Efficient solvent- and metal-free Sonogashira protocol catalysed by 1,4-diazabicyclo(2.2.2) octane (DABCO)

Article abstract of DOI:10.1039/b821134p

An efficient metal-free Sonogashira coupling protocol catalysed by DABCO is reported, where very good conversions and selectivities to the cross-coupling product were obtained under mild reaction conditions. The reported solvent-, phosphane- and metal-free protocol, that uses a cheap base as catalyst, considerably improves the green credentials of the reaction.

Full text of DOI:10.1039/b821134p

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Efficient solvent- and metal-free Sonogashira protocol catalysed by
1,4-diazabicyclo(2.2.2) octane (DABCO)
Rafael Luque*† and Duncan J. Macquarrie
Received 25th November 2008, Accepted 12th January 2009
First published as an Advance Article on the web 25th February 2009
DOI: 10.1039/b821134p
An efficient metal-free Sonogashira coupling protocol catalysed by DABCO is reported, where very
good conversions and selectivities to the cross-coupling product were obtained under mild reaction
conditions. The reported solvent-, phosphane- and metal-free protocol, that uses a cheap base as
catalyst, considerably improves the green credentials of the reaction.
cost-efficient.15 Alternative systems to the Sonogashira coupling
Introduction
have been reported,16,17 including a few metal-free coupling
protocols.9,18
The Sonogashira coupling between alkynes and aryl or alkenyl
halides (Scheme 1) is an easy, efficient, and long-established route
in organic synthesis. This reaction has been widely reported as an
interesting option for the creation of C–C bonds.1
Recent studies of Leadbeater et al. showed that only small traces
of palladium as impurities in basic salts (e.g. Na2CO3) were able to
catalyse C–C bond forming reactions, yielding the cross-coupled
product in a free transition metal reaction.19 Consequently, the
choice of the base in the process seems to be a critical step in
order to design novel and more efficient catalytic systems able to
provide improved yields and selectivities combined with shorter
reaction times and milder and more environmentally friendly
conditions.
Scheme 1 Product distribution of the Sonogashira coupling.
We have previously investigated the use of triethylamine (TEA)
as the base, combined with the use of hetero◦geneous palladium
complexes under mild reaction conditions (70 C).20 However, the
reported protocol required long reaction times, and an unusual
inversion in selectivity to the homocoupling product was observed
when the coupling of bromoaryls was attempted. Other bases
including diethylamine (DEA)21 and piperidine5–7 have been widely
employed in this coupling.
Many research efforts have been devoted to the development
of new catalytic systems due to the wide range of target com-
pounds that can be synthesized, including: natural products,2
pharmaceuticals3 and molecular organic materials.4 The Sono-
gashira coupling is generally carried out in organic solvents
including toluene, dioxane and THF with the requirement of a
stoichiometric amount of base and usually a Pd(0)-Cu(I) catalytic
system.1a
Compared to these bases, 1,4-diazabicyclo(2.2.2) octane
(DABCO) has been regarded as an unhindered and nucleophilic
base, and considered one of the best amine catalysts due to its
unique properties.22–26 DABCO was reported as a stronger Lewis
base than TEA and DEA when employed in the Baylis–Hillman
coupling between arylaldehydes and activated ketones/enones.22
DABCO seemed to promote the Baylis–Hillman reaction by itself
whereas the addition of TEA or DEA as bases required catalytic
amounts of L-proline to allow the successful coupling of the
starting materials. Furthermore, DABCO has also been employed
as a ligand in C–C coupling reactions catalysed by Pd(OAc)224 and
CuI.25 Encouraged by these promising results, we were prompted
to investigate the effect of DABCO in a solvent-, phosphane- and
transition metal-free protocol under mild conditions. To the best
of our knowledge, this is the first report of the direct utilisation of
DABCO as a catalyst in the Sonogashira reaction.
In an attempt to improve and broaden the applications of the
catalytic systems to fine and green chemistry, elimination of CuI5–7
(to avoid homocoupling reactions between terminal alkynes),
solventless reactions,8 microwave enhanced couplings8–10 and sol-
vent-, copper- and phosphane-free protocols have been
considered.11 However, despite the need for improvements in
the green credentials of the reactions in the synthesis of
pharmaceuticals,12 many of the reported protocols still rely on
homogeneous catalysis.1a,13,14 These homogeneous experiments
often involve time-demanding purification-isolation procedures to
achieve high purity in the product as well as to ensure the removal
of traces of undesired metals (e.g. Pd, Cu). Compared to their
homogeneous counterparts, stable heterogeneous systems may
offer high purities in products at relatively comparable reaction
rates, combined with high reusabilities that significantly improve
the green credentials of the reactions as well as making them more
Results and discussion
Green Chemistry Centre of Excellence, The University of York, Heslington,
York, UK YO10 5DD. E-mail: q62alsor@uco.es; Fax: +44 1904 432705;
Tel: +44 1904 434456
Preliminary experiments
† Current address: Departamento de Qu´ımica Orga´nica, Universidad de
Co´rdoba, Edificio Marie Curie (C-3), Ctra Nnal IV, Km 396, E14014,
Co´rdoba (Spain).
We initially performed a range of experiments in order to optimise
the different reaction parameters using a DABCO promoted
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Sonogashira coupling between phenylacetylene and an electron-
deficient reagent (e.g. 3-iodobenzotrifluoride) as a model reaction,
using a palladium system and reaction conditions similar to
those previously reported.20 The results obtained for the coupling
reaction at different temperatures (ranging from 70 ◦C to 100 ◦C)
showed an increase in the conversion values, with an increase in the
temperature, reaching a maximum at 100 ◦C. Almost quantitative
conversion (>95%) of starting material was achieved at 100 ◦C
in less than 1 h, preserving a complete selectivity to the cross-
coupled product regardless of the temperature increase. These
preliminary results were a remarkable improvement on those
previously reported by our group.20
Heterogeneous activity tests were carried out in order to
ascertain the truly heterogeneous nature of the materials under
the reaction conditions. For this purpose, both ICP analysis of the
filtrate after the reaction and a hot filtration test were carried out.
The reaction mixture was filtered off and extracted in order to
quantify any leached palladium in solution. No palladium was
detected by ICP, implying that the concentration of Pd in solution
(if any) was below the detection limit (<0.5 ppm).
A hot filtration test was also performed. Small liquid aliquots
were poured out from reactions, typically at the halfway point
(around 20–30 min), rapidly filtered off and added to another
mixture with fresh starting materials (SM). In general, no signifi-
cant increase in the conversion of the SM was observed after 6 h
in the liquid aliquot taken from the first reaction. In contrast,
the conversion remarkably increased in the unfiltered fraction
containing the solid catalyst, thus indicating that the solid catalyst
remained active.
Fig. 1 Metal-free Sonogashira coupling of iodobenzotrifluoride and
phenylacetylene using different basic molecules as catalysts. Phenylacety-
lene : iodobenzotrifluoride : base ratio = 1 : 1 : 2, 100 ◦C, 12 h.
be promoted by a previous substrate activation through stirring
the starting material with DABCO under mild heating (50–70 ◦C).
In order to improve these promising results, several parameters
were investigated including the temperature and time of reaction,
the quantity of DABCO and the use of microwave irradiation
instead of conventional heating.
Samples from the poured liquid mixture containing DABCO,
taken after 12 h, displayed a substantial increase in catalytic
activity compared to that initially observed. Further experiments
without catalysts were then needed to fully understand this
unusual increase in catalytic activity.
Effect of the reaction time
◦
Setting the temperature of the systems at 130 C, the conversion
increased in systems with longer reaction times as expected.
Almost quantitative conversion of starting material was found
after 48 h (Table 2, entries 1 and 3), with a remarkable increase
in conversion for electron-donating susbtituted arenes (Table 2,
entries 8 and 9).
Metal-free Sonogashira couplings
Firstly, a comprehensive study of a range of basic molecules
employed in the reaction, including: TEA (1), diisopropylethy-
lamine (DIEA, 2), 1,1,3,3-tetramethylguanidine (TMG, 3), 1,3 -
dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU, 4), 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU, 5) and DABCO (6) was
performed. The results are summarised in Fig. 1.
Interestingly, no significant increase in conversion was found in
the Sonogashira coupling with the exception of DABCO for which
a 35% conversion was found after 12 h at 100 ◦C (Fig. 1, entry
6). These promising results pointed to a conversion increase in the
systems directly related to the addition of DABCO that may act
as a catalyst in the Sonogashira coupling.
Effect of the reaction temperature
The results in Fig. 2 show that an increase in temperature up to
130 ◦C considerably increased the conversions of starting materials
in the systems, reaching around 60–80% after 12 h for a range of
selected electron-poor reagents. In contrast, poor conversions were
achieved in arenes with electron-donating substituents (e.g. 28%
conversion for m-iodoanisole). Temperatures above 130 ◦C did not
offer significantly higher yields of cross-coupling products (Fig. 2).
Effect of the quantity of DABCO
DABCO was consequently employed as the catalyst in the
Sonogashira coupling. Results have been included in Table 1.
Blank runs (without DABCO) gave virtually no coupling activ-
ity. Only poor conversions (ranging from 15 to 40%) were found
for the DABCO catalysed Sonogashira cross-coupling reaction
using electron-deficient reagents under similar conditions to those
employed with the palladium systems. When substituted arenes
with electron-donating groups were used, virtually no conversion
was obtained (Table 1, entries 6 and 7). These reactions seemed to
A 1 : 1 : 2 ratio of phenylacetylene : arene : DABCO was initially
employed in the coupling reaction. Smaller quantities of DABCO
considerably decreased the conversion in the systems (e.g. a 1 : 1 : 1
ratio provided a 78% conversion in the coupling of phenylacetylene
and 3-iodobenzotrifluoride compared to a 93% for a 1 : 1 : 2
ratio, Table 2, entry 1). Interestingly, an increase in the quantity
of DABCO (e.g. 1 : 1 : 3 or 1 : 1 : 4) did not seem to have
a remarkable effect on the reaction in terms of conversion and
selectivity to cross-coupling products, regardless of the arene and
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Table 1 Metal-free Sonogashira cross-coupling reaction experiments
between phenylacetylene and different electron-deficient aryl halides using
DABCO as catalysta
Table 2 DABCO catalysed Sonogashira cross-coupling reaction between
phenylacetylene and various substituted aryl iodides under optimised
conditionsa
Entry Starting material (SM) Conversion (%)b B : A ratioc Time/h
Entry
1
Starting material (SM)
Conversion (%)b
B : A ratioc
85 : 15
1
2
3
35
15
40
90 : 10
99 : 1
99 : 1
12
12
12
>90 (86)
2
3
73
95 : 5
99 : 1
90 (84)
4
27
70 : 30
12
4
84 (77)
95 : 5
5
6
7
15
<10
<5
99 : 1
99 : 1
99 : 1
12
12
12
5
6
82 (76)
80 : 20
99 : 1
<20
7
77 (72)d
90 : 10
a Re◦action conditions: phenylacetylene : SM : DABCO ratio = 1 : 1 : 2,
100 C. b Based on the conversion in moles of starting material. Determined
by GC-analysis with dodecane as the internal standard. c Determined by
GC-analysis of the crude product.
8
9
61
55
49
99 : 1
99 : 1
99 : 1
10
11
<40
95 : 5
a Reaction conditions: phenylacetylene : SM : DABCO ratio = 1 : 1 : 2,
130 ◦C, 48 h. b Based on the conversion in moles of starting material.
Determined by GC-analysis using dodecane as internal standard. Isolated
yields (where appropriate) are given in brackets. c Determined by GC-
analysis of the crude product. d 60 h reaction time.
Fig. 2 Metal-free Sonogashira coupling of iodobenzotrifluoride and
phenylacetylene at different temperatures using DABCO as catalyst.
Phenylacetylene : iodobenzotrifluoride : DABCO ratio = 1 : 1 : 2,
12 h.
the substituent(s). Therefore, a 1 : 1 : 2 ratio was selected as
optimum for the coupling reaction.
with already proven reports in palladium heterogeneously catal-
ysed Sonogashira protocols.15a,28 The efficiency of the microwave
metal-free DABCO catalysed Sonogashira coupling is clearly
demonstrated in Table 3. Under the optimised conditions (1 mmol
phenylacetylene, 1 mmol arene, 4 mmol DABCO, 200 W, 120 ◦C,
1 h), quantitative conversion of starting material was achieved in
most cases, except for electron-donating substituted arenes that
Microwave irradiation vs. conventional heating
Microwaves have been proved to be a very useful tool in organic
synthesis, allowing fast and homogeneous heating that can consid-
erably improve yields and even selectivities in catalysed reactions,27
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Table 3 DABCO catalysed Sonogashira cross-coupling reaction under
microwave irradiationa
Entry Starting material (SM) Time/h Conversion (%)b B : A ratioc
1
2
3
1
87 (80)
95 : 5
99 : 1
99 : 1
1.5
1
75
83
Scheme 2 Plausible reaction pathways for the metal-free DABCO-catal-
4
5
1
1
77
80
99 : 1
95 : 5
ysed Sonogashira coupling.
bond in the alkyne (route 1). The intermediate formed, relatively
stabilised through conjugation with the aromatic ring, may then
attack the aryl halide to yield the cross-coupling product through
elimination of HI.
An alternative pathway (route 2) may involve nucleophilic
substitution of the aryl halide through the iodine and then attack
of the newly generated nucleophilic carbon in the aromatic ring
(after DABCO leaving) to another molecule of alkyne, to give the
substituted diphenylacetylene.
6
7
2
2
66
49
99 : 1
99 : 1
We believe that the reaction progresses mainly through route
1 as the rate determining step is likely to be the nucleophilic
attack, by the intermediate formed after the initial DABCO attack,
of another molecule (either the aryl iodide or the alkyne, rate
determining step, Scheme 2). This rate determining step will be
expected to be slower in route 2 compared to that of route 1 due
to nucleophilic substitution in the aryl halide being favoured (the
iodine has a big electronic cloud that facilitates the nucleophilic
attack leaving as HI), as well as the stability of the route 2
intermediate formed after nucleophilic substitution (DABCO-aryl
halide, rate determining step, Scheme 2). Electron-withdrawing
substituents in the aromatic ring may help to reduce electron
density in the aromatic ring, thus destabilising the positive charge
generated in the C–I bond and facilitating the attack of the
nucleophile.
a Reaction conditions: phenylacetylene : SM : DABCO ratio = 1 : 1 : 2, 200
W, 140 ◦C (maximum temperature reached). b Based on the conversion in
moles of starting material. Determined by GC-analysis using dodecane as
internal standard. Isolated yields (where appropriate) are given in brackets.
c Determined by GC-analysis of the crude product.
required longer irradiation times to achieve comparable results
(Table 3, entries 6 and 7).
Most importantly, the selectivities to the cross-coupling prod-
ucts were improved under microwave conditions, being >95% in
all cases (Table 3).
Then, why did the other basic molecules, including DBU and
TMG, not work in the coupling? In general, the nucleophilicity
and steric constraints of a base are critical parameters to consider
in order to employ it as a catalyst or a promoter in a catalysed
reaction.23 Aliphatic amines including TEA, DIEA and TMG are
good nucleophiles but, comparatively, pyrrolidines and piperidines
are generally accepted as more nucleophilic and stable. DBU is
a non-nucleophilic base.29 Strictly speaking DMPU cannot be
considered as a base but it has been reported to have Lewis
basicity when employed as a catalyst in some reported reactions.30
Furthermore, compared to the other bases where steric effects may
be present, DABCO has two sterically unhindered N groups due
to its particular conformation that are readily available to act as
nucleophilic centres in a range of reactions (e.g. etherifications).31
In particular, the presence of a second N withdraws electronic
density from the neighbouring N atom through an inductive effect,
making it less nucleophilic and basic and consequently a better
Discussion of results and proposed reaction mechanism
Following studies by Leadbeater et al.19 in which they report that
the metal impurities (e.g. Pd) in basic salts were able to catalyse
C–C bond forming reactions, a sample of the reaction mixture was
further checked for Pd traces in solution. ICP and AAS gave no
Pd content in solution, implying that the Pd concentration (if any)
was below the detection limit of the techniques (<0.5 ppm).
We tried to rationalise these interesting findings by proposing
a plausible mechanism for the DABCO catalysed Sonogashira
coupling in the absence of metal catalysts (Scheme 2). These
mechanisms may be supported by the fact that the coupling does
not work very well with electron-donating substituents in the
aromatic ring as they may partially stabilise the positive charge
generated in the C–I bond through an inductive effect.
Thus, in the proposed reaction mechanism, DABCO acts as
a nucleophile attacking the electrophilic carbon from the triple
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leaving group in the reaction.29 These properties may also explain
the fact that the transition metal-free reaction takes place with
DABCO but not with the other investigated bases.
The rates of the reaction are, in any case, slower than those
obtained in the Sonogashira coupling catalysed by transition metal
(supported) catalysts.
2-Hydroxydiphenylethyne
Product from entry 7, Table 2: white solid, yield 72%. Mp 45–
48 ◦C. Flash chromatography: c-Hex–Et2O 90 : 10. GC-MS: 82
(10), 165 (50), 194 (100). 1H NMR (300 MHz, CDCl3): 7.22–7.55
(m, 5H), 6.88–7.15 (m, 4H). 13C NMR (75 MHz, CDCl3): 156.2,
131.5, 131.4, 130.0, 128.6, 128.1, 122.3, 120.1, 114.9, 109.7, 96.7,
83.1.
Blank experiments in the absence of DABCO gave no significant
coupling yields (<10%) after 48 h. Chemicals were obtained from
Aldrich, Merck and Lancaster and used as received. The reactions
were highly reproducible and provided very similar results under
identical conditions when run several times.
Every single reaction was run with new glassware and stirrer
bars to avoid cross-contamination and/or potential contribution
of traces of metals in the solution. All reagents were also tested
for traces of Pd and other transition metals by means of AAS
(Hitachi) and ICP (Perkin-Elmer 40 instrument) and showed no
detectable traces of metals (<0.5 ppm).
Microwave experiments were conducted on a CEM-Discover
model with PC control and monitored by sampling aliquots of the
reaction mixture that were subsequently analysed by GC/GC-
MS. Experiments were conducted in a closed vessel (pressure
controlled) with continuous stirring. The microwave method was
generally power controlled where the samples were irradiated with
the required power output (200 W) and temperatures ranged
between 120–140 ◦C depending on the reaction.
Further experiments are currently underway to ascertain
whether the reaction follows proposed route 1 or 2.
Conclusions
1,4-Diazabicyclo(2.2.2) octane (DABCO) was found to act as
a catalyst in a novel metal-free Sonogashira coupling using a
range of substituted aryl iodides in a solvent- and phosphane-free
protocol. Very good to excellent conversions and selectivities were
obtained to the cross-coupling products after 24 h+ reaction under
conventional heating and the optimised conditions. Microwave
irradiation provided comparative yields with improved selectivities
with much reduced reaction times (typically 1–2 h). We envisage
our novel and exciting results may lead to the development of
future metal-free protocols with the use of alternative bases as
catalysts in coupling processes.
Experimental
In a typical metal-free DABCO catalysed Sonogashira coupling,
a two-necked flask equipped with a condenser (or alternatively a
microwave tube) was charged with the alkyne (phenylacetylene,
1 mmol), DABCO (2 mmol, 224 mg), dodecane (1 mmol, 140 mL)
and the desired substituted iodoarene (1 m◦mol). The reaction
mixture was heated/microwaved at 100–130 C for the required
period of time (up to 48 h and 2 h for reactions run under
conventional heating and microwave irradiation, respectively) and
the colour of the solution gradually turned to deep red-brown.
Samples were taken periodically from the reaction mixture and
the reaction stopped after 72 h. After cooling down, the extracted
sample was filtered off, extracted with dichloromethane (DCM)
and analysed by GC/GC-MS. The use of an internal standard
technique allows the reaction yields to be determined. All the
Sonogashira and dimerisation products were characterised by GC-
MS analysis using an Agilent 6890 N GC model equipped with a
7683B series autosampler, fitted with a DB-5 capillary column
and an FID detector, and compared with reported analytical
data.5,20,32
Chemicals were bought from Aldrich, Lancaster and Acros
Organics and used as purchased.
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