Metal-free synthesis of polysubstituted pyrroles by (diacetoxyiodo)benzene- mediated cascade reaction of 3-alkynyl amines

Article abstract of DOI:10.1002/asia.201100474

Free metal jacket: A regioselective metal-free synthesis of polysubstituted pyrroles has been developed through a (diacetoxyiodo)benzene-mediated cascade reaction of alkynyl amines in moderate to good yield (see scheme).

Full text of DOI:10.1002/asia.201100474

COMMUNICATION
DOI: 10.1002/asia.201100474
Metal-Free Synthesis of Polysubstituted Pyrroles by
(Diacetoxyiodo)Benzene-Mediated Cascade Reaction of 3-Alkynyl Amines
Dong-Liang Mo,[a] Chang-Hua Ding,[a] Li-Xin Dai,[a] and Xue-Long Hou*[a, b]
In memory of Emanuel Vogel
Pyrrole has been found extensively as a subunit in phar-
macologically active natural and unnatural products.[1] Its
derivatives are important building blocks in the syntheses of
various heterocyclic compounds.[2] Accordingly, synthetic
methods to develop pyrroles have received continuing atten-
tion of synthetic chemists since their discovery. Nowadays, a
variety of protocols for the construction of pyrroles are
available.[3] The classical Hantzsch procedure,[4] Paal–Knorr
reaction,[5] various cycloaddition reactions,[6] and aza-Claisen
rearrangements,[7] have been widely applied for the synthe-
ses of pyrrole derivatives. Recent developed strategies based
upon metal catalysis such as the Buchwald–Hartwig cou-
deduced that the amination product of acetylene would be
afforded if amines were used as nucleophiles. Herein, we
report our preliminary results of the simple and efficient
PhIACTHNUGTRNENG(U OAc)2-mediated reaction for the regioselective synthe-
sis of polysubstituted pyrroles from alkynyl amines under
metal-free conditions.[13]
Initially, phenylacetylene was treated with TsNH2, benzyl-
amine, or N-Methyl benzylamine, respectively, in the pres-
ence of PhIACTHNUGTRENUNG(OAc)2 in AcOH at 708C. Unfortunately, only
the formation of a-acyloxy ketone, instead of the amination
product, was observed. In order to avoid the nucleophilic
attack of the OAc anion, we considered that an intramolecu-
lar amination of the alkyne could be the solution. To this
end, alkynyl amine 1a was treated with two equivalents of
(diacetoxyiodo)benzene in acetic acid at 708C for 12 hours
under argon. Surprisingly, we did not observe the expected
dihydropyrrole, but more appealing 2,3,4-trisubstituted pyr-
roles 2a and 3a were obtained in 34% and 8% yields, re-
spectively [Eq. (1)]. The structure of pyrrole 2a was deter-
mined by NOESY experiments. Interestingly, almost the
same yield was afforded when the reaction ran under aero-
bic conditions.
pling reaction,[8] metal-catalyzed cyclizations,[9] and C H
À
bond amination[10] provide even more synthetic approaches.
However, some of them suffer from either multistep syn-
thetic operations, low chemical yield, low regioselectivity, or
the use of toxic or expensive metals. An efficient synthetic
procedure for the regioselective construction of pyrroles is
still to be explored. Recently, we have developed a hyperva-
lent iodine-mediated oxidation of terminal alkynes to afford
a-acyloxy ketones by using OAc (Ac=acetyl) as the nucleo-
phile.[11,12] In this reaction, the key steps involve the forma-
tion of the alkynyliodonium salt from terminal alkyne with
PhIACHTUNGTRENNUNG(OAc)2 and the subsequent Michael-type addition of
AcOH to the alkynyliodonium salt. Based on the results, we
[a] D.-L. Mo, Dr. C.-H. Ding, Prof. L.-X. Dai, Prof. X.-L. Hou
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
345 Lingling Road, Shanghai, 200032 (China)
Fax : (+86)21-54925100
E-mail: xlhou@sioc.ac.cn
Encouraged by these promising results, the influence of
the reaction parameters was examined (Table 1). A survey
of solvents indicated that the reaction is sensitive to the re-
action medium. Only the starting material 1a was recovered
using N,N-dimethylformamide (DMF) or tetrahydrofuran
(THF) as solvents (Table 1, entries 1 and 2), while the reac-
tion proceeded sluggish in the solvents such as acetonitrile,
[b] Prof. X.-L. Hou
Shanghai-Hong Kong Joint Laboratory in Chemical Synthesis
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201100474.
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Chem. Asian J. 2011, 6, 3200 – 3204

Table 1. Optimization of Reaction Conditions.[a]
tion of alkynyl amine 1 with tosyl (Ts) and triflate (Tf)
groups provided 2a in high yield (Table 2, entries 1 and 2),
but no isolable product was detected if alkynyl amine 1
bearing PhCO and PhNHCO groups were used (Table 2, en-
tries 3 and 4). With alkynyl amine 1 having a Ts group as
the substituent, different hypervalent iodine compounds
Entry
Equiv. of PhI
(OAc)2
Solvent
T [8C]
2a:3a [%][b]
were also screened. The employment of PhI
ACHTUNGTRNEUN(NG COOtBu)2 and
PhI(COOPh)2 as oxidants diminished the yield of pyrrole
AHCTUNGTRENNUNG
1
2
3
4
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
3.0
1.0
2.0
4.0
DMF
THF
CH3CN
DCE
tBuOH
DCE
HOAc
HOAc
HOAc
HOAc
HOAc
HOAc
HOAc
HOAc
70
70
70
70
70
70
70
25
50
100
120
70
70
70
–
–
2a (Table 2, entries 5 and 6 vs 1). Other hypervalent iodine
sources such as Koser’s reagent and phenyliodine bis(tri-
fluoroacetate) (PIFA) gave inferior results (Table 2, en-
tries 7 and 8).
Under the optimized conditions, we explored the sub-
strate scope of the reaction, and the results are summarized
in Table 3. Treatment of various 2,2-disubstituted 3-alkynyl
12:–
25:–
11:–
49:–
75:10
14:–
36:9
10:50
–:64
16:trace
34:8
64:15
5
6[c]
7
8
9
10
11
12
13
14
amines 1 with PhIACTHNUTRGNEUNG(OAc)2 furnished the corresponding 2,3,4-
trisubstituted pyrroles 2 regioselectively in good isolated
yields (Table 3, entries 1–6 and 10). The protocol is also
viable for preparing 2,3-disubstituted pyrroles with high re-
[a] Reaction conditions: 1a (0.2 mmol), solvent (3.0 mL). [b] Yield of iso-
lated product. [c] HOAc (5.0 equiv) was added.
gioselectivity using 2-substituted-3-alkynyl amines
1
(Table 3, entries 7–9). The alkynyl amines 1 with electron-
rich aromatic substituents afforded the pyrroles 2 in higher
yield than electron-deficient aromatic substituents (Table 3,
entries 2 vs 3 and 7 vs 8). Pyrrole 2e possessing cyclohexane
and pyrrole 2j bearing benzocycloheptane were obtained
readily from alkynyl amines 1e and 1j in 56% and 65%
yields, respectively (Table 3, entries 5 and 10). Two re-
gioisomers were obtained in the case of alkynyl amines 1k
and 1l with two different substituents at the 2-position
(Table 3, entries 11 and 12); these regioisomers are separa-
ble by silica-gel column chromatography. When the sub-
strate with disubstituted alkyne was used, no reaction oc-
curred and the substrate was recovered.
1,2-dichloroethane (DCE), or tBuOH (Table 1, entries 3–5).
The product 2a was obtained in 49% yield when five equiv-
alents of HOAc were added to DCE (Table 1, entry 6). Con-
sequently, when HOAc was chosen as the solvent, the yield
of trisubstituted pyrrole 2a increased to 75% and pyrrole
3a was formed in 10% yield (Table 1, entry 7). The amount
of (diacetoxyiodo)benzene also has a significant impact on
the yield of pyrroles 2a and 3a (Table 1, entry 7, and 12–
14). Using three equivalents of (diacetoxyiodo)benzene
proved to be optimal for the transformation (Table 1,
entry 7). The investigation on the effect of the temperature
showed that 2a was obtained in higher yield at 708C while
pyrrole 3a became the major product when temperature in-
creased to 1008C (Table 1, entries 7–11).
During the study of this reaction, we isolated dihydropyr-
role 4a in 8% yield using PhIACHTNUGTRENUNG(COOPh)2 as the oxidant
(Table 2, entry 6), which was deduced as a reaction inter-
mediate. To confirm this, the dihydropyrrole 5a was pre-
pared according to the literature procedure[14] and was sub-
The substituents on the nitrogen atom of alkynyl amines
showed their critical role in the reaction (Table 2). The reac-
jected to 1.2 equivalents of PhIACHTNUGTRENUNG(OAc)2 in acetic acid at 708C
[Eq. (2)]. Trisubstituted pyrroles 6a and 2a were obtained
in 29% and 5% yield, respectively, meanwhile the dihydro-
pyrrole 4a was formed in 53% yield by allylic oxidation of
the dihydropyrrole 5a. Further studies indicated that the di-
hydropyrrole 4a can be readily transformed into the trisub-
stituted pyrrole 2a in 31% yield, with the formation of
three byproducts 3a, 7a, and 8a [Eq. (3)]. These results sug-
gest that the dihydropyrrole 4a should serve as the precur-
sor of the pyrrole 2a.
Table 2. Effect of R Group of 1 and Oxidant on the Reaction.[a]
Entry
1, R
Oxidant
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(OAc)2
PhI(COOtBu)2
PhI(COOPh)2
2:4 [%][b]
1
2
3
4
5
6
7
8
Ts
Tf
G
75:–
69:–
ACHTUNGTRENNUNG
[c]
C(O)Ph
C(O)NHPh
Ts
Ts
Ts
Ts
R
–
–
[c]
N
G
69:–
45:8
–
AHCTUNGTRENNUNG
PhI(OH)OTs
PIFA
5:–
[a] Reaction conditions:
1 (0.2 mmol), oxidant (0.6 mmol), HOAc
(3.0 mL). [b] Yield of isolated product. [c] Complex mixture.
Chem. Asian J. 2011, 6, 3200 – 3204
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X.-L Hou et al.
COMMUNICATION
Table 3. Substrate Scope of the PhIACHTUNGTRNEUNG
(OAc)2-Mediated Reaction of Alkynyl Amines.[a]
Entry
1
1
Product
Yield [%][b]
75
1a
1b
2a
2b
2
3
82
68
1c
2c
4
1d
2d
72
5
6
1e
1 f
2e
2 f
56
45
7
8
1g
1h
2g
2h
78
68
9
1i
2i
55
65
10
11
1j
1k
2j
2k
71ACHTUNGTRENNUNG
(3:1)[c]
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Chem. Asian J. 2011, 6, 3200 – 3204

Metal-Free Synthesis of Polysubstituted Pyrroles
Table 3. (Continued)
Entry
1
Product
Yield [%][b]
12
1l
2l
49ACHTUNGTRENNUNG
(1:1)[c]
[a] Reaction conditions: 1 (0.2 mmol), PhI
by H NMR spectroscopy.
(OAc)2 (0.6 mmol), HOAc (3.0 mL). [b] Yield of isolated product. [c] The ratio of two isomers was determined
1
In summary, we have developed an efficient metal-free re-
gioselective synthesis of polysubstituted pyrroles in good
When compounds 9 and 10 were treated with 1.2 equiva-
yields by a PhIACHTNGUTERNNU(G OAc)2-mediated reaction of alkynyl amines,
lents of PhI
(OAc)2 under the reaction conditions, as shown
and some complex pyrrole structures can be constructed. A
plausible reaction mechanism was proposed. Further investi-
gations on the detailed reaction mechanism and applications
of the reactions in organic synthesis are in progress.
in Table 3, no reaction occurred and the starting material
was recovered [Eqs. (4) and (5)]. It was suggested that the
double bond of the dihydropyrrole 4a might be initially oxi-
dized by PhIACHTUNGTRENNUNG(OAc)2 during the conversion into the pyrrole
2a. With the above-experimental results and literature re-
ports,[12,15,16] a speculated mechanism was proposed using al-
kynyl amine 1a as an example (Scheme 1). Reaction of ter-
Experimental Section
General procedure for preparation of polysubstituted pyrrole 2
In a Schlenk tube, alkynyl amine 1 (0.2 mmol) was added to a stirring
mixture of PhIACHTNUGTRENUNG(OAc)2 (192 mg, 0.6 mmol), and acetic acid (1.5 mL) under
an atmosphere of air. Then, the mixture was heated to 708C for 12 h
until the substrate 1 disappeared, as monitored by TLC. The mixture was
diluted with dichloromethane (5 mL) and water (5 mL) was added. The
aqueous solution was extracted with dichloromethane (5 mLꢁ3) and the
organic layer was combined and washed with NaHCO3 (5 mLꢁ2), dried
with anhydrous Na2SO4. The solvent was removed under reduced pres-
sure and the residue was purified by flash chromatography on silica gel
using petroleum ether/EtOAc (10:1–5:1) as an eluent to give product 2.
Scheme 1. Speculated Mechanism for the PhI
of Alkynyl Amine 1a. PIDA=PhI(OAc)2.
ACHTUNGRTEN(NUNG OAc)2-Mediated Reaction
AHCTUNGTRENNUNG
Acknowledgements
Financial support from the Major Basic Research Development Program
(2011CB808700), National Natural Science Foundation of China
(20932008, 20872161, 20821002), Chinese Academy of Sciences, the Ex-
ternal Cooperation Program of CAS (GJHZ200816), the Technology
Commission of Shanghai Municipality and the Croucher Foundation of
Hong Kong are greatly acknowledged.
minal alkyne 1a with PhIACTHNUTRGNE(UGN OAc)2 forms the alkynyliodonium
salts Int-A.[12] An intramolecular aza-Michael-type addition
affords the Int-B,[16] the migration of the proton gives rise to
the Int-C. The transformation of the Int-C into pyrrole 2a
may proceed through a PhIACTHNUTRGNE(NUG OAc)2-mediated oxidation pro-
cess and 1,2-rearrangement.[17] The exact reaction mecha-
nism still needs more experimental evidence.
Keywords: amines
compounds · pyrrole · regioselectivity
·
cascade reaction
·
hypervalent
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X.-L Hou et al.
COMMUNICATION
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Received: May 21, 2011
Published online: September 26, 2011
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Chem. Asian J. 2011, 6, 3200 – 3204