Mild Cobalt(III)-Catalyzed C–H Hydroarylation of Conjugated C=C/C=O Bonds

Article abstract of DOI:10.1002/adsc.201700097

An efficient cobalt(III)-catalyzed C-2-selective indole C–H hydroarylation of acrolein, enones, and glyoxylates was achieved under mild reaction conditions. The versatile cobalt(III) catalyst displayed excellent positional selectivity with the assistance

Full text of DOI:10.1002/adsc.201700097

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DOI: 10.1002/adsc.201700097
Mild Cobalt(III)-Catalyzed C–H Hydroarylation of Conjugated
C=C/C=O Bonds
Jie Li,a,* Zhao Zhang,+a Wenbo Ma,+b Mengyao Tang,a Dawei Wang,c
and Liang-Hua Zoua
a
School of Pharmaceutical Sciences, Jiangnan University. Lihu Road 1800, 214122 Wuxi, Jiangsu, Peopleꢀs Republic of
China
Fax : (+86)-510-8532-9042; phone: (+86)-510-8591-2653; e-mail: jjackli@jiangnan.edu.cn
Antibiotics Research and Re-evaluation Key Laboratory of Sichuan Province, Sichuan Industrial Institute of Antibiotics,
b
Chengdu University, 610052 Chengdu, Sichuan, Peopleꢀs Republic of China
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material
c
Engineering, Jiangnan University, Lihu Road 1800, 214122 Wuxi, Jiangsu, Peopleꢀs Republic of China
+
Z.Z. and W.M. contributed equally to this work.
Received: January 24, 2017; Revised: March 7, 2017; Published online: && &&, 0000
Supporting information for this article is available under http://dx.doi.org/10.1002/adsc.201700097.
Abstract: An efficient cobalt(III)-catalyzed C-2-se- approach provides an expedient route to indolyl-sub-
lective indole C–H hydroarylation of acrolein, stituted aldehydes, ketones, and esters with wide
enones, and glyoxylates was achieved under mild re- functional group tolerance. In addition, it also fea-
action conditions. The versatile cobalt(III) catalyst tures high step- and atom-economy.
displayed excellent positional selectivity with the as-
sistance of pyrimidinyl, pyridyl, and pyrazolyl direct-
ing groups, thus overcoming the inherent C-3 selec- Keywords: C–H functionalization; cobalt(III) cataly-
tivity of an electrophilic indole derivatization. This sis; hydroarylation; indoles; positional selectivity
Introduction
first proved to be feasible by Hartwig.[6] Further ad-
vances were recently achieved by Ma,[7] Shi[8] and
Indoles are among the most pharmaceutically useful Chang[9] with the introduction of appropriate directing
heterocycles, as found in diverse biologically active groups. Various functionalizations on the ortho- and
compounds. Therefore, the synthesis of indole deriva- meta-positions around the N-atom of the indole core
tives has attracted considerable recent attention.[1] were realized through C–H activation. Despite these
Generally, the C-3 position of indole is the favored major advances, transition metal-catalyzed selective
site for electrophilic substitution, while various C-3 C-2 C–H hydroarylation of electron-deficient olefins/
decorated indoles can be successfully constructed by aldehydes still remained challenging. The catalytic ad-
simple applications of the Friedel–Crafts reaction.[2] ditions of inert C H bonds to these a,b-unsaturated
À
Hence, the C-2 C–H functionalized product was deliv- acceptors have recently experienced increasing atten-
ered only when the C-3 position is unavailable, except tion.[10] While most of the achievements were restrict-
for sporadic examples.[3] Since transition metal-cata- ed to the use of expensive 4d transition metal cata-
lyzed C–H functionalization has become a powerful lysts, a significant breakthrough was achieved using
tool for organic synthesis in recent decades, site selec- high-valent cobalt complexes as the appropriate cata-
tivity is predictable by the introduction of hetero- lysts, as reported by Kanai and co-workers,[11] and fur-
ACHTUNGTRENNUNG
atom-containing directing groups.[4] In this process, ther major advances were accomplished in the recent
many research efforts have led to remarkable prog- past.[12] However, among a number of analogous
ress for transition metal-catalyzed C-2 selective C–H Cp*Co(III) complexes, only [Cp*Co(benzene)](PF6)2
functionalization on indoles with the assistance of N- displayed high efficacy for the addition of 2-arylpyri-
directing groups.[5] Remarkably, regioselective C–H dines onto multiple bonds in a,b-unsaturated enones
functionalization at the C-7 position of indoles was and imines (Scheme 1a).[11a] Very recently, a remark-
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able two-step sequence consisting of cobalt(III)-cata-
lyzed C–H hydroarylation of a,b-unsaturated enones
and subsequent diastereoselective addition of the
cobalt enolate to aldehydes was developed, as was re-
ported by Ellman (Scheme 1b).[13] However, the sub-
strate scope of the reaction needs to be further ex-
tended. For example, some important polar electro-
philes, such as a,b-unsaturated aldehydes and glyoxy-
lates, could not be employed in these processes; only
rare examples of their hydroarylation have been ac-
complished by rhodium(III)[10a,14] and manganese(I)[15]
catalysis via C–H functionalization (Scheme 1c).
Based on our previous studies on direct C–H hydroar-
ylation of polar p-electrophiles by ruthenium(II)[16]
and rhodium(III)[17] catalysis, we herein report on
a mild cobalt(III)-catalyzed C-2 selective C–H alkyla-
tion of indoles with C=C/C=O multiple bonds to de-
liver indole substituted aldehydes, ketones, and esters
in an atom-economical mode (Scheme 1d).[18]
Results and Discussion
Our initial investigation focused on the cobalt(III)-
catalyzed C-2 selective C–H hydroarylation of acrole-
in for the synthesis of b-indolylpropanal 3a (Table 1).
We were pleased to observe that the desired product
3a was isolated in 84% yield when employing combi-
nations of 5.0 mol% of Cp*CoI2(CO), 10.0 mol% of
AgSbF6 and stoichiometric of KOPiv (Table 1,
entry 1). Reducing the loading of KOPiv resulted in
significantly reduced yield of the desired product 3a
(entry 2). In contrast, different catalyst loadings exert-
ed a minor influence (entries 3 and 4). This allowed
Scheme 1. Cobalt(III)/manganese(I)-catalyzed C–H hydroar-
ylation of polar p-electrophiles (2-py=2-pyridyl).
Table 1. Optimization of the cobalt(III)-catalyzed C–H hydroarylation of acrolein 2.[a]
Entry
[Co] (mol%)
Additive
Base
Yield [%][b]
1
2
3
4
5
6
7
8
9
Cp*CoI2(CO) (5.0)
Cp*CoI2(CO) (5.0)
Cp*CoI2(CO) (2.5)
Cp*CoI2(CO) (10.0)
Cp*CoI2(CO) (1.0)
Cp*CoI2(CO) (2.5)
–
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
–
AgSbF6
AgSbF6
AgSbF6
KOPiv
KOPiv
KOPiv
KOPiv
KOPiv
KOPiv
KOPiv
–
84
70[c]
89
91
64
81
0
Cp*CoI2(CO) (2.5)
–
0 (40)[d]
0
–
[a]
General reaction conditions: 1a (0.25 mmol), 2 (0.45 mmol), [Cp*CoI2(CO)] (2.5 mol%), AgSbF6 (5.0 mol%), KOPiv
(0.5 mmol, 2.0 equiv.), TFE (1.0 mL), under argon, 708C, 16 h.
Isolated yield of 3a.
KOPiv (0.25 mmol, 1.0 equiv.).
Isolated yield of 3a’ (TFE=trifluoroethanol; 2-pym=pyrimidin-2-yl).
[b]
[c]
[d]
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for efficient catalysis to occur at a catalyst loading as decorated propanal 3m in rather modest yield, as did
low as 1.0 mol%, delivering 3a in 64% yield, which the 1-phenyl-1H-pyrazole.
highlighted again the outstanding efficacy of
The efficient cobalt(III) catalyst was subsequently
Cp*CoI2(CO). To our surprise, the reaction occurred tested in the C-2 selective hydroarylation of a,b-unsa-
well in the absence of silver(I) sources (entry 6), turated ketones 5 (Scheme 3), aliphatic enones gave
while the opposite result was observed when the the b-indolyl ketones 6aa–6ad in moderate to good
cobalt complex was omitted in the reaction (entry 7). yields under the standard reaction conditions. Howev-
Importantly, only C-3 substituted product 3a’ was ob- er, the reactivity of aryl vinyl ketones showed a signifi-
tained when the reaction was run without the assis- cant deterioration under the optimized reaction con-
tance of carboxylate, which highlighted again the in- ditions, thus delivering the oxa-Michael adducts with
herent property of indole and the importance of car- TFE as the main products. In contrast, a rhodium(III)
boxylate assistance for the selective C–H functionali-
zation (entries 8 and 9).[19]
With the optimized reaction conditions in hand,
a variety of indoles was investigated for direct C–H
hydroarylation of acrolein 2 (Scheme 2). Different
functional groups substituted on the C-3, C-4, C-5, C-
6 or C-7 positions were well tolerated by our cobalt-
ACHTUNGTRENNUNG(III) catalysis, delivering the C-2 selective alkylated
products in high yields (3a–3l). Beyond that, the elec-
tronic nature and steric hindrance of the substituent
displayed a very minor influence on the reactivity. It
is worth noting that the N-pyridylindole 1m gave the
Scheme 2. Cobalt(III)-catalyzed C–H hydroarylation of
acrolein 2.
Scheme 3. Cobalt(III)-catalyzed C–H hydroarylation of a,b-
unsaturated ketones 5.
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catalyst furnished the corresponding hydroarylated
product 6ae with high efficacy. The aryl vinyl ketones
of 5f and 5g yielded the desired products in moderate
yields when a stoichimetric amount of BF3·OEt2 was
used as the additive. To our delight, various heteroar-
yl vinyl ketones, such as benzofuran, furan and thio-
phene, demonstrated favorable reactivities for C–H
hydroarylation. It is worth noting that the conjugated
alkene 5k was also successfully employed as the Mi-
chael acceptor, exclusively generating the E-olefin
6ak as the product. In addition, the cobalt(III) cata-
lyst was further proved to be appropriate to arenes
7a–7c. Interestingly, the dialkylated product was
mainly obtained with the assistance of pyrimidine, as
compared to pyridine. Intriguingly, pyrrole also deliv-
ered a trialkylated product 7c’, which was not expect-
ed, albeit the yield was as low as 18%.
Moreover, we next explored the cobalt(III) catalyst
for the C–H hydroarylation of C=O bonds. Very re-
cently, Ackermann and co-workers developed a man-
ganese(I)-catalyzed C–H hydroarylation of C=Het
bonds, thus highlighting its unique position selectivity
on indoles.[15] Our studies demonstrated that
2.5 mol% of Cp*CoI2(CO), along with catalytic
amounts of AgSbF6 and KOPiv enabled the efficient
C-2 selective C–H addition to glyoxylate 8 under mild
reaction conditions (Scheme 4). Various functional
groups, such as bromo, chloro, ester, and ketone sub-
stituents, were well tolerated by our cobalt(III) cata-
lyst. Notably, a more sterically hindered methyl or
propionate group at the C-3 position of indole also Scheme 4. Cobalt(III)-catalyzed C–H hydroarylation of
glyoxylate 8.
Scheme 5. Mechanistic studies.
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underwent the C–H hydroarylation process, albeit the of a concerted metalation deprotonation (CMD), mi-
product 9na was delivered in a rather modest yield, as gratory insertion of conjugated alkenes, and subse-
was also observed when employing pyrrole as the sub- quent proto-demetalation sequence to generate the
strate. However, the C–H additions with trifluoroacet- alkylated products (Scheme 7).
A
Finally, we tried to illustrate the synthetic potential
on the C-3 position of indole in low yields (see the of the protocol by removing the N-pyrimidyl substitu-
Supporting Information).
In order to unravel the mode of action of the reac-
À
tion, we performed the cobalt(III)-catalyzed C H
bond functionalization with 2a in the presence of D2O
as the co-solvent. Obvious evidence of H/D scram-
bling on both the reisolated starting material [Dn]-1a
and desired product [Dn]-6aa was observed, which il-
lustrates that the cobalt(III)-catalyzed C–H cobalta-
tion step is reversible, as was proved again when em-
ploying [D2.5]-1a as the starting material (Scheme 5a).
Furthermore, a minor kinetic isotope effects of kH/D
=
À
1.1 was found, which demonstrates that the C H
bond cobaltation is not the rate-determining step
(Scheme 5b).
Thereafter, we became interested in disclosing the
reactivity of different a,b-unsaturated carbonyl com-
pounds and glyoxylate for their direct C–H alkylation
with 1a (Scheme 6). The results of a set of intermolec-
ular competition experiments revealed that acrolein 2
and glyoxylate 8 are more reactive than aliphatic
enones, while the aryl vinyl ketones are inherently
less reactive.
Based on our experimental findings and previous
mechanistic explorations,[20] we propose that the for-
mation of key intermediate 11 through cobalt(III)-
catalyzed C–H activation can be rationalized in terms Scheme 7. Catalytic cycle for the C–H hydroarylation.
Scheme 6. Competition experiments between 2, 5 and 8.
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2H); 13C NMR (CDCl3, 100 MHz): d=201.7, 158.2, 139.9,
137.0, 129.2, 123.0, 122.1, 119.9, 117.1, 114.3, 106.5, 43.6,
22.5; IR (ATR): n=2716, 1709, 1558, 1435, 744 cmÀ1; HR-
MS (ESI): m/z=252.1131 [M+H+], calcd. for C15H14N3O
[M+H+]: 252.1131
Typical Procedure B for Direct C–H Hydroarylation
of a,b-Unsaturated Ketones
A suspension of indole 1 (0.50 mmol, 1.00 equiv.), methyl
vinyl ketone 5a (1.0 mmol, 2.00 equiv.), [Cp*CoI2(CO)]
(6.0 mg, 2.5 mol%), AgSbF6 (8.6 mg, 5.0 mol%), and KOPiv
(70.0 mg, 2.0 equiv.) in anhydrous TFE (2.0 mL) was stirred
at 708C for 16 h under an atmosphere of argon. At ambient
temperature, the solvent was evaporated under vacuum and
the remaining residue was purified by column chromatogra-
phy on silica gel (n-hexane/EtOAc 5:1) to afford product
6aa as a colorless solid; yield: 98 mg (74%); mp 66–688C.
1H NMR (CDCl3, 400 MHz): d=8.78 (d, J=4.8 Hz, 2H),
8.29 (d, J=8.1 Hz, 1H), 7.52 (d, J=7.2 Hz, 1H), 7.25–7.20
(m, 1H), 7.20–7.16 (m, 1H), 7.14 (t, J=4.8 Hz, 1H), 6.45 (s,
1H), 3.47–3.39 (m, 2H), 2.95–2.84 (m, 2H), 2.17 (s, 3H);
13C NMR (CDCl3, 100 MHz): d=207.9, 158.2, 158.2, 140.6,
137.0, 129.2, 122.8, 122.0, 119.8, 117.1, 114.2, 106.1, 43.5,
30.0, 23.8; IR (ATR): n=2919, 1716, 1559, 1419, 1247,
744 cmÀ1; HR-MS (ESI): m/z=266.1284 [M+H+], calcd. for
C16H16N3O [M+H+]: 266.1288.
Scheme 8. Removal of the directing group.
ent (Scheme 8). According to the previous protocol,[21]
only the product 13 was obtained when 6aa was treat-
ed with NaOEt (4.0 equiv.) in DMSO, Li proposed
a formal 1,4-shift mechanism for this process.[18a]
Thereafter, we devised a two-step reaction sequence,
which led to the b-indolyl ketal 14 in good yield.[10l]
Conclusions
In conclusion, we have developed a highly C-2 selec-
tive C–H alkylation of indoles with conjugated al-
kenes and aldehydes under cobalt(III) catalysis. Since
acrolein and enones have been recognized as effective
electrophiles and widely utilized in Diels–Alder reac-
tions, as well as Michael addition reactions, this pres-
ent work highlights their reactivities for catalytic
C(sp2)–H addition, thus providing an expedient route
to b-indolyl aldehydes and ketones. Beyond that, the
mild cobalt(III)-catalyzed C–H hydroarylation of
glyoxylate could deliver indolyl acetates with high ef-
ficacy. Detailed mechanistic studies revealed the pri-
ority of different conjugated C=C/C=O bonds and
suggested that the C–H cobaltation is not the rate-de-
termining step.
Typical Procedure C for Direct C–H Hydroarylation
of Glyoxylate
A suspension of indole 1 (0.50 mmol, 1.00 equiv,), glyoxylate
8
(1.0 mmol,
2.00 equiv,),
[Cp*CoI2(CO)]
(6.0 mg,
2.5 mol%), AgSbF6 (8.6 mg, 5.0 mol%), and KOPiv (7.0 mg,
10.0 mol%) in anhydrous TFE (2.0 mL) was stirred at 508C
for 16 h under an atmosphere of argon. At ambient temper-
ature, the solvent was evaporated under vacuum and the re-
maining residue was purified by column chromatography on
silica gel (n-hexane/EtOAc 3:2) to afford product 9aa as
a
yellow solid; yield: 144 mg (97%); mp 113–1158C.
1H NMR (CDCl3, 400 MHz): d=8.71 (d, J=4.9 Hz, 2H),
8.59 (d, J=8.4 Hz, 1H), 7.60 (d, J=7.7 Hz, 1H), 7.38–7.30
(m, 1H), 7.29–7.20 (m, 1H), 7.11 (t, J=4.9 Hz, 1H), 6.80 (s,
1H), 5.86 (d, J=10.3 Hz, 1H), 5.51 (d, J=10.3 Hz, 1H),
4.12–3.93 (m, 2H), 0.97 (t, J=7.1 Hz, 2H); 13C NMR
(CDCl3, 100 MHz): d=171.7, 158.1, 157.4, 137.1, 137.0,
128.9, 124.6, 122.7, 121.0, 116.7, 115.7, 111.9, 69.2, 61.4, 14.0;
Experimental Section
IR (ATR): n=2995, 1740, 1578, 1429, 1247, 1063, 670 cmÀ1
;
Typical Procedure A for Direct C–H Hydroarylation
of Acrolein
HR-MS (ESI): m/z=347.1119 [M+H+], calcd. for
C16H16N3O3 [M+H+]: 347.1121.
A suspension of indole 1 (0.50 mmol, 1.00 equiv.), acrolein 2
(1.0 mmol, 2.00 equiv.), [Cp*CoI2(CO)] (6.0 mg, 2.5 mol%),
AgSbF6 (8.6 mg, 5.0 mol%), and KOPiv (70.0 mg, 2.0 equiv.)
in anhydrous TFE (2.0 mL) was stirred at 708C for 16 h
under an atmosphere of argon. At ambient temperature, the
solvent was evaporated under vacuum and the remaining
residue was purified by column chromatography on silica
gel (n-hexane/EtOAc 5:1) to afford product 3a as a slightly
yellow solid; yield: 112 mg (89%); mp 86–878C. 1H NMR
(CDCl3, 400 MHz): d=9.84 (t, J=1.3 Hz, 1H), 8.77 (d, J=
4.8 Hz, 2H), 8.32 (d, J=8.3 Hz, 1H), 7.52 (d, J=7.2 Hz,
1H), 7.27–7.16 (m, 2H), 7.14 (t, J=4.8 Hz, 1H), 6.47–6.46
(m, 1H), 3.50 (t, J=7.3 Hz, 2H), 2.90 (td, J=7.6, 1.3 Hz,
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21602083), National Natu-
ral Science Foundation of Jiangsu Province (Grant No.
BK20160160), and the Fundamental Research Funds for the
Central Universities (JUSRP51703A).
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