Rhodium(III)-catalyzed oxidative bicyclization of 4-arylbut-3-yn-1-amines with internal alkynes through C-H functionalization

Article abstract of DOI:10.1039/c5cc05093f

A new Rh(III)-catalyzed oxidative bicyclization through C-H functionalization is presented. This reaction allows the selective assembly of diverse benzo[g]indoles from 4-arylbut-3-yn-1-amines and internal alkynes via a sequence of aromatic C(sp²)-H functionalization, cyclodimerization and nucleophilic cyclization.

Full text of DOI:10.1039/c5cc05093f

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Rhodium(III)-Catalyzed Oxidative Bicyclization of 4-Arylbut-3-yn-
1-amines with Internal Alkynes Through C-H Functionalization
Rui Pi, a Ming-Bo Zhou, a Yuan Yang, a Cai Gao, a Ren-Jie Song a and Jin-Heng Li*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
new Rh(III)-catalyzed oxidative bicyclization through C-H
functionalization is presented. This reaction allows the selective
assembly of diverse benzo[g]indoles from 4-arylbut-3-yn-1-amines
and internal alkynes via
a
sequence of aromatic C(sp2)-H
functionalization, cyclodimerization and nucleophilic cyclization.
Ring-fused naphthalenes,1 including benzo[g]indoles (Scheme 1),2
are an important class of polycyclic hydrocarbons in organic
synthesis, chemical biology, pharmaceutical discovery, and
materials science. As a result, much attention has been attracted to
the development of new efficient methods to build ring-fused
naphthalenes.1-6 Traditional approaches for such compound
synthesis are derived from the preexisting naphthalene skeletons
via several steps.1-3 In recent years, transition-metal-catalyzed
tandem annulation reactions and especially tandem annulation
strategy between aromatic compounds and alkynes through aryl
C(sp2)-H functionalization have emerged as an efficient, convergent
method to assemble naphthalenes4-7 and ring-fused naphthalenes.8
However, the majority of these transformations are limited by the
use of alkynes only as the 2-carbon synthons,5,6,8 and approaches to
ring-fused naphthalenes are quite rare.8 In 1998, Kisch and co-
workers developed a novel HCl-facilitated rhodium-catalyzed aryl
C(sp2)-H functionalization and [4+2] cyclodimerization of arylalkynes
for building naphthalene skeletons, in which one arylalkyne
molecule was used as the 4-carbon synthon and the other
arylalkyne molecule as the 2-carbon synthon.7a Miura and co-
Scheme
1
Selected examples of important benzo[g]indole
compounds.
cross-dimerization to predominate over the conceivable homo-
dimerization.7b During the cyclodimerization process (Pathway I,
Scheme 2),7 the rhodium hydride species first formed from the Rh(I)
species and a hydrogen cation (H+) would subsequently undergo the
insertion of a C-C triple bond in an arylalkyne to the Rh-H bond and
geometrical isomerization via a zwitterion to give intermediate A.
ortho-Metalation of intermediate A with the liberation of HX
produces rhodacycle B, followed by selective insertion of another
alkyne molecule to the rhodium-aryl or -alkenyl bond and reductive
elimination afford naphthalenes.
[RhI]
[Rh]
+ H+
H
H
H
R
H
R
R
R
+
[Rh]
Rh
refs 4
Pathway I
H
H
[RhIII
]
A
B
Pathway II
workers have reported
a new rhodium/phosphine/amine·HBr
R
catalyst system for the highly chemoselective synthesis of
multisubstituted naphthalenes by aryl C(sp2)-H functionalization
and [4+2] cyclodimerization of two different internal alkynes; the
catalytic conditions tolerated various internal alkynes and made the
TsN
RhIII
R
[O]
R
RhIII
RhIII
[RhIII
]
R = CH2CH2NHTs
TsN
D
2
E
C
Our work
TsHN
R3
R4
TsN
[{Cp*RhCl2}2]
AgOTf, O2
[{Cp*RhCl2}2]
AgOTf, O2
aState Key Laboratory of Chemo/Biosensing and Chemometrics, College of
Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China.
Fax: 0086731 8871 3642; Tel: 0086731 8882 2286; E-mail: jhli@hnu.edu.cn
Prof. Dr. J.-H. Li
R2
CF3CO2H, MeOH
120 oC, 22 h
CF3CO2H, MeOH
120 oC, 22 h
R4
R2
NHTs
R3
R2
H
R2
3
1
bState Key Laboratory of Applied Organic Chemistry Lanzhou University, Lanzhou
730000, China
4
Scheme 2 Rhodium-catalyzed annulation of arylalkynes.
† Electronic Supplementary Informaꢀon (ESI) available: [details of any
supplementary information available should be included here]. See
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In contrast, Rh(III)-catalyzed oxidative aromatic C(sp2)-H promoted (entry 4). Notably, the RhIII catalyst and AgOTf play a
functionalization and annulation reactions with alkynes are initiated crucial role in the reaction, as omittance of any one of these species
via the direct insertion of Rh(III) species to the aromatic C(sp2)-H leads to no detectable products 3aa (entries 5 and 6). Other Ag salts,
bond leading to intermediate C (Pathway II, Scheme 2).5,6,8 On this namely AgSbF6, AgOAc and AgCO2CF3, were less efficient than
basis, we speculated that a RhIII oxidative catalysis might trigger AgOTf (entries 7-9). Use of Cu(OTf)2 instead of AgOTf showed
novel C-H functionalization and cyclodimerization reactions of activity for the reaction, albeit giving a lower yield (entry 10).
arylalkynes through different quenching from the RhI catalysis to However, Sc(OTf)3 was ineffective (entry 11). These results support
provide intermediate E, which would react with a nucleophile to that Ag salts and Cu salts act as a promoter to activate the RhIII
afford ring-fused naphthalene skeletons. Herein, we report the first species, not as a Lewis acid. Screening on the effect of acids
Rh(III)-catalyzed oxidative bicyclization of 4-arylbut-3-yn-1-amines confirmed that the role of CF3CO2H is to improve the reaction
with internal alkynes through C-H functionalization; this reaction (entries 1, 12 and 13 and Table S1 in the Supporting Information).
proceeds by a sequence of aromatic C(sp2)-H functionalization, The yield of 3aa decreased dramatically when N2 or Cu(OAc)2 was
cyclodimerization and nucleophilic cyclization and represents an used to replace O2 (entries 14 and 15).
practical method to access benzo[g]indoles (Scheme 2).
R1
Table 1 Screening of the optimal reaction conditionsa
N
R3
NHR1
Conditionsa
+
R4
R4
H
1
R2
R2
R3
2
3
TsN
Ph
TsN
TsN
R2 = Me, 3ba, 52%
R2 = Br, 3ca, 58%
R2 = Cl, 3da, 60%
R2 = NO2, 3ea, 40%
Ph
Br
+
Isolated yield [%]
Ph
R2
Ph
Entry Variation from the standard conditions
Br
Ph
R1
N
3aa
66
65
28
21
0
4a
8
Ph
3fa/3fa', 55% (>20:1)
Ph
1b
none
TsN
TsN
TsN
at 130 oC
at 100 oC
10
2
trace
15
Ph
3
4c
Ph
Ph
Ph
[{Cp*RhCl2}2] (10 mol%)
Ph
3ga, 55%
R1 = Ac, 3ia, 48%
R1 = Bn, 3ja, trace
R1 = H, 3ka, trace
Ph
3ha, 48%
Without [{Cp*RhCl2}2]
5
0
TsN
without AgOTf
AgSbF6 instead of AgOTf
AgOAc instead of AgOTf
AgCO2CF3 instead of AgOTf
Cu(OTf)2 instead of AgOTf
Sc(OTf)3 instead of AgOTf
without CF3CO2H
0
0
6
7
TsN
3ab, 50%
35
19
18
40
trace
46
10
27
21
5
TsN
TsN
trace
trace
10
8
9
R
R = H, 3ad, 63% (>20:1)
R = Me, 3ae, 68% (1.5:1)
R = Cl, 3af, 71% (1:1)
10
11
12
13
14
15
trace
9
R = Ac, 3ag, 47% (2.7:1)
3ac, 52% (1.5:1)
3ah
, 73% (2.2:1)
3ai
, 68%
TfOH instead of CF3CO2H
N2 (1 atm) instead of O2
Cu(OAc)2 (1 equiv) instead of O2
trace
trace
trace
Scheme 3 Bicyclization of 4-arylbut-3-yn-1-amines (1) with internal
alkynes (2). Reaction conditions: 1 (0.2 mmol), 2 (1.5 equiv),
a
[{Cp*RhCl2}2] (5 mol%), AgOTf (4 mol%), CF3CO2H (1 equiv), O2 (1
atm), MeOH (anhydrous, 2 mL), 120 oC, 22 h. The regioselective
ratio based on unsymmetrical alkynes 2 as the 2-carbon synthons is
given in parenthesis. Two main side-products 4 and 5 were
observed.
a
Reaction conditions: 1a (0.2 mmol), 2a (1.5 equiv), [{Cp*RhCl2}2] (5
mol%), AgOTf (4 mol%), CF3CO2bH (1 equiv), O2 (1 atm), MeOH
(anhydrous, 2 mL), 120 oC, 22 h. Other side-products, including 4-
methyl-N-(4-oxo-4-phenylbutyl)benzenesulfonamide (5a; 15%) from
hydration of alkyne 1a, were observed. c Side-product 5a in 36% yield.
The scope of this cross-bicyclization reaction with regard to 4-
arylbut-3-yn-1-amines reacting with different internal alkynes was
first investigated by using the optimal reaction conditions (Scheme
3). The substituents, namely Me, Br, Cl and NO2 groups, on the aryl
ring in 4-arylbut-3-yn-1-amines 1b-h were well-tolerated. For
example, treatment of 4-Br- or 4-Cl-susbtitutted substrates 1c or 1d
with 1,2-diphenylethyne (2a), [{Cp*RhCl2}2], AgOTf, CF3CO2H and O2
afforded the desired benzo[g]indoles 3ca-da in moderate yields,
which may provide opportunities for further additional
modifications of the product. Notably, 3-Br-susbtituted substrate 1f
led to a mixture of regioselective bicyclization products 3af/3af’.
Gratifyingly, substrate 1h with a phenyl group on the 1 position was
also compatible with the optimal conditions and gave 3ha in 48%
yield. Substrate 1i with a Ac group instead of the Ts group also
We started our optimization investigation with the bicyclization
reaction
between
4-methyl-N-(4-phenylbut-3-yn-1-
yl)benzenesulfonamide (1a) and 1,2-diphenylethyne (2a) (Table 1).
When a combination of [{Cp*RhCl2}2] (5 mol%) with AgOTf (4 mol%),
CF3CO2H (1 equiv) and O2 (1 atm) in the medium MeOH at 120 oC
for 22 h was employed, the cross-bicyclization product 3aa9 was
furnished in the highest yield (66%) with two side-products, the
homo-bicyclization product 4a and hydration product 5a, from
substrate 1a in 8% and 15% yields, respectively (entry 1). While a
higher reaction temperature gave the same results with those at
120 oC (entry 2), a lower reaction temperature had a negative effect
(entry 3). However, the yield of 3aa decreased sharply when using
10 mol% of [{Cp*RhCl2}2] because the side-reactions were
2 | J. Name., 2012, 00, 1-3
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TsN
afforded the expected product 3ia in moderate yield. However,
substrates 1j and 1k with a Bn group or two free hydrogen atoms
on the nitrogen atom were not viable for the bicyclization reaction
and lea to no formation of 3ja and 3ka.
NHTs
Ph
[{Cp*RhCl2}2] (5 mol%)
AgOTf (4 mol%)
O
(1)
+
CF3CO2H (1 equiv), O2
MeOH, 120 oC, 22 h
Ph
Ph
3aa, 0%
Ph
H
N
2a
5a
TsN
The optimal conditions were found to be applicable to various
internal alkynes 2b-i (3ab-ai). Using symmetrical internal alkynes,
1,2-di-p-tolylethyne (2b) and hex-3-yne (2i), to react with substrate
1a, [{Cp*RhCl2}2], AgOTf, CF3CO2H and O2 successfully furnish 3ab
and 3ai in high yields. For unsymmetrical internal alkynes 2c-h, a
mixture of regioselective products were observed. For example, (4-
methylphenyl)phenylethyne (2c) was converted into 3ac with >20:1
regioselectivity.9 Aryl-substituted prop-1-ynes 2d-g were suitable
substrates and the substitution effect of the aryl group had a
fundamental influence on the regioselectivity (3ad-ag). Gratifyingly,
hex-2-yne (2h), an aliphatic internal alkyne, also led to 3ah with
73% yield and 2.2:1 regioselectivity.
Ts
Ph
[{Cp*RhCl2}2] (5 mol%)
AgOTf (4 mol%)
(2)
+
CF3CO2H (1 equiv), O2
MeOH, 120 oC, 22 h
Ph
Ph
Ph
H
6a
2a
3aa
, 0%
Intermolecular KIE Experiment
Intramolecular KIE Experiment
H
NHTs
H
NHTs
D
D
NHTs
H
H
H
H
D
D
+
D
H
D
H
H
1a
21% yield (1 h), kH/kD = 3.0
1a-D1
65% yield (22 h), kH/kD = 2.3
1a-D5
Scheme 4 Control experiments.
In conclusion, we have developed a Rh(III)-catalyzed oxidative
bicyclization reaction of 4-arylbut-3-yn-1-amines with internal
alkynes via C(sp2)-H functionalization, cyclodimerization and
nucleophilic cyclization cascades, which enables a variety of ring-
fused naphthalenes with excellent functional-group tolerance. In
contrast to the catalytic cycle of the RhI catalysis, this RhIII catalysis
is initiated by C-H functionalization and quenched through
nucleophilic cyclization, which may be useful for the construction of
diverse polycyclic structures in organic synthesis and medicinal
chemistry. Further studies on the mechanism and applications of
this bicyclization strategy are currently under way in our laboratory.
As shown in Table 2, the homo- bicyclization of 4-methyl-N-(4-
arylbut-3-yn-1-yl)benzenesulfonamides 1a-g were examined. 4-
Phenylalkyne 1a was treated with [{Cp*RhCl2}2], AgOTf, CF3CO2H
and O2 smoothly, providing the desired homo-bicyclization product
4a in 65% yield with 25% yield of the hydration product 5a (entry 1).
Alkynes 1b-d and 1f-g bearing a 4-MeC6H4, a 4-BrC6H4, a 4-ClC6H4, a
3-BrC6H4 or a 3,5-diMeC6H4 group at the 4 position, successfully
delivered 4b-d and 4f-g in moderate yields (entries 2-6).9 Notably,
4-arylalkyne 1f with a Br group at the meta position gave a mixture
of regioselective isomers 4f/4f’ based on alkyne 1f (entry 5).
a
Table 2 Homo-bicyclization of 4-arylalkynes(1)
Notes and references
‡ We thank the Natural Science Foundation of China (Nos.
21472039 and 21172060), Hunan Provincial Natural Science
Foundation of China (No. 13JJ2018) and Specialized Research
Fund for the Doctoral Program of Higher Education (No.
20120161110041) for financial support.
1
(a) G. Collin, H. Höke and H. Greim, Naphthalene and
Hydronaphthalenes in Ullmann's Encyclopedia of Industrial
Chemistry, Wiley-VCH, Weinheim, 2003; (b) M. Malacria,
Chem. Rev., 1996, 96, 289; (c) M. Randić, Chem. Rev., 2003,
Isolated Yield [%]
Entry
R2(1)
H, 1a
4
5
4a, 65 (>20:1)
5a, 25
5b, 26
5c, 38
5d, 32
5f, 26
5g, 33
1
2
103
,
3449; (d) R. G. Taylor, Polycyclic Aromatic
4b, 67 (>20:1)
4c, 57 (>20:1)
4-Me, 1b
4-Br, 1c
Hydrocarbons: Chemistry and Carcinogenicity, Cambridge
University Press, Cambridge, UK, 1991; (e) H. Knölker and K.
R. Reddy, Chem. Rev., 2002, 102, 4303; (f) J. Jacob, Polycyclic
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E. Anthony, Angew. Chem. Int. Ed., 2008, 47, 452; (i) R. G.
Harvey, Polycyclic Aromatic Hydrocarbons, Wiley-VCH, New
3
4-Cl, 1d
4d, 59 (>20:1)
4
5b
3-Br, 1f
4f (8-Br)/4f’ (6-Br), 62
4g, 60 (>20:1)
3,5-diMe, 1g
6
a
For reaction conditions, see Table
1
and Scheme 3. The
York, NY, 1997; (j) R. A. Pascal, Jr., Chem. Rev., 2006, 106
4809.
,
regioselective ratio based on unsymmetrical alkynes as the 2-carbon
synthons is given in parenthesis. b The regioselectivity ratio of 4f/4f’ is
2.5:1 based on alkyne 1f as the 4-carbon synthon.
2
For selected papers, see: (a) E.-M. Karg, S. Luderer, C.
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To understand the mechanism for this bicyclization reaction
(Pathway II, Scheme 2), some control experiments were performed
(Scheme 4). Substrates 5a and 6a could not be converted into the
expected product 3aa, suggesting that they were not intermediates
for this bicyclization reaction [Eq (1) and Eq (2)].10 The
intermolecular (kH/kD = 3.0) and intramolecular (kH/kD = 2.3) kinetic
isotope effect experiments support that the C(sp2)-H
functionalization is the rate-limiting step.5
This journal is © The Royal Society of Chemistry 20xx
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For selected papers on the synthesis of benzo[g]indoles, see:
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4
For selected papers for tandem annulation not via C-H
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CCDC 1063383 (3aa), CCDC 1063384 (3ad) and CCDC
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4 | J. Name., 2012, 00, 1-3
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