Rhodium(III)-catalyzed direct C-2 olefination of unactivated indoles utilizing OH/NH2 as directing group

Article abstract of DOI:10.1002/adsc.201300930

Oxidative C-2 olefination of indoles/pyrrole via rhodium(III)-catalyzed direct C-H bond activation is reported. Phenolic OH and aniline NH2 units were revealed to be effective chelating groups to activate the aryl C-H bond. The reactions proceeded with ex

Full text of DOI:10.1002/adsc.201300930

UPDATES
DOI: 10.1002/adsc.201300930
Rhodium
ACHTUNGTRENNUNG
Indoles Utilizing OH/NH2 as Directing Group
Chen-Yu Tang,a Yuan Tao,a Xin-Yan Wu,a and Feng Shaa,*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, Peopleꢀs Republic of China
Fax : (+86)-021-6425-2758; phone: (+86)-021-6425-2011; e-mail: shaf@ecust.edu.cn
Received: October 19, 2013; Published online: February 5, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300930.
Abstract: Oxidative C-2 olefination of indoles/pyr-
À
role via rhodium
activation is reported. Phenolic OH and aniline
NH2 units were revealed to be effective chelating
G
groups to activate the aryl C H bond. The reactions
proceeded with excellent selectivity and high func-
tional group tolerance, furnishing 2-vinylindoles/2-
vinylpyrroles in good yields.
Keywords: directing groups; hydroxy group; in-
doles; olefination; rhodium
Scheme 1. Hydroxy group- or primary amino group-directed
Oxidative olefination reactions, pioneered by Fuji- olefination reactions.
wara and Moritani (Heck-type reactions), have been
recognized as atom- and step-economic synthetic
tools. Compared with traditional Heck coupling, this
On the other hand, 2-vinylindoles are important
strategy possesses significant advantages to achieve building blocks for various medical intermediates and
complex target skeletons from simple starting materi- functional materials.[10] As an effective construction
als with no preactivation of arenes.[1] For the purpose method, direct olefination on the indole ring with
of acquiring high regioselectivity and reactivity, the high selectivities has received considerable attention
chelation-assisted strategy has been widely employed and related works have been accomplished by Miura
in olefination reactions with the aid of directing and Satoh, Carretero, and others.[11,12] However, the
groups. Examples utilizing directing groups such as olefination at the C-2 position of 2,3-unsubstituted in-
carboxyl,[1e,2]
carbonyl,[3]
pyridyl,[4]
amide,[1f–I,5] doles is less well addressed since the lower nucleophi-
imine[1j,6] and ester[7] have been widely reported by licity of C-2 (compared with C-3) makes its olefina-
Yu, Miura and Satoh, Glorius, Ackermann, van Leeu- tion a challenging task.[11,12] Herein, we wish to dis-
wen, and others. Notably, the simple hydroxy or pri- close a RhACTHNUGRTENUNG(III)-catalyzed C-2 olefination of indoles
mary amino group appears to be even more desirable employing simple OH/NH2 group as the directing
due to its easy availability and promising value in the group [Scheme 1, Eq. (2)].
further transformations.[8] However, since OH and
Due to the high selectivity and functional group tol-
NH2 groups are highly sensitive and therefore need to erance provided by only low loadings of Rh com-
have protecting groups appended, reports employing
the hydroxy or primary amino group as a directing
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
group in olefination reactions are very rare and the ried out utilizing [RhCp*Cl2]2 as the catalyst in the
process remains rather challenging [Scheme 1, Eq. presence of CuACTHNUGTRNEUNG(OAc)2·H2O in DMF at 258C under
(1)].[9]
a nitrogen atmosphere. Fortunately, the desired 2-ole-
finated product (E)-ethyl 3-[1-(2-hydroxyphenyl)-1H-
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Chen-Yu Tang et al.
Table 1. Optimization of the reaction conditions.[a]
loading of [RhCp*Cl2]2 (entry 16). However, employ-
ment of an even lower amount of catalyst reduced the
yield of 3a (entries 17 and 18).
With the optimized conditions in hand, we next ex-
plored the substrate scope of this reaction with
a series of substituted N-(2-hydroxyphenyl)indoles
1 and alkenes 2. As shown in Table 2, various indoles
1 bearing substituents on C-3 to C-7 positions were
successfully allowed to react with ethyl acrylate (2a)
to give the corresponding coupling products 3a–i in
good to excellent yields (70–97%). Substituents on
the indole moiety of substrates 1 such as F, Br, NO2,
alkyl and alkoxy were smoothly introduced, which are
attractive due to their potential value to be further
transformed to other useful functional groups.[8] It
seems that the weak electron-withdrawing and elec-
tron-donating groups in the indoles had negligible ef-
fects on the olefination reaction. However, the pres-
ence of a strong electron-withdrawing substituent
such as the nitro group (6-nitro-substituted indole 1f)
decreased the reactivity of product 3f to some extent
(71%, Table 2). Bromo and fluoro substituents were
well tolerated under the optimal conditions, demon-
strating the high functional group tolerance of this Rh
catalysis. Interestingly, the presence of a methyl group
on the C-3 position of indole 1i (R1 =3-Me) did not
reduce the yield, giving product 3i in 84% yield. This
Entry
U
Solvent[d] Temperature Yield
[8C]
[%][e]
1
2
3
4
5
6
7
8
DMF
DMF
DMF
DMF
DMF
DMSO
toluene 80
dioxane 80
DCE
25
40
60
80
100
80
32
69
78
93
80
trace
74
81
51
38
82
95
80
78
95
95
90
87
9
80
80
10
11
12
13[b]
14[c]
15
16
17
18
MeCN
t-AmOH 80
DMA
DMA
DMA
DMA
DMA
DMA
DMA
80
80
80
80
80
80
80
À
experimental fact strongly indicated that the C H
rhodation occurred on the C-2 position of substrate
1 and demonstrated that the 3-methyl substituent did
not show much of an ortho-steric effect (Figure 1, B-
1). In contrast, the starting material 1h equipped with
a methyl group on the C-7 position of indole ring led
to 3h in a lower yield (70%, Table 2). The reduced
yield of 3h might be attributed to the steric hindrance
[a]
Unless otherwise specified, the reactions were conducted
utilizing 1.0 equiv. of 1a (0.3 mmol), 1.5 equiv. of 2a,
[RhCp*Cl2]2 and 2.1 equiv. of CuACHTNUGTRENUNG(OAc)2·H2O in 2.5 mL
of solvent under a nitrogen atmosphere.
The reaction was carried out under an air atmosphere.
The reaction was carried out using 30 mol% of Cu-
[b]
[c]
between the
H atom and the 7-methyl group
(Figure 1, B-2) which disturbed the planarity of the
intermediate rhodacycle B and then deteriorated the
reaction yield (Figure 1, compare B-2 with B-3).[1h]
Indole derivatives 1j (R2 =F) and 1k (R2 =Me)
bearing fluoro and methyl substituents on the phenol-
ic moiety could also react with 2a successfully, provid-
ing C-2 olefinated products 3j and 3k in 87% and
ACHTUNGTRENNUNG(OAc)2·H2O under an oxygen atmosphere.
Abbreviations:
DMSO=dimethyl sulfoxide; DCE=1,2-dichloroethane;
DMA=N,N-dimethylacetamide.
Isolated yield of 3a based on 1a.
[d]
[e]
DMF=N,N-dimethylformamide;
indol-2-yl]acrylate (3a) was obtained in 32% yield 84% yields, respectively (Table 2). Further studies re-
(Table 1, entry 1). On raising the temperature, the re- vealed that acrylates 2 from alcohols other than etha-
action gave better yields of product 3a (69–93%, en- nol could also give coupling products 3l–o in 83–96%
tries 2–5); up to 93% yield of 3a could be obtained at yields. Moreover, vinyl ketones 1-phenylprop-2-en-1-
under 808C (entry 4). Subsequent solvent examina- one (2f) and but-3-en-2-one (2g) could be smoothly
tion demonstrated that DMA was a better choice, employed and afforded the corresponding products
giving 3a in a higher yield of 95% (entries 6–12). The 3p and 3q in good yields.
further results demonstrated that the reaction could
Having the successful experience in realizing the 2-
also be conducted under an air atmosphere with 3a olefination of indoles, we further intended to explore
furnished in 80% yield (entry 13). Upon decreasing the C-2 olefination of pyrroles bearing two H atoms
the amount of CuACHTNUTRGNE(UNG OAc)2·H2O to 30 mol% under an which may undergo hydrorhodation on both positions
oxygen atmosphere, the reaction furnished 3a in a rel- ortho to the N atom. Notably, both mono- and bis-
ative lower yield (78%, entry 14). Gratifyingly, a 95% olefinated pyrroles are important skeletons widely
yield of 3a could be obtained with an only 2 mol% found in natural products and material chemistry,[11]
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RhodiumACTHUNRTGNEUNG(III)-Catalyzed Direct C-2 Olefination of Unactivated Indoles
Table 2. Hydroxy group-directed C-2 olefination of indoles.[a,b]
Product
Yield [%]
95
Product
Yield [%]
87
3a
3j
3b
3c
84
97
3k
3l
84
96
3d
3e
3f
90
87
71
3m
3n
3o
86
90
83
3g
3h
90
70
3p
3q
77
69
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Chen-Yu Tang et al.
Table 2. (Continued)
Product
Yield [%]
Product
Yield [%]
3i
84
[a]
Unless otherwise specified, the reactions were conducted utilizing 1.0 equiv. of 1 (0.3 mmol), 1.5 equiv. of 2, 2 mol% of
[RhCp*Cl2]2 and 2.1 equiv. of Cu
Isolated yield of 3 based on 1.
Np-2=2-naphthalenyl.
ACHTUNGTREN(NUNG OAc)2·H2O in 2.5 mL of DMA at 808C for 10 h under a nitrogen atmosphere.
[b]
[c]
Figure 1. The effects of methyl group in the formation of
rhodacycle intermediate B.
indicating that the preparation of mono- or bis-olefi-
nated pyrroles with selectivity is of great importance.
Thus, control experiments under different conditions
were conducted. Under the standard conditions, the
reaction of N-(2-hydroxyphenyl)pyrrole (4a) with ac-
rylate 2a gave bis-olefinated product 5a as the major
product in 56% yield, together with 29% of mono-
olefinated product 5b [Eq. (3)].
Scheme 2. Rh-catalyzed direct C-2 olefination of N-(2-hy-
droxyphenyl)pyrrole. Conditions I: 4a (1.0 equiv.), 2a
(3.0 equiv.), [RhCp*Cl2]2 (2 mol%) and CuACHTNUGTRNEUNG(OAc)2·H2O
(4.2 equiv.) in DMA at 808C for 24 h. Conditions II: 4a
(1.0 equiv.), 2a (1.0 equiv.), [RhCp*Cl2]2 (2 mol%) and
CuACTHNUTRGNE(UNG OAc)2·H2O (2.1 equiv.) in DMA at 508C for 5 h.
uct 5b in 84% isolated yield by decreasing the
amount of 2a to 1.0 equiv. and lowering the tempera-
ture to 508C [Scheme 2, Eq. (5)].
Moreover, applications of the cross-coupling prod-
ucts were explored. As shown in Eq. (6), the intramo-
lecular oxa-Michael reation of 2-vinylindole 3q was
conducted in the presence of Et3N (20 mol%) in
CH2Cl2 at room temperature. The reaction proceeded
smoothly and afforded the annulation product 1-(6H-
benzoACHTUGNTREN[NNUG 5,6]ACHTUNRTGENGN[UN 1,4]oxazinoACHTNUGERTN[NUGN 4,3-a]indol-6-yl)propan-2-one
(6a) in excellent yield (94%). It should be noted that
the l,4-benzoxazine heterocyclic system has served as
a rich source for a variety of pharmaceutical agents.[14]
As predicted, enhancement of the amount of acry-
late 2a to 3.0 equiv. afforded bis-olefinated pyrrole 5a
as the sole product in 91% isolated yield [Scheme 2,
Eq. (4)]. Interestingly, it was found possible to control
the selectivity to achieve the mono-olefination prod-
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UPDATES
RhodiumACTHUNRTGNEUNG(III)-Catalyzed Direct C-2 Olefination of Unactivated Indoles
Table 3. Amino group-directed C-2 olefination of indoles.[a]
the utilization of tert-butyl acrylate (2c), the olefinat-
ed product was isolated in a slightly lower yield
(63%, entry 6).
Surprisingly, the reaction of 2-naphthalenyl acrylate
(2e) with N-(2-aminophenyl)indole (7a) did not ter-
minate at the olefination stage to give the expected
olefinated product. Instead, the C-2 olefination prod-
uct/intermediate might subsequently undergo an in-
tramolecular aza-Michael addition to furnish the an-
nulated compound 9a directly as the single product
[68%, Scheme 3, Eq. (7)]. Similarly, phenyl acrylate
2h was also examined and furnished 5,6-
Entry
R1 (7)
R2 (2)
Yield [%][b]
dihydroindoloACTHNUTRGNEUNG[1,2-a]quinoxaline 9b in 51% yield
1
2
3
4
5
6
7
H (7a)
CO2Et (2a)
2a
2a
2a
76 (8a)
78 (8b)
75 (8c)
80 (8d)
82 (8e)
63 (8f)
79 (8g)
[Scheme 3, Eq. (8)]. The differences observed for
these aryl acrylates and alkyl acrylates (Table 3) with
respect to aza-Michael reactions are likely caused by
the differences of the stereoelectronic effects of the
acrylate intermediate/product. Notably, no direct an-
nulation product was observed when N-(2-hydroxy-
phenyl)indole 1a was reacted with 2e (Table 2), which
may lie in the difference in nucleophilicity between
the aniline NH2 group and the phenolic OH group.
On the basis of the experimental results and known
4-OBn (7b)
4-Me (7c)
5-OMe (7d)
5-F (7e)
7a
2a
CO2-t-Bu (2c)
CO2Bn (2d)
7a
[a]
The reactions were conducted utilizing 1.0 equiv. of 7
(0.3 mmol), 1.5 equiv. of 2, 5 mol% of [RhCp*Cl2]2 and
2.1 equiv. of Cu
.
(OAc)2 H2O in 2.5 mL of DMA at 808C
for 12 h under a nitrogen atmosphere.
Isolated yield of 8 based on 7.
[b]
À
Rh-catalyzed C H bond activation reactions, a plausi-
ble catalytic cycle for the reaction was proposed
(Scheme 4).[1a,d,13] Coordination of the oxygen atom of
Having successfully applied the OH group as the the phenolic moiety (from 1a) to the RhX3 species
dierecting group of the olefination, we envisioned gave RhACTHUNRTGNE(GNU III) species A. Similar with the previous
that the NH2 group may also be applied for the con- works on C-2 olefination[12] and alkylation[15] of in-
À
struction of 2-olefinated indole products of great po- doles, subsequent direct C H bond activation at the
tential value. With this notion in mind, we then exam- C-2 position generated the six-membered rhodacycle
ined the reaction of N-(2-aminophenyl)indoles 7 with intermediate B. Subsequent alkene insertion led to
alkenes 2. To our delight, the coupling products 8 the seven-membered rhodacycle C.[5c,16] Finally, b-H
could also be obtained in good yields (63–82%, elimination of intermediate C offered product 3a and
Table 3). Unexpectedly, substrates 7 equipped with the resulting RhX species was oxidized by the CuX2
electron-donating methoxy or electron-withdrawing salt to regenerate RhX3 species.
fluoro substituents on the C-5 could furnish both cor-
In summary, we have developed an RhACTHNGUTERNNU(G III)-cata-
responding products 8d and 8e in higher yields (80% lyzed direct C-2 olefination of unactivated indoles/
À
and 82%, entries 4 and 5). In addition, in the case of pyrrole via the C H activation strategy. Notably,
Scheme 3. Reactions of 7a with 2-naphthalenyl acrylate (2e) and phenyl acrylate (2h).
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UPDATES
Chen-Yu Tang et al.
3054, 2979, 1684, 1627, 1277, 1180 cmÀ1; HR-MS (EI): m/z=
307.1212, calcd. for C19H17NO3 (M+): 307.1208.
Acknowledgements
We are grateful for the financial support from National Natu-
ral Science Foundation of China (Project Nos. 21102043), the
China Postdoctoral Science Foundation (Project Nos.
20110490070, and 2012T50401) and the Fundamental Re-
search Funds for the Central Universities.
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Scheme 4. Plausible mechanism.
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Experimental Section
Typical Procedure
Under a nitrogen atmosphere, 2-(1H-indol-1-yl)phenol (1a)
(62.8 mg, 0.3 mmol), [Cp*RhCl2]2 (3.7 mg, 0.006 mmol) and
CuACHTUNGTRENNUNG(OAc)2·H2O (125.7 mg, 0.63 mmol) were added to a 25-
mL Schlenk tube equipped with a magnetic stirrer. Then
2.5 mL of DMA and ethyl acrylate (2a) (45.0 mg,
0.45 mmol) were added. The reaction mixture was stirred at
808C. When the reaction was complete as monitored by
TLC, the reaction mixture was filtered through a short
column of silica gel and eluted with CH2Cl2. The filtrate was
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(95%); mp 122.7–122.88C. H NMR (400 MHz, CDCl3): d=
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UPDATES
RhodiumACTHUNRTGNEUNG(III)-Catalyzed Direct C-2 Olefination of Unactivated Indoles
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