Stereoselective synthesis of 2,3-dihydropyrroles from terminal alkynes, azides, and α,β-unsaturated aldehydes via N-sulfonyl-1,2,3-triazoles

Article abstract of DOI:10.1021/ja407166r

A stereoselective method for synthesis of trans-2,3-disubstituted 2,3-dihydropyrroles is reported. N-Sulfonyl-1,2,3-triazoles prepared from terminal alkynes generate α-imino rhodium carbene complexes, which when combined with α,β-unsaturated aldehydes produce trans-2,3- disubstituted dihydropyrroles. The method can be successfully applied to a one-pot process starting from terminal alkynes.

Full text of DOI:10.1021/ja407166r

Communication
pubs.acs.org/JACS
Stereoselective Synthesis of 2,3-Dihydropyrroles from Terminal
Alkynes, Azides, and α,β-Unsaturated Aldehydes via N‑Sulfonyl-1,2,3-
triazoles
Tomoya Miura,* Takamasa Tanaka, Kentaro Hiraga, Scott G. Stewart,† and Masahiro Murakami*
Department of Synthetic Chemistry and Biological Chemistry, Kyoto University, Katsura, Kyoto 615-8510, Japan
S
* Supporting Information
Table 1. Denitrogenative Reaction of Triazole 1a with (E)-
Crotonaldehyde (4a): Screening of Rhodium(II) Catalysts
a
ABSTRACT: A stereoselective method for synthesis of
trans-2,3-disubstituted 2,3-dihydropyrroles is reported. N-
Sulfonyl-1,2,3-triazoles prepared from terminal alkynes
generate α-imino rhodium carbene complexes, which
when combined with α,β-unsaturated aldehydes produce
trans-2,3-disubstituted dihydropyrroles. The method can
be successfully applied to a one-pot process starting from
terminal alkynes.
b
yield (%)
entry
Rh2(L)4
5aa
32
6aa
1
2
3
4
5
6
Rh2(OCOC7H15)4
Rh2(OCOtBu)4
34
18
19
48
16
3
49
he 2,3-dihydropyrrole ring system is a valuable structural
1
Rh2(OCO-1-Ad)4
Rh2[(S)-DOSP]4
Rh2[(S)-PTPA]4
Rh2[(S)-NTTL]4
67
Tmotif found in a number of biologically active compounds.
46
In addition, 2,3-dihydropyrroles have been widely employed as
important intermediates in the synthesis of natural products2 and
other complex molecules.3 Thus, the development of efficient
methods for their synthesis from readily accessible starting
materials is highly desired.4 Now, we report a sequential
procedure for the diastereoselective synthesis of trans-2,3-
disubstituted 2,3-dihydropyrroles from terminal alkynes, N-
sulfonyl azides, and α,β-unsaturated aldehydes (Figure 1).
40
74 (80)
a
Conditions: 1a (0.20 mmol), 4a (0.22 mmol), and 4 Å MS (40 mg)
were heated in toluene (1 mL) at 120 °C for 1 h in the presence of
b
Rh2(L)4 (2.0 μmol). 1H NMR yield using CHBr2CHBr2 as an
internal standard, with isolated yield in parentheses.
to the production of 4-oxazolines, as recently reported by Fokin
et al.10 As a continuation of our previous studies on the utilization
of N-sulfonyl-1,2,3-triazoles for synthetic purposes, we next
examined the use of α,β-unsaturated aldehydes as the reaction
partner. Thus, we initially prepared 1-methanesulfonyl-4-phenyl-
1,2,3-triazole (1a) from phenylacetylene (2a) and methanesul-
fonyl azide (3a) according to the procedure using copper(I)
thiophene-2-carboxylate (CuTC).5b Triazole 1a (0.20 mmol)
was reacted with (E)-crotonaldehyde (4a, 0.22 mmol) in the
presence of Rh2(OCOC7H15)4 (1.0 mol %) and 4 Å molecular
sieves (MS) in refluxing toluene (Table 1, entry 1). Triazole 1a
was completely consumed in 1 h, giving a mixture of 2,3-
dihydropyrrole (5aa, 32% NMR yield) and 4,5-dihydro-1,4-
oxazepine (6aa, 34% NMR yield). The relative stereochemistry
at the 2,3-positions of 5aa was unambiguously assigned as trans
by a single-crystal X-ray analysis. Next, a variety of ligands on
rhodium(II) (Figure 2) were examined in terms of product
selectivity, which showed a significant dependence on the ligands
(entries 2−6). In particular, when the sterically bulky chiral
ligand (S)-NTTL11 was employed, formation of 6aa was
suppressed (3% NMR yield) and trans-5aa was obtained in
80% isolated yield as a racemic mixture. Production of the five-
membered-ring compound 5aa is in sharp contrast to the
Figure 1. Construction of 2,3-dihydropyrroles starting from terminal
alkynes, N-sulfonyl azides, and α,β-unsaturated aldehydes.
N-Sulfonyl-1,2,3-triazoles, which can be easily prepared from
terminal alkynes by copper-catalyzed 1,3-dipolar cycloaddition
with N-sulfonyl azides,5 have recently received much attention as
precursors of α-imino metal carbenes.6 The generated metal
carbene species are electrophilic in nature to accept various
nucleophiles, including those having acidic protons like water7
and allylic alcohols.8 In our previous report, the addition of allylic
alcohols was immediately followed by a Claisen rearrangement to
afford α-allyl-α-amino-ketones. On the other hand, the nitrogen
atom of the α-imino group exhibits a higher nucleophilic
character than the related α-oxo metal carbenes. This
nucleophilicity, when combined with the electrophilic character
of the carbene carbon, enables the compound to incorporate
unsaturated compounds such as nitriles, alkynes, allenes,
isocyanates, furans, and indoles to produce the corresponding
N-heterocycles.9 For example, the reaction with aldehydes leads
Received: July 13, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja407166r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Scheme 1. Proposed Mechanism for the Formation of
Products 5 and 6 from Triazole 1a and α,β-Unsaturated
Aldehyde 4
Figure 2. Chiral Rh(II) catalysts examined in the optimization studies.
reaction of rhodium α-oxo-carbene intermediates with α,β-
unsaturated aldehydes, which furnished alkenyl-substituted
epoxides without participation of the α-oxo moieties.12
A pair of (E)- and (Z)-isomers of cinnamaldehyde (4b) was
then used to compare the stereochemical outcomes. The two
isomers were independently subjected to the reaction conditions
(Rh2[(S)-NTTL]4, 120 °C, 1 h), and both isomers furnished
exclusively the trans-2,3-disubstituted dihydropyrrole 5ab in high
yield as a racemic mixture (eq 1).
the α,β-unsaturated aldehyde 4 to the electrophilic carbene
center of A occurs to furnish the zwitterionic intermediate B. The
anionic rhodium releases an electron pair, which induces the
imino nitrogen to attack on either the α- or γ-carbon of the
oxonium ion. Attack on the γ-carbon forms 4,5-dihydro-1,4-
oxazepine 6 (path a), whereas attack on the α-carbon forms the
4-oxazoline intermediate 7 (path b). The C−O bond of the N,O-
aminal moiety of 7 is selectively cleaved, probably due to the
higher electronegativity of oxygen and the higher electron-
donating ability of nitrogen,15 giving the zwitterionic inter-
mediate C. Double bond isomerization occurs readily with the
delocalized conjugate cation moiety of C, which converges into a
form of the more stable (E)-isomer. Finally, the enolate moiety of
(E)-C intramolecularly couples with the delocalized conjugate
cation moiety through conformation E, which experiences less
gauche interactions along the axis of the developing carbon−
carbon bond, to give the trans-configured 2,3-dihydropyrrole 5.
Although a [3,3] sigmatropic rearrangement presents an
alternative option for the mechanistic pathway, the ionic
mechanism mentioned above is favored on the basis of the
non-stereospecificity between the isolated enantiomerically
enriched 4-oxazoline intermediate and the product trans-5aa,13
and the production of the racemic dihydropyrroles in the other
cases (vide infra).
The scope of α,β-unsaturated aldehydes 4 was examined using
Rh2[(S)-NTTL]4 as the catalyst (Table 2). (E)-β-Monosub-
stituted substrates 4c−g, possessing a wide variety of alkyl and
aryl groups, reacted cleanly with triazole 1a to afford products
5ac−ag in yields ranging from 71% to 90% (entries 1−5). In
addition, the mono(dimethyl acetal) of fumaraldehyde (4h) and
fumaraldehydic acid methyl ester (4i) successfully participated in
the annulation reaction (entries 6 and 7). As in the case of (E)-
and (Z)-cinnamaldehyde (4b) (eq 1), both (E)- and (Z)-isomers
of hex-2-enal (4c) selectively gave the trans isomer of the product
5ac (entries 1 and 8). The reaction of (Z)-3-bromoacrylaldehyde
(4j) was followed by the subsequent E1cB process, resulting in
the formation of 2-benzoylpyrrole 8 (entry 9). Acyclic α,β-
disubstituted substrates 4k−n were also effectively converted
When the reaction of triazole 1a with (E)- and (Z)-4b was
carried out for 1 h at room temperature in chloroform, the
corresponding (E)- and (Z)-2-styryl-4-oxazolines 7ab, which
retained their original double-bond geometries, were isolated
respectively.13 When each isolated 7ab was independently
heated in refluxing toluene for 1 h, both isomers exclusively
afforded the trans-isomer of 2,3-dihydropyrrole 5ab (eq 2).10
The rearrangement reaction of the 4-oxazolines 7ab was
monitored at 50 °C by 1H NMR. When starting from (E)-7ab,
only the remaining (E)-7ab and the trans-product 5ab were
detected throughout the reaction course. On the other hand,
when starting from (Z)-7ab, the (E)-7ab was detected in
addition to the (Z)-7ab and the trans-product 5ab. These results
suggested that the (Z)-7ab isomerized to (E)-7ab prior to the
rearrangement process.
On the basis of these results and the previous study on the
formation of 4-oxazolines from aldehydes,10 we propose a
mechanism for the formation of products 5 and 6 from triazole
1a and the α,β-unsaturated aldehyde 4 as depicted in Scheme 1.
Initially, a reversible ring−chain tautomerization of the triazole
1a generates α-diazo imine 1a′,14 which reacts with rhodium(II)
in an irreversible manner to afford an α-imino rhodium carbene
A with extrusion of molecular nitrogen. Nucleophilic addition of
B
dx.doi.org/10.1021/ja407166r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 2. Rh(II)-Catalyzed Denitrogenative Annulation of
Triazole 1a with Various α,β-Unsaturated Aldehydes 4c−p
Table 3. Rh(II)-Catalyzed Denitrogenative Annulation of
Various Triazoles 1b−i with (E)-Cinnamaldehyde (4b)
a
a
triazole 1
b
entry
R1
R4
product 5 yield (%)
1
2
3
4
5
6
7
8
4-MeO-C6H4
Me
Me
Me
Me
1b
1c
1d
1e
1f
5bb
5cb
5db
5eb
5fb
91
87
73
48
89
83
88
77
4-CF3-C6H4
3-thienyl
nPr
c
Ph
(CH2)2TMS
4-Tol
Ph
1g
1h
1i
5gb
5hb
5ib
Ph
4-MeO-C6H4
4-Br-C6H4
Ph
a
Conditions: 1a (0.20 mmol), 4b (0.22 mmol), and 4 Å MS (40 mg)
were heated in toluene (1 mL) at 120 °C for 1 h in the presence of
b
Rh2[(S)-NTTL]4 (2.0 μmol). Isolated yield (average of two runs).
c
Using 4b (0.60 mmol), Rh2[(S)-NTTL]4 (5.0 μmol), and 4 Å MS
(10 mg) in toluene (0.2 mL) for 2 h.
Table 4. One-Pot Synthesis of 2,3-Dihydropyrroles 5 Starting
a
from Phenylacetylene (2a)
α,β-unsaturated
azide 3
R4
Me
Me
aldehyde 4
b
entry
R2
R3
product 5 yield (%)
1
2
3
4
5
3a
3a
3g
3a
3a
Me
Ph
Ph
Ph
Me
H
4a
4b
4b
4l
5aa
5ab
5gb
5al
68
70
74
69
68
H
4-Tol
Me
H
Me
H
c
Me
4a
5aa
a
Conditions: 2a (0.20 mmol), 3 (0.20 mmol), 4 (0.22 mmol), CuTC
(20 μmol), Rh2[(S)-NTTL]4 (2.0 μmol), and 4 Å MS (40 mg) in
toluene (1 mL) were stirred at rt for 6 h, then heated at 120 °C for 1 h.
a
b
c
Conditions: 1a (0.20 mmol), 4 (0.22 mmol), and 4 Å MS (40 mg)
were heated in toluene (1 mL) at 120 °C for 1 h in the presence of
Isolated yield (average of two runs). On a 10 mmol scale using 4a
(15 mmol).
b
Rh2[(S)-NTTL]4 (2.0 μmol). Isolated yield (average of two runs).
c
d
e
E:Z = 9:91. E:Z = 5:>95. Using 4 (0.40 mmol).
With the detailed study on the transformation of the triazoles 1
into the 2,3-dihydropyrroles 5 finalized, we next carried out a
one-pot synthesis of compounds 5 from terminal alkyne 2 in
order to demonstrate the practical convenience of the present
transformation (Table 4). Phenylacetylene (2a, 0.20 mmol), N-
sulfonyl azides 3 (0.20 mmol), α,β-unsaturated aldehydes 4 (0.22
mmol), CuTC (10 mol %), Rh2[(S)-NTTL]4 (1.0 mol %), 4 Å
MS, and toluene (1 mL) were placed in a reaction vessel, and the
reaction mixture was simply stirred at room temperature. Both 2a
and 3 were consumed after 6 h. The reaction mixture was
subsequently stirred at 120 °C for an additional 1 h. After
chromatographic separation, the compounds 5 were isolated in
overall yields ranging from 68% to 74% (entries 1−4). An
experiment using 1.0 g of 2a (10 mmol) also gave a comparable
result (entry 5).
into the products 5ak−an (entries 10−13). 1-Cyclohexene-1-
carbaldehyde (4o) gave the bicyclic compound 5ao in only 35%
yield due to low product selectivity 5/6 (entry 14).16 Acrolein
(4p) was also a suitable substrate, furnishing the product 5ap in
73% yield (entry 15). The products 5ad, 5ae, 5af, 5ag, and 5ap
were analyzed by chiral HPLC and determined to be racemic.
Variation of triazoles 1 was also examined in the reaction with
(E)-cinnamaldehyde (4b) (Table 3). Triazoles 1b−d, possessing
aryl groups at the 4-position, afforded the corresponding
products 5bb−db in yields ranging from 73% to 91% (entries
1−3). The reaction of the alkyl-substituted triazole 1e gave the
product 5eb in moderate yield, due to 1,2-hydride migration
occurring with the rhodium carbene intermediate to form N-
mesyl-pent-2-en-1-imine (entry 4).17 The annulation reaction
was amenable with respect to the R4 substituent on the sulfonyl
group to give the products 5fb−ib in high yields (entries 5−8).
The synthetic utility of the dihydropyrrole products was
exemplified by further transformations. Deprotective aromatiza-
C
dx.doi.org/10.1021/ja407166r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
J.; Gutzeit, H. O.; Knolker, H.-J. Angew. Chem., Int. Ed. 2009, 48, 8042.
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̈
Scheme 2. Synthetic Derivatization of 2,3-Dihydropyrroles
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tion took place on treatment with 1,8-diazabicycloundec-7-ene
(DBU) through an E1cB/prototropy sequence (Scheme 2).10
Tetrahydropyrrole (2,3-disubstituted pyrrolidine) 10 was
obtained when 5aa was hydrogenated using Crabtree’s catalyst.
Furthermore, 2,3,5-trisubstituted pyrrolidine 12 was diastereo-
selectively synthesized through a sequence of hydration under
acidic conditions, Horner−Wadsworth−Emmons olefination,
and an aza-Michael reaction.18
In summary, we have disclosed an interesting and useful
reactivity of α-imino rhodium carbenes toward α,β-unsaturated
aldehydes, providing an efficient method for the diastereo-
selective synthesis of trans-2,3-disubstituted 2,3-dihydropyrroles
from terminal alkynes.
(7) Miura, T.; Biyajima, T.; Fujii, T.; Murakami, M. J. Am. Chem. Soc.
2012, 134, 194.
(8) Miura, T.; Tanaka, T.; Biyajima, T.; Yada, A.; Murakami, M. Angew.
Chem., Int. Ed. 2013, 52, 3883.
(9) (a) Horneff, T.; Chuprakov, S.; Chernyak, N.; Gevorgyan, V.;
Fokin, V. V. J. Am. Chem. Soc. 2008, 130, 14972. (b) Miura, T.;
Yamauchi, M.; Murakami, M. Chem. Commun. 2009, 1470. (c) Grimster,
N.; Zhang, L.; Fokin, V. V. J. Am. Chem. Soc. 2010, 132, 2510.
(d) Chattopadhyay, B.; Gevorgyan, V. Org. Lett. 2011, 13, 3746.
(e) Chuprakov, S.; Kwok, S. W.; Fokin, V. V. J. Am. Chem. Soc. 2013,
135, 4652. (f) Schultz, E. E.; Sarpong, R. J. Am. Chem. Soc. 2013, 135,
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135, 6802.
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, spectral data for the new compounds,
and details of the X-ray analysis. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
tmiura@sbchem.kyoto-u.ac.jp
murakami@sbchem.kyoto-u.ac.jp
■
(10) Zibinsky, M.; Fokin, V. V. Angew. Chem., Int. Ed. 2013, 52, 1507.
(11) (a) Muller, P.; Allenbach, Y.; Robert, E. Tetrahedron: Asymmetry
̈
2003, 14, 779. (b) Ghanem, A.; Gardiner, M. G.; Williamson, R. M.;
Muller, P. Chem.Eur. J. 2010, 16, 3291. (c) DeAngelis, A.; Boruta, D.
̈
Notes
T.; Lubin, J.-B.; Plampin, J. N., III; Yap, G. P. A.; Fox, J. M. Chem.
Commun. 2010, 46, 4541.
The authors declare no competing financial interest.
†S.G.S. is on leave from School of Chemistry and Biochemistry,
The University of Western Australia
(12) Davies, H. M. L.; DeMeese, J. Tetrahedron Lett. 2001, 42, 6803.
(13) The isolated (E)-7ab was racemic. On the other hand, the
reaction of triazole 1a with (E)-crotonaldehyde (4a) under analogous
conditions afforded enantiomerically enriched 4-oxazoline (73% ee), in
accordance with Fokin’s report.10 However, the isolated 4-oxazoline
facilely underwent racemization even at room temperature. These
results indicate that the ring opening of the N,O-aminal moiety and
recyclization occur in a reversible way to racemize 4-oxazolines.
(14) McKinney, M. A.; Patel, P. P. J. Org. Chem. 1973, 38, 4059.
(15) For an aza-Cope/Mannich reaction triggered by the C−O bond
cleavage of the N,O-aminal moiety of 5-vinyloxazolidines, see:
(a) Overman, L. E.; Kakimoto, M.; Okawara, M. Tetrahedron Lett.
1979, 20, 4041. (b) Johnson, B. F.; Marrero, E. L.; Turley, W. A.;
Lindsay, H. A. Synlett 2007, 893. (c) Carballo, R. M.; Purino, M.;
Ramírez, M. A.; Martín, V. S.; Padron, J. I. Org. Lett. 2010, 12, 5334.
́
(16) The byproduct 6ao was easily deprotected in situ to afford 1,4-
oxazepine derivative in 20% yield.
(17) (a) Miura, T.; Funakoshi, Y.; Morimoto, M.; Biyajima, T.;
Murakami, M. J. Am. Chem. Soc. 2012, 134, 17440. (b) Selander, N.;
Worrell, B. T.; Fokin, V. V. Angew. Chem., Int. Ed. 2012, 51, 13054.
(18) (a) Collado, I.; Ezquerra, J.; Vaquero, J. J.; Pedregal, C.
Tetrahedron Lett. 1994, 35, 8037. (b) Cheng, T.; Meng, S.; Huang, Y.
Org. Lett. 2013, 15, 1958.
ACKNOWLEDGMENTS
■
We thank Dr. Yuuya Nagata (Kyoto University) for helping with
X-ray analysis. This work was supported by MEXT (Grant-in-Aid
for Scientific Research on Innovative Areas Nos. 22105005 and
24106718, Scientific Research (B) No. 23350041, Young
Scientists (A) No. 23685019), JST (ACT-C), and Takeda
Science Foundation. S.G.S. is grateful for the JSPS Invitation
Fellowship for Research in Japan.
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dx.doi.org/10.1021/ja407166r | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX