Synthesis of Tetrasubstituted Pyrroles from Homopropargylic Amines via a Sonogashira Coupling/Intramolecular Hydroamination/Oxidation Sequence

Article abstract of DOI:10.1002/adsc.201500350

A one-pot reaction of homopropargylic amines and aryl iodides was developed in the presence of a palladium (Pd) catalyst and generated a series of tetrasubstituted pyrrole derivatives in good to high yields. This process involved a Sonogashira, intramolecular hydroamination, cyclization and oxidation sequence of reactions.

Full text of DOI:10.1002/adsc.201500350

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DOI: 10.1002/adsc.201500350
Synthesis of Tetrasubstituted Pyrroles from Homopropargylic
Amines via a Sonogashira Coupling/Intramolecular
Hydroamination/Oxidation Sequence
a
a
a
a
a,
Chan Wang, Kaimeng Huang, Junying Wang, Hongkai Wang, Lingyan Liu, *
a
a,b,
Weixing Chang, and Jing Li *
State key Laboratory of Elemento-Organic Chemsitry, Nankai University, Weijin Road 94#, Nankai District, Tianjin
a
300071, Peopleꢀs Republic of China
E-mail: liulingyan@nankai.edu.cn or lijing@nankai.edu.cn
The Synergetic Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300071, Peopleꢀs Republic of
China
b
Received: April 7, 2015; Revised: June 13, 2015; Published online: September 10, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500350.
Abstract: A one-pot reaction of homopropargylic
amines and aryl iodides was developed in the pres-
ence of a palladium (Pd) catalyst and generated
a series of tetrasubstituted pyrrole derivatives in
good to high yields. This process involved a Sonoga-
shira, intramolecular hydroamination, cyclization
and oxidation sequence of reactions.
Suzuki reaction of a protected homopropargylic alco-
[4]
hol [Scheme 1, Eq. (1)]. A PtI -catalyzed cycloiso-
2
merization reaction of a 1,5-enyne alcohol has also
[5]
been studied [Scheme 1, Eq. (2)]. In view of this, we
herein have designed a 1,5-enyne amine (homopro-
pargylic amine) which contains three functional
groups, an amino group, a double bond and a triple
bond, with the aim to develop a new enyne cycloiso-
merization reaction with amino groups being involved
[Scheme 1, Eq. (3)]. However, a novel multisubstitut-
ed pyrrole compound 3, was unexpectedly obtained in
Keywords: cascade reaction; homopropargylic
amines; pyrroles
1
2% yield when performing the Sonogashira coupling
reaction of the homopropargylic amine and iodoben-
zene for the preparation of the internal 1,5-enyne
The cascade reaction concept has been rapidly devel- amine [Scheme 1, Eq. (4)]. The structural assignment
oped over the last decade, and attracted more and of cascade reaction product 3 was confirmed by X-ray
[6]
more chemistsꢀ attention due to its simple and effi- analysis (Figure 1).
cient synthetic route for the construction of enriched Herein, the expected Sonogashira coupling product
and complicated compounds from easily available was not observed [Eq. (3)]. Moreover, two molecules
starting materials. Among them, the cascade reaction of aryl iodide reacted in one pot yielding tetrasubsti-
of alkynes catalyzed by palladium is one of the most tuted pyrroles. In other words, this reaction actually
[
1]
important reactions. A number of methodologies involves a three-component cascade reaction. More-
has been reported for aminoalkyne substrates, due to over, pyrroles or substituted pyrroles are important
their double functionality (amino group and triple building blocks for the construction of numerous nat-
[
2]
bond). To the best of our knowledge, the reported ural products and others with pharmaceutical applica-
[7,8]
aminoalkynes mainly include two types, a-aminoal- tions.
In particular, 1,2,3,5-tetrasubstituted pyrroles
kynes and b-aminoalkynes. For the a-aminoalkynes, are biologically active and have been shown to display
the intermolecular cascade reactions are generally antibacterial, antiviral (also anti HIV-1), anti-inflam-
studied. In comparison, the intramolecular reactions matory, and antioxidant activity and to inhibit cyto-
[3]
[9]
are more often reported for the b-aminoalkynes.
kine-mediated diseases. Based on the above inter-
This is because the b-aminoalkyne is more likely to esting results and the very few cascade reaction re-
undergo 5-endo cyclization in the presence of Pd cat- ports about the b-aminoalkynes, we then conducted
alysis. Despite all this, reports about the cascade reac- a thorough study of this novel cascade reaction.
tion of b-aminoalkynes are sparse in the literature.
Considering that the double bond of compound
Our research group has developed a Pd-catalyzed se- 1 was not involved in this cascade reaction, we select-
quential Sonogashira–carbopalladative cyclization– ed simple homopropargylic amine 4a and iodoben-
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Chan Wang et al.
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Scheme 1. The cascade cyclization reactions of various alkynes developed in our group.
zene 5a as model substrates. Various Lewis bases the best results in the presence of a Pd(PPh ) Cl /CuI
3
2
2
were initially examined (Table 1). It was found that catalytic system (Table 1, entry 2).
the organic bases displayed better activity than inor- Afterwards, the catalyst was investigated. As shown
ganic bases (entries 1–4 vs. 5 and 6), and Et N gave in Table 2, no reaction occurred when using only CuI
3
and PPh without a Pd catalyst (Table 2, entry 1).
3
Moreover, in the presence of other Pd catalysts and
phosphine ligands, the reaction results were unsatis-
factory in the absence of CuI (Table 2, entries 3–7).
With Pd(PPh ) as catalyst, this cascade cyclization re-
3
4
action generated the pyrrole 6aa in 40% yield
(
entry 8). Overall, the Pd(PPh ) Cl /CuI pair was the
3
2
2
best catalytic system ( Table 2, entry 2). Upon closer
investigation of the product structure, we found that
this reaction involved an oxidation–dehydrogenation
process. Moreover, tracing this reaction system by
1
H NMR, it was observed that there were characteris-
tic H-1, H-2a and H-2b dihydropyrrole peaks
(
Figure 2, A). But after work-up with brine and ex-
traction with EtOAc, the characteristic peaks of a di-
hydropyrrole disappeared with the appearance of the
pyrrole H-2’ peak (Figure 2, B). This may be due to
the instability of the dihydropyrrole in air.
Based on the above results, the reaction actually in-
volves two-steps in one pot. Thus, several oxidants or
Figure 1. The ORTEP drawing of product 3.
2
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COMMUNICATIONS Synthesis of Tetrasubstituted Pyrroles from Homopropargylic Amines
Table 1. The effect of Lewis base on the cascade reaction of
homopropargylic amine and iodobenzene.
Table 2. The screening of catalysts in the cascade reaction of
[a]
[a]
homopropargylic amine and iodobenzene.
[
b]
Entry
Lewis Base
piperidine
Yield [%]
[b]
Entry
Catalyst (mol%)
Yield [%]
[
c]
1
2
3
4
5
6
23
43
35
31
11
0
1
2
3
4
5
6
7
8
CuI (5)/Ph P (40)
0
3
Et N
3
(Ph P) PdCl (5)/CuI (10)
43
35
20
<10
13
13
40
3
2
2
DBU
piperidine
[d]
Pd(dppf)Cl
2
(5)
PdCl (5)/Cy P (20)
2
3
K CO3
2
PdCl (5)/Ph P (40)
2
3
t-BuOK
Pd(OAc) (10)/Ph P (20)
2
3
[a]
Reaction conditions: under an N atmosphere, 4a (44 mg,
.2 mmol, 1.0 equiv.), Lewis base (5.0 equiv.), PhI
Pd
Pd(PPh ) (5)/CuI (10)
3 4
dba
2
3
(5)
2
0
(
(
122 mg, 0.6 mmol, 3.0 equiv.), catalyst and DMF
0.35 mL) were sequentially added into a Schlenk tube.
[
[
a]
b]
Standard reaction procedure.
Isolated yield.
The reaction was carried out under the given reaction
conditions and the products were subsequently detected
by TLC.
Isolated yield.
PhI 1.5 equiv. at 608C.
[
[
[
b]
c]
d]
Et N to (i-Pr) NH increased the yield of the corre-
3 2
sponding multisubstituted pyrrole product from 65%
to 72% (entry 6). Therefore, the optimal conditions
were determined to be: 1) 5 mol% Pd(PPh ) Cl /
The solvent is THF (558C).
3
2
2
1
0 mol% CuI, 5 equiv. (i-Pr) NH, in DMF at 808C;
2
additives were screened in later stage reactions in the and 2) 1.5 equiv. DDQ at room temperature.
presence of Pd(PPh ) Cl /CuI with Et N as a Lewis With the optimal reaction conditions in hand, vari-
3
2
2
3
base (Table 3). As a result, DDQ (2,3-dichloro-5,6-di- ous homopropargylic amines and substituted iodoben-
cyano-1,4-benzoquinone) displayed advantageous re- zenes were then investigated to diversify the general
sults over other oxidants (Table 3, entry 5 vs. en- multisubstituted pyrrole scaffold (Table 4). It was
tries 1–4). Furthermore, changing the Lewis base found that all substrates were tolerated in this cascade
1
Figure 2. The H NMR spectra of dihydropyrrole (A) and pyrrole (B).
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Chan Wang et al.
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Table 3. The effect of additives on the cascade reaction of responding product in good yield (Table 4, entries 25
[a]
homopropargylic amine and iodobenzene.
and 26).
Encouraged by the above good yields with aryl io-
dides, we attempted to employ aryl chlorides or aryl
bromides which are more readily available. Gratify-
ingly, in the case of phenyl bromide 7a, the aryl ho-
mopropargylic amines with non-substituted (4a) or
electron-donating substituted (Me) aryl groups on the
N atom (4b) gave good yields (6aa, 77% and 6ba,
7
6%), whereas for the homopropargylic amines with
electron-withdrawing substituted (F or Cl) aryls on
the N atom (4d and 4e), the yield of corresponding
cascade cyclization products was low (6da, 34% and
6ea, 26%) (Figure 3). Using phenyl chloride, no target
compound was obtained under the standard reaction
conditions and the starting material decomposed, in-
stead.
Subsequently, several additional experiments were
conducted to investigate the mechanism of this reac-
tion (Scheme 2). Surprisingly, using 4-iodopyridine 5k
and homopropargylic amine 4a, the reaction could
only give the Sonogashira coupling product 9 in 71%
yield without any further cyclization products 8
[
b]
Entry
Additive
Yield [%]
[
[
c]
c]
1
2
3
4
5
6
CuBr2
0
Cu(OAc)2
TBAI
12
28
17
65
72
Ag O
2
DDQ
DDQ
[
d]
[
[
[
[
a]
b]
c]
d]
Standard procedure.
Isolated yield.
THF, 558C.
Using (i-Pr) NH.
[
Scheme 2, Eq. (5)]. But by treating the compound 9
2
with 4-methyliodobenzene under the standard cycliza-
tion reaction conditions, various 1,2,4,5-substituted
reaction and produced tetrasubstituted pyrroles in
good to high yields. Specifically, the best results were
obtained using iodobenzene or 4-nitroiodobenzene
and N-(1-phenylbut-3-ynyl)aniline (Table 3, entries 1
and 5), which were smoothly converted into the de-
sired products 6aa and 6ae in 86% yield. Moreover,
the iodobenzenes with electron-withdrawing substitu-
ents gave higher yields of the target compounds than
those with electron-donating substituents (entries 2
and 3 vs. 4–7) in the case of the substrate 4a in this
cascade reaction. This regularity was also embodied
in the case of cinnamyl homopropargylic amine 4i
(
entries 23 and 24). Additionally, 2-thiophenyl iodide
and the large naphthalenyl iodide were also shown to
be suitable substrates in this cascade reaction (en-
tries 8 and 9). When using 4-methyliodobenzene, the
substrates 4 with electron-donating substituted aro-
matic rings on the N atom exhibited higher efficiency
than those with electron-withdrawing substituents
(
Table 4, entries 10 and 11 vs. 12–14). Similarly, the
homopropargylic amine 4g with a large naphthalenyl
group on the N atom could also smoothly react with
the aryl iodide (entry 15). Additionally, several other
different substrates were also tolerated (Table 4, en-
tries 16–22), especially the ortho-substituted aryl io-
dides, 2-FC H I and 2-MeC H I (entries 17 and 18).
6
4
6
4
Notably, using the alkyl-substituted homopropargylic
1
2
amines (R =cyclohexyl or R =n-Bu) also allowed Figure 3. The cascade cyclization reaction of homopropar-
the reaction to proceed smoothly and afford the cor- gylic amines and phenyl bromide.
2
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COMMUNICATIONS Synthesis of Tetrasubstituted Pyrroles from Homopropargylic Amines
[
a]
Table 4. The scope of homopropargylic amines and substituted iodobenzenes.
1
2
3
[b]
Entry
4
R
R
5
R
Product 6
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
4f
4g
4d
4e
4a
4h
4b
4d
4e
4i
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-MeC H4
Ph
Ph
Ph
cinnamyl
cinnamyl
cyclohexyl
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
5a
5b
5c
5d
5e
5f
5g
5h
5i
5b
5b
5b
5b
5b
5b
5d
5j
5k
5f
5a
5a
5a
5c
5d
5a
5a
Ph
4-Me-C H
4-MeO-C H
4-F-C H
4-NO -C H
3-Cl-C H
3-CN-C H
thiophenyl
naphthyl
4-Me-C H
4-Me-C H
4-Me-C H
4-Me-C H
4-Me-C H
4-Me-C H
4-F-C H
2-F-C H
2-Me-C H
3-Cl-C H
Ph
Ph
Ph
4-MeO-C H
4-F-C H
Ph
Ph
6aa
6ab
6ac
6ad
6ae
6af
6ag
6ah
6ai
6bb
6cb
6db
6eb
6fb
6gb
6dd
6ej
6ak
6hf
6ba
6da
6ea
6ic
86
72
57
76
86
84
79
83
57
70
76
62
59
63
75
78
79
60
67
72
64
57
33
79
82
74
6
4
6
4
6
4
2
6
4
6
4
6
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
4-Me-C H
4-MeO-C H
4-F-C H4
4-Cl-C H
4-Br-C H
naphthyl
4-F-C H4
4-Cl-C H
Ph
Ph
4-Me-C H
4-F-C H4
4-Cl-C H
Ph
Ph
4-F-C H4
6
4
6
4
6
4
6
4
4
4
4
4
6
6
6
4
6
6
4
6
6
6
6
4
6
4
6
4
6
4
6
6
4
6
4
6
6
4
6
4
4i
4j
4k
6id
6ja
6ka
6
4
6
n-Bu
[a]
Reaction conditions: to a dried Schlenk tube were added the homopropargylic amine (0.2 mmol, 1 equiv.), CuI (3.8 mg,
.02 mmol, 10 mol%) and (PPh ) PdCl (7 mg, 5 mol%), and the reaction vessel was evacuated and backfilled with nitro-
0
3
2
2
gen three times. Then the substituted iodobenzene (0.6 mmol, 3 equiv.), diisopropylamine (101 mg, 1 mmol, 5 equiv.) and
DMF solvent (0.35 mL) were added under a nitrogen atmosphere. The reaction mixture was stirred at 808C for about
0.5 h until complete disappearance of 3 (monitored by TLC). After cooling the mixture to room temperture, DDQ
(0.3 mmol, 1.5 equiv.) was added and the mixture stirred at room temperature for additional 0.5–1 h until completion of
the reaction (monitored by TLC).
Isolated yield.
[b]
pyrroles 10 could be generated in 42% yield [Eq. (6)]. pyrrole 12 were generated in 35% and 12% yield, re-
Nevertheless, combining 4-iodopyridine 5k with the spectively [Eq. (8)]. It should be noted that this reac-
homopropargylic amine 4a and concomitantly 4- tion became complicated with no intramolecular hy-
methyl iodobenzene 5b in one-pot resulted in only droamination occurring in the absence of aryl iodide
trace amounts of cascade cyclization product 10 (5% under the standard reaction conditions. It was sur-
yield) and a great amount of iodobenzene was recov- mised that the terminal alkyne is difficult to use to
ered which simultaneously generated the Sonogashira conduct the intramolecular hydroamination using our
coupling product 9 in 37% yield [Eq. (7)]. The reason protocol.
remains unclear at present. Moreover, using (E)-4-
methyl-N-(1-phenylhex-1-en-5-yn-3-yl)benzenesulfo-
Based on all above results, one possible cascade re-
action mechanism was proposed (Scheme 3). The ho-
namide 4l and 4-methyl iodobenzene 5b as substrates, mopropargylic amine 4 firstly undergoes standard oxi-
the Sonogashira coupling product 11 and the dihydro- dative addition (I), transmetallation (III) and reduc-
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Chan Wang et al.
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Scheme 2. Control experiments.
tive elimination to give the Sonogashira product IV in Experimental Section
3
3
the presence of Pd/Cu with R I 5. The active R PdI
II) species continues to activate the internal alkyne General Procedure for Pyrroles 6
(
to give p-complex (V), which is accompanied by in-
tramolecular hydroamination cyclization to afford the
cycloadduct (VI) and followed by reductive elimina-
tion to give dihydropyrrole VII as well as regenerate
To a dried Schlenk tube was added the homopropargylic
amine (0.2 mmol, 1 equiv.), CuI (3.8 mg, 0.02 mmol,
1
0 mol%) and (PPh ) PdCl (7 mg, 5 mol%), and the reac-
3 2 2
tion mixture was evacuated and backfilled with nitrogen
three times. Then the substituted iodobenzene (0.6 mmol,
[
1d,10]
Pd(0).
The unstable dihydropyrrole VII is directly
oxidized by DDQ and results in the target molecule:
tetrasubstituted pyrrole (TM 6).
3
equiv.), diisopropylamine (101 mg, 1 mmol, 5 equiv.) and
DMF solvent (0.35 mL) were added under a nitrogen atmos-
In summary, we have developed a new cascade cyc-
lization reaction of homopropargylic amines catalyzed
by palladium. This reaction involves Sonogashira–in-
tramolecular hydroamination–oxidation steps, and is
actually a three-component multistep cascade reac-
phere. The reaction mixture was stirred at 808C for about
0
.5 h until complete disappearance of 3 (monitored by
TLC). After cooling the mixture to room temperture, DDQ
0.3 mmol, 1.5 equiv.) was added and the mixture stirred at
room temperature for additional 0.5–1 h until completion of
(
tion. Through this reaction, numerous variously tetra- the reaction (monitored by TLC). The mixture was passed
substituted pyrrole derivatives were obtained in good through a short kieselguhr column by using CH Cl , extract-
2
2
to high yields.
ed with EtOAc, and washed with brine and distilled water.
The combined organic extracts were dried over MgSO , con-
4
centrated under vacuum and purified by flash chromatogra-
2
800
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COMMUNICATIONS Synthesis of Tetrasubstituted Pyrroles from Homopropargylic Amines
Scheme 3. The possible mechanism for this cascade cyclization reaction.
phy on silica gel (gradient elution of EtOAc/petroleum
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3
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