0.0125m (Ts=4-toluenesulfonyl,
Ns=4-nitrobenzenesulfonyl, Ms=methylsulfonyl, Bn=benzyl). [b] Yield
of the isolated product. [c] The d.r. value was determined by 1H NMR
spectroscopy. [d] The amount of 1p recovered is given in parentheses.
AHCTUNGTRENNUNG
using [AuACHTUNGTRENNUNG(tBuXPhos)ACHTUNRTGEGNUN(N NCMe)]ACTHUNGTREN[NUGN SbF6] (XPhos=2-dicyclohex-
ylphosphino-2’,4’,6’-triisopropylbiphenyl) as the catalyst, tet-
racyclic compound 5b could be formed smoothly in 79%
yield. The structure of compound 5b was confirmed by use
of NMR spectroscopic data and X-ray diffraction analysis
(see the Supporting Information).[11]
porting Information for details). As for substrate 1q, having
a methyl group at the C2-position of indole, the correspond-
ing conjugated diene 3q was formed in 38% yield rather
than the cyclized product.
To elucidate the cycloisomerization mechanism, deuteri-
um-labeling experiments were performed as shown in
Scheme 5. Alkylidenecyclopropane [D1]-1a, bearing a D
atom at the indole C2-position, produced cyclized product
As shown in Scheme 4, carrying out the reaction of conju-
gated diene 3a in the presence of [RhClACHTNURTGNE(UNG PPh3)3] (5 mol%)/
PPh3 (15 mol%) in toluene (0.025m) led to the desired
product 2a in 88% yield. It should be mentioned here that
trifluoroacetic acid (TFA) can also promote this reaction,
suggesting that the Rh complex may play a role as a Lewis
acid in this cyclization process (see Table SI-3 in the Surp-
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13669

X-Y. Tang, Y. Wei, M. Shi and D.-H. Zhang
ed the corresponding products
[D2]-2a and [D3]-2a in 53%
(>90% D) and 86% (>80%
D) yields, respectively.
A plausible mechanism for
this reaction is outlined in
Scheme 6 on the basis of the
aforementioned deuterium-la-
beling and control experi-
ments.[12] Initial insertion of
the metal at the distal position
of alkylidenecyclopropane 1a
gives metallacyclobutene B,
followed by isomerization to
form intermediate C through a
trimethylenemethane (TMM)-
like transition state. The inter-
Scheme 3. Further transformations of product 2a.
mediate
C
undergoes b-H
elimination to give rhodium–
hydrogen species D, which can
isomerize to produce p-allylic
RhIII complex E. Conjugated
diene 3a can be obtained from
intermediate E through reduc-
Scheme 4. Transformations of 3a and 1q in the presence of rhodium(I) catalysts.
tive elimination along with re-
generation of the RhI complex.
There are two competing path-
ways for cyclization in the
Pictet–Spengler reaction, since
both the indole C2- and C3-
positions are nucleophilic.[13]
Through indole C3-position
attack on the conjugated
diene, spiroindolenine inter-
mediate F is formed, which
can further undergo a 1,2-alkyl
shift to afford the six-mem-
bered-ring intermediate G. Al-
ternatively, nucleophilic attack
of the indole C2-position on
the diene moiety leads directly
to intermediate G, followed by
deprotonation to give 2a (see
Table SI-3 in the Supporting
Information). Furthermore, we
carried out a deuterium-label-
ing experiment by using alkyli-
denecyclopropane [D1]-3a, in
which the indole C2-position
Scheme 5. Isotopic labeling experiments.
of 3a was deuterated, as the
substrate under the standard
2a in 78% yield with 0% deuterium incorporation. As for
substrate [D2]-1a, containing two deuterium atoms at the al-
lylic position, the desired product [D1]-2a was obtained in
74% yield with >65% deuterium incorporation at its
methyl carbon atom. Carrying out the reaction of indole de-
rivatives 1a or 3a in the presence of D2O (50 equiv) afford-
conditions (Scheme 7, for details, see Table SI-1 in the Sup-
porting Information). The observed KIE (kH/kD ꢀ1.55) was
rather insignificant, indicating that the deprotonation step
may not be the rate-limiting step.
To clearly understand the mechanism of the rhodium(I)-
catalyzed reaction and subsequent cyclization step in the
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Polycyclic Indole Derivatives
COMMUNICATION
calculations were performed at
the B3LYP level with the
Gaussian 09 program.[14] The
LANL2DZ basis set and pseu-
dopotential were used for the
rhodium atom, and the 6-
31G(d) basis set was used for
all other atoms. The formation
of reactant complex A pro-
motes oxidative addition to
afford square-pyramidal rhoda-
cyclobutane B, which is
6.0 kcalmolÀ1 lower in energy
than the reactant complex and
has an associated transition
state, TS1, at 13.0 kcalmolÀ1.
Intermediate B rearranges to
afford intermediate C via tran-
sition state TS2, with an
energy barrier of 27.2 kcal
molÀ1. Passing through transi-
tion state TS3 with an energy
barrier of 14.3 kcalmolÀ1, inter-
mediate
elimination to give rhodium–
C
undergoes b-H
Scheme 6. A plausible reaction mechanism.
hydrogen species D. The rhodi-
um-hydrogen species
D can
isomerize to produce p-allylic
RhIII complex E via TS4 with
an energy barrier of 18.6 kcal
molÀ1. Passing through TS5,
the p-allylic RhIII complex E
undergoes reductive elimina-
tion to yield the product com-
Scheme 7. Deuterium-labeling experiment for the rhodium(I)-catalyzed cycloisomerization of alkylidenepro-
panes 3a and [D1]-3a.
Pictet–Spengler reaction, we have theoretically investigated
the reaction pathways, as shown in Scheme 8 and
Scheme SI-1 in the Supporting Information, respectively. All
plex H with an energy barrier of 16.1 kcalmolÀ1. We also
theoretically investigated the reaction mechanism via rho-
À
dium(I)-catalyzed allylic C H activation (for details, see
Scheme 8. DFT studies on the rhodium(I)-catalyzed reaction.
Chem. Eur. J. 2013, 19, 13668 – 13673
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13671

X-Y. Tang, Y. Wei, M. Shi and D.-H. Zhang
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Scheme SI-1 in the Supporting Information). However, the
À
reaction mechanism via rhodium(I)-catalyzed allylic C H
À
activation involves a C C bond-cleavage step with a very
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pathway shown in Scheme 8 is more reasonable. The calcu-
lation results for the subsequent cyclization step in the
Pictet–Spengler reaction suggest that the direct indole C2-
attack is favored (for details, see Scheme SI-2 in the Sup-
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In conclusion, we have developed a new rhodium(I)-cata-
lyzed cycloisomerization reaction of nitrogen-tethered in-
doles and alkylidenecyclopropanes to provide easy access to
tetrahydro-b-carboline derivatives in good yields. The reac-
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and control experiments, and is also supported by DFT cal-
culations. Further applications of this chemistry and more
detailed mechanistic investigations are underway in our lab-
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Experimental Section
General procedure for the rhodium(I)-catalyzed cycloisomerization of
indole–alkylidenecyclopropanes: Under an argon atmosphere, indole–al-
kylidenecyclopropane 1 (0.1 mmol, 1.0 equiv) was dissolved in toluene
(8.0 mL, 0.0125m) in an Schlenk tube, and [RhClACTHUNRTGNEUNG(PPh3)3] (5 mol%) and
PPh3 (15 mol%) were added. The reaction mixture was then stirred at
1108C until the reaction was complete. The solvent was removed under
reduced pressure and the residue was purified by a flash column chroma-
tography (SiO2) to give the corresponding product 2 in moderate to good
yields.
Acknowledgements
We thank the Shanghai Municipal Committee of Science and Technology
(11JC1402600), the National Basic Research Program of China (ACTHNUTRGNEU(GN 973)-
2009CB825300), and the National Natural Science Foundation of China
(20872162, 20672127, 21121062, 20732008 and 21102166) for financial
support.
Keywords: alkylidenecyclopropanes · cycloisomerization ·
indoles · polycyclic indoles · rhodium · tetrahydrocarbolines
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Polycyclic Indole Derivatives
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Received: June 19, 2013
Published online: September 17, 2013
Chem. Eur. J. 2013, 19, 13668 – 13673
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13673