One-pot synthesis of 2,5-dihydropyrroles from terminal alkynes, azides, and propargylic alcohols by relay actions of copper, rhodium, and gold

Article abstract of DOI:10.1002/chem.201405357

Relay actions of copper, rhodium, and gold formulate a one-pot multistep pathway, which directly gives 2,5-dihydropyrroles starting from terminal alkynes, sulfonyl azides, and propargylic alcohols. Initially, copper-catalyzed 1,3-dipolar cycloaddition of

Full text of DOI:10.1002/chem.201405357

DOI: 10.1002/chem.201405357
Communication
&
Synthetic Methods
One-Pot Synthesis of 2,5-Dihydropyrroles from Terminal Alkynes,
Azides, and Propargylic Alcohols by Relay Actions of Copper,
Rhodium, and Gold
Tomoya Miura,* Takamasa Tanaka, Kohei Matsumoto, and Masahiro Murakami*[a]
Dedicated to Professor Iwao Ojima (SUNY) on the occasion of his 70th birthday
one[4] with regard to the position of unsaturation. Of particular
Abstract: Relay actions of copper, rhodium, and gold for-
note is that multiple reactions occur stepwise by simply apply-
mulate a one-pot multistep pathway, which directly gives
ing the requisite substrate/catalyst to terminal alkynes. A
2,5-dihydropyrroles starting from terminal alkynes, sulfonyl
single execution of work-up/purification procedures gave sub-
azides, and propargylic alcohols. Initially, copper-catalyzed
stantial expansion of the structural, as well as functional com-
1,3-dipolar cycloaddition of terminal alkynes with sulfonyl
plexities.
azides affords 1-sulfonyl-1,2,3-triazoles, which then react
The direct synthesis of various 2,5-dihydropyrroles in one
with propargylic alcohols under the catalysis of rhodium.
pot starting from terminal alkynes is summarized in Table 1. 1-
The resulting alkenyl propargyl ethers subsequently un-
Arylethynes 1a–d having both electron-donating and -with-
dergo the thermal Claisen rearrangement to give a-allen-
drawing substituents reacted facilely with tosyl azide (2a) and
yl-a-amino ketones. Finally, a gold catalyst prompts 5-
propargylic alcohol 3a to give the corresponding 2,5-dihydro-
endo cyclization to produce 2,5-dihydropyrroles.
pyrroles 4a–d in yields ranging from 56 to 83% (entries 1–4).
Mesyl azide (2b) also successfully participated in the reaction
(entry 5). Multisubstituted 2,5-dihydropyrroles 4 f and g could
Functionalized pyrrolidines constitute a core structural element
broadly found in natural products and pharmaceutically active
substances.[1] Dihydropyrroles with an unsaturation available
for further functionalization are valuable precursors for their
synthesis.[2] Therefore, the development of efficient methods
for the synthesis of dihydropyrroles from readily available start-
ing materials has been an active area of research.[3] We have
reported the synthesis of 2,3-dihydropyrroles from terminal al-
kynes, sulfonyl azides, and a,b-unsaturated aldehydes.[4] a-
Imino carbenoid species are involved therein as the key reac-
tive intermediate of electrophilic nature.[5] Herein, we report
a new synthetic protocol constituting 2,5-dihydropyrroles from
three components including propargylic alcohols (Figure 1).
Relay actions of copper, rhodium, and gold catalysts[6] formu-
late a one-pot multistep pathway, complementing the previous
be regioselectively synthesized by using a,a-disubstituted and
a,a,g-trisubstituted propargylic alcohols 3b and c, respectively
(entries 6 and 7).
From the experimental viewpoint, the transformation con-
sists of three operational procedures and a single work-up/pu-
rification procedure. In the first procedure, 1-sulfonyl-1,2,3-tria-
zoles were generated from terminal alkynes and sulfonyl
azides by the authentic method developed by Fokin;[7] termi-
nal alkynes 1 (1.0 equiv) were treated with sulfonyl azides 2
(1.0 equiv) in the presence of copper(I) thiophene-2-carboxyl-
ate (10 mol%, CuTC) in 1,2-dichloroethane (DCE) at room tem-
perature for 6 h. In the second procedure, propargylic alcohols
3 (1.5 equiv) and rhodium(II) pivalate dimer (1.0 mol%) were
added to the same reaction vessel, which was heated at 1008C
for 30 min under microwave (MW) irradiation[8] to prompt OÀH
insertion and subsequent sigmatropic rearrangement.[9–12] The
resulting a-allenyl-a-amino ketones were then cyclized in the
third procedure; [Au(JohnPhos)(CH3CN)][SbF6] (5.0 mol%, John-
Phos=(2-biphenyl)di-tert-butylphosphine)[13] was added to the
reaction vessel, which was further stirred at room temperature
for 3 h. Finally, the reaction mixture was subjected to a work-
up/purification procedure to give 2,5-dihydropyrrole products
4.
Figure 1. Constitution of 2,5-dihydropyrroles from terminal alkynes, tosyl
azide, and propargylic alcohols.
The rhodium-catalyzed OÀH insertion reaction in the second
procedure was not interfered by the copper catalyst used in
the first procedure for 1,3-dipolar cycloaddition. Neither the
gold-catalyzed cyclization reaction occurring in the third proce-
dure was interfered by the copper and rhodium catalysts used
in the first and second procedures. As a result, the whole mul-
tistep transformation can be carried out in a same reaction
[a] Prof. Dr. T. Miura, T. Tanaka, K. Matsumoto, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University, Katsura, Kyoto 615-8510 (Japan)
E-mail: tmiura@sbchem.kyoto-u.ac.jp
murakami@sbchem.kyoto-u.ac.jp
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201405357.
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Propargylic alcohol 3 undergoes nucleophilic addi-
tion to the electrophilic carbenoid carbon of A to
give zwitterionic intermediate B. Then, a proton
shifts onto the imino nitrogen with release of rhodiu-
m(II), forming the (Z)-isomer of alkenyl propargyl
ether C. It is probably spontaneous at 1008C for C to
undergo the thermal Claisen-type [3,3]-sigmatropic
rearrangement via the six-membered chairlike transi-
tion state D (see below) to produce a-allenyl a-
amino ketone 6.[9–11] The third procedure follows the
gold-catalyzed cyclization reaction developed by
Krause.[3d] The allenyl group is activated by gold to
induce intramolecular addition of the amino group in
a 5-endo mode, producing 2,5-dihydropyrrole 4.[15]
To delineate the scope of the present method, the
transformation consisting of the second rhodium(II)-
catalyzed and third gold(I)-catalyzed procedures was
examined by using the propargylic alcohol 3a and
various isolated 1-tosyltriazoles 5 (Table 2). Triazoles
5aa–ga possessing electron-donating and -withdraw-
ing aryl groups at the fourth positions all gave the
corresponding products in good to high yields (en-
tries 1–6 in Table 2). A similar result was obtained
when the reaction of entry 1 was carried out on
a larger scale by using 2.0 mmol of 5aa. Even alkyl-
substituted triazoles 5ha–ja successfully gave 2,5-di-
hydropyrroles 4k–m, despite of the possibility of 1,2-
Table 1. Sequential one-pot reaction starting from terminal alkynes 1.
Entry 1-Alkyne 1
Azide 2
R2
Alcohol 3
R3 C(R4)2
Product Yield [%][d]
4
R1
1
2
3
4
5
6
7
Ph
p-Tol
1a p-Tol 2a
1b p-Tol 2a
1c p-Tol 2a
H
H
H
H
H
H
CH2
CH2
CH2
CH2
CH2
CMe2
3a 4a
83
83
82
56
86
73[e]
73
3a 4b
3a 4c
3a 4d
3a 4e
3b 4 f
p-MeOC6H4
p-MeO2CC6H4 1d p-Tol 2a
Ph
Ph
Ph
1a Me
2b
1a p-Tol 2a
1a p-Tol 2a Me cyclohexylidene 3c 4g
[a] 1 (0.20 mmol), 2 (0.20 mmol), CuTC (20 mmol), 1,2-dichloroethane (DCE, 1 mL). [b] 3
(0.30 mmol), [Rh2(tBuCO2)4] (2.0 mmol), DCE (1 mL). [c] [Au(JohnPhos)(CH3CN)][SbF6]
(10 mmol), DCE (1 mL). [d] Isolated yield (average of two runs). [e] Using [Au-
(JohnPhos)(CH3CN)][SbF6] (20 mmol) for 15 h.
vessel,[14] necessitating only a one-time execution of a work-
up/purification procedure, which would be most costly in gen-
eral, consuming a significant amount of time and solvents.
Scheme 1 depicts a more detailed mechanistic explanation
for the sequential procedures mentioned above. The first op-
erational procedure is for the copper(I)-catalyzed 1,3-dipolar
cycloaddition.[7b] The resulting triazole 5 equilibrates with a-
diazo imine 5’, which in the second procedure reacts with the
rhodium(II) catalyst to generate a-imino rhodium-carbene
complex A.[5]
hydride shift occurring with the intermediate rhodium(II) car-
bene complex (entries 7–9 in Table 2).[16] However, the reaction
Table 2. Sequential reaction of various triazole 5 with propargyl alcohol
(3a).[a]
Entry
Triazole 5
R1
Product 4
Yield [%][b]
R2
1
2
3
4
5
6
7
8
Ph
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
p-Tol
Me
iPr
Me3Si(CH2)2
p-MeOC6H4
p-BrC6H4
5aa
5ba
5ca
5ea
5 fa
5ga
5ha
5ia
4a
4b
4c
4h
4i
4j
4k
4l
4m
4n
4e
4o
4p
4q
4r
89
89
90
84
85
90
80[c]
71[c]
81[c]
51[d]
86
95
81
94
90
p-MeOC6H4
p-EtO2CC6H4
p-CF3C6H4
3-thienyl
Et
n-Hex
BzO(CH2)4
EtO
Ph
Ph
Ph
Ph
9
5ja
10
11
12
13
14
15
5ka
5ab
5ac
5ad
5ae
5af
Ph
[a] 1) 5 (0.20 mmol), 3a (0.30 mmol), [Rh2(tBuCO2)4] (2.0 mmol), DCE
(1 mL); 2) [Au(JohnPhos)(CH3CN)][SbF6] (10 mmol), DCE (1 mL). [b] Isolated
yield (average of two runs). [c] Using 3a (0.60 mmol). [d] 1) 3a
(1.0 mmol), 10 min; 2) [Au(JohnPhos)(CH3CN)][SbF6] (20 mmol).
Scheme 1. Plausible mechanism for the one-pot synthesis of 4 from 1, 2,
and 3.
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of the ethoxy-substituted triazole 5ka was slower to give the
product 4n in 51% yield,[9a,17] probably due to the less electro-
philic nature of the carbenoid carbon (entry 10). The reaction
was efficient with respect to the R2 substituent on the sulfonyl
group (entries 11–15 in Table 2).
The reaction scope was also examined with respect to prop-
argylic alcohols 3 (Scheme 2). g-Substituted alcohols 3d and
e were regioselectively converted to 3-substituted 2,5-dihydro-
pyroles 4s and t, respectively. On the other hand, the reaction
of a,a-disubstituted alcohols 3 f gave 5,5-disubstituted 2,5-di-
hydropyroles 4u. These results are consistent with the occur-
rence of [3,3]-sigmatropic rearrangement during the transfor-
mation.
Scheme 3. Reaction with enantiomerically pure alcohol 3g. 1) [Rh2(tBuCO2)4]
(1.0 mol%), DCE, 1008C/MW, 30 min; 2) [Au(JohnPhos)(CH3CN)][SbF6]
(5.0 mol%), DCE, RT, 3 h.
Scheme 2. Sequential reaction of triazole 5aa with various substituted prop-
argylic alcohols 3d–f.
When enantiomerically pure a-substituted alcohol 3g was
used, a 3:1 mixture of trans/cis diastereoisomers was obtained
(Scheme 3). Notably, the enantiomeric purity was retained with
the both diastereoisomers (trans 99% ee, cis 99% ee), the abso-
lute configurations of which were not determined. When the
sequential transformation was intercepted before the addition
of the gold catalyst, a diastereomeric mixture of a-allenyl-a-
amino ketones G and H (3:1) was obtained. Based on these re-
sults, we assume that the alkenyl moiety of the intermediate
alkenyl propargylic ether partially isomerizes under the reac-
tion conditions before the [3,3]-sigmatropic rearrangement,
giving a 3:1 mixture of Z/E isomers. The Z and E isomers both
undergo the rearrangement via the six-membered chairlike
transition states E and F, respectively, with the a-methyl sub-
stituent taking the pseudoequatorial position. The chirality of
3g is transferred to the axial chirality of the allenyl group.
Then, the external allenic p bond coordinates to gold(I) from
the face cis to the Ht. The nitrogen intramolecularly adds to
the activated p bond from the face opposite to the gold(I) in
a five-endo mode to form the zwitterionic species K and L, re-
spectively. Finally, protodemetalation affords 4v with genera-
tion of a catalytically active gold(I) complex.[3d,18] Thus, the orig-
inal central chirality of 3g is first transferred to the axial chirali-
ty of the allenyl group, and finally back to the central chirality
of the fifth position of the dihydropyrroles 4v.
Further derivatization of the obtained dihydropyrroles are
exemplified in Equation (1). Treatment of 4a with 1,8-diazabi-
cycloundec-7-ene (DBU) induced a sequence of E1cB and pro-
totropy to produce pyrrole 7. The carbon–carbon double bond
was stereoselectively dihydroxylated by a simple dihydroxyla-
tion protocol by using K2OsO2(OH)2, giving diol 8 in 87% yield
(6:1 d.r.). Monosubstituted pyrrolidine 9 was readily synthe-
sized by hydrogenation using the Crabtree’s catalyst.
In summary, we have developed the one-pot multistep pro-
cedure for the synthesis of 2,5-dihydropyrroles starting from
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Communication
[4] T. Miura, T. Tanaka, K. Hiraga, S. G. Stewart, M. Murakami, J. Am. Chem.
Soc. 2013, 135, 13652.
terminal alkynes. The three different catalysts operate in relay
without any serious catalyst deactivation,[19] making this se-
quential transformation work out.
[5] For reviews on synthetic transformations of triazoles, see: a) A. V. Gule-
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[6] For one-pot sequential reactions involving gold catalysts, see: a) M. D.
Milton, Y. Inada, Y. Nishibayashi, S. Uemura, Chem. Commun. 2004,
2712; b) J. T. Binder, S. F. Kirsch, Org. Lett. 2006, 8, 2151; c) ꢅ. Aksin-
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[7] a) E. J. Yoo, M. Ahlquist, S. H. Kim, I. Bae, V. V. Fokin, K. B. Sharpless, S.
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[8] We used a microwave synthesizer in to safely handle a low-boiling-
point solvent at high temperature. When a toluene solution of triazole
5aa and propargylic alcohol 3a (1.5 equiv) was simply heated at 1008C
in the presence of [Rh2(tBuCO2)4] (1.0 mol%) for 30 min without micro-
wave irradiation, 6aa was formed also in high yield.
[9] It is possible to isolate the intermediate a-allenyl-a-amino ketones. For
the previous reports on the rhodium-catalyzed reaction of triazoles
with propargylic alcohols forming a-allenyl-a-amino ketones, see: a) T.
Miura, T. Tanaka, T. Biyajima, A. Yada, M. Murakami, Angew. Chem. Int.
Ed. 2013, 52, 3883; Angew. Chem. 2013, 125, 3975; b) S. Chuprakov, B. T.
Worrell, N. Selander, R. K. Sit, V. V. Fokin, J. Am. Chem. Soc. 2014, 136,
195.
Experimental Section
Typical procedure for the one-pot synthesis of 2,5-dihydropyr-
roles from terminal alkynes (Table 1, entry 1): Phenylethyne (1a,
20.4 mg, 0.20 mmol), TsN3 (2a, 39.4 mg, 0.20 mmol), CuTC (3.8 mg,
20 mmol), and 1,2-dichloroethane (1 mL) were added to an oven-
dried 0.5–2 mL Biotage microwave vial equipped with a stirrer bar
in a glovebox. The vial was sealed with a Teflon pressure cap. The
reaction mixture was stirred at RT for 6 h. Then, propargylic alcohol
3a (16.8 mg, 0.30 mmol) and [Rh2(tBuCO2)4] (1.2 mg, 2.0 mmol) in
1,2-dichloroethane (1 mL) were added. The mixture was heated at
1008C for 30 min under microwave irradiation. After the resulting
mixture was cooled, the solution of [Au(JohnPhos)(CH3CN)][SbF6]
(7.7 mg, 10 mmol)) in 1,2-dichloroethane (1 mL) was added. The re-
action mixture was stirred at RT for 3 h, and it was concentrated
under reduced pressure. The resulting residue was purified by
preparative thin-layer chromatography (CHCl3/ethyl acetate 30:1)
to give the 2,5-dihydropyrrole 4a (55.7 mg, 0.17 mmol, 83%) as
a white solid.
Acknowledgements
This work was supported by MEXT (Grant-in-Aid for Scientific
Research on Innovative Areas nos. 22105005 and 24106718,
Young Scientists (A) no. 23685019, Scientific Research (B) no.
23350041) and JST (ACT-C).
Keywords: copper · gold · rhodium · terminal alkynes ·
triazoles
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[13] C. Nieto-Oberhuber, S. Lꢂpez, M. P. MuÇoz, D. J. Cꢇrdenas, E. BuÇuel, C.
Nevado, A. M. Echavarren, Angew. Chem. Int. Ed. 2005, 44, 6146; Angew.
Chem. 2005, 117, 6302.
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Communication
[14] We carried out the all-in-one-pot reaction by mixing all substrates and
catalysts at once and heating the mixture. However, no reaction oc-
curred, and terminal alkynes remained unchanged.
[15] T. J. Brown, D. Weber, M. R. Gagnꢆ, R. A. Widenhoefer, J. Am. Chem. Soc.
2012, 134, 9134.
[16] a) T. Miura, Y. Funakoshi, M. Morimoto, T. Biyajima, M. Murakami, J. Am.
Chem. Soc. 2012, 134, 17440; b) N. Selander, B. T. Worrell, V. V. Fokin,
Angew. Chem. Int. Ed. 2012, 51, 13054; Angew. Chem. 2012, 124, 13231.
[17] J. S. Alford, H. W. L. Davies, J. Am. Chem. Soc. 2014, 136, 10266.
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1790; b) T. Miura, M. Shimada, P. de Mendoza, C. Deutsch, N. Krause, M.
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[19] When the isolated a-allenyl-a-amino ketone 6a was treated with [Au-
(JohnPhos)(CH3CN)][SbF6] (0.5 mol%) at RT for 5 min, the corresponding
2,5-dihydropyrroles 4a was formed in 68% yield (NMR) and the starting
material was recovered in 26% yield (NMR). The same reaction in the
presence of both the gold catalyst and [Rh2(tBuCO2)4] (1.0 mol%) gave
a similar result, suggesting no interference of the rhodium(II) catalyst
on the gold(I)-catalyzed cyclization reaction.
Received: September 21, 2014
Published online on && &&, 0000
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Communication
COMMUNICATION
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Synthetic Methods
T. Miura,* T. Tanaka, K. Matsumoto,
M. Murakami*
&& – &&
One-Pot Synthesis of 2,5-
Trio works! Relay actions of copper,
rhodium, and gold formulate a one-pot
multistep pathway, which directly gave
2,5-dihydropyrroles starting from termi-
nal alkynes, sulfonyl azides, and propar-
gylic alcohols. A single execution of
work-up/purification procedures gives
substantial expansion of the structural,
as well as functional complexities (see
scheme).
Dihydropyrroles from Terminal
Alkynes, Azides, and Propargylic
Alcohols by Relay Actions of Copper,
Rhodium, and Gold
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Chem. Eur. J. 2014, 20, 1 – 6
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!