Autotandem catalysis: Synthesis of pyrroles by gold-catalyzed cascade reaction

Article abstract of DOI:10.1021/ol5024695

A novel synthesis of substituted pyrroles by a gold(I)-catalyzed cascade reaction has been developed. The reaction proceeded with an autotandem catalysis consisting of an initial addition of gold-acetylide to an acetal moiety and was followed by gold-catalyzed 5-endo-dig cyclization and aromatization. Gold catalysts play a dual role in activating nucleophilicity or electrophilicity of terminal acetylenes by forming gold-acetylides or by π-coordination. The formal (3 + 2) annulation of two components provided a variety of substituted pyrroles in a modular fashion.

Full text of DOI:10.1021/ol5024695

Letter
pubs.acs.org/OrgLett
Autotandem Catalysis: Synthesis of Pyrroles by Gold-Catalyzed
Cascade Reaction
Hirofumi Ueda, Minami Yamaguchi, Hiroshi Kameya, Kenji Sugimoto,† and Hidetoshi Tokuyama*
Graduate School of Pharmaceutical Sciences, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan
S
* Supporting Information
ABSTRACT: A novel synthesis of substituted pyrroles by a gold(I)-
catalyzed cascade reaction has been developed. The reaction
proceeded with an autotandem catalysis consisting of an initial
addition of gold−acetylide to an acetal moiety and was followed by
gold-catalyzed 5-endo-dig cyclization and aromatization. Gold
catalysts play a dual role in activating nucleophilicity or electro-
philicity of terminal acetylenes by forming gold−acetylides or by π-coordination. The formal (3 + 2) annulation of two
components provided a variety of substituted pyrroles in a modular fashion.
yrroles are an important nitrogen-containing heterocycle
Scheme 1. Formation of Indolizinones by Gold-Catalyzed
Cascade Double Cyclization Developed in Our Group
P
often observed in biologically significant natural products,1
pharmaceutical substances,2 and functional materials.3 There-
fore, the development of a novel construction of pyrrole has
inspired synthetic chemists, and numerous methods have been
reported to date. However, most of the well-known classical
methods, such as the Paal−Knorr4 and Hantzsh5 pyrrole
syntheses, have limited utility concerning functional group
compatibility because these methods usually require relatively
harsh conditions such as heating in the presence of a strong acid.
However, many transition metal-catalyzed formations of
substituted pyrroles have also been developed in the past
decade.6 Because of mild reaction conditions, these reactions
enable a wide substrate scope and have enjoyed high synthetic
utility for the construction of substituted pyrroles. In particular,
gold catalysts have recently received considerable attention for
their low catalyst loading and excellent functional group tolerance
owing to mild reaction conditions.7,8
Scheme 2. Working Hypothesis of Intermolecular Reaction
Against this background, we recently reported a highly efficient
construction of indolizinones 2 by a gold-catalyzed cascade
double cyclization of linear yneamide 1 (Scheme 1). The utility of
this reaction was fully demonstrated by our total syntheses of
(−)-rhazinilam 3 and (−)-rhazinicine 4.9 The proposed
mechanism is initiated by intramolecular nucleophilic addition
of nitrogen to the activated alkyne by π-coordination of cationic
gold catalyst, affording an enamide intermediate. Then, after
protonolysis of the gold−carbon bond, the resultant enamide
should undergo cyclization; the subsequent elimination of
methanol and aromatization should lead to indolizinone 2. We
demonstrated the intermediacy of the enamide species by
isolating the enamide as a mixture of E/Z isomers.9 We observed
that a gold catalyst was indispensable to promote not only
enamide formation but also pyrrole ring formation from the
enamide intermediate.9
pyrrole synthesis (Scheme 2). Herein, we report a new synthesis
of substituted pyrroles via a gold-catalyzed cascade reaction. The
reaction was found to proceed by autotandem catalysis,10 in
which one gold catalyst catalyzed two consecutive processes by
two different modes of activation of terminal alkynes: σ-activation
by forming gold−acetylides to activate its nucleophilicity and
activation of its electrophilicity by π-coordination.
At the outset, we examined the reaction of N-(2,2-
dimethoxyethyl)benzamide 7a and phenylacetylene 8a under
the standard conditions established for the intramolecular
Because the indolizinone skeleton is regarded as a substituted
pyrrole, we envisioned that the application of the intramolecular
process using the linear substrates 1 to an intermolecular reaction
using acetals 7 and terminal acetylenes 8 would lead to a new
Received: August 20, 2014
Published: September 9, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
C−F with bulky biphenyl moieties13 (Table 1, entries 9−12), C
and F proved to be superior to the others and to B. In addition,
catalyst G,14 which bore a N-heterocyclic carbene ligand, gave 9a
in a yield similar to those obtained with C−F (Table 1, entry 13).
At this point, we carefully analyzed the side products of the
reaction of entry 12 in Table 1 and isolated pyrrole derivative 10
in 13% yield; this compound should be generated by a gold-
catalyzed overreaction of 9a with 8a. The generation of 10 was
substantially suppressed under diluted conditions (0.25 M);
more importantly, the yield of 9a was increased up to 77% (Table
1, entry 14). Finally, the reaction time could be shortened when
diisopropyl acetal 8b was used instead of 8a (Table 1, entry 15).
In addition, the reaction could be conducted with reduced
catalyst loading (3 mol %) at higher temperature (140 °C) when
catalyst G was used (Table 1, entry 16). Ultimately, we selected
two sets of conditions, entries 15 and 16 (Table 1), as optimal
conditions.
Encouraged by these results, we then directed our attention
toward the substrate scope. Subjection of various terminal
arylalkynes 8b−m to condition set A (acetal 7b, catalyst F, and
AgOTf in toluene at 110 °C) and to condition set B (acetal 7a,
catalyst G, and AgOTf in xylene at 140 °C) revealed that a broad
range of functional groups is compatible with these reaction
conditions (Table 2). Arylalkynes 8b−e, bearing electron-
donating substituents, such as methoxy, methyl, and protected
Scheme 3. Unexpected Regiochemistry in the First Trial of
Intermolecular Reaction
Table 1. Optimization of Gold-Catalyzed Pyrrole Formation
entry
catalysts
solvent
time (h)
yield (%)
a
1
Au(PPh3)NTf2
Au(PPh3)NTf2
PPh3AuCl, AgNTf2
PPh3AuCl, AgPF6
PPh3AuCl, AgBF4
PPh3AuCl, AgOTf
A, AgOTf
1,4-dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
xylene
3
10
13
15
19
23
37
29
40
53
48
41
53
46
77
78
77
a
2
5
3
3.5
4
5
5
1.5
6
1
7
1
8
B, AgOTf
1
9
C, AgOTf
2
10
11
12
13
D, AgOTf
45 min
Table 2. Substrate Scope of Acetylenes
E, AgOTf
1
F, AgOTf
1
G, AgOTf
1
b
14
F, AgOTf
2
bc
,
15
16
F, AgOTf
1
d
G, AgOTf
10 min
a
b
c
Au(PPh3)NTf2 was prepared by premixing PPh3AuCl with AgNTf2.
Conditions: F (10 mol %), AgOTf (10 mol %) in toluene (0.25 M).
The corresponding diisopropylacetal 7b was used as the substrate.
d
Conditions: G (3 mol %), AgOTf (3 mol %) in xylene (0.5 M) at 140
°C.
reaction (Scheme 3).9 Heating of a 1,4-dioxane solution of 7a
(0.5 M) and five equivalents of 8a in the presence of Gagosz’s
11
catalyst, Au(PPh3)NTf2, at 110 °C gave a pyrrole as a single
isolable product in low yield. Surprisingly, the structure of the
product was not that of 3-phenylpyrrole derivative 5a, which we
expected on the basis of the intramolecular reaction; the product
was instead 2-phenylpyrrole derivative 9a. The structure of 9a
1
was unambiguously determined on the basis of its H NMR
spectrum.12
Although we obtained unexpected results, we optimized the
reaction to maximize the yield of the pyrrole product (Table 1).
The yield of 9a was slightly improved when toluene was used as a
solvent (Table 1, entry 2). Reactions using combinations of
PPh3AuCl and various silver salts (Table 1, entries 3−6) revealed
that AgOTf gave the best result to provide 9a in 37% yield (Table
1, entry 6). We then surveyed a series of catalysts bearing a variety
of ligands (Table 1, entries 7−13). A comparison of catalysts
bearing triarylphosphine ligands (Table 1, entries 6−8) revealed
that catalyst B, which bore a para-trifluoromethyl group,
improved the yield (Table 1, entry 8). Among a series of catalysts
a
Conditions A: Substrate 7b, F/AgOTf (10 mol %) in toluene (0.25
M) at 110 °C. Conditions B: Substrate 7a, G/AgOTf (3 mol %) in
xylene (0.5 M) at 140 °C. Conditions: G/AgOTf (5 mol %) at 140
°C. Conditions: Substrate 7b, G/AgOTf (5 mol %) in xylene (0.25
M) at 120 °C. A 3-substituted pyrrole was obtained in 7% yield.
b
c
d
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Letter
Scheme 4. Syntheses of Tri- and Tetrasubstituted Pyrroles
Table 3. Control Reactions Using N-Methyl Amide
yield (%)
entry
catalysts
19
20
1
2
3
4
F/AgOTf
AgOTf
F
0
63
0
37
89
90
0
none
0
Scheme 6. Plausible Mechanism of Autotandem Gold
Catalysis in the Cascade Reaction
amino groups, and a halogen, such as a bromo group at the para
position of the benzene ring, gave the corresponding pyrroles
9b−e in good to high yields (Table 2, entries 1−4).
However, alkynes 8f and 8g, which bear electron-withdrawing
groups, exhibited lower reactivity and afforded pyrroles 9f and 9g
in modest yields (Table 2, entries 5 and 6). Substrates 8h−l,
which bear ortho- and meta-disubstituted phenyl and naphthyl
groups, also gave the desired products 9h−l in good to high yields
(Table 2, entries 7−11). Finally, an alkyl-substituted alkyne, such
as 1-hexyne 8m, was observed to be feasible in the reaction and
under condition set B; the corresponding pyrrole 9m, which is
associated with a small amount of the 3-substituted regioisomers,
was obtained in moderate yield (Table 2, entry 12).
Significantly, further functionalized acetal fragments were
found to be applicable to the reaction for providing multi-
substituted pyrroles via a slight optimization of the reaction
conditions (Scheme 4).9 Thus, reaction of acetal 11, which was
readily prepared from (−)-alanine,15 proceeded smoothly under
a modified version of condition set B (i.e., microwave irradiation
in the presence of catalytic KHSO4) to give 1,2,5-trisubstituted
pyrrole derivative 12 in good yield. In addition, acetal 13, which
was synthesized from glycine in a few steps, gave the
corresponding 1,2,4-trisubstituted pyrrole derivative 14 in
modest yield under a modified version of condition set B.
Furthermore, reaction of acetal 15 with 8c afforded 1,2,3,5-
tetrasubstituted pyrrole derivative 16.
To gain mechanistic insights, we conducted several model
reactions. First, two modified substrates were subjected to
condition set A to identify the initial reaction site. The reaction of
amide 17 with no acetal moiety gave only a trace amount of
enamide 18, with recovery of the starting material 17 in 93% yield
(Scheme 5). This result indicated that the initial process was not a
C−N bond-forming reaction by nucleophilic addition of a
nitrogen atom to the activated alkyne, as that which occurs in the
intramolecular reaction (Scheme 1). However, the reaction of
amide 19, whose nitrogen was protected by a methyl group,
resulted in clean formation of acetylene adduct 20, strongly
suggesting that a gold−acetylide16 should be generated from the
cationic gold catalyst and terminal alkyne, which would add to an
oxonium ion formed from the acetal (Table 3, entry 1). The
generation of a gold−acetylide is supported by recent literature16
and by the results of control experiments reported in Table 3.
Thus, reactions using AgOTf or catalyst F did not provide 20
(Table 3, entries 2 and 3).
A plausible mechanism based on the control reactions is shown
in Scheme 6.17 The reaction should be initiated by formation of
gold−acetylide 21 to activate the nucleophilicity of the terminal
acetylene, which undergoes addition to oxonium ion 2210h to give
the alkyne adduct 23. Then, 5-endo-dig cyclization8c,d,g should
occur via activation of the electrophilicity of alkyne moieties by π-
coordination of the gold catalyst. Finally, protonolysis of the
carbon−gold bond 25, followed by aromatization, furnishes the
corresponding pyrroles 9, with regeneration of the cationic gold
catalyst. As a whole, our proposed mechanism involves
autotandem catalysis,10 in which the same catalyst catalyzes in
two different modes of activation of acetylene; this mechanism
thus clearly explains the different regiochemical outcomes
between the intramolecular reactions that we previously
reported9 and the intermolecular reaction developed in this
study.
Scheme 5. Control Reaction Using Benzamide Lacking Acetal
Moiety
In summary, we have developed a new addition−cyclization
sequence for the synthesis of multisubstituted pyrroles; this
sequence is promoted by autotandem gold catalysis. The
advantage of this reaction is its versatility and convergency to
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Letter
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* Supporting Information
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Experimental details and procedures, compound characterization
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AUTHOR INFORMATION
Corresponding Author
■
̀
Chem. 2010, 75, 510. (f) Benedetti, E.; Lemiere, G.; Chapellet, L.-L.;
*(H.T.) E-mail: tokuyama@mail.pharm.tohoku.ac.jp.
Present Address
†(K.S.) Graduate School of Medicine and Pharmaceutical
Sciences, University of Toyama, Toyama, 930-0887, Japan.
Penoni, A.; Palmisano, G.; Malacria, M.; Goddard, J.-P.; Fensterbank, L.
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was financially supported by the Cabinet Office,
Government of Japan through its Funding Program for Next
Generation World-Leading Researchers (LS008), a Grant-in-aid
for Scientific Research (A) (26253001) and Young Scientists (B)
(21790004 and 26860004), JSPS, and the Sumitomo Founda-
tion.
M.-P.; Valdes
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, C. Org. Lett. 2014, 16, 2264. (h) Yu, Y.; Yang, W.;
Rominger, F.; Hashmi, A. S. K. Angew. Chem., Int. Ed. 2013, 52, 7586.
(i) Wang, T.; Shi, S.; Pflasterer, D.; Rettenmeier, E.; Rudolph, M.;
̈
Romeinger, F.; Hashmi, A. S. K. Chem.−Eur. J. 2014, 20, 292.
DEDICATION
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(11) Mezailles, N.; Ricard, L.; Gagosz, F. Org. Lett. 2005, 7, 4133.
■
(12) For the structural determination based on 1H NMR, see
Supporting Information.
Professor Amos B. Smith, III on the occasion of his 70th birthday.
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