Gold-catalyzed synthesis of functionalized pyridines by using 2H-azirines as synthetic equivalents of alkenyl nitrenes

Article abstract of DOI:10.1002/anie.201402470

2H-Azirines are easily synthesized from the corresponding ketones and, despite possessing a C=N bond embedded in a strained three-membered cycle, they are sufficiently stable to be isolated, stored, and manipulated. 2H-Azirines can be regarded as valuable

Full text of DOI:10.1002/anie.201402470

Angewandte
Chemie
DOI: 10.1002/anie.201402470
Heterocycles
Hot Paper
Gold-Catalyzed Synthesis of Functionalized Pyridines by Using
2H-Azirines as Synthetic Equivalents of Alkenyl Nitrenes**
Agnes Prechter, Guilhem Henrion, Pierre Faudot dit Bel, and Fabien Gagosz*
Dedicated to Professor Albert Padwa
Abstract: 2H-Azirines are easily synthesized from the corre-
=
sponding ketones and, despite possessing a C N bond
embedded in a strained three-membered cycle, they are
sufficiently stable to be isolated, stored, and manipulated.
2H-Azirines can be regarded as valuable synthetic equivalents
of alkenyl nitrenes, however, reactions capitalizing on the
cyclic strain of the heterocyclic motif and involving the
À
cleavage of the C N single bond remain surprisingly limited.
A gold-catalyzed reaction that allows the formation of
polysubstituted functionalized pyridines from easily accessible
2-propargyl 2H-azirine derivatives was developed. This trans-
formation, which corresponds to an unprecedented intramo-
lecular transfer of an alkenyl nitrene to an alkyne, proceeds
with low catalyst loading, is efficient, and exhibits a high
functional-group tolerance and a wide substrate scope.
T
he unusual heterocyclic motifs 2H-Azirines[1] (1) are mostly
known as reactive intermediates in the Neber rearrange-
ment,[2] a useful transformation that allows the installation of
an amino group at positions a to a ketone. 2H-Azirines can be
easily synthesized from the corresponding ketones 2 by
following a reliable two-step procedure [Scheme 1, Eq. (1)].
=
While they possess an intriguing structure featuring a C N
bond embedded in a strained three-membered cycle, they are
sufficiently stable to be isolated, stored, and manipulated.
Of the various transformations of 2H-azirines that have
been reported to date,[1,3] those capitalizing on the cyclic
strain of the heterocyclic motif and involving the cleavage of
Scheme 1. 2H-Azirines as synthetic equivalents of nitrenes in metal-
mediated transformations and the design of a gold-catalyzed transfer
of alkenyl nitrenes to alkynes.
À
the C N single bond remain surprisingly limited. 2H-Azirines
can, however, be regarded as valuable synthetic equivalents
of alkenyl nitrenes (3)[1] and therefore deserve more consid-
eration in view of the recent developments in metal-mediated
nitrene chemistry.[4] To date, only two modes of reactivity
involving 2H-azirines as synthetic equivalents of nitrenes
have been described: a) the direct interaction of 1 with
a metal, thus leading to the formation of a metal–nitrene
complex of type 4 [Scheme 1, Eq. (2)][5]; and b) the reaction
between a metal–carbene complex (5) and 1 to produce an
azadiene such as 6 [Scheme 1, Eq. (3)].[6] In this context, and
following our continuing interest in gold catalysis,[7,8] we
considered the possibility of performing an alternative and
unprecedented alkenyl nitrene transfer to alkynes[9] by taking
advantage of the p-acid and electron-donor properties of gold
complexes [Scheme 1, Eq. (4)].[7] In such a process, the
nucleophilic addition of 1 to a gold-activated alkyne (7)
would furnish the aziridinium species 8, which would then
evolve into the corresponding reactive a-imino gold–carbene
intermediate 9. We report herein the successful implementa-
tion of this mechanistic design to the synthesis of function-
alized pyridines[10] 11 from 2-propargyl 2H-azirine derivatives
10 [Scheme 2, Equation (5)].[11] These substrates could be
obtained in an efficient and modular manner following the
synthetic routes shown in Equations (6) and (7) in Scheme 2.
Our investigations started with 2H-azirine 10a. The gold-
catalyzed cycloisomerization of 10a to give pyridine 11a was
chosen as a model to evaluate the feasibility of the alkenyl
nitrene transfer to an alkyne in its intramolecular version.
[*] Dr. A. Prechter, G. Henrion, P. Faudot dit Bel, F. Gagosz
Dꢀpartement de Chimie, UMR 7652 and 7653 CNRS
Ecole Polytechnique, 91128 Palaiseau (France)
E-mail: gagosz@dcso.polytechnique.fr
[**] We are deeply appreciative of financial support from Ecole
Polytechnique to A.P. and G.H. and from ANR to P.F., and we thank
Rhodia Chimie Fine (Dr. F. Metz) for a generous gift of HNTf2.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402470.
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possessing bulky ligands of the XPhos family were employed
(Table 1, Entries 5 and 6). The best result was obtained with
[(tBuXPhos)Au]NTf2 (13), which allowed the complete con-
version of 10a in 3 h and the formation of 11a in an excellent
yield of 95% (Table 1, Entry 6). An additional improvement
was subsequently achieved: working at a lower catalyst
loading of 2 mol% in refluxing 1,2-dichloroethane reduced
the reaction time to 1.5 h while maintaining the efficiency of
the process (Table 1, Entry 7). Under these experimental
conditions, pyridine 11a could be isolated in an optimal yield
of 90%. Additional control experiments were also carried out
to show that the transformation could not be performed by
using AgNTf2 or HNTf2 as the catalyst, or under simple
thermal conditions (Table 1, Entries 9–12).[14]
The optimal catalytic conditions noted in Table 1, Entry 7
(2 mol% of 13 in (CH2)2Cl2 at 858C) were then applied to
a range of 2-propargyl 2H-azirine derivatives (10a–w) to
Scheme 2. A synthetic route to functionalized pyridines.
Compound 10a, which could be conveniently obtained by determine the scope of the reaction. As seen from the results
a four-step sequence from tert-butyl acetoacetate,[12] was first collected in Table 2, the transformation proved to be
treated with the simple [(Ph3P)Au]NTf2 complex[13] (5 mol%) extremely general and the corresponding functionalized
in CDCl3 at 508C and the reaction was monitored by 1H NMR pyridines 11a–w could be obtained in good to excellent
spectroscopy (Table 1, Entry 1). We were pleased to observe yields (62–99%). The substrates were completely consumed
the slow but clean conversion of 10a into the desired pyridine in generally less than 10 h with the exception of terminal
11a, which gradually accumulated to reach a moderate yield alkynes (10v,w), for which an extended reaction time was
of 40% after 3 h. A series of other gold complexes possessing required. The reaction could be performed with substrates
various electronic and steric properties were then screened in possessing aryl groups with either electron-donating (11d,r,s)
an attempt to increase both the rate and the yield of the or electron-withdrawing (11e,f) substituents, heteroaromatic
reaction. The main results of this study are compiled in moieties such as a furan (11n,p) or thiophene (11j), or alkyl
Entries 2–8 of Table 1. While [(JohnPhos)Au]NTf2, chains (11k–n). Substitutions were tolerated at the alkyne
[(IPr)Au]NTf2 and the phosphite-based gold(I) complex 12 terminus (C(6)), at the propargylic position (C(4)), and on the
proved to be less catalytically active (Table 1, Entries 2–4), azirine motif (C(2)) of the substrates, thus allowing the
a remarkable improvement was made when complexes formation even of tetrasubstituted pyridines (11t,u).
The transformation also proved to be compatible with the
presence of various commonly employed functionalities such
Table 1: First attempts at cyclization with azirine 10a and optimization
as esters (11a–w), halogen atoms (11g–i), ketals (11l), nitro
of the catalytic system.[a]
groups (11 f), and alkenes (11m). A series of general com-
ments can be made regarding the reactivity of the substrates.
First, it is notable that the reaction is selective and that despite
the use of regular 1,2-dichloroethane, no trace of amino
ketone products resulting from a Neber rearrangement[2]
could be observed. Nor could by-products resulting from
a potentially competitive gold-catalyzed nucleophilic addition
of the tert-butyloxycarbonyl group on the alkyne be
detected.[15]
Entry Catalyst[b]
Solvent
T
t
Conv.[c] Yield[d]
[8C] [h]
[%]
[%]
1
2
3
4
5
6
7
[Ph3PAu]NTf2
[(JohnPhos)Au]NTf2 CDCl3
[(IPr)Au]NTf2
[(ArO)3PAu]NTf2
[(XPhos)Au]NTf2
[(tBuXPhos)Au]NTf2 CDCl3
[(tBuXPhos)Au]NTf2 (CH2Cl)2 85 1.5 100 95
CDCl3
50
50
50
50
50
50
3
3
3
3
3
3
42
33
18
35
90
40
28
17
32
82
CDCl3
CDCl3
CDCl3
While no general rule can be established, it is also
interesting to note that substrates such as 10k–m, which
[e]
possess alkyl substituents both at the alkyne terminus (C(6)
)
100 95
[f]
[g]
and on the azirine moiety (C(2)), tend to react comparatively
more slowly (> 10 h) than those possessing at least one
aromatic substituent at the same positions (compare, for
instance, 11k with 11a or 11n with 11p).[16] In line with this
observation, substrates 10p–s, which bear aromatic groups at
both C(2) and C(6), exhibited the highest reactivity with
conversions being complete in only 1 hour. Finally, it should
be noted that substrates 10t and 10u, which have substituents
at the propargylic position, were reacted as inseparable 1:1
mixtures of diastereoisomers. For 10t, the two diastereoiso-
mers, while leading to the formation of the same pyridine 11t,
were shown to react with different kinetics. One of the
(90%)[h]
8
9
10
11
12
AuCl3
AgNTf2
HNTf2
–
CD3CN
CDCl3
CDCl3
CDCl3
toluene
50
50
50
50
110
3
4
4
4
4
8
7
[i]
–
–
–
–
–
–
–
–
[i]
[i]
[i]
–
[a] Substrate concentration: 0.07m. [b] Catalyst loading 5 mol% except
for Entry 7. [c] Determined by 1H NMR spectroscopy. [d] Yield measured
by NMR spectroscopy. [e] Ar=2,6-(tBu)2-C6H3. [f] Catalyst loading
2 mol%. [g] Substrate concentration: 0.1m. [h] Yield of isolated product.
[i] No conversion observed by 1H NMR spectroscopy. Tf=triflate,
XPhos=2-dicyclohexylphosphino-2’,4’,6’-triisopropylbiphenyl.
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Table 2: Substrate scope.[a]
diasteroisomer completely reacted within 1 hour while 12 h
were required for the other.[17] At the extreme limit, one of
the diastereoisomers of 10u did not react at all.[18] In the
special case of substrate 10x (Scheme 3), the two diastereo-
isomers 10xsyn and 10xanti could be separated and their
relative configurations were assigned by X-ray diffraction
analysis of 10xsyn. This allowed us to more conveniently
compare their reactivities and to study the influence of the
propargylic susbtituent on the course of the reaction.
Scheme 3. Reactions of diastereoisomers 10xsyn and 10xanti
.
A slow but selective reaction was observed in the case of
diastereoisomer 10xsyn, which delivered the expected pyridine
11x in 66% yield after 12 h of reaction (69% conversion).[17]
The transformation of diastereoisomer 10xanti was more
favorable and complete conversion was observed after 9 h.
However, the reaction proved to be surprisingly unselective
and another pyridine 11x’ was formed along with 11x (72%
global yield; 11x/11’x = 1.8:1).[19] This difference in reactivity,
with the unusual migration of the aromatic group from
position C(4) in 10xanti to position C(5) in 11x’, provides
significant insights into the possible reaction mechanism and
the reactive intermediates that could be involved.
A proposed general mechanism[20] for the formation of
pyridines 11 from propargylic 2H-azirine derivatives 10 is
given in Scheme 4. The transformation could be initiated by
an electrophilic activation of the alkyne by the gold complex,
which would induce the 5-endo nucleophilic addition of the
azirine fragment. This would generate the aziridinium species
14,[21] the formation of which should be favored in the
presence of R1 groups with the ability to stabilize the incipient
positive charge. Some potential back-donation from gold may
also help in stabilizing intermediate 14, which would thus be
better represented as species A. As proposed in our
mechanistic design [Scheme 1, Eq. (4)], 14 would then
evolve into an a-imino gold–carbene intermediate of type
15[9] following a gold-assisted ring-fragmentation process
(Path A). A subsequent 1,2-shift of a migrating group (MG;
MG = H for 10a–w or 3,4,5-(MeO)3-C5H2 for 10xanti) from
position C(4) to C(5) would generate the charge-delocalized
[a] Substrate concentration: 0.1m. Yields of isolated products. Reaction
times are given in parentheses. [b] Based on the reacting diastereo-
isomer.
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Keywords: azirine · gold · heterocycles ·
.
homogeneous catalysis · pyridine
[1] For reviews on 2H-azirines: a) A. F. Khlebnikov, M. S. Novikov,
Tetrahedron 2013, 69, 3363; b) F. Palacios, A. M. O. de Retana,
E. M. de Marigorta, J. M. de Los Santos, Org. Prep. Proced. Int.
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Heterocyclic Chemistry II, Vol. 1A (Ed.: A. Padwa), Pergamon,
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[2] a) P. W. Neber, K. Hartung, W. Ruopp, Chem. Ber. 1925, 58,
1234; b) W. F. Berkowitz, Org. React. 2011, 78, 321.
[3] For selected recent transformations involving azirines:
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2013, 125, 2268; Angew. Chem. Int. Ed. 2013, 52, 2212; b) A. F.
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dron 2013, 69, 4546; c) M. J. Alves, V. C. M. Duarte, H. Faustino,
A. G. Fortes, N. Micaelo, Tetrahedron: Asymmetry 2013, 24,
1063; d) N. V. Rostovskii, M. S. Novikov, A. F. Khlebnikov, S. M.
Korneev, D. S. Yufit, Org. Biomol. Chem. 2013, 11, 5535.
[4] G. Dequirez, V. Pons, P. Dauban, Angew. Chem. 2012, 124, 7498;
Angew. Chem. Int. Ed. 2012, 51, 7384.
Scheme 4. Proposed mechanism.
[5] Selected examples: a) S. Chiba, G. Hattori, K. Narasaka, Chem.
Lett. 2007, 36, 52; b) A. Padwa, T. Stengel, Tetrahedron Lett.
2004, 45, 5991; c) S. Jana, M. D. Clements, B. K. Sharp, N. Zheng,
Org. Lett. 2010, 12, 3736; d) T. Izumi, H. Alper, Organometallics
1982, 1, 322. See also: e) D. A. Candito, M. Lautens, Org. Lett.
2010, 12, 3312; Ref. [3d].
[6] Selected examples: Ref. [3a,b]; a) A. F. Khlebnikov, V. A.
Khlebnikov, S. M. Korneev, M. S. Novikov, N. V. Rostovskii,
Tetrahedron 2013, 69, 4292; b) A. F. Khlebnikov, M. S. Novikov,
N. V. Rostovskii, I. A. Smetanin, D. S. Yufit, Tetrahedron Lett.
2012, 53, 5777; c) A. Mashida, K. Miki, K. Ohe, K. Okamoto, M.
Watanabe, Synlett 2013, 24, 1541.
[7] For selected reviews on gold catalysis: a) A. Fꢀrstner, Chem.
Soc. Rev. 2009, 38, 3208; b) E. Jimꢁnez-NfflÇez, A. M. Echavar-
ren, Chem. Rev. 2008, 108, 3326; c) Z. Li, C. Brower, C. He,
Chem. Rev. 2008, 108, 3239; d) A. Arcadi, Chem. Rev. 2008, 108,
3266; e) D. J. Gorin, F. D. Toste, Chem. Rev. 2008, 108, 3351;
f) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; g) A. Fꢀrstner,
P. W. Davies, Angew. Chem. 2007, 119, 3478; Angew. Chem. Int.
Ed. 2007, 46, 3410; h) A. S. K. Hashmi, G. J. Hutchings, Angew.
Chem. 2006, 118, 8064; Angew. Chem. Int. Ed. 2006, 45, 7896.
[8] For selected recent contributions from our group: a) C. Gron-
nier, G. Boissonnat, F. Gagosz, Org. Lett. 2013, 15, 4234; b) G.
Henrion, T. E. J. Chavas, X. Le Goff, F. Gagosz, Angew. Chem.
2013, 125, 6397; Angew. Chem. Int. Ed. 2013, 52, 6277; c) Z. Cao,
F. Gagosz, Angew. Chem. 2013, 125, 9184; Angew. Chem. Int. Ed.
2013, 52, 9014; d) B. Bolte, F. Gagosz, J. Am. Chem. Soc. 2011,
133, 7696; e) S. Kramer, Y. Odabachian, J. Overgaard, M.
Rottländer, F. Gagosz, T. Skrydstrup, Angew. Chem. 2011, 123,
5196; Angew. Chem. Int. Ed. 2011, 50, 5090.
[9] Ref. [8a]; a) A. Wetzel, F. Gagosz, Angew. Chem. 2011, 123,
7492; Angew. Chem. Int. Ed. 2011, 50, 7354; b) B. Lu, Y. Luo, L.
Liu, L. Ye, Y. Wang, L. Zhang, Angew. Chem. 2011, 123, 8508;
Angew. Chem. Int. Ed. 2011, 50, 8358; c) Y. Xiao, L. Zhang, Org.
Lett. 2012, 14, 4662; d) Z.-Y. Yan, Y. Xiao, L. Zhang, Angew.
Chem. 2012, 124, 8752; Angew. Chem. Int. Ed. 2012, 51, 8624.
[10] A low yield (5%) for a pyridine formed from a bis-homopro-
pargylic azide under gold catalysis has been reported. See
Ref. [9d].
intermediate 16, which would then ultimately evolve into
pyridine 11 with regeneration of the gold catalyst.
However, this mechanism alone does not explain the
different product selectivity obtained for the reactions of
diastereoisomers 10xsyn and 10xanti (Scheme 3). Following
Path A, the same a-imino gold–carbene intermediate 15
should indeed be formed from 10xsyn and 10xanti, and identical
results should therefore be obtained from both epimers. An
alternative mechanistic scenario that accounts for our exper-
imental observations can be envisaged.[22] Instead of being
produced following a sequential reaction pathway, intermedi-
ate 16 could alternatively be the result of a more concerted
process in which the ring fragmentation and the 1,2-shift
proceed concomitantly via a transition state of type 17
(Path B). In such a scenario, one can easily conceive that, in
the case of diastereoisomers 10xsyn and 10xanti, the difference
in relative stereochemistry at C(3) and C(4) would directly
impact the geometry of the corresponding intermediates 14
and therefore the energy levels of the transition states 17, thus
leading to the migration of either a hydrogen atom or
a trimethoxyphenyl group from C(4) to C(5)
.
In conclusion, we have developed a new gold-catalyzed
reaction that enables the formation of polysubstituted
functionalized pyridines from easily accessible 2-propargyl
2H-azirine derivatives. This transformation, which corre-
sponds to an unprecedented intramolecular transfer of an
alkenyl nitrene to an alkyne, proceeds with low catalyst
loading, is efficient, and exhibits a high functional-group
tolerance and a wide substrate scope. Additional investiga-
tions aimed at clarifying the reaction mechanism, as well as
further studies on the development of new metal-mediated
processes involving 2H-azirines, are currently underway.
[11] A combination of a strained ring and an alkyne has been used
previously for the gold-catalyzed synthesis of heterocycles, for
instance: a) A. Hashmi, P. Sinha, Adv. Synth. Catal. 2004, 346,
432; b) A. L. Blanc, K. Tenbrink, J.-M. Weibel, P. Pale, J. Org.
Received: February 15, 2014
Published online: && &&, &&&&
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Chem. 2009, 74, 5342; c) N. Kern, A. Blanc, J.-M. Weibel, P. Pale,
Chem. Commun. 2011, 47, 6665.
[12] See the Supporting Information.
[18] The unreactive diastereoisomer of 10u is probably that possess-
ing the same relative stereochemistry to that found in 10xsyn. The
formation of the corresponding intermediate 14 (Scheme 4)
should be disfavoured by a strong steric interaction arising from
the cis relationship between the ethyl group at C(4) and the tert-
butyl ester at C(3). The complete inertness of this diastereoisomer
of 10u when compared to 10xsyn can be explained by considering
that an ethyl group would possess a lower capacity than an
aromatic group to stabilize an incipient positive charge during
a concerted process leading from 14 to 16 (Path B) and involving
the migration of an hydrogen atom(MG=H).
[13] N. Mꢁzailles, L. Ricard, F. Gagosz, Org. Lett. 2005, 7, 4133.
À
À
[14] Azirines are known to cleave at the C N or C C single bonds
under thermal conditions. See Ref. [1a].
[15] For selected examples: a) F. Istrate, A. Buzas, I. D. Jurberg, Y.
Odabachian, F. Gagosz, Org. Lett. 2008, 10, 925; b) A. Buzas, F.
Gagosz, Org. Lett. 2006, 8, 515; c) J.-E. Kang, E.-S. Lee, S.-I.
Park, S. Shin, Tetrahedron Lett. 2005, 46, 7431; d) J.-E. Kang, S.
Shin, Synlett 2006, 717.
[16] The possible coordination of the gold complex to the pyridine
products, a process that should compete with the activation of
the alkyne moiety, could explain in part why the transformation
of substrates 10k–m required longer reaction times. Such
a coordination should indeed be more favorable in the case of
the more basic 2,6-dialkyl substituted pyridines 11k–m.
[17] For the less reactive diastereoisomer of 10t and for 10xsyn, the
formation of the corresponding intermediate 14 (Scheme 4)
should be disfavoured by a strong steric interaction arising from
the cis relationship between the aromatic group at C(4) and the
tert-butyl ester at C(3). This would account for the longer reaction
time observed.
[19] The electron-rich 3,4,5-trimethoxyphenyl group possesses
a greater migratory aptitude than a simple phenyl group and
can therefore compete with the hydrogen atom at C(4) for the 1,2-
shift step, thus leading to 16 (Scheme 4).
[20] A. Hashmi, Angew. Chem. 2010, 122, 5360; Angew. Chem. Int.
Ed. 2010, 49, 5232.
[21] A mechanism involving an initial activation of the azirine moiety
by the gold complex to form an intermediate nitrene–gold
complex can be ruled out. Indeed, no reaction occurred when
a control experiment was performed with a substrate lacking the
alkynyl chain.
[22] Both mechanisms could operate since one does not exclude the
other.
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Communications
Heterocycles
A. Prechter, G. Henrion, P. Faudot dit Bel,
F. Gagosz*
&&&& — &&&&
Gold-Catalyzed Synthesis of
Functionalized Pyridines by Using2H-
Azirines as Synthetic Equivalents of
Alkenyl Nitrenes
Ringing the changes: A series of easily
accessible 2-propargyl 2H-azirine deriva-
tives were efficiently converted into the
corresponding functionalized pyridines in
the presence of a gold catalyst. This
transformation, which exhibits a high
functional-group tolerance and a wide
substrate scope, corresponds to the
formal intramolecular transfer of an
alkenyl nitrene to an alkyne.
6
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