Catalytic asymmetric transannulation of NH-1,2,3-triazoles with olefins

Article abstract of DOI:10.1002/anie.201306706

A convenient one-pot asymmetric synthesis of 2,3-dihydropyrroles from in situ generated triflated triazoles and olefins is described that further expands the utility of azavinyl carbene chemistry and provides access to an important class of cyclic enamides. Mechanistic investigations support the involvement of triflated cyclopropylaldimine intermediates in the formation of 2,3-dihydropyrrole. To the best of our knowledge, this is the first example of a chiral Bronsted acid catalyzed rearrangement of cyclopropylimines into enantioenriched 2,3-dihydropyrroles. Enantioenriched dihydropyrroles can be generated by a rhodium(II)-catalyzed asymmetric transannulation between sulfonyl-1,2,3-triazoles and electron-rich styrenes. Mechanistic investigations support the formation of cyclopropane intermediates which undergo ring expansion to 2,3-dihydropyrroles in the presence of a chiral Bronsted acid catalyst.

Full text of DOI:10.1002/anie.201306706

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DOI: 10.1002/anie.201306706
Azavinyl Carbenes
Catalytic Asymmetric Transannulation of NH-1,2,3-Triazoles with
Olefins**
Sen Wai Kwok, Li Zhang, Neil P. Grimster, and Valery V. Fokin*
Abstract: A convenient one-pot asymmetric synthesis of 2,3-
dihydropyrroles from in situ generated triflated triazoles and
olefins is described that further expands the utility of azavinyl
carbene chemistry and provides access to an important class of
cyclic enamides. Mechanistic investigations support the
involvement of triflated cyclopropylaldimine intermediates in
the formation of 2,3-dihydropyrrole. To the best of our
knowledge, this is the first example of a chiral Brønsted acid
catalyzed rearrangement of cyclopropylimines into enantioen-
riched 2,3-dihydropyrroles.
azoles include 1-sulfonylated derivatives, which can be
synthesized in multigram amounts, isolated, and safely
stored,[8e] or can be generated in situ from stable NH-triazoles
1 with triflic anhydride under mild conditions (Scheme 1).[6d]
In the presence of RhII catalysts, N-triflyltriazoles formed in
this way are readily converted into azavinyl carbenes, which
readily react with a variety of olefins 2 to yield highly
enantioenriched cyclopropane carboxaldehydes 3.
However, when 4-methoxystyrene was subjected to the
reaction, only 2,3-dihydropyrroles 4 were obtained in excel-
lent yield and moderate enantioselectivity. While formation
of 2,3-dihydrofurans by the reaction of a-diazocarbonyl
compounds with electron-rich olefins is known,[10,11] the
analogous transformation of a-diazoimines[6d,7j] had not
been reported prior to our recent disclosure.[6d]
T
he multifaceted reactivity of diazo carbonyl compounds[1] is
illustrated by their many uses in both research and industrial
laboratories, ranging from photolithography in the manufac-
ture of computer chips[2] to the synthesis of pharmaceuticals[3]
and production of insecti-
cides.[4] In contrast to a-diazo-
carbonyl compounds, the
reactivity
and
synthetic
potential of the related a-
diazoimines has remained rel-
atively unexplored, in large
part because of the dearth of
safe and convenient routes to
access this class of com-
pounds.[5] Recently, we[6] and
others[7] have demonstrated
that
transition-metal/aza-
vinyl carbenes can be
accessed directly from certain
electron-deficient
1,2,3-tri-
azoles,[8] likely as a result of
the ring-chain tautomerism of
Scheme 1. Reactions of N-triflyl-Rh-azavinyl carbenes with olefins. Tf=triflate, EDG=electron-donating
the latter.[9] The reactive tri- group.
[*] Dr. S. W. Kwok, Dr. L. Zhang, Dr. N. P. Grimster, Prof. Dr. V. V. Fokin
Department of Chemistry, The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
E-mail: fokin@scripps.edu
Here, we describe our studies of this novel transformation
and results of a mechanistic investigation of possible path-
ways that give rise to this useful class of heterocycles.[12] To
improve the enantioselectivity of this transformation, a panel
of chiral dirhodium catalysts was evaluated in the reaction of
4-phenyl-NH-1,2,3-triazole and 4-methoxystyrene (Table 1).
Similar to the trends observed in the RhII-catalyzed cyclo-
propanations,[6c,d] increasing the steric volume of the dirho-
dium catalysts improved the enantiomeric excess of product
4aa (entries 1–5), with the [Rh2(S-NTTL)4] catalyst providing
the highest enantioselectivity (72% ee, entry 5).[6d] Intrigu-
ingly, increasing steric demands further through the introduc-
tion of a 4-bromo substituent into the naphthalene ring
(entry 6) did not significantly affect the enantioselectivity.
Homepage: http://www.scripps.edu/fokin
Prof. Dr. V. V. Fokin
Moscow Institute of Physics and Technology
9 Institutskiy per., Dolgoprudny
Moscow Region, 141700 (Russian Federation)
[**] This work was supported by the National Institute of General
Medical Sciences, National Institutes of Health (GM087620), and
Ministry of Science and Education of Russian Federation
(14.A12.31.0005). We thank Prof. Hisashi Yamamoto (Univ. of
Chicago) for his kind gift of chiral phosphoric acids.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306706.
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Table 1: Screening of chiral catalysts.[a]
Entry
Chiral catalyst
Yield [%][b]
ee [%]
1
2
3
4
5
6
7
8
9
[Rh2(S-NTA)4]
[Rh2(S-NTL)4]
[Rh2(S-NTPA)4]
[Rh2(S-NTV)4]
[Rh2(S-NTTL)4]
[Rh2(S-4-Br-NTTL)4]
[Rh2(S-PTTL)4]
[Rh2(S-TFPTTL)4]
[Rh2(S-4-Me-PTTL)4]
[Rh2(S-BPTTL)4]
68
74
67
61
92
40
15
56
34
33
23
20
44
55
72
70
46
70
66
64
10
[a] Unless otherwise specified, all reactions were performed initially at
À308C and were then allowed to warm up to room temperature.
[b] Yields of isolated product. DTBMP=2,6-di-tert-butyl-4-methylpyri-
dine.
Scheme 2. Substrate scope of [Rh2(S-NTTL)4]-mediated synthesis of
2,3-dihydropyrroles.
Furthermore, the enantioselectivity remained essentially
unchanged with [Rh2(S-TFPTTL)4], a catalyst that Hashi-
moto and co-workers used to significantly improve the
enantioselectivity of RhII-catalyzed nitrene transfers[13]
(entry 8). These observations suggested that the enantiose-
lectivity of the reaction was only partially determined by the
catalyst.
Intrigued by the apparent limit of enantioselectivity, we
examined the electronic and steric effects of substrates on the
process. To this end, substituents on both the 1,2,3-triazole
ring and the olefin partner were varied (Scheme 2). Interest-
ingly, incorporation of an electron-withdrawing substituent at
the C4-position of the triazole ring, for example, 4-trifluor-
omethyl (4ab) and 4-cyanophenyl (4ac), reduced the ee value
of the products by about 10%, while introduction of a less
electronegative group, such as 3-chlorophenyl, returned the
ee value to approximately the same level as seen for the
parent reaction (4ad, 74% ee). In contrast, the presence of an
electron-donating substituent, such as 4-methoxyphenyl, at
the C4-position drastically reduced the enantiomeric excess of
the product (4ae; 15% ee). The strong dependence of the
enantioselectivity on the electronic properties of the sub-
strates was further illustrated by the reaction of 4-ethyl-
carboxy-NH-triazole with 4-methoxystyrene, which delivered
a racemic mixture of 4af in 38% yield. These results indicated
that NH-triazoles containing either strongly electron-donat-
ing or strongly electron-deficient substituents at the C4-
position were less efficient reaction partners with electron-
rich olefins under the current set of experimental conditions.
To examine steric effects, 4-methoxystyrene was replaced
with the bulkier, but still electron-rich, 2-methoxystyrene.
Reaction between 4-phenyl-NH-triazole and 2-methoxystyr-
ene led to a reduction in the ee value by approximately 10%
(4ag versus 4aa). The steric and electronic effects on the
enantioselectivity were also shown to be cumulative: the
reaction of 4-cyanophenyl-NH-triazole with 2-methoxystyr-
ene reduced the ee value of the product to 49% (4ah versus
4ac).
To gain insight into the mechanism of the reaction and to
improve both the enantioselectivity and yield of this process,
we carried out a series of experiments aimed at establishing
the role that the rhodium catalyst plays in the formation of the
2,3-dihyrdopyrroles. Subsequent to the generation of the
rhodium carbene II, two pathways can account for the
formation of the observed product (Scheme 3). In pathway A,
the carbene reacts with the electron-rich olefin to yield
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hydride (Scheme 4). Impor-
tantly, in these 1H NMR studies,
the presence of
7 was not
detected prior to the complete
conversion of 5 into 6. These
results suggest that an analo-
gous cyclopropane intermediate
may exist during the formation
of the triflated 2,3-dihydropyr-
roles (Pathway A, Scheme 3).
The ring expansion of the
cyclopropane intermediate was
examined using cyclopropylal-
dimine 9, which was synthe-
sized, isolated, and character-
ized. We were pleased to
observe that no 2,3-dihydropyr-
role products were formed when
9 was heated in CDCl3 at 608C
for 12 h, either in the presence
Scheme 3. Proposed pathways for the formation of 2,3-dihydropyrroles.
cyclopropylaldimine III. The presence of the
strongly electron-withdrawing triflate com-
bined with the electron-donating methoxy
group facilitates the cleavage of the cyclo-
propane ring, and generates a stabilized
zwitterionic intermediate IV, which collapses
into the corresponding 2,3-dihyrdopyrrole. In
this mechanism, the chiral rhodium catalyst
exerts stereocontrol over the final product by
determining the enantioselectivity of the
intermediate cyclopropane. The subsequent
rearrangement leads to the erosion of enan-
Scheme 4. Rearrangement of cyclopropyl-N-octylsulfonylimine 6 into 2,3-dihydropyrrole 7.
tiomeric excess of the final product, the
extent of which is dependent on the relative
À
rate of rotation about the Ca Cb bond versus
the rate of ring closure of the open-chain intermediate IV. In
pathway B, the electron-rich alkene reacts with the Rh-
carbene species to form the rhodium-bound zwitterionic
intermediate V. Cyclization of Vand release of the Rh catalyst
from VI affords the desired product. The chiral rhodium
catalyst would thus be expected to directly control the final
stereoselectivity of this reaction.
The general instability of cyclopropyl-N-triflylimine inter-
mediates, combined with the heterogeneity of the reaction
mixture, complicated NMR studies. To circumnavigate these
problems, we focused on detecting the formation of 2,3-
dihydropyrroles prepared by the reaction of the more stable
4-phenyloctylsulfonyltriazole (5) and 4-methoxystyrene. The
[Rh2(S-NTTL)4]-catalyzed reaction was performed in CDCl3
at 608C, and was monitored by 1H NMR spectroscopy
(Scheme 4). These experiments demonstrated that 2,3-dihy-
dropyrroles were formed in two consecutive steps: initially,
triazole 5 was converted into cyclopropylimine 6, which
subsequently rearranged to 2,3-dihydropyrrole 7. The inter-
mediacy of 6 was further confirmed by isolation and
spectroscopic characterization of its derivative 8 (88% ee),
which was obtained by reduction of 6 with lithium aluminum
Scheme 5. Acid-catalyzed ring expansion of cyclopropylaldimine 9.
or absence of a dirhodium catalyst (Scheme 5), thus eliminat-
ing the possibility that either the catalyst or heat is responsible
for the rearrangement of cyclopropylaldimine. Pioneering
studies by Cloke[14] and further investigations by Stevens[12]
established that cyclopropylimines rearrange to 2,3-dihydro-
pyrroles under acidic conditions. In addition, related acid-
mediated ring expansions of cyclopropyl aldehydes and
ketones to yield 2,3-dihydrofurans are also well docu-
mented.[15] We, therefore, suspected that trace amounts of
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1
a sulfonic acid, generated by hydrolysis of the starting
material, sulfonyltriazole, might be responsible for the
observed rearrangement. To test this hypothesis, methane-
sulfonic acid (MsOH, 0.01 equiv) was added to a solution of
a racemic mixture of 9 in CDCl3, which resulted in the rapid
(< 5 min) formation of racemic 10rac at room temperature
(Scheme 5). This result establishes the kinetic competency of
alkylsulfonic acids in catalyzing the ring expansion of cyclo-
12 by H NMR spectroscopy and chiral HPLC indicated the
presence of cis and trans stereoisomers, thus implicating the
À
rotation around the Ca Cb bond as the cause. This bond
rotation limits the transfer of chirality from the presumably
highly enantioenriched cyclopropane to the 2,3-dihydropyr-
role.[16] The apparent upper limit of the enantioselectivity
(ca. 70% ee) observed in the chiral catalyst screen (Table 1) is
likely a result of the erosion of the enantioselectivity through
rapid bond rotation during the rearrangement.
propylaldimine 9 to 2,3-dihydropyrroles 10rac
.
We further hypothesized that the introduction of a chiral
Brønsted acid may induce an enantioselective rearrangment.
Indeed, subjecting a racemic mixture of 9 to a catalytic
amount of chiral phosphoric acid (HA) generated enantioen-
riched 2,3-dihyrdopyrroles 10 (58% ee; Scheme 6).
The convenient one-pot asymmetric synthesis of 2,3-
dihydropyrroles from in situ generated triflated triazoles and
olefins described here further expands the utility of azavinyl
carbene chemistry and provides access to an important class
of cyclic enamides. Mechanistic investigations support the
involvement of triflated cyclopropylaldimine intermediates in
the formation of 2,3-dihydropyrrole. To the best of our
knowledge, this is the first example of a chiral Brønsted acid
catalyzed rearrangement of cyclopropylimines into enan-
tioenriched 2,3-dihydropyrroles. Manipulation of the prod-
ucts should enable asymmetric synthesis of cyclic amines,
amino acid analogues,[17] and complex polycyclic architec-
tures[18] found in natural products and pharmaceutically
useful compounds. These studies, along with computational
investigations, are currently underway.
Scheme 6. Chiral Brønsted acid catalyzed ring expansion of cyclo-
propylaldimine 9.
Experimental Section
Typical procedure for the synthesis of 2,3-dihydropyrroles as exem-
plified by the synthesis of 4aa: [Rh2(S-NTTL)4] (2.5 mg, 0.0017 mmol,
0.5 mol%), 2,6-di-tert-butyl-4-methylpyridine (DTBMP) (84 mg,
0.41 mmol, 1.2 equiv), phenyl-NH-1,2,3-triazole (50 mg, 0.34 mmol,
1.0 equiv), and anhydrous chloroform (2 mL) were added to an oven-
dried reaction vessel fitted with a magnetic stirrer bar, and sealed with
a septum under a dry nitrogen atmosphere. 4-Methoxystyrene
(138 mg, 137 mL, 1.03 mmol, 3.0 equiv) was then added, and the
resulting purple suspension was cooled to À308C while stirring. After
2–3 min, neat triflic anhydride (102 mg, 60 mL, 0.36 mmol, 1.05 equiv)
was added in one portion with a glass syringe. The color of the
reaction mixture changed immediately from purple to green. The
reaction mixture was allowed to warm from À308C to room
temperature overnight, then quenched with saturated aqueous
NaHCO3 (4 mL), and extracted with EtOAc (3 ꢀ 10 mL). The
combined organic layers were dried over MgSO4, filtered, and
concentrated under reduced pressure. The crude product was purified
by flash column chromatography over silica gel with 2% EtOAc in
hexanes as the eluting solvent to recover the DTBMP (83 mg, 99%).
The solvent gradient was gradually raised to 10% EtOAc in hexanes
to elute the product. Concentrating the desired fractions under
reduced pressure afforded 4aa as a colorless oil (120 mg, 92%).
The results of these mechanistic investigations are con-
sistent with the involvement of a cyclopropylaldimine inter-
mediate in the reaction, with the resultant 2,6-di-tert-butyl-4-
methylpyridinium triflate acting as the possible source of the
Brønsted acid. Proton activation, however, may have limited
importance in the rearrangement of triflated cyclopropyli-
mines, because of the increased electron-withdrawing power
of the trifluoromethanesulfonyl group compared with alkyl-
sulfonyl groups. While we could not obtain spectroscopic data
to unambiguously identify triflated cyclopropylimines in the
reaction mixtures, we were able to probe this intermediate by
replacing 4-methoxystyrene with deutero-4-methoxystyrene
11 and generating the corresponding deutero-2,3-dihydropyr-
roles 12 under standard reaction conditions (Scheme 7).
Characterization of the isolated deuterium-labeled products
Received: July 31, 2013
Published online: February 14, 2014
Keywords: azavinyl carbenes · dihydropyrroles · enamides ·
.
rhodium · triazoles
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