Microwave-assisted aminocarbonylation of ynamides by using catalytic [Fe3(CO)12] at low pressures of carbon monoxide

Article abstract of DOI:10.1002/chem.201100447

The microwave-assisted aminocarbonylation of ynamides at low pressures of CO is reported. A new class of (E)-acrylamides that are potentially suitable for several applications has been regioselectively synthesized after microwave irradiation for only 20 min by using eco-friendly [Fe₃(CO) ₁₂] as the catalyst precursor and triethylamine as the ligand. This transformation is atom economic as all reactants are used in stoichiometric quantities. Furthermore, the transformation is efficiently applied to the alkoxycarbonylation of alkynes as well. Moreover, running these reactions under microwave irradiation allows the simplification of the reaction conditions with remarkable reductions in time, temperature and gas pressure.

Full text of DOI:10.1002/chem.201100447

FULL PAPER
DOI: 10.1002/chem.201100447
Microwave-Assisted Aminocarbonylation of Ynamides by Using Catalytic
[Fe3(CO)12] at Low Pressures of Carbon Monoxide
Marianna Pizzetti, Adele Russo, and Elena Petricci*[a]
Abstract: The microwave-assisted ami-
nocarbonylation of ynamides at low
pressures of CO is reported. A new
class of (E)-acrylamides that are poten-
tially suitable for several applications
has been regioselectively synthesized
after microwave irradiation for only
and triethylamine as the ligand. This
transformation is atom economic as all
reactants are used in stoichiometric
quantities. Furthermore, the transfor-
mation is efficiently applied to the alk-
oxycarbonylation of alkynes as well.
Moreover, running these reactions
under microwave irradiation allows the
simplification of the reaction condi-
tions with remarkable reductions in
time, temperature and gas pressure.
Keywords: acrylamides
·
carbon
monoxide · carbonylation · iron ·
microwave chemistry · ynamides
20 min
by
using
eco-friendly
[Fe3(CO)12] as the catalyst precursor
Introduction
er, if compared to the plethora of methodologies reported
for the carbonylation of aryl derivatives,[10,12,13,15,16,20–23] only
a few studies have been reported on the carbonylation of
the alkyne moiety. The first example of the palladium cata-
lyzed carbonylation of alkynes was related by Tsuji and
Nogi in 1966[24] but it was only in the 1980s that a large
range of applications for this reaction became possible, after
the introduction of new families of Pd catalyst.[25] Other
noble-metal-based catalysts have been proposed but their
usefulness is limited by low selectivity and availability, and
high cost and toxicity.[15d,26,27] Common Pd-catalyzed proce-
dures for the carbonylation of terminal alkynes mainly give
2-substituted acrylamides (the Markovnikov product) under
high CO pressures (20–150 bar), after long reaction times
(24–48 h), and few of the protocols are regioselective.[18,25–29]
In the search for more economical and environmentally
friendly catalysts, the first-row transition metals, such as
iron, copper, zinc, and manganese, can be considered as a
good alternative. Iron catalysis has recently been considered
as a promising area within the homogeneous catalysis com-
munity.[30,31] Iron–carbonyl compounds have been increasing-
ly used in organic synthesis in recent years.[32–35] In 1997, Per-
iasamy and co-workers first reported the possibility of using
stoichiometric [HFe3(CO)11]À for the transformation of al-
kynes into cyclobutenediones[34] and, five years later, they
described the use of iron–carbonyl salts for the synthesis of
succinimides from alkynes.[35] More recently, Beller and co-
workers published a catalytic version of this reaction.[36]
Starting from these works in combination with our interest
in new eco-friendly catalysts for microwave-assisted carbon-
ylation reactions that use carbon monoxide as a benign
source of carbon,[15b,37] we were particularly intrigued by the
possibility of a microwave-assisted catalytic carbonylation of
alkynes and ynamides. The group of alkynes used here[38]
seemed the most interesting substrates as the 3-amidoacryl-
Microwave dielectric heating has emerged over the last 20
years as an efficient, useful new technique for materials,
ACHTUNGTRENNUNG
medicinal and organic chemistry processes.[1–4] The greatest
strengths of microwave-assisted organic synthesis (MAOS)
are the dramatic reduction in reaction times, improvement
in yields, and enhanced product purity. Essentially, all reac-
tors for MAOS have been designed to resist the pressure
generated by the solvent and can be considered to be small
autoclaves in which reactions with pressurized gas can be
carried out.[5] Several examples of the application of micro-
wave dielectric heating to implement hydrogenation,[6–8] hy-
droformylation,[6,9–11] and alkoxy-[10,12,13] and aminocarbon-
ACHTUNGTRENNUNG
ylation[13] have been reported. Inside the microwave cavity,
these reactions proceed at lower pressures of gaseous re-
agents and in shorter reaction times compared with tradi-
tional heating procedures.
The development of new eco-friendly, efficient, and selec-
tive transformations is a hot topic in organic chemistry.[14]
Carbonylation is a widely applied, atom economic reaction
that provides esters, ketones, carboxylic acids, amides, and
heterocyclic compounds in a very efficient manner.[15,16] The
Reppe reaction with acetylene has been used for a long
time for the synthesis of acrylic glass[17] and several proce-
dures for the carbonylation of alkene[18] and alkyne[18,19] de-
rivatives with different catalysts have been studied. Howev-
[a] M. Pizzetti, A. Russo, Dr. E. Petricci
Dipartimento Farmaco Chimico Tecnologico
Universitꢀ degli Studi di Siena
Via A. Moro, 53100 Siena (Italy)
Fax : (+39)0577-2343-33
E-mail: petricci@unisi.it
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100447.
AHCTUNGTREGaNNUN mides produced are a new class of molecule with several
Chem. Eur. J. 2011, 17, 4523 – 4528
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4523

potential applications. Consequently, we carried out a study
to select the best reaction conditions for this transformation.
A few weeks ago, Beller and co-workers disclosed his results
on the general selective iron-catalyzed aminocarbonylation
of simple substituted alkynes.[39] Therefore, we report herein
the preliminary achievements simultaneously obtained by
our group.
amines[31c] were tested to generate the catalytically active
species “amine–[Fe(CO)4]”.[33–35,42] By using 1,8-diazabicy-
cloundec-7-ene (DBU) or N,N-diisopropylethylamine
(DIPEA) and 2 (1.2 equiv), almost the same yield of 3 was
observed (entries 4 and 5, Table 1). The addition of DMF or
NH3 produced a dramatic reduction in reaction yields (en-
tries 7 and 8, Table 1), whereas triethylamine (TEA,
4 equiv) gave 3 in 70% isolated yield after microwave irra-
diation for 15 min at 908C, in the presence of CO (1.3 bar)
and nPrNH2 (1.2 equiv; entry 6, Table 1).
Results and Discussion
Seeking the optimal reaction conditions, different sol-
vents, temperatures, times, and pressures of CO were tested.
Dioxane, toluene, and DMF did not promote the reaction
(entries 1–3, Table 2). THF proved to be the best solvent for
this transformation at 908C (entry 4, Table 2).
Ynamide 1 and n-propylamine 2 were chosen as model sub-
strates for the optimization of the Reppe reaction
(Scheme 1). First, the conditions reported for the synthesis
Table 2. Optimization of reaction conditions: solvent, temperature, time,
and pressure of CO.[a]
Solvent
T [8C]
t
CO [bar]
Yield [%]
1
2
3
4
5
6
7
8
9
DMF
dioxane
toluene
THF
THF
THF
THF
THF
THF
THF
90
90
90
90
70
70
110
90
90
90
90
90
15 min
15 min
15 min
15 min
15 min
3ꢂ15 min
15 min
20 min
20 min
20 min
20 min
24 h
1.3
1.3
1.3
1.3
1.3
1.3
1.3
1.3
2.7
9
20
15
25
70
Scheme 1. Microwave-assisted iron-catalyzed monocarbonylation of
ACHTUNGTRENNUNGynamides.
5
5 (PhNHBoc 40)
of succinimides[36] were applied using microwave heating
(908C) at a lower pressure of CO (1.3 vs. 10 bar); surprising-
ly, after 15 min of irradiation, 3 was obtained regioselective-
ly in 20% isolated yield, with no trace of the corresponding
succinimide (entry 1, Table 1).[40]
45
81
80
75
83
61
10
11[b]
12[c]
THF
THF
–
1.3
[a] Reaction conditions: solvent (3 mL),
1
(0.6 mmol), nPrNH2
Table 1. Iron-catalyzed carbonylation of ynamide 1 with amine 2: auxilia-
ries and catalysts.
(0.72 mmol), TEA (2.4 mmol), [Fe3(CO)12] (0.12 mmol), CO, MW.
[b] [Fe3(CO)12] (6 equiv) was used. [c] Traditional heating procedures
used.
Entry
Catalyst
Amine/Auxiliary
Yield [%]
1[a]
2[b]
3[a]
4[c]
5[c]
6[c]
7[c]
8[c]
[Fe(CO)5]
–
A
nPrNH2 (6 equiv)
nPrNH2 (6 equiv)
nPrNH2 (6 equiv)
nPrNH2 (1 equiv)/DIPEA (4 equiv)
nPrNH2 (1 equiv)/DBU (4 equiv)
nPrNH2 (1 equiv)/TEA (4 equiv)
nPrNH2 (1 equiv)/DMF (4 equiv)
20
–
Working at a lower temperature (708C), only slight con-
version was observed (entry 5, Table 2), although microwave
irradiation for longer time periods allowed tert-butyloxycar-
bonyl (Boc)–aniline (40%) to be recovered with several
poorly identified byproducts (entry 6, Table 2). At higher
temperatures, a lower conversion was observed (entry 7,
Table 2). No improvement in reaction yield was observed if
the reaction was run at a higher pressure of CO, probably
because of the over coordination of Fe by CO (entries 9 and
10, Table 2). The optimal conditions for this transformation,
therefore, are microwave irradiation for 20 min, at 1.3 bar of
CO, in THF at 908C (entry 8, Table 2). A comparable yield
of 3 was also obtained by using [Fe3(CO)12] (6 equiv), dem-
onstrating that the complex may act both as the catalyst and
the CO source (entry 11, Table 2).[34,35] Finally, applying tra-
ditional heating procedures for the aminocarbonylation of 1
at the same CO pressure to that used under microwave
heating, a reduction of the overall conversion was observed,
as well as the formation of the succinimide byproduct that
was never isolated when using microwave dielectric heating
(entry 12, Table 2). Microwave irradiation not only dramati-
cally reduces reaction times, but also allows a better conver-
50
53
52
70
30
45
N
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
[d]
R
nPrNH2 (1 equiv)/NH3 (4 equiv)
[a] Reaction conditions: THF (3 mL), 1 (0.6 mmol), nPrNH2 (3.6 mmol),
catalyst (0.06 mmol), CO (1.3 bar), MW, 908C, 15 min. [b] Reaction con-
ditions: THF (3 mL), 1 (0.6 mmol), nPrNH2 (3.6 mmol), CO (1.3 bar),
MW, 908C, 15 min. [c] Reaction conditions: THF (3 mL), 1 (0.6 mmol),
nPrNH2 (0.72 mmol), auxiliary (2.4 mmol), [Fe3(CO)12] (0.12 mmol), CO
(1.3 bar), MW, 908C, 15 min. [d] NH3 (30%) solution in MeOH.
To exclude possible contamination of the microwave reac-
tor by previous transformations carried out with different
catalysts, the reaction was initially run without a metal cata-
lyst. As indicated by entry 2, Table 1, no reaction was ob-
served in the absence of an iron–carbonyl complex. Triiron
dodecacarbonyl was investigated as the catalytic precursor
because it is less toxic and easier to handle than [Fe(CO)5],
giving 3 in an improved yield (entry 3, Table 1).[41] With the
aim to reduce the amount of nucleophile required, different
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Chem. Eur. J. 2011, 17, 4523 – 4528

Aminocarbonylation of Ynamides
FULL PAPER
sion to be observed by using a lower pressure of CO (1.3 vs.
10 bar).[39] This cannot be related to any specific effect of
microwaves but our results are completely in agreement
with well-documented studies on microwave-assisted reac-
tions in the presence of gaseous reagents; hydroformylation,
hydrogenation, and carbonylation reactions can all be run
inside a microwave cavity by using lower pressures of gas,
giving the possibility of running these transformations in a
safer manner.[4,5,10,11]
The scope of the catalyst system was demonstrated by the
microwave-assisted carbonylation of various ynamides in the
presence of different nucleophiles (Table 3). Starting materi-
als (1,[43] 6, and 7) were prepared in good yields by a cross-
coupling reaction between a protected amine, 4a–c, and (2-
bromoethynyl)triisopropylsilane (5) followed by deprotec-
tion with tetra-n-butylammonium fluoride (TBAF;
Scheme 2).[43]
Scheme 2. Synthesis of ynamides. KHMDS=potassium hexamethyldisila-
zane.
It is worth noting that all of the ynamide derivatives
showed similar reactivity in the presence of different nucleo-
philes. Only with phenylethylamine, the corresponding ac-
rylamide was obtained in a lower yield. In any case, the
anti-Markovnikov compound was the only product isolated
and no trace of the Z isomer or the succinimide derivative
was observed. By using MeOH as the nucleophile, (E)-3-
amidoacryl ester 14 was ob-
tained in good yield, demon-
Table 3. Iron-catalyzed monocarbonylation of ynamides with different nucleophiles (Nu).[a]
Ynamide Nu Product
strating a possible extension of
this methodology to the alkoxy-
carbonylation reaction.
Yield
[%]
To verify that this unprece-
dented monocarbonACTHUNRGTNEyNUG lation of
1
2
1
1
Ph
(CH2)2NH2
8
9
70
90
ynamides was not limited to
this substrate, differently substi-
tuted simple alkynes (15a–d)
were submitted to the same re-
action conditions. We were
pleased to observe that the cor-
CyNH2
pip
3
4
1
6
10
11
75
71
responding
(E)-acrylamides
were obtained regioselectively
in acceptable yields (Table 4).
nPrNH2
nPrNH2
However,
methoxyphenyl
with
acetylene
p-
or
N
phenylacetylene, a 10% yield
of the corresponding succini-
mide was always isolated if a
primary amine was used as the
nucleophile (entries 1, 2, 4, 5,
and 7, Table 4). The same (E)-
5
7
12
81
acrylamide/succinimide
ratio
was observed if the carbonyla-
tion reaction was run by tradi-
tional heating inside a stainless
steel autoclave for 24 h, but a
slightly lower overall conver-
sion was noted, further demon-
strating the efficiency of micro-
wave dielectric heating in this
transformation.
6
7
1
Ph
(CH2)2NH2
13
14
67
78
7[b]
MeOH
Enantiomerically pure (S)-1-
phenylethanamine gave amide
[a] Reaction conditions: THF (3 mL), ynamide (0.6 mmol), amine (0.72 mmol), TEA (2.4 mmol), [Fe3(CO)12]
(0.12 mmol), CO (1.3 bar), MW, 908C, 20 min; pip=piperidine, Cy=cyclohexyl. [b] Reaction conditions: THF
(3 mL), ynamide (0.6 mmol), MeOH (0.72 mmol), TEA (2.4 mmol), [Fe3(CO)12] (0.18 mmol), CO (1.3 bar),
MW, 908C, 20 min.
16e as
a single enantiomer
(entry 5, Table 4). If MeOH
was used as the nucleophile, al-
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4525

E. Petricci et al.
Table 4. Iron-catalyzed monocarbonylation of terminal alkynes with different nucleophiles.[a]
tion with the ynamide to give
intermediate 17 (or more likely
the dimer iron complex 18;
Scheme 3).[44,45] After insertion
of CO between Fe and C, the
nucleophilic attack of nPrNH2
occurs with simultaneous hy-
dride transfer to Fe. TEA re-
generates the [Et3N][Fe(CO)4]
complex releasing (E)-acrylam-
R
Nucleophile
nPrNH2
CyNH2
pip
Product
Yield
[%]
1
2
3
4
p-OMePh 15a
p-OMePh 15a
p-OMePh 15a
p-OMePh 15a
p-OMePh 15a
16a
16b
16c
16d
78[b]
58[b]
44
ide 21.
A possible assisting
effect of the carbamate group
could avoid the formation of
the
succinimide
deriva-
tive.[35,36,39,44c,e]
Ph
N
55[b]
Conclusion
We have developed the first
catalytic microwave-assisted
aminocarbonylation of yna-
mides with eco-friendly
5
6
7
(S)-1-phenylethanamine
nPrNH2
16e
16 f
16g
45[b]
CH
U
68 (90)[c]
58[b]
[Fe3(CO)12] using TEA as the
only ligand, in the presence of a
stoichiometric amount of nucle-
ophile and under low pressures
of CO. The 3-amido acryla-
mides obtained thus are a new
class of molecules that are po-
tentially suitable for several ap-
plications. The same procedure
can easily be applied to termi-
nal alkynes to give (E)-acryla-
mides and cinnamides regiose-
lectively in only 20 min, at low
Ph 15c
nPrNH2
8
tBu 15d
CyNH2
MeOH
16h
16i
54 (95)[c]
9[d]
p-OMePh 15a
62
10[d]
Ph 15c
MeOH
16j
57
[a] Reaction conditions: THF (3 mL), 15 (0.6 mmol), Nu (0.72 mmol), TEA (2.4 mmol), [Fe3(CO)12]
(0.12 mmol), CO (1.3 bar), MW, 908C, 20 min. [b] 10% of the succinimide derivative was isolated. [c] Con- pressures of CO (1.3 bar). As
version determined by GC-MS analysis. [d] Reaction conditions: THF (3 mL), 15 (0.6 mmol), MeOH
(0.72 mmol), TEA (2.4 mmol), [Fe3(CO)12] (0.18 mmol), CO (1.3 bar), MW, 908C, 20 min.
indicated by preliminary results
using MeOH as the nucleo-
koxycarbonylation occurs in good yield (entries 9 and 10,
Table 4). It is worth noting that the anti-Markovnikov com-
pound was again isolated as the only isomer in the presence
of N- or O-nucleophiles.
However, unlike Bellerꢃs observations, the microwave-as-
sisted monocarbonylation of alkynes worked well under the
following conditions: higher catalyst loadings, in the absence
of ligands, with limited amounts of the nucleophile (1.2 vs.
10 equiv), after shorter times and under lower CO pressure.
If the reaction was carried out in the commercially available
80 mL tube, 1 g of ynamide 1 was conveniently transformed
into the desired acrylamide 3.
A possible pathway for the iron-catalyzed carbonylation
that tries to rationalize the high stereoselectivity of the addi-
tion is outlined in Scheme 3. First, the active complex
Scheme 3. Proposed mechanism for the iron-catalyzed monocarbonyla-
tion of ynamides.
“amine–[Fe(CO)4]”[33c,42] is formed, followed by coordina-
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Aminocarbonylation of Ynamides
FULL PAPER
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Once more, running reactions with gases in a microwave
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respect to classical autoclaves with remarkable reductions in
time, temperature and gas pressure, which allows the reac-
tion to proceed in a more eco-compatible manner.
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General procedure for the carbonylation of ynamides: In a MW vessel
(10 mL), [Fe3(CO)12] (60 mg, 0.12 mmol) was subjected to 2 cycles of
vacuum then N2. Dry THF (3 mL) and triethylamine (334 mL, 2.4 mmol)
were added to give a red-brown solution. n-PrNH2 (59 mL, 0.72 mmol)
and 1 (130 mg, 0.6 mmol) were then added to the mixture. The MW tube
was then subjected to two cycles of vacuum then CO inside the micro-
wave cavity, pressurized with CO (1.3 bar) and irradiated at 250 W (value
previously set on the microwave oven) for 20 min at 908C. The solvent
was removed in vacuo and the product purified by column chromatogra-
phy (AcOEt/PE, 3:7) to give 3 (147 mg, 81%). 1H NMR (400 MHz,
CDCl3): d=8.29 (d, J=13.6 Hz, 1H), 7.43–7.39 (m, 2H), 7.36–7.34 (m,
1H), 7.11 (d, J=8 Hz, 1H), 5.16 (bs, 1H), 4.58 (d, J=13.6 Hz, 1H),
3.19–3.15 (m, 2H), 1.43 (s, 11H), 0.85 ppm (t, J=7.4 Hz, 3H); 13C NMR
(100 MHz, CDCl3): d=168.3 (s, CO), 149.2 (s, CO), 142.2 (s, CH), 134.8
(s, C), 128.5 (s, 2ꢂCH), 124.7 (s, CH), 120.1 (s, 2ꢂCH), 118.1 (s, CH),
60.9 (s, CH2), 43.2 (s, CH2), 31.4 (s, CH2), 23.0 (s, CH2), 19.5 (s, CH2),
12.7 (s, 2ꢂCH3); MS (ES): m/z: 305 [M+1]+; 327 [M+Na]+.
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Acknowledgements
This work was funded by MIUR (Rome) within the FIRB project
(RBFR08TTWW). We really thank Professor Maurizio Taddei for stimu-
lating scientific discussions and interesting suggestions.
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Received: February 9, 2011
Published online: March 25, 2011
4528
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Chem. Eur. J. 2011, 17, 4523 – 4528