Gold-catalyzed heterogeneous aerobic dehydrogenative amination of α,β-unsaturated aldehydes to enaminals

Article abstract of DOI:10.1002/anie.201308260

Although enaminals (β-enaminals) are very important compounds and have been utilized as useful synthons for various important compounds, they have been synthesized through non-green and/or limited procedures until now. Herein, we have successfully develop

Full text of DOI:10.1002/anie.201308260

Angewandte
Chemie
DOI: 10.1002/anie.201308260
Heterogeneous Gold Catalysis
Gold-Catalyzed Heterogeneous Aerobic Dehydrogenative Amination
of a,b-Unsaturated Aldehydes to Enaminals**
Xiongjie Jin, Kazuya Yamaguchi, and Noritaka Mizuno*
Abstract: Although enaminals (b-enaminals) are very impor-
tant compounds and have been utilized as useful synthons for
various important compounds, they have been synthesized
through non-green and/or limited procedures until now.
Herein, we have successfully developed a green synthetic
procedure using a heterogeneous catalyst. In the presence of
gold nanoparticles supported on manganese-oxide-based octa-
hedral molecular sieves OMS-2 (Au/OMS-2), dehydrogenative
amination of a,b-unsaturated aldehydes with amines pro-
ceeded efficiently, with the corresponding enaminals isolated in
moderate to high yields (50–97%). The catalysis was truly
heterogeneous, and Au/OMS-2 could be reused. Furthermore,
the formal Wacker-type oxidation of a,b-unsaturated alde-
hydes to enaminones has been realized.
E
naminals (b-enaminals) are important compounds that
have been utilized for the synthesis of various heterocyclic
compounds such as furans, pyrans, pyrroles, pyridines, pyr-
azoles, isothiazoles, and isoxazoles.[1,2] In addition, they are
key synthons for several bioactive compounds, including
noeuromycin, AKT (protein kinase B) inhibitor-IV, and
AZD4877 (a kinesin spindle protein inhibitor and potential
anticancer agent).[1,2] To date, several synthetic procedures for
enaminals have been developed,[2,3] and the following two
procedures have most commonly been utilized: 1) Dehydra-
tive condensation of amines with malondialdehyde generated
in situ by the hydrolysis of 1,1,3,3-tetramethoxypropane
(Procedure A in Scheme 1). 2) Hydroamination of propar-
gylic aldehydes (Procedure B in Scheme 1). Procedure A is
limited to the synthesis of b-aminoacroleins; b-substituted
enaminals cannot be synthesized by the dehydrative con-
densation of ketoaldehydes with amines, owing to the higher
reactivity of aldehyde functional groups to the condensation,
consequently giving enaminones as the major products.[3e]
Although procedure B is efficient for the synthesis of a wide
variety of enaminals, the development of alternative synthetic
procedures for enaminals would be of great significance
considering the limitation in availability (synthesis) of starting
propargylic aldehydes. Direct dehydrogenative amination of
Scheme 1. Synthesis of enaminals.
readily available a,b-unsaturated aldehydes would be a can-
didate for an efficient synthetic route to enaminals (Proce-
dure C in Scheme 1).
Dehydrogenative amination or amidation of alkenes,
namely aza-Wacker-type oxidation, is a valuable method for
2
[4–6]
À
the construction of C(sp ) N bonds.
To date, there have
been several reports of palladium-catalyzed amidation, and
less-basic amides, such as sulfonamides, carbamates, and
lactams, have been utilized as amide nucleophiles in most
cases.[4] In comparison with amidation, amination using
simple amines as nucleophiles is a relatively less-developed
reaction,[5,6] likely because of the severe deactivation of
homogeneous metal-based catalysts by the strong coordina-
tion of amines. With regard to the amination of a,b-
unsaturated carbonyl compounds, the groups of Ishii[5e,h] and
Ying[5g] have recently reported excellent palladium-catalyzed
procedures. However, a,b-unsaturated carbonyl compounds
suitable for these systems are limited to a,b-unsaturated
esters and ketones,[7] and the scope of amines is also limited to
only aniline derivatives.[5e,g,h] Thus, the dehydrogenative
amination of a,b-unsaturated aldehydes to enaminals has
never been previously reported. To realize dehydrogenative
amination to synthesize structurally diverse enaminals, the
design of efficient catalysts is required. We focused on
heterogeneous catalysts in particular because of their easy
separation and reusability. More importantly, we envisaged
that heterogeneous catalysts would possess higher durability
against oxidative conditions, heating, and strong coordination
of nitrogen nucleophiles in comparison with their homoge-
neous counterparts.
[*] X. Jin, Dr. K. Yamaguchi, Prof. Dr. N. Mizuno
Department of Applied Chemistry, School of Engineering
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 113-8656 (Japan)
E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp
[**] This work was supported in part by the Grants-in-Aid for Scientific
Researches from Ministry of Education, Culture, Sports, Science
and Technology.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308260.
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Herein, we disclose for the first time a gold-catalyzed
dehydrogenative amination of a,b-unsaturated aldehydes to
enaminals using air (molecular oxygen) as the sole oxidant
(Procedure C in Scheme 1). In the presence of gold nano-
particles on manganese-oxide-based octahedral molecular
sieves OMS-2[8] (Au/OMS-2; average particle size of gold =
4.1 nm; see the Supporting Information, Figure S1), the
amination of various combinations of a,b-unsaturated alde-
hydes and amines proceeded efficiently, with the correspond-
ing enaminals isolated in moderate to high yields (50–97%).
The catalysis in the present amination was truly heteroge-
neous, and the Au/OMS-2 catalyst could be reused several
times. The procedure developed herein provides a highly
efficient synthetic route to enaminals and reveals new
oxidation catalysis of gold.[9,10]
Next, we investigated the scope of the present Au/OMS-2-
catalyzed dehydrogenative amination of a,b-unsaturated
carbonyl compounds. Under the optimized reaction condi-
tions, various structurally diverse enaminals could be synthe-
sized from a,b-unsaturated aldehydes and amines (Schemes 2
and S1). All products could be readily isolated (see the
Supporting Information), and the yields of isolated products
are summarized in Scheme 2.
Initially, the dehydrogenative amination of cinnamalde-
hyde (1a) with pyrrolidine (2a) to 3-phenyl-3-(1-pyrroli-
dinyl)-2-propenal (3aa) was carried out in the presence of
various kinds of supported metal catalysts (such as gold,
palladium, ruthenium, rhodium, and copper on various
supports; given in the format: metal/support). The reaction
was carried out at 508C in air (1 atm). The amination barely
proceeded in the absence of catalysts or in the presence of Pd/
Al2O3, Cu/Al2O3, Ru/Al2O3, or Rh/Al2O3 (Table S1, entries 1–
4 and 13).[7] In contrast, 44% yield (as determined by GC
analysis) of the desired amination product 3aa was obtained
when using Au/Al2O3 (Table S1, entry 5). Among the sup-
ports examined, OMS-2 was the best support; the yield of 3aa
increased up to 83% (as determined by GC analysis) using
Au/OMS-2 (Table S1, entry 8).[11] Amination with a physical
mixture of Au/TiO2 and OMS-2 gave almost the same yield of
3aa as with Au/TiO2 (Table S1, entry 11), and OMS-2 alone
could not promote the amination (Table S1, entry 12). Thus,
highly dispersed gold nanoparticles on the surface of OMS-2
would play an important role on the present amination.[12]
To verify whether the observed catalysis was derived from
solid Au/OMS-2 or leached metal species (gold and/or
manganese), the Au/OMS-2 catalyst was removed by hot
filtration, and the reaction was carried out with the filtrate
under the same reaction conditions described in Table S1. The
amination was completely stopped by removal of the catalyst
(Figure S2). Furthermore, it was confirmed by inductively
coupled plasma atomic emission spectroscopic (ICP-AES)
analysis that gold and manganese species were hardly present
in the filtrate (Au: < 0.002%, Mn: < 0.00001%). These
results can rule out any contribution to the observed catalysis
from metal species that had leached into the reaction solution,
and the observed catalysis for the present amination is
intrinsically heterogeneous.[13] After the amination of 1a with
2a, the Au/OMS-2 catalyst could be easily retrieved from the
reaction mixture by simple filtration with > 97% recovery.
The retrieved catalyst could be reused at least five times for
the same reaction without a significant loss in its high catalytic
performance. Even at the fifth reuse experiment, 76% yield
(as determined by GC analysis) of 3aa was still obtained,
although decreases in the reaction rates for the reuse experi-
ments were observed (for example, 0.20 mm hÀ1 for the fifth
reuse experiment vs. 0.27 mm hÀ1 for the first run with the
fresh catalyst; Figure S3).
Scheme 2. Aerobic dehydrogenative amination of a,b-unsaturated alde-
hydes. Reaction conditions: 1 (0.5 mmol), 2 (1.0 mmol), Au/OMS-2
(3.6 mol%), THF (1.9 mL), H2O (0.1 mL), 508C, air (1 atm). Yields
(based on 1) of isolated products are shown. Major byproducts were
amidation products 4 (<5% yield, except for 4ea). See the Supporting
Information for the E/Z ratios of products. [a] 4ea was obtained as an
amidation byproduct (24% yield). [b] THF (2 mL), without addition of
H2O. [c] 5 fa (see Scheme 4) was obtained as a byproduct (28% yield).
[d] 2 (2.0 mmol), THF (1.8 mL), H2O (0.2 mL). [e] THF (1.8 mL), H2O
(0.2 mL). [f] Toluene (2 mL). [g] 608C. [h] 708C.
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Various substituted cinnamaldehydes efficiently reacted
with 2a to give the corresponding enaminals (Scheme 2,
entries 1–5). An aliphatic a,b-unsaturated aldehyde was also
useful for the present amination (Scheme 2, entry 6). In this
case, a moderate yield of 5 fa (enaminone), derived from the
isomerization of the corresponding enaminal, was formed as
a byproduct (see below; see also Scheme 4). Gratifyingly,
acrolein, which is susceptible to both polymerization and
oxidation under oxidative conditions, could be utilized as
a substrate for the present amination (Scheme 2, entries 7 and
10–28). Aside from aldehydes, a,b-unsaturated ketones could
be efficiently aminated at the b-position, although four
equivalents of 2a were required (Scheme 2, entries 8 and 9).
Considering the significant utility of b-aminoacrolein
moieties in organic synthesis,[1,2] we mainly used acrolein as
a coupling partner to examine the amine scope. Various
aliphatic amines efficiently reacted with acrolein to give the
corresponding b-aminoacroleins (Scheme 2, entries 10–18).
Benzylic amines and aniline derivatives were excellent
amination reagents for acrolein (Scheme 2, entries 19–22).
With regard to diallylamine, the allyl group remained intact
after the reaction (Scheme 2, entry 23). Furan and pyridine
derivatives were also compatible with the present amination
(Scheme 2, entries 24 and 25). Ester and amide groups
survived under the reaction conditions (Scheme 2, entries 26
and 27). A secondary amine containing an alcohol group also
reacted well with acrolein (Scheme 2, entry 28). The amina-
tion of 1a with aliphatic and benzylic amines also gave the
corresponding enaminals (Scheme 2, entries 29 and 30).
Notably, we could successfully synthesize 3aa in 67%
yield (as determined by GC analysis) starting from cinnamy-
lalcohol (6a) by a one-pot sequential reaction of the oxidative
dehydrogenation of 6a to 1a, followed by amination with 2a
(Scheme 3). Thus, Au/OMS-2 could act as an efficient
heterogeneous catalyst for both the oxidative dehydrogen-
ation of alcohols and the amination of a,b-unsaturated
aldehydes.
Scheme 4. One-pot synthesis of enaminones from a,b-unsaturated
aldehydes and 2a. Reaction conditions: 1 (0.5 mmol), 2a (1.0 mmol),
Au/OMS-2 (3.6 mol%), THF (1.9 mL), H2O (0.1 mL), 508C, 1.5 h, air
(1 atm). Then, the catalyst was filtered off, followed by the addition of
2a (2.0 mmol) and H2O (0.1 mL) to the filtrate. For 1a, the filtrate was
stirred at 608C for an additional 24 h. For 1 f, 508C for an additional
3 h. Yields (based on 1) of isolated products are shown.
(Scheme 4). To the best of our knowledge, this is the first
example of (formal) Wacker-type oxidation of a,b-unsatu-
rated aldehydes.[14]
For the amination of 1a with 2a, the reaction rate and the
final yield of 3aa were almost unchanged, even in the
presence of stoichiometric amounts of radical scavengers such
as 2,6-di-tert-butyl-4-methylphenol (BHT) or 2,2,6,6-tetrame-
thylpiperidine 1-oxyl (TEMPO; Figure S4), which indicates
that radical species were not involved in the present
amination. The reaction profiles for the present Au/OMS-2-
catalyzed amination showed that a b-aminoaldehyde (aza-
Michael addition product) was initially produced, followed by
formation of the corresponding enaminal (Figure S5). In
addition, we confirmed that Au/OMS-2 could catalyze the
dehydrogenation of 1-methyl-4-piperidone (b-aminoketone)
to the corresponding enaminone (Scheme S2).
These experimental results indicate that the present Au/
OMS-2-catalyzed amination of a,b-unsaturated aldehydes
possibly proceeds through the following sequential reactions:
Initially, aza-Michael addition of a secondary amine to an a,b-
unsaturated aldehyde proceeds to give a b-aminoaldehyde
(step 1 in Scheme 5). Then, the oxidative dehydrogenation of
the b-aminoaldehyde takes place to give the corresponding
enaminal (step 2 in Scheme 5),[6,15] which is efficiently pro-
moted by Au/OMS-2. This mechanism is completely different
from the traditional palladium-catalyzed amination, which is
supposed to proceed through migratory insertion of an amine
into a palladium–alkene complex, followed by b-hydride
elimination.[5e,g,h]
As described above, the corresponding enaminone 5 fa,
which can be regarded as a Wacker-type oxidation product,
was formed as a byproduct for the reaction of 2-octenal (1 f)
with 2a. We found that enaminones (Wacker-type oxidation
products) could be synthesized from a,b-unsaturated alde-
hydes and amines. The overall conversion of a,b-unsaturated
aldehydes to enaminones was accomplished as a one-pot
procedure by simply adding amines and water to the reaction
solution after the Au/OMS-2 catalyzed amination was com-
pleted. For example, enaminones 5aa and 5 fa could be
efficiently synthesized from 1a and 1 f, respectively
Scheme 3. One-pot synthesis of 3aa from 6a and 2a. Reaction
conditions: 1) 6a (0.5 mmol), Au/OMS-2 (3.6 mol%), toluene (2 mL),
1008C, 1 h, O2 (1 atm). Then, 2) 2a (1.0 mmol) was added to the
reaction mixture followed by stirring at 508C for an additional 6 h.
Yield was determined by GC analysis.
Scheme 5. Proposed reaction path for the present Au/OMS-2 catalyzed
dehydrogenative amination of a,b-unsaturated aldehydes and formal
Wacker-type oxidation.
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[7] We have confirmed that palladium-based catalysts were not
effective for the amination of a,b-unsaturated aldehydes. For
example, when the reaction of 1a with 2a was carried out using
the PdCl2(CH3CN)2/heteropolyacid/hydroquinone catalyst
developed by Ishii et al.[5e,f] under the conditions described in
Table S1 (dimethylformamide was used as a solvent instead of
THF), the desired product 3aa was not obtained. In this case, 1a
was mostly decomposed to benzaldehyde (retro-aldol reaction;
18% yield) and other unidentified byproducts; see: D. Enders,
T. V. Nguyen, Tetrahedron Lett. 2012, 53, 2091.
In the presence of excess amounts of amine, an enaminal
could react further with the amine to form transient iminium
species A (step 3 in Scheme 5), which can isomerize to
species A’ (step 4 in Scheme 5). The hydrolytic decomposi-
tion of species A’ affords the corresponding enaminone as
a final Wacker-type oxidation product (step 5 in Scheme 5).
In summary, we have successfully developed a novel
heterogeneous dehydrogenative amination of a,b-unsatu-
rated aldehydes catalyzed by Au/OMS-2. Various structurally
diverse enaminals could be synthesized. Furthermore, formal
Wacker-type oxidation of a,b-unsaturated aldehydes to
enaminones has been realized for the first time. The present
system provides a convenient approach to synthetically
important enaminals and enaminones, starting from readily
available a,b-unsaturated aldehydes and amines.
[8] It has been reported that OMS-2 can act as an efficient catalyst
or support for several oxidation reactions; see: a) Y.-C. Son,
V. D. Makwana, A. R. Howell, S. L. Suib, Angew. Chem. 2001,
113, 4410; Angew. Chem. Int. Ed. 2001, 40, 4280; b) T. Oishi, K.
Yamaguchi, N. Mizuno, ACS Catal. 2011, 1, 1351; c) K.
Yamaguchi, H. Kobayashi, T. Oishi, N. Mizuno, Angew. Chem.
2012, 124, 559; Angew. Chem. Int. Ed. 2012, 51, 544; d) K.
Yamaguchi, Y. Wang, T. Oishi, Y. Kuroda, N. Mizuno, Angew.
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[9] J. Mielby, S. Kegnæs, P. Fristrup, ChemCatChem 2012, 4, 1037.
[10] Several research groups have reported the gold-catalyzed
amidation of alcohols, but the synthesis of enaminals (amina-
tion) has not been mentioned; see: a) Y. Wang, D. Zhu, L. Tang,
S. Wang, Z. Wang, Angew. Chem. 2011, 123, 9079; Angew. Chem.
Int. Ed. 2011, 50, 8917; b) J.-F. Soulꢂ, H. Miyamura, S.
Kobayashi, J. Am. Chem. Soc. 2011, 133, 18550; c) J. Zhu, Y.
Zhang, F. Shi, Y. Deng, Tetrahedron Lett. 2012, 53, 3178.
[11] Although the presence of small amounts of water had a positive
effect on the amination, the corresponding amidation byproduct
4aa was formed significantly in the presence of large amounts of
water (Table S2). At least two equivalents of 2a were required to
obtain high yields of 3aa (Table S3). For the present amination,
a broad range of solvents could be utilized, and tetrahydrofuran
was the best solvent among them (Table S4).
[12] Au/OMS-2 showed high catalytic activity, even under Ar
atmosphere (Table S1, entry 9), which suggests that step 2 in
Scheme 5 is efficiently promoted by the presence of OMS-2.[8b]
[13] R. A. Sheldon, M. Wallau, I. W. C. E. Arends, U. Schuchardt,
Acc. Chem. Res. 1998, 31, 485.
[14] Kaneda and co-workers have recently reported that electron-
deficient internal alkenes could be oxidized to the corresponding
ketones by a palladium-catalyzed formal Wacker-type oxidation
using molecular oxygen as the sole oxidant; a,b-unsaturated
aldehydes were not used as substrates; see: T. Mitsudome, S.
Yoshida, T. Mizugaki, K. Jitsukawa, K. Kaneda, Angew. Chem.
2013, 125, 6077; Angew. Chem. Int. Ed. 2013, 52, 5961.
Received: September 20, 2013
Published online: November 29, 2013
Keywords: aldehydes · amines · dehydrogenative amination ·
.
gold · heterogeneous catalysis
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