One-pot synthesis of 2,3-difunctionalized indoles: Via Rh(III)-catalyzed carbenoid insertion C-H activation/cyclization

Article abstract of DOI:10.1039/c7ob01977g

Reported herein is the first Rh(iii)-catalyzed carbenoid insertion C-H activation/cyclization of N-arylureas and α-diazo β-keto esters. The redox-neutral reaction has the following features: good to excellent yields, broad substrate/functional group tolerance, exclusive regioselectivity, and no need for additional oxidants or additives, which render this methodology as a more efficient and versatile alternative to the existing methods for the synthesis of 2,3-difunctionalized indoles.

Full text of DOI:10.1039/c7ob01977g

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ARTICLE
One-pot synthesis of 2,3-difunctionalized indoles via Rh(III)-
catalyzed carbenoid insertion C–H activation/cyclization
Honggui Lv, ‡ a,b,c Jingjing Shi, ‡ b Bo Wu,a Yujuan Guo,a Junjun Huang a and Wei Yi*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Reported herein is the first Rh(III)-catalyzed carbenoid insertion C–H activation/cyclization of N-
arylureas and α-diazo β-keto esters. The redox-neutral reaction features no need of additional oxidants
or additives, good to excellent yields, broad substrate/functional group tolerance, and exclusive
regioselectivity, rendering this methodology as a more efficient and versatile alternative to the existing
methods towards the synthesis of 2,3-difunctionalized indoles.
www.rsc.org/
Introduction
C–H activation/cyclization has attracted much attention. As a result,
Highly functionalized indoles are one of the most important recently remarkable advances have been made on this topic.5-6
structural motifs that ubiquitously present in numerous natural However, by reviewing these development, one common feature was
products, pharmaceuticals and bioactive compounds.1 Consequently, found that the introduction of an additional additive was absolutely
the development of general, efficient, and sustainable methods necessary for such transformations. Clearly, it is still highly
towards their construction has attracted increasing interest in modern desirable to explore and develop new, simple and efficient transition-
organic chemistry. In this context, recently popular transition-metal- metal-catalyzed redox-neutral carbenoid insertion C–H activation
catalyzed C–H activation/cyclization has emerged as a powerful, reactions for efficient construction of highly functionalized indoles
versatile and step-economical approach for the synthesis of highly without relying on additional oxidants or additives to induce the
functionalized indoles.2-4 For example, In 2008, Fagnou and catalytic turnover.
coworkers reported the first example of highly efficient
On the other hand, dimethylamino urea group (NH-CONMe2)
rhodium(III)–catalyzed C–H activation/cyclization of acetanilides has recently been widely used as versatile directing group (DG) in
with alkynes for oxidative synthesis of privileged 2,3- the field of transition-metal-catalyzed C–H functionalization7
difunctionalized indoles.3a Following this pioneering work, and up to because of its distinctive properties, such as easy to prepare and
now, Jiao,3b-c Li,3d-f Glorius, 3g-i Ackermann, 3j-l Huang,3m-o Wang,3p remove, good coordination ability, and facile derivatization.
Shi3q and others4 have independently made significant progress in However, the coupling partner of NH-CONMe2-assisted C–H
the construction of highly functionalized indoles by employing the functionalization is mostly limited to alkenes,7a-c alkynes,4h,4q,7d-f aryl
similar C–H activation strategy. Among them, nonetheless, several halides,7g arylboronic acids,7h-i amidating reagents,7j and
developed methods3b,3f,3g still suffer from one or more of the organometallic compounds.7k To the best of our knowledge, the
following disadvantages, such as relatively poor regioselectivity with combination of NH-CONMe2-assisted arene C–H activation and
unsymmetrical internal alkynes, and the compulsive requirement for intermolecular coupling with diazo compounds has remained
a stoichiometric amount of external oxidants.
underexplored, and thus, such a fascinating and useful coupling is
To address the aforementioned limitations, one emerging worth further development.
strategy to use the innovative diazo compounds that act
This work:
O
NMe2
R2
simultaneously as both cross-coupling partners and internal oxidants
for the synthesis of indole framework via transition-metal-catalyzed
H
N
O
O
[Cp*Rh(III)]
N
NMe2
R1
DCE, 50 oC, 18 h
(1)
+
R2
R3
R1
O
N2
2
H
R3
O
1
3
(α-diazo β-keto esters)
a.Key Laboratory of Molecular Clinical Pharmacology & Fifth Affiliated Hospital,
Guangzhou Medical University, Guangzhou, Guangdong 511436, P. R. China
E-mail: yiwei@gzhmu.edu.cn or yiwei.simm@simm.ac.cn
(N-arylureas)
(2,3-difunctionalized indoles)
Inspired by the above information and in continuation of our
Fax: +86-20-37104196; Tel: +86-20-37104196
b.VARI/SIMM Center, Center for Structure and Function of Drug Targets, CAS-Key
Laboratory of Receptor Research, Shanghai Institute of Materia Medica, Chinese
Academy of Sciences, Shanghai 201203, P. R. China
interest in the transition-metal-catalyzed carbenoid insertion C–H
functionalization,8 we herein report a new and mild Rh(III)-
catalyzed carbenoid insertion C–H activation/cyclization of readily
available N-arylureas and α-diazo β-keto esters for one-pot
streamlined synthesis of 2,3-difunctionalized indoles (eqn (1)). The
simple catalytic system, only releasing water and nitrogen as by-
products, and subsequently convenient deprotection of the DG part,
c. Department of chemistry, Shanghai University, 99 Shang-Da Road, Shanghai
200444, P. R. China
‡ H. Lv and J. Shi contributed equally to this work.
Electronic Supplementary Information (ESI) available: Detailed experimental procedure
and characterization of all new products (1H, 13C and 19F NMR spectra). See
DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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render this methodology as a more efficient, highly regioselective,
environmentally benign, and atom-economical protocol for the
synthesis of important highly functionalized indoles.
coupling partner (Scheme 1). To our delight, we fVoieuwndArtitchleaOt ntlihnee
DOI: 10.1039/C7OB01977G
present methodology was amenable to the presence of a variety of
functional groups, such as methyl (3b, 3c, 3f and 3n-o), chloro (3d
and 3j), ester (3e and 3l), isopropyl (3g), methoxy (3h), fluoro (3i),
bromo (3k), and ketone (3m) substituents, and delivering the desired
indole products in good to excellent yields (78-92%). The electronic
nature and the position of the substituents on the benzene ring in N-
arylurea substrate had no obvious influence on the reaction outcome,
and in the present study, N-arylureas bearing both electron-donating
and -withdrawing groups either at the ortho- (3b), meta- (3c-e), or
para- (3f-m) position showed comparable reaction efficiency with
exclusive regioselectivity. In addition, we were please to find that,
the disubstituted N-arylureas, such as 1n and 1o, were suitable
substrates, coupling with 2a to give the corresponding indole
products in reasonably good yields (3n and 3o, 82 and 89%,
respectively). Of note, the coupling reaction also tolerated the
naphtalene-1-amine substrate, which produced the interesting 3p in
83% yield. Interestingly, changing the NH-CONMe2 DG to NH-
CONHMe group also showed good compatibility with this catalytic
system since the predicted products 3q-s were smoothly obtained in
good isolated yields (80-85%). Taken together, these results further
illustrated the remarkable robustness of this coupling in the
developed Rh(III) catalysis.
Results and discussion
Table 1. Optimization of reaction conditionsa
O
H
NMe2
OMe
O
O
N
NMe2
N
catalyst
+
OMe
solvent, T, 18 h
O
N2
2a
H
3a
1a
O
Entry
1
Catalyst system (mol%)
Solution
MeOH
T(oC)
85
Yieldb(%)
30
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*RhCl2]2 (5)/AgSbF6 (20)
[Cp*CoI2]2 (5)/AgSbF6 (20)
Ph3PAuCl (5)/ AgSbF6 (20)
[Cp*IrCl2]2 (5)
2
3
THF
Toluene
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
85
85
85
85
85
85
85
85
50
r.t
45
40
80
45
68
0
4
5
6
7
8
36
0
9
[Cp*IrCl2]2 (5)/Cu(OAc)2 (20)
[Cp*Rh(MeCN)3][SbF6]2(5)
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*Rh(MeCN)3][SbF6]2 (5)
[Cp*Rh(MeCN)3][SbF6]2 (2.5)
10
11
12c
13
89
60
88
73
O
R
H
N
50
50
O
O
[Cp*Rh(MeCN)3][SbF6]2 (5 mol%)
N
R
+
R1
OMe
DCE, 50 oC, 18 h
R1
O
N2
2a
H
aReaction conditions: 1a (0.15 mmol, 1.0 equiv.), 2a (0.225 mmol, 1.5 equiv.), catalyst
(X mol%), solution (0.5 mL), 18 h, under air. bIsolated yields. cReacted for 24 h.
R = NMe2 or NHMe
OMe
1a-s
3a-s
O
O
O
O
O
NMe2
NMe2
NMe2
OMe
R1
NMe2
Given the successful history of [Cp*Rh(MeCN)3](SbF6)2 in the
field of C–H activation from us8a-c,9 and others,10 at the outset of this
study, we thus chose it as the active Rh(III) catalyst and employed
simple N,N-dimethyl-N'-phenyl urea (1a) and methyl 2-diazo-3-
oxobutanoate (2a) as the model substrate for the reaction
R1
N
N
N
N
O
R1
OMe
Me
OMe
O
OMe
O
O
O
3m: 78%
R1 = Me, 3f: 92%
R1 = i-Pr, 3g: 90%
R1 = Me, 3c: 87%
R1 = Cl, 3d: 85%
R1 = H, 3a: 89%
R1 = Me, 3b: 82%
R1 = OMe, 3h: 93%
R1 =F, 3i: 85%
R1 = COOMe, 3e: 82%
o
development (Table 1). As predicted, the reaction occurred at 80 C
R1 = Cl, 3j: 88%
R1 = Br, 3k: 90%
R1 =COOMe, 3l: 80%
O
O
NMe2
N
NMe2
NMe2
OMe
in MeOH to afford the desired product 3a in 30% isolated yield
(entry 1). The switching of solvent from MeOH to various other
reaction media revealed that DCE was optimal, which the isolated
yield of 3a could be increased to 80% (entries 1-4). Further
O
N
N
OMe
O
OMe
O
O
3p: 83%
3n: 82%
3o: 89%
O
O
O
screening
of
catalyst
system
showed
that
single
NHMe
NHMe
OMe
NHMe
N
N
N
[Cp*Rh(MeCN)3](SbF6)2 species was more beneficial than other
well-known Rh(III), Co(III) and Ir(III) species and recently
developed Au(I) catalysis11 in promoting the transformation (entries
4-9). Subsequently, the effect of the reaction temperature was
explored, and finding that the coupling at 50 oC gave most satisfying
result (entries 10-11), revealing that a balance between the
decomposition of the substrate and the formation of the product
might exist in the reaction. No distinct difference was observed
when the reaction time was extended to 24 h (entry 12). In addition,
lowering the catalyst loading to 2.5 mol% led to a significant
decrease in the product yield (entry 13). Based on these optimization
studies, we have concluded that 1a (0.15 mmol) and 2a (0.225 mmol)
Cl
F
OMe
OMe
O
O
O
3q: 85%
3r: 80%
3s: 83%
Scheme 1 Scope of the N-arylureas. Reaction conditions: 1 (0.15 mmol, 1.0
equiv.), 2a (0.225 mmol, 1.5 equiv.) and Rh catalyst (0.005 mmol, 5 mol%),
in DCE (0.5 mL) at 50 oC for 18 h under air. Isolated yields.
Encouraged by the above results, next we were interested in
probing the substrate scope with respect to α-diazo β-keto esters.
Thus, a series of α-diazo β-keto esters bearing different substituents
were evaluated with the current catalytic system. As illustrated in
Scheme 2, the reaction all proceeded successfully under the
optimized conditions to provide the desired products in good yields.
Notably, 2-diazo-N,N-dimethyl-3-oxobutanamide, dimethyl 1-diazo-
2-oxo-propylphosphonate also could served as suitable substrates as
exemplified by the synthesis of 4d and 4e, respectively. The reaction
conditions were also compatible for coupling with α-diazo β-
o
in the presence of [Cp*Rh(MeCN)3](SbF6)2 (5 mol%) at 50 C in
DCE for 18 h under air offers the best conditions for this carbenoid
insertion C–H activation reaction.
Having the optimized conditions in hand, we first examined the
substrate scope of N-arylureas by employing 2a as the model
2 | J. Name., 2012, 00, 1-3
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aldehyde esters. To the best of our knowledge, few examples of C–H
ARTICLE
To gain more insight into the reaction mechanism of this
View Article Online
DOI: 10.1039/C7OB01977G
functionalization with α-diazo β-aldehyde esters as the coupling carbenoid insertion C–H activation/cyclization process, several
partners have been reported so far. Gratifyingly, we found that the experimental investigations were performed (Scheme 4). First, 1a
coupling of 1a with methyl α-diazo β-aldehyde ester 2j furnished the was treated with [Cp*Rh(MeCN)3](SbF6)2 in DCE/D2O in the
desired product 4i with an impressive yield of 83%.
absence of α-diazo β-keto ester 2 to check the reversibility of the
C−H activation step (Scheme 4a). After stirring at 50 oC for 18 h, 90%
of 1a was recovered and no obvious deuterium incorporation was
detected by ESI-HRMS analysis (<10% H/D exchange), revealing
that the C–H bond activation step might be irreversible. Next, the
isotope-labeling experiment was carried out to further explore the C–
H activation process (Scheme 4b). Treatment of 2a with an
equimolar amount of 1a and [D5]-1a for 1 h under low conversion
gave a relatively big value of kH/kD = 4 : 1, indicating that the C–H
bond cleavage probably involved in the turnover-limiting step.
Finally, the intermolecular competition experiment between
differently substituted N-arylureas was conducted to delineate the
electronic preference of the reaction (Scheme 4c). The ratio of
products (3h/3l = 2.12 : 1) showed that the electron-rich substrates
are more active for the reaction to generate the corresponding indole
product, suggesting that the C–H activation process might follow an
internal electrophilic substitution (IES)-type mechanism.12
On the basis of these experimental observations and literature
precedents,6-7,13 a plausible catalytic cycle is proposed in Scheme 5.
First, [Cp*Rh(MeCN)3](SbF6)2 is coordinated with 1a through an
irreversible C-H activation and subsequent cyclometallation occurs
to form six-membered rhodacycle intermediate A by IES-type
mechanism. The coordination of diazo compound 2a to intermediate
A generates the rhodium-carbene intermediate B with the emission
of nitrogen. Subsequently, the migratory insertion of the carbene into
the Rh-C bond give seven-membered rhodacyclic intermediate C,
which is protonated to deliver intermediate D and release the Rh(III)
catalyst simultaneously for the next catalytic cycle. Finally,
O
NMe2
R1
H
O
O
[Cp*Rh(MeCN)3][SbF6]2 (5 mol%)
DCE, 50 oC, 18 h
N
N
NMe2
+
R1
R2
O
N2
2b-j
H
R2
4a-i
1a
O
O
O
O
O
NMe2
NMe2
NMe2
NMe2
N
N
N
N
OMe
OMe
NMe2
P
O
R1
O
O
O
O
4d: 80%
4c: 85%
R1 = OEt, 4a: 88%
R1 = OBn, 4b: 86%
4e: 87%
O
O
N
O
O
NMe2
NMe2
NMe2
Ph
NMe2
N
N
N
H
O
OMe
OEt
OMe
OMe
O
O
O
O
4i: 83%
4f: 87%
4g: 71%
4h: 89%
Scheme 2 Scope of the α-diazo β-keto esters. Reaction conditions: 1a (0.15
mmol, 1.0 equiv.), 2 (0.225 mmol, 1.5 equiv.) and Rh catalyst (0.005 mmol,
5 mol%), in DCE (0.5 mL) at 50 oC for 18 h under air. Isolated yields.
Due to the prevalence in nature and the broad utility in
pharmaceutical industry and synthetic chemistry demonstrated by
the free-NH indole skeleton, we attempted to remove the carbamoyl
group. As shown in Scheme 3, the deprotection was easily achieved
by heating 3a with Bu4N+F- in THF for 10 h to provide free-NH
indole 5 in 91% yield. Additionally, treatment of 4i with anhydrous
o
NaOH in EtOH at 80 C for 24 h and subsequent neutralization by
1M of HCl afforded the free-NH indole 6 in 85% yield.
O
tautomerization of intermediate D delivers in situ an enol
NMe2
OMe
H
intermediate E,6a,6d,6h,14 which undergoes the elimination of water by
intramolecular condensation to provide the final product 3a.
Bu4N+F-
N
N
THF, reflux, 10 h
OMe
O
O
3a
5: 91%
O
NMe2
[Cp*Rh(III)]
O
H
N
NMe2
protonolysis
C-H activation
NH
NaOH,EtOH, 80 oC, 24 h
and then HCl (1M)
O
H
O
N
Me2N
N
H
OH
D
1a
O
O
OMe
OMe
O
H
6: 85%
4i
N
NMe2
C
H
N
tautomerization
NMe2
Scheme 3 Removal of the carbamoyl group.
O
MeO
O
O
NMe2
Rh
Rh
Cp*
A
Cp*
O
H/D
N
H
N
a)
O
NH
O
O
OH
NMe2
NMe2 [Cp*Rh(MeCN)3][SbF6]2 (5 mol%)
OMe
O
2a
O
DCE (0.5 mL), D2O (0.1 mL)
50 oC, 18 h
N2
H/D
E
H
1a
O
OMe
90% recovery, <10% D
H
coordination
migratory insertion
N
NMe2
cyclization
H2O
b)
O
O
N2
O
O
Rh
OMe
O
H
N
3a
H
N
O
NMe2
B
Cp*
D5
N2
2a, 0.225 mol
NMe2
+
NMe2
N
H4 / D4
O
O
O
H
standard conditions, 1 h
Scheme 5 proposed catalytic cycle.
OMe
[D5]-1a, 0.15 mol
KIE = 4:1
O
1a, 0.15 mol
MeO
c)
O
O
Conclusions
MeOOC
O
O
NMe2
N2
In summary, we have developed the first Rh(III)-catalyzed
carbenoid insertion C–H activation/cyclization of diverse N-
arylureas with α-diazo β-keto esters for one-pot cascade synthesis of
important 2,3-difunctionalized indoles. This transformation could
proceed smoothly in a very simple catalytic system under air with
N
2a, 0.225 mol
+
R
standard conditions, 1 h
N
H
H
O
N H
O
OMe
O
NMe2
NMe2
-OMe/-CO2Me = 2.12 : 1
1h, 0.15 mol
1l, 0.15 mol
Scheme 4 Mechanistic experiments.
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5
6
For selected reviews, see: (a) Q. Xiao, Y. Zhang and J. Wang, Acc. Chem. Res.,
View Article Online
the remarkable features of good to excellent yields, no need of any
additional oxidants or additives, excellent substrate/functional group
tolerance, and exclusive regioselectivity. Through a series of
mechanistic studies, a plausible reaction pathway was rationally
deducted. Moreover, efficient removal of the carbamoyl moiety to
build the valuable free-NH indoles has been successfully illustrated
by subsequently convenient deprotection. Taken together, we believe
that the present protocol could serve as benign alternative to the
existing methods for building the highly functionalized indoles and
also should have the potential for broad synthetic utility. Further
investigation on the scope, mechanism, and application of this
versatile reaction are currently underway in our laboratories.
2013, 46, 236; (b) Y. Xia, Y. Zhang and J. Wang, ACS Catal., 2013, 3, 2586; (c)
DOI: 10.1039/C7OB01977G
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank NSFC (81502909) for financial support of this study.
Notes and references
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