Site- and Regioselective Monoalkenylation of Pyrroles with Alkynes via Cp?CoIII Catalysis

Article abstract of DOI:10.1021/acs.orglett.6b02997

A site-, regio-, syn-, and monoselective alkenylation of dimethylcarbamoyl-protected pyrroles proceeded using a catalytic amount of [Cp?Co(CH₃CN)₃](SbF₆)₂ and KOAc. A variety of internal alkynes with several fun

Full text of DOI:10.1021/acs.orglett.6b02997

Letter
pubs.acs.org/OrgLett
Site- and Regioselective Monoalkenylation of Pyrroles with Alkynes
via Cp*CoIII Catalysis
Ryo Tanaka,† Hideya Ikemoto,‡ Motomu Kanai,‡ Tatsuhiko Yoshino,*,†,§ and Shigeki Matsunaga*,†,§
†Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
‡Graduate School of Pharmaceutical Sciences, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
§ACT-C, Japan Science and Technology Agency, Sapporo 060-0812, Japan
S
* Supporting Information
ABSTRACT: A site-, regio-, syn-, and monoselective alkenylation of dimethylcarba-
moyl-protected pyrroles proceeded using a catalytic amount of [Cp*Co(CH3CN)3]-
(SbF6)2 and KOAc. A variety of internal alkynes with several functional groups and a
terminal alkyne afforded hydropyrrolation products in a selective manner in good to
excellent yield. The site-selectivity (C2/C5 selectivity) observed for C3-substituted
pyrroles is noteworthy because Cp*RhIII-catalyzed conditions afforded only a
moderate yield and low selectivity. The conditions described here provide general and
straightforward access to unsymmetrically mono- and disubstituted pyrrole
derivatives.
yrrole, one of the simplest nitrogen-containing heterocycles,
Pis a common structural motif in many natural and unnatural
biologically active compounds.1 Therefore, the development of
efficient synthetic methods of pyrrole derivatives will accelerate
drug discovery and other biological studies. Typical pyrrole
synthesis requires condensation of the corresponding nitrogen
sources and carbonyl compounds,2 as represented by Parr−
Knorr synthesis.3 The availability of the starting materials thus
often limits the diversity of accessible structures.
Recent progress in transition-metal-catalyzed C−H bond
functionalization reactions4 has opened up alternative routes to
substituted pyrrole-containing molecules.5−8 For installation of
the alkenyl moiety, oxidative alkenylation using alkenes has been
intensively studied with Pd5 and other metal catalysts.6 Direct
addition of aromatic C−H bonds to alkynes, the hydroarylation
reaction, is another attractive method to introduce alkenyl
groups due to the high atom-economy9 and availability of various
alkynes.10 Transition-metal-catalyzed hydropyrrolation reactions
Figure 1. Selectivity issues on hydropyrrolation of alkynes.
of electron-rich nonactivated alkynes, however, is less well
studied than other hydroarylation reactions.7 This seemingly
by Schipper and Fagnou were only applied to a specific pyrrole
bearing the same ester groups at the C3 and C4 positions.7c
simple transformation has formidable selectivity issues: mono/
Accordingly, a general catalytic system for the selective
hydropyrrolation of internal and terminal alkynes is still in high
demand.
diselectivity, regioselectivity of alkyne insertion, syn/anti
selectivity, and site-selectivity of pyrrole C−H bonds (Figure
1). Most of the reported reaction conditions were only optimized
for indoles or other substrates and suffer from selectivity issues
and/or lack of substrate generality. For example, Yoshikai’s
conditions using a low-valent cobalt catalyst7d and Zeng’s
conditions using a RuII catalyst7f afforded a bisalkenylated
product using only a nonsubstituted pyrrole. π−Acidic metal
catalyzed reactions using N-alkyl- and N-arylpyrroles generally
suffer from low selectivity and rarely afford alkenylated
products.7g,10a Some Ru catalysts were reported to promote
branch-selective hydropyrrolation, but only terminal alkynes
have been utilized.7b,e Cp*RhIII-catalyzed conditions developed
During the course of our studies on Cp*CoIII-catalyzed C−H
bond functionalization,11 we reported an alkenylation reaction
and alkenylation/annulation reaction of indoles with alkynes.11b
In addition, Chen and Yu reported a Cp*CoIII-catalyzed
hydroarylation reaction using various aromatic compounds,
including indole.12 Nevertheless, a hydropyrrolation reaction
under high-valent cobalt catalysis13−15 has not yet been
Received: October 5, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02997
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
investigated. In this paper, we report a Cp*CoIII-catalyzed
hydropyrrolation reaction of internal and terminal alkynes with
high site-, regio-, syn-, and monoselectivity, while Cp*RhIII-
catalyzed conditions afforded only low C2/C5 selectivity when
unsymmetrically substituted pyrroles were used.
Scheme 1 shows the substrate scope of the Cp*CoIII-catalyzed
hydropyrrolation. The electronic property of the alkynes hardly
a
Scheme 1. Substrate Scope
Optimization studies using dimethylcarbamoyl-protected
pyrrole 1a7c,11b and alkyne 2a are summarized in Table 1. We
a
Table 1. Optimization of Reaction Conditions
b
b
entry
X (mol %)
solvent
temp (°C)
yield (%)
3aa/4aa
1
2
3
4
5
6
7
5
DCE
80
80
80
80
80
80
80
80
60
rt
67
88
91
47
63
13
6
14/1
17/1
18/1
12/1
12/1
9/1
5
PhCl
5
toluene
THF
5
5
dioxane
TFE
5
5
HFIP
nd
c
8
2.5
2.5
2.5
2.5
toluene
toluene
toluene
toluene
>95
>95
14
18/1
>20/1
18/1
18/1
c
9
c
10
d
e
11
60
93
a
The reactions were run using 1a (0.36 mmol) and 2a (0.30 mmol),
[Cp*Co(CH3CN)3](SbF6)2 5, and KOAc in indicated solvent (0.2
b
1
M). Determined by H NMR analysis of the crude mixture using
c
1,1,2,2-tetrachloroethane as an internal standard. 1a (0.72 mmol) and
2a (0.60 mmol) were used. 1a (0.60 mmol) and 2a (0.72 mmol)
were used. Combined isolated yield of 3aa and 4aa.
d
e
selected [Cp*Co(CH3CN)3](SbF6)2 5 as a catalyst precursor14a
due to its user-friendly nature.16 As expected, selective C2-
monoalkenylation proceeded to afford monoalkenylated prod-
duct 3aa in 67% yield and high selectivity in the presence of 5 mol
% of 5 and KOAc in DCE at 80 °C (entry 1). Although a small
amount of isomeric product 4aa was observed (14/1), no other
isomers or bis-alkenylated product was identified. Toluene was
the best solvent (entries 2−7), and the yield was improved to
91% (entry 3). Ether-type solvents and fluorinated alcohols were
inefficient. The catalyst loading was decreased to 2.5 mol %
without any loss of the yield (entry 8). Although the reaction also
proceeded smoothly at 60 °C, it was very sluggish at room
temperature (entries 9, 10). It is noteworthy that monoalkeny-
lated product 3aa was selectively obtained in 93% isolated yield
even when a slight excess amount of alkyne 2a was used (entry
11). The second alkenylation of 3 did not proceed probably due
to unfavorable metallacycle formation that would cause severe
steric repulsion between the alkenyl moiety and the directing
group (Figure 2).16
a
All the indicated yields are combined yields of 3 and 4 after isolation.
1
The ratios in parentheses are those of 3/4 determined by H NMR
b
analysis of the crude mixture. 5 mmol scale, 46 h.c1 (0.36 mmol), 2
(0.30 mmol), and PivOH (50 mol %) were used, and the reaction was
run at 110 °C. E/Z ratio >20/1.
affected the reactivity, and substrates with various functional
groups on the phenyl group afforded the products in high yields
(3aa−3ag). Both aryl and alkyl groups on the alkynes were also
compatible to afford the products in good yields (3ah, 3ai). A
gram-scale reaction successfully afforded 3aa in 91% yield with
high selectivity although a longer reaction time was required. On
the other hand, several alkynes and pyrroles were less reactive
under the above optimized conditions (Conditions A). Addi-
tional studies revealed that the addition of a catalytic amount of
pivalic acid to promote the protonation step in the catalytic cycle
improved the reactivity.17 Under the newly optimized conditions
(Conditions B), diphenylacetylene 2j and 1-(2-naphthyl)-1-
propyne 2k afforded the hydropyrrolation products in good
yields and selectivity. When pyrroles bearing C3-substituents
were used as a substrate, the less hindered C5-position was
selectively alkenylated to give 3ba, 3ca, and 3da in 69−90%
yields along with a tiny amount of the corresponding C2-
alkenylated products. Terminal alkynes were also applicable by
increasing the reaction temperature to 110 °C and the amount of
pivalic acid to 50 mol %; alkenylation product 3al was obtained in
Figure 2. Unfavorable second alkenylation.
B
DOI: 10.1021/acs.orglett.6b02997
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
82% yield, but a small amount of the Z-isomer was observed in
this case.
The high site-selectivity observed for 3-substituted pyrroles
under Cp*CoIII catalysis is remarkable because the correspond-
ing rhodium catalysis did not work well for these substrates under
our optimized conditions or Schipper’s conditions,7c as shown in
Table 2. Almost no reaction proceeded between 1c and 2a when
Table 2. Comparison of Cobalt and Rhodium Catalysis
Figure 3. Plausible catalytic cycle.
metallacycle III′ would be hampered by steric repulsion between
the Cp* ligand and the substituent X. Insertion of alkyne (IV)
and protodemetalation by AcOH or PivOH would generate the
catalyst with the release of product 3. Alkenylcobalt IV would be
less stable than the corresponding alkenylrhodium species, and
therefore only a catalytic amount of carboxylic acid efficiently
would promote the final protonation step while the Cp*RhIII-
catalyzed conditions required excess amounts of PivOH to
achieve a good yield.
In summary, site-, regio-, syn-, and monoselective alkenylation
reaction of dimethylcarbamoyl-protected pyrroles with alkynes
catalyzed by [Cp*Co(CH3CN)3](SbF6)2 5 was developed. In
addition to excellent functional group compatibility and
generality, higher site-selectivity of the substituted pyrroles
were observed compared with previously reported conditions
using a Cp*RhIII catalyst, probably due to the smaller ionic radius
of cobalt.
e
entry
conditions
1 (−X)
yield (%)
3/3′/4
a
d
1
2
3
4
5
Conditions B
1c (−CO2Et)
1c (−CO2Et)
1c (−CO2Et)
1d (−Ac)
90
100/6/4
b
Rh instead Co
trace
−
c
e
Rh (ref 7c)
40
100/59/−
100/3/4
100/17/−
a
d
Conditions B
69
c
e
Rh (ref 7c)
1d (−Ac)
17
a
1 (0.30 mmol), 2 (0.36 mmol), 5 (5 mol %), KOAc (5 mol %), and
PivOH (10 mol %) in dioxane (0.2 M), 80 °C, 20 h. 1 (0.30 mmol),
2 (0.36 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5 mol %), KOAc (5 mol
%), and PivOH (10 mol %) in dioxane (0.2 M), 80 °C, 20 h. 1 (0.30
mmol), 2 (0.33 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5 mol %),
PivOH (5 equiv) in DCE (0.4 M), 90 °C, 24 h. Isolated yield.
b
c
d
e
1
Determined by H NMR analysis of the crude mixture.
5 was replaced with [Cp*Rh(CH3CN)3](SbF6)2 under our
optimized conditions B (entry 2). Although Schipper’s
conditions using [Cp*Rh(CH3CN)3](SbF6)2 and an excess
amount of pivalic acid afforded the products in moderate yield,
the C5/C2 selectivity was only 100/59 (entry 3). A similar
tendency was observed for 3-acetylpyrrole derivative 1d (entries
4, 5). Although moderate selectivity was observed under the
rhodium catalysis in this case, only 17% combined yield was
obtained. These differences in the site-selectivity between
Cp*CoIII and Cp*RhIII catalysis probably reflect the difference
in the ionic radius between cobalt and rhodium. Steric repulsion
between the Cp* ligand and the substituent (X) would be
enhanced by the smaller ionic radius of cobalt.11e,14q The
requirement for a large amount of pivalic acid under Cp*RhIII
catalysis might indicate higher stability of the alkenylrhodium
intermediate and slower protodemetalation compared with
alkenylcobalt intermediate.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b02997.
■
S
Experimental procedures, characterization data, and copy
of NMR spectrum (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: tyoshino@pharm.hokudai.ac.jp.
*E-mail: smatsuna@pharm.hokudai.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Finally, removal of the dimethylcarbamoyl group of 3aa was
accomplished to afford NH-free pyrrole 6aa in 89% yield by
heating with KOH in aqueous ethanol (eq 1).7c
This work was supported in part by ACT-C from JST, JSPS
KAKENHI Grant Number JP15H05802 in Precisely Designed
Catalysts with Customized Scaffolding, JSPS KAKENHI Grant
Number JP15H05993, and Asahi Glass Foundation. H.I. thanks
JSPS for the fellowship.
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D
DOI: 10.1021/acs.orglett.6b02997
Org. Lett. XXXX, XXX, XXX−XXX