Synthesis of Enamides by Ruthenium-Catalyzed Reaction of Alkyl Azides with Acid Anhydrides in Ionic Liquid

Article abstract of DOI:10.1002/cctc.201500935

Enamides were synthesized by a ruthenium-catalyzed one-pot, one-step procedure from alkyl azides and acid anhydrides. The substrate scope includes not only secondary azides, but also primary aliphatic ones to give a wide range of enamides containing vario

Full text of DOI:10.1002/cctc.201500935

DOI: 10.1002/cctc.201500935
Communications
Synthesis of Enamides by Ruthenium-Catalyzed Reaction
of Alkyl Azides with Acid Anhydrides in Ionic Liquid
Han Kyu Pak, Junghoon Han, Mina Jeon, Yongjin Kim, Yearang Kwon, Jin Yong Park,
Young Ho Rhee,* and Jaiwook Park*[a]
Enamides were synthesized by a ruthenium-catalyzed one-pot,
one-step procedure from alkyl azides and acid anhydrides. The
substrate scope includes not only secondary azides, but also
primary aliphatic ones to give a wide range of enamides con-
taining various functional groups. This one-step procedure was
based on the newly discovered activity of Severin’s dirutheni-
um complex ([Cp^RuCl2]2: Cp^=h5-1-methoxy-2,4-di-tert-butyl-
3-neopentylcyclopentadienyl) for the transformation of alkyl
azides into the corresponding NÀH imine intermediates in
ionic liquids. The formation of ruthenium tetrazene complexes
was observed in the stoichiometric reaction of Severin’s
complex with alkyl azides, which acted as the catalyst for the
formation of NÀH imine intermediates.
and subsequent reduction in the presence of acyl donors
(Scheme 1a). Recently, Reeves and co-workers developed
a direct and redox-free synthesis of enamides from ketones,
ammonia, and acetic anhydride with the aid of Ti(OiPr)4
(Scheme 1b).[13] Although this is a one-pot process under mild
conditions, the use of an excess amount of Ti(OiPr)4 requires
a complicated workup procedure to remove titanium waste
from the reaction mixture. As an innovative method overcom-
ing the problem of stoichiometric reductants or additives, our
group developed the synthesis of enamides from alkyl azides
and acid anhydrides[14] that is based on Ru catalysis to trans-
form alkyl azides into NÀH imines (Scheme 1c).[15] Diruthenium
complex 1 shown in Scheme 1c needs light to be activated,
and the resulting activated species is deactivated by acetic
acid. Thus, Scheme 1c is a two-step process requiring the ac-
cumulation of the intermediate NÀH imines before acylation
with acetic anhydride. This two-step process limits the sub-
strate scope, because unstable NÀH imine intermediates suffer
from side reactions such as rearrangements and self-condensa-
Enamides are key functional groups in numerous natural prod-
ucts and drug candidates[1] and serve as versatile intermediates
in organic synthesis.[2] They have been utilized in pericyclic,[3]
radical,[4] photochemical,[5] and transition-metal-catalyzed reac-
tions,[6] including the well-known enantioselective hy-
drogenation reaction for the synthesis of chiral
amines.[7] For these extensive applications, a number
of methods have been devised for the synthesis of
enamides.[8] Conventional ones include the addition
of Grignard reagents to nitriles followed by reaction
with acyl donors,[9] transition-metal-catalyzed cross-
coupling reactions of vinyl electrophiles with
amides,[10] direct condensation of amides with alde-
hydes and ketones,[11] and reductive acylation of ke-
toximes.[12] However, these methods frequently suffer
from functional group tolerance, difficulties in prepar-
ing substrates, low yields, and/or the requirement of
stoichiometric reductants or additives.
One of the most commonly employed procedures
Scheme 1. Methods for the synthesis of N-acyl enamides through NÀH imine intermedi-
is the reductive acylation of ketoximes, which re-
quires the transformation of ketones into ketoximes
ates.
tion reactions.[16] Particularly, aliphatic NÀH aldimines are too
unstable to afford the corresponding enamides.
[a] H. K. Pak, J. Han, M. Jeon, Y. Kim, Y. Kwon, J. Y. Park, Prof. Y. H. Rhee,
Prof. J. Park
Department of Chemistry
Aliphatic NÀH aldimines will be valuable intermediates for
the synthesis of peptide natural products and bioactive mole-
cules that contain an enamide functionality as an important
structural motif.[1,8a,11d] Our search for new catalyst systems that
can provide aliphatic NÀH aldimines and retain catalytic activi-
ty under conditions for the subsequent acylation[17] led to the
finding of chloride-bridged diruthenium complex 2.[18] Complex
POSTECH (Pohang University of Science and Technology)
Pohang 790-784 (Korea)
Fax: (+82)54-279-3399
Homepage: yhrhee.postech.ac.kr
Homepage: oml.postech.ac.kr
E-mail: yhrhee@postech.ac.kr
pjw@postech.ac.kr
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500935.
2 has shown catalytic activity for the transformation of benzyl
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azide into hydrobenzamide. Although its substrate scope is de-
scribed to be limited to primary benzylic azides, we were at-
tracted by the probable intermediacy of NÀH benzaldimine for
the formation of hydrobenzamide.[19] Herein, we wish to report
a newly discovered activity of 2 in ionic liquids; we found that
efficient synthesis of the enamides involving aliphatic NÀH al-
dimine intermediates is possible. In fact, a wide range of enam-
ides can be prepared not only from primary aliphatic azides
but also from secondary azides by a one-pot, one-step process
(Scheme 1d).
amine (Table 1, entries 8–11). Analogous RuIII complexes and
other commercially available RuII complexes showed much
lower catalytic activity than 2 (Table 1, entries 12–16). Geomet-
rical selectivity was not so high: E/Zꢀ58:42–74:26.
With the conditions optimized in Table 1, we explored the
substrate scope for aliphatic azides (Table 2). Various functional
Table 2. Synthesis of enamides from aliphatic azides.[a–c]
The transformation of 1-octyl azide (3a) into N-(oct-1-enyl)a-
cetamide (4a) was examined under various conditions
(Table 1). The yield of 4a was only 32% in the first attempt in
Table 1. Transformation of n-octyl azide into N-(oct-1-enyl)acetamide.[a]
Entry Catalyst
Solvent
Conversion
of azide [%][b] [%]
Yield[b]
(E/Z)
1
2
3
4
2
2
2
2
THF
dioxane
[bmim]Cl
>99
>99
74
32
41
60
(65:35)
(68:32)
(58:42)
(60:40)
[bmim]BF4 >99
52
5
6
7
8
2
2
2
2
2
2
2
[bmim]PF6
[hmim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
[omim]Cl
17
>99
>99
>99
>99
>99
73
<5
29
5
trace
90
99
65[c] (74:26)
90[d] (60:40)
70[e] (64:36)
44[f] (66:34)
trace
29
trace
trace
trace
(62:38)
(58:42)
9
[a] Reaction conditions: A solution of 3 (0.25 mmol), 2 (2.0 mol%), Et3N
(0.50 mmol), and Ac2O (0.50 mmol) in [omim]Cl (1.0 mL) was stirred at
708C for 3 h. MOM=methoxymethyl. [b] Reactions conditions: A solution
of 5 (0.25 mmol), 2 (2.0 mol%), Et3N (0.50 mmol), and Ac2O (0.50 mmol)
in [omim]Cl (1.0 mL) was stirred at 708C for 12 h. [c] Yields of isolated
products are given; E/Z ratios are given in parentheses. [d] 1 h at 608C.
10
11
12
13
14
15
16
(CpRuCl2)2
(Cp*RuCl2)2
CpRuCl(PPh3)2
[(p-cymene)RuCl2]2 [omim]Cl
RuCl2(PPh3)3 [omim]Cl
(61:39)
4
29
[a] Reaction conditions: A solution of n-octyl azide (0.25 mmol), catalyst
(2.0 mol%), Et3N (0.50 mmol), and Ac2O (0.50 mmol) in solvent (1.0 mL)
was stirred at 708C for 3 h. Cp=h5-cyclopentadienyl, Cp*=h5-pentame-
thylcyclopentadienyl. [b] Determined by 1H NMR spectroscopy by using
dibromomethane as an internal standard. [c] At 508C. [d] 2 (1.0 mol%).
[e] Ac2O (1.2 equiv.). [f] Et3N (2.0 mol%).
groups were compatible with the catalyst system: chloride
(see product 4b), methoxymethyl ether (see product 4c), and
ester groups (see product 4d). b-Methoxy-substituted enamide
4e was obtained in good yield, which is a useful precursor for
the synthesis of b-amino alcohols.[12h,20] b-Chloro enamides 4 f
and 4g were obtained in moderate to good yields and have
been used as important building blocks in the synthesis of
chartellines.[21]
THF (Table 1, entry 1). However, considering the scope limita-
tion described in the previous report,[14] it was an unexpected
and promising result. This finding supports the idea that the
catalytic activity of 2 for the generation of octan-1-imine was
retained to some extent under conditions for acylation. The
yield increased slightly in dioxane (Table 1, entry 2). A consider-
able increase in the yield was observed for the reaction per-
formed in the ionic liquid (butylmethylimidazolium chloride
([bmim]Cl)) (Table 1, entry 3). Although changing the chloride
anion with tetrafluoroborate or hexafluorophosphate was not
rewarding (Table 1, entries 4 and 5), increasing the length of
the alkyl substituent improved the yield significantly (Table 1,
entry 6). Remarkably, 4a was formed in quantitative yield in oc-
tylmethylimidazolium chloride ([omim]Cl) (Table 1, entry 7). The
yield decreased on lowering the reaction temperature and on
reducing the amount of catalyst 2, acetic anhydride, or triethyl-
Acyclic 4h and cyclic b-branched enamide 4i were formed
in high yields. Enamides 4j–l conjugated with aryl rings and
enamides 4m and 4n conjugated with heteroaromatic rings
were obtained in almost quantitative yields. The electronic ef-
fects of the substituents in the aromatic rings of 4j–l were not
significant for enamide formation.
In the transformation of linear secondary aliphatic azides 5,
corresponding enamides 6 were formed as mixtures of re-
gioisomers and geometric isomers. From 2-azidooctane, trisub-
stituted enamide 6a was obtained as a 90:10 E/Z mixture in
72% combined yield with minor regioisomer 6a’ in 23% yield.
Conjugation to the phenyl ring was not beneficial to the reac-
tion efficiency or to the product selectivity in the transforma-
tion of (2-azidopropyl)benzene; 6b was obtained as a 53:47 E/
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Z mixture in 47% combined yield with minor regioisomer 6b’
in 13% yield. Enamides were obtained quantitatively in com-
bined yields in the transformation of cyclic secondary aliphatic
azides 5c and 5d, although the regioselectivity was low. There
was negligible regioselectivity in the formation of cyclohexeny-
lacetamides 6c and 6c’, whereas moderate regioselectivity was
observed in the formation of the cyclopentenylacetamides to
give tetrasubstituted enamide 6d as the major product.
Notably, the E/Z mixtures of N-acetyl enamides 4j–l formed
in the reactions of (2-azidoethyl)benzene derivatives were con-
verted quantitatively into the E isomers by heating the mix-
tures at 110 8C for 24 h (Scheme 2).[22] It is also notable that E
Table 3. Transformation of (1-azidoethyl)benzene into N-(1-phenylvinyl)-
acetamide.[a]
Entry
2
Solvent
Ac2O
[equiv.]
Et3N
[equiv.]
Yield
[%][b]
[mol%]
1
2
3
4
5
6
7[c]
8
2.0
2.0
2.0
2.0
2.0
1.0
0.10
2.0
[omim]Cl
[hmim]Cl
[bmim]Cl
[bmim]BF4
[bmim]PF6
[bmim]Cl
[bmim]Cl
THF
2.0
2.0
2.0
2.0
2.0
1.2
1.2
2.0
2.0
2.0
2.0
2.0
70
70
99
99
73
99
91[d]
99
2.0
0.020
0.0020
2.0
[a] Reaction conditions: A solution of (1-azidoethyl)benzene (0.25 mmol),
2, Et3N, and Ac2O in solvent (1.0 mL) was stirred at 708C for 3 h. [b] Deter-
1
mined by H NMR spectroscopy by using dibromomethane as an internal
standard. [c] (1-Azidoethyl)benzene (1.0 g, 7.0 mmol). [d] Yield of isolated
product.
Table 4. Synthesis of enamides from benzylic azides.[a,b]
Scheme 2. Thermal isomerization of N-acyl enamides.
isomers 4o–q were formed exclusively by the use of benzoic
anhydride instead of acetic anhydride under the conditions of
Table 2. However, breaking the conjugation to the phenyl ring
as in 4r diminished the selectivity, and slow decomposition or
incomplete isomerization was observed in the heat treatment
of E/Z mixtures of other enamides. The thermal isomerization
of cyclopentenylacetamide 6d’ into 6d in toluene at 1108C
was successful, but slow decomposition of 6c and 6c’ was ob-
served at 110 8C in the presence of triethylamine.
[a] Reaction conditions: A solution of benzylic azide 7 (0.25 mmol), 2
(1.0 mol%), Et3N (2.0 mol%), and Ac2O (0.30 mmol) in [bmim]Cl (1.0 mL)
was stirred at 708C for 3 h. [b] Yields of isolated products are given; E/Z
ratios are given in parentheses. [c] Reaction performed in THF.
The transformation of benzylic azides, in contrast to that of
aliphatic azides, was more facile in [bmim]Cl than in ionic liq-
uids having longer alkyl chains such as hexylmethylimidazoli-
um chloride ([hmim]Cl) and [omim]Cl (Table 3, entries 1–3). The
reaction efficiency in [bmim]BF4 was as good as that in
[bmim]Cl (Table 3, entry 4), whereas that in [bmim]PF6 was me-
diocre (Table 3, entry 5). Enamide 8a was formed in quantita-
tive yield even with 1.0 mol% of 2, 1.2 equivalents of acetic an-
hydride, and 2.0 mol% of triethylamine (Table 3, entry 6). A
large-scale reaction proceeded smoothly with 0.10 mol% of 2
and 0.20 mol% of Et3N to give 8a in 91% yield (Table 3,
entry 7). Interestingly, THF was also a good reaction medium
for the transformation (Table 3, entry 8).
8g), halide (see products 8h–j), thioether (see product 8k),
amide (see product 8l), and cyano groups (see product 8m).
Noticeably, the chloromethyl group labile toward nucleophiles
in 8n and the acetal group labile toward acids in 8o remained
intact during the transformation. Naphthyl (see product 8p)
and heteroaromatic enamides (see product 8q–s) were also
obtained in good yields. b,b-Disubstituted derivative 8t was
formed in quantitative yield. b-Monosubstituted derivative 8u
was produced as a mixture of geometrical isomers in quantita-
tive yield. Benzocyclic enamides 8v–y were obtained from the
corresponding benzocyclic azides in good to excellent yields.
Notable examples are 9a and 9b, which cannot be afforded
by conventional enamide synthesis by employing carbonyl
substrates and/or nucleophilic reagents.
A wide range of benzylic azides were successfully trans-
formed into the corresponding enamides (Table 4). A small
steric effect was observed in the formation of 8b having a 2-
methylphenyl group, whereas there was no clear trend of the
electronic effect in the formation of 8d–g. Various functional
groups were compatible with the reaction conditions: alkoxy
(see product 8e), ester (see product 8 f), nitro (see product
Interestingly, diruthenium complex 2 was first reported by
Severin’s group,[23] and the formation of ruthenium tetrazene
complex 12 was also reported by the group in the reaction of
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benzyl azide with 2.[18] We observed the formation of a diaste-
reomeric mixture of analogous tetrazene complexes 11 in the
reaction of 2 with 7a (Scheme 3). In fact, 11 acted as the cata-
from secondary azides. The one-step synthetic process is much
more convenient than any other known method, is highly effi-
cient, and provides a wide range of enamides containing vari-
ous functional groups. A large-scale reaction was demonstrat-
ed in nonflammable ionic liquid to diminish safety concerns
associated with the use of organic azides as substrates.[25]
Experimental Section
General procedure
A flame-dried J-Young flask filled with argon gas was charged with
ruthenium catalyst 2 (6.3 mg, 0.10 mol%), dried and degassed
[bmim]Cl (5.0 mL), 7a (1.0 g, 7.0 mmol), Et3N (2.0 mL, 0.20 mol%),
and Ac2O (790 mL, 1.2 equiv.). The mixture was stirred at 708C for
24 h and was then extracted with dichloromethane (15 mL”3).
The organic layer was dried with anhydrous sodium sulfate, con-
centrated, and purified by column chromatography to give N-(1-
phenylvinyl)acetamide (1.0 g, 91%).
Scheme 3. Formation of Ru–tetrazene complexes and their activities.
lyst for the conversion of 7a into NÀH ketimine 13, which was
1
characterized by H NMR spectroscopy and converted into N-
Acknowledgements
(1-phenylvinyl)acetamide (8a) in quantitative yield in a subse-
quent reaction with acetic anhydride.[14] However, 11 was
intact under conditions involving heating at 708C in the pres-
ence of acetic anhydride. In contrast, in the presence of trie-
thylamine at 708C 11 decomposed into a complex mixture, in
which ketimine 13 was not observed.[24]
This work was supported by the National Research Foundation of
Korea (NRF) grant funded by the Korea government (MSIP)
(2015R1A2A2A01008130).
Keywords: azides
ruthenium
· enamides · imines · ionic liquids ·
On the basis of the results in Scheme 3, we propose a path-
way involving ruthenium tetrazene complex A as the key inter-
mediate for the formation of enamides (Scheme 4). Tetrazene
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Received: August 22, 2015
Published online on October 30, 2015
ChemCatChem 2015, 7, 4030 – 4034
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