Palladium(II)-Catalysed Aminocarbonylation of Terminal Alkynes for the Synthesis of 2-Ynamides: Addressing the Challenges of Solvents and Gas Mixtures

Article abstract of DOI:10.1002/cssc.201601601

2-Ynamides can be synthesised through Pd^IIcatalysed oxidative carbonylation, utilising low catalyst loadings. A variety of alkynes and amines can be used to afford 2-ynamides in high yields, whilst overcoming the drawbacks associated with previ

Full text of DOI:10.1002/cssc.201601601

Accepted Article
Title: Palladium(II) Catalysed Oxidative Aminocarbonylation for the
Synthesis of 2-Ynamides: Addressing the Challenges of Solvents
and Gas Mixtures
Authors: Nicola Hughes, Clare Brown, Andrew Irwin, Qun Cao, and
Mark Muldoon
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COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium(II) Catalysed Aminocarbonylation of Terminal Alkynes for the
Synthesis of 2-Ynamides: Addressing the Challenges of Solvents and Gas
Mixtures
N. Louise Hughes, Clare L. Brown, Andrew A. Irwin, Qun Cao and Mark J.
Muldoon*
School of Chemistry and Chemical Engineering, Queen’s University of Belfast, Stranmillis Road, Belfast, David Keir
Building, BT9 5AG, Northern Ireland
Web: www.markmuldoon.com Email. m.j.muldoon@qub.ac.uk
Received:
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######
Abstract. 2-Ynamides can be synthesised via Pd(II)
catalysed oxidative carbonylation, utilizing low catalyst
loadings. A variety of alkynes and amines can be used to
afford 2-ynamides in high yields, whilst overcoming the
drawbacks associated with previous oxidative methods
which rely on dangerous solvents and gas mixtures.
copper catalyst for the crossdehydrogenative coupling
of terminal alkynes with formamides (Fig. 1, C).7 In
this case a large excess of the formamide reagent was
required.
Keywords: carbonylation; oxidation; alkynes;
palladium; solvents
2-Ynamides are valuable building blocks in the
synthesis of heterocycles 1 and biologically active
molecules.2 There are a number of methods for the
preparation of 2-ynamides. Examples of non-catalytic
approaches include the coupling of alkynyl carboxylic
acids with amines, using N-hydroxysuccinimide and
the coupling reagent N,N'-Dicyclohexylcarbodiimide
(DCC)in 1,4-dioxane.1b Terminal alkynes, amines and
CO have been utilised to prepare 2-ynamides by
Hoberg and Riegel, however they employed nickel(II)
complexes as a stoichiometric reagent. 3 Ideally, 2-
ynamides should be prepared using catalytic methods
and there are a variety of examples that have been
reported. Pd/Cu catalyst systems have been used for
the reaction of carbamoyl chlorides with terminal
4
alkynes, however the use of such acid chloride
derivatives is undesirable. There have been a number
of more recent reports exploring catalytic alternatives.
Dong and co-workers demonstrated a catalytic system
comprised of bromoalkynes and amines utilizing
Pd2(dba)3, Xphos, Cs2CO3 as the catalyst and
Co2(CO)8 as the carbonyl source (Fig. 1, A).5 Lee and
co-workers reported the use of alkynyl carboxylic
acids and amines with CO gas, using Ag2O as a base
and oxidant (Fig. 1, B).6 Ye and co-workers used a
Figure 1. Examples of synthesis of2-ynamides.
Due to our previous work in Pd(II) catalytic
oxidations, 8 we were particularly interested in the
oxidative carbonylation methods, which use Pd
catalysts with O2 as the oxidant and CO gas as the
1
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10.1002/cssc.201601601
ChemSusChem
carbonyl source. In general, Pd catalysed oxidative
carbonylation reactions offer potentially advantageous
storing them for short periods and testing for the
presence of peroxides.18 It is worth noting that 1,4-
synthetic methods,9 but such reactions have been less dioxane is often reported in the academic literature as
well developed compared to other areas of Pd(II)
oxidation chemistry, such as alcohol oxidation.10
We recently developed a system for the synthesis
of 2-alkynoates, via oxidative carbonylation of
a solvent or co-solvent in oxidation reactions and it
could be that the presence of peroxides is a factor in
aiding the reactivity. There are a number of Pd(II)
catalysed oxidation systems which have discussed the
alkynes and alcohols.11 Moving on from this work, the role of peroxides resulting from 1,4-dioxane.19 While
aim of this study was to improve the synthesis of 2-
ynamides. There are a number of previous reports
which use alkynes, secondary amines and CO to
produce 2-ynamides. This route is desirable asalkynes
and amines are commercially available and
inexpensive, and these reagents are used without
additional functionalisation in order to activate them.
While CO is an abundant and inexpensive source of
the carbonyl group. In 2001, Gabriele and co-workers
reported a method which utilised a simple PdI2 catalyst
system (Fig. 1, D). 12 In a study focused on the
preparation of 2-alkynoates (where the nucleophile is
an alcohol), Yamamoto and co-workers showed that
their catalyst system (PdCl2 and triphenylphospine in
DMF) could also be used for the synthesis of 2-
such systems may be academically interesting, we
believe that the use of ethereal solvents for aerobic
oxidation reactions is not something which would be
adopted by industry. In fact, we believe that the use of
such an oxygen sensitive solvent hinders this area of
catalysis. In this study we have employed ethyl acetate,
which is classed as a ‘recommended’ solvent in the
solvent selection guides developed by the
pharmaceutical industry.17
There are additional safety issues for aerobic
reactions and the safe use of O2 has not been discussed
in these previous oxidative carbonylation studies.
Although there is no doubt that O2 is the most
economical and sustainable terminal oxidant it does
pose safety hazards, particularly when used on a larger
ynamides, using diethylamine.13 Recently, Xia and co- scale. The flammability of organic solvents is one
workers reported a Pd(II) system with an N- important consideration and employing limiting
heterocyclic carbene ligand (Fig. 1, E).14 Bhanage and oxygen concentrations (LOC)is one way to try and use
co-workers reported a heterogeneous system that used
Pd on carbon in 1,4-dioxane, with
tetrabutylammonium iodide [NBu4]I as an additive.15
O2 safely. The LOC of ethyl acetate at 100 °C is 9.4
vol% O2 at a pressure of 1 bar absolute and 9.9 vol%
O2 at a pressure of 20 bar absolute.20 An additional
More recently the same group reported the use of Pd issue with carbonylation reactions is that CO is a
on carbon with KI as an addtive in acetonitrile. 16 In flammable gas. The lower flammability limit (LFL)
this report they utilised tertiary amines (i.e. the
reaction involved a N-dealkylation step), which means
that in the case of unsymmetrical amines a mixture of
products is formed.
(also referred to as the lower explosion limit (LEL)) of
carbon monoxide in air is 11.5 vol% at 100 °C and
atmospheric pressure. 21 The upper flamability limit
(UFL) (or upper explosion limit (UEL)) is 75 vol% at
100 °C and atmospheric pressure. In the case of the
We felt that the work to-date had aspects which
needed addressed as their conditions would method by Gabriele and co-workers (Fig. 1, D), they
significantly hamper the wider application of these
catalytic methods. In particular, the reliance on
hazardous solvents is an issue. DMF was used by
Yamamoto and co-workers,13 while the three papers
which focussed on 2-ynamide synthesis using
secondary amines all employed 1,4-dioxane. Solvents
utilized a gas mixture of CO and air in a 4:1 ratio at a
total pressure of 20 atm.12 It would appear that this
ratio is in-line with the UFL (i.e. the fuel rich region).
However, industry would prefer to operate with LFL
conditions (fuel lean region). First of all, it is known
that with increasing pressure the LFL values decrease
are crucial to the sustainability and safety of chemical somewhat but UFL values increase and do so to a
reactions and so it is important when optimising a
catalytic method, to critically evaluate the solvent
selected. Solvent selection guides, based on safety, vapour phase is vented (or if there is a leak) then the
greater extent.22 Additionally, if UFL conditions are
used then there is an inherent danger because when the
health, and environment criteria, produced by some of
the world’s largest pharmaceutical companies, classed
gas mixture will passthrough the flammable/explosive
region as it mixes with the air. If LFL conditions are
DMF and 1,4-dioxane as‘hazardous’ and ‘problematic’ used then the gas will not form ignitable compositions
and stated that such solvents should be avoided. 17
Furthermore, etherealsolvents such as 1,4-dioxane are
arguably completely incompatible for carrying out
oxidation reactions in a safe and scalable manner. It is
well known that such solvents readily form peroxide
speciesand laboratories have to take precautions when
using such solvents in general, for example only
when it mixes with air.23 In this study we have used 5
bar of CO and 30 bar of an O2:N2 (8:92) gas mixture.
20
As recently highlighted by Stahl and co-workers,
there is a lack of such safety data under the types of
reaction conditions that are used in aerobic catalytic
reactions, therefore the conditions we have used have
not been studied exactly. However, we have based our
2
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10.1002/cssc.201601601
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conditions on the aforementioned LOC and LFL data,
observed in the synthesis of 2-alkynoates. The
importance of Pd salt purity and its effect on catalytic
performance has been discussed recently,26 and we
believe it is worth highlighting such factors in order to
aid reproducibility.
which should mean the system is safer. Additionally,
the conditions also demonstrate that catalysts can
operate effectively under such conditions.
We commenced the reaction optimisation using
phenylacetylene and diethylamine as the model
reaction, in the presence of PdI2 (0.2 mol%), and
[NBu4]I (2.5 mol%). The reaction was run at 80 °C, in
EtOAc, for 6 hours with a 1:2 ratio of alkyne to amine.
We initially screenedboth 8% O2 in N2 and air (30 bar)
with CO (5 bar), finding that both gave almost
identical results. This indicated that the systemwasnot
limited in O2 when operating under these more dilute
conditions. This is important as if the system is limited
in O2 then Pd(0) aggregation and catalyst deactivation
will take place. Moving on from this, we screened a
variety of palladium salts, as shown in Table 1.
Having chosen to utilise Pd(OAc)2 for the
remainder of the studies, we then went on to study the
effect of ligands. For the synthesis of 2-alkyonates the
system was greatly affected by the presence of ligands
and we found optimal performance with
tetramethylethylenediamine (TMEDA). However, in
these aminocarbonylation studies we found that
ligands (such as TMEDA and phenanthroline) were
not necessary and did not lead to any enhanced
performance under the conditions studied. It could be
that in this case, the amine substrate acts as an
adequate ligand. The lack of ligand and indeed the
loading is very similar to the conditions by Gabriele
and co-workers who used 0.2 mol% PdI2 (Fig. 1, D).12
However, it contrasts the recent work by Xia and co-
workers found that under their conditions, superior
performance was obtained when they employed an N-
heterocyclic carbene ligand (Fig. 1, E) and the catalyst
loading was 1 mol% catalyst.14 We are pleased, to see
that we can avoid the use a ligand and indeed higher
catalyst loadings, obviously an advantage from both
economical and green perspectives.
Table 1. Influence of palladium(II) salts on catalytic
performance.
In many previous reports of Pd oxidative
carbonylations, iodide salts have been a key ingredient,
therefore we examined the effect of such additives, as
shown in Table 2.
Table 2. Effect of additives on catalytic system.
The reaction proceeded with all the anions tested,
however the best results were observed with
carboxylate anions such as acetate, and pivalate. This
contrasts with what we observed in the previous 2-
alkynoate studies; with catalyst activity only obtained
when carboxylate anions were used.11 Acetate has
previously been shown to be a key component in
oxidative carbonylation reactions for the synthesis of
2-alkynoates, either as the palladium salt or as an
additive in the form of NaOAc.13,24 The observation
that the 2-ynamide reaction proceeds readily without
the presence of carboxylate anions would suggest that
the role of the acetate in the 2-alkynoate reactions is
for deprotonation of the alcohol; something previously
shown in Pd(II) catalysed alcohol oxidation
reactions. 25 We also noted the importance of the
Pd(OAc)2 salt purity, something which we also
Although KI was used by Gabriele and co-workers
(Fig. 1, E)12 and Xia and co-workers (Fig. 1, E),14 it
can be seen that we observed little of the desired
2-ynamide when using KI under our reaction
conditions. We presume that the high activity with
[NBu4]Iis due to the increasedsolubility in our chosen
reaction solvent, ethyl acetate. We alsofound the same
3
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10.1002/cssc.201601601
ChemSusChem
behaviour with the previously mentioned 2-alkynoate
system.11 We tested various loadings of [NBu4]I,
comparing 1.25 mol%, 2.5 mol% and 5 mol%, to find
2.5 mol% to be the optimal. It is also worth noting that
at present there is not a well proven explanation or
consensus on the importance of such iodide salts in
these oxidative carbonylation reactions and further
work is needed to develop a better understanding.
We testedother parametersof the reaction system,
including solvent, base and reaction temperature. The
results are shown in Table 3.
Table 3. Comparison of reaction systemparameters.
Figure 2. Substrate scope utilising various alkynes and
amines. Yields reported are isolated yields. Further details
are given in the SI.
In conclusion, we have developed a method for the
synthesis of 2-ynamides via oxidative carbonylation
which utilizes low loadings of Pd(II) and does not
require the use of ligands. Importantly, the method
avoids the use of dangerous 1,4-dioxane, the solvent
which was previously the solvent of choice for these
reactions. The use of [NBu4]I enables us to employ the
industrially recommended solvent ethyl acetate. We
have used oxygen as the terminal oxidant and
demonstrated that the catalyst could operate under
safer conditions with lower O2 concentrations. These
factors are important if we are to hope that such
catalytic methods are to be exploited on a larger scale.
It wasshown that ethyl acetate evenout-performed the
commonly employed 1,4-dioxane under these
conditions (c.f. entries 2 and 3). K3PO4 was used by
Xia and co-workers ((Fig. 1, E),14 however under our
conditions we observed that the addition of this base
decreased the yield (entry 4). A longer reaction time
(entry 7) was also tested, however the results obtained
were almost identical to that after 6 hours. We found
80 °C to be the optimal temperature; at 60 °C the
activity was greatly reduced, and 100 °C led to a
reduction in selectivity. We compared a substrate ratio
of 1:1 (entry 8), but found that this causeda significant
drop in product yield.
Once the reaction system had been optimised, we
tested the system on a variety of alkynes and amines.
The results can be seen in Figure 2. We could use both
activated and unactivated alkynes. Alkynes containing
both electron withdrawing and electron donating
substituents worked well. The systemwasproven to be
heteroatom tolerant as can be seen with 5 and 6.
Sterically hindered alkynes such as 2-ethynyl-1,3,5-
trimethylbenzene 9 produced a good yield.
Experimental Section
Reactions were performed in 45 mL high-pressure reactors
which are made of Hastelloy C276 and fitted with a safety
pressure relief valve. The reaction mixture was placed in a
glass liner, equipped with a magnetic stirrer. To the glass
liner, tetrabutylammonium iodide (2.5 mol%, 0.05 mmol,
0.0185 g) and Pd(OAc)2 (0.2 mol%, 0.004 mmol, 0.0009 g)
from a stock solution in ethyl acetate (4 mL) were added.
This was followed by the addition of alkyne (2 mmol) and
amine (4 mmol). The glass liner was placed in a reactor and
then pressurized with 5 bar of carbon monoxide gas,
4
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10.1002/cssc.201601601
ChemSusChem
followed by O2:N2 (8:92) to give a total reaction pressure of
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aqueous layer was separated and back extracted with ethyl
acetate twice. The combined organic layers were dried over
magnesium sulphate, filtered and concentrated under
reduced pressure. The product was purified by silica gel
flash column chromatography, and the appropriate fractions
combined and concentrated under reduced pressure. The
product was then dried under high vacuum.
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For facilities and funding, thanks to Queen’s University
Belfast (QUB), the Department of Employment and
Learning (DEL), Northern Ireland, Queen’s University
Belfast Ionic Liquid Laboratories (QUILL), and the
Engineering and Physical Sciences Research Council
(EPSRC). For insightful discussions we are grateful to Dr
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