Heck arylation of allyl alcohol catalyzed by Pd(0) nanoparticles

Article abstract of DOI:10.1016/j.tet.2017.03.034

Pd(0) nanoparticles ca. 2 nm in diameter were obtained by the reduction of PdCl₂ and Pd(OAc)₂ in water at 80 °C in the presence of a PVP-stabilizing polymer. Pd(0) NPs were successfully used in the Heck coupling of allyl alcohol with iodo- and bromobenzenes. Iodobenzenes reacted under solventless conditions or in DMF solution producing 3-arylpropanals and 2-arylpropanals as the main products. The same products were obtained in the reaction of bromobenzene in TBAB as the reaction medium. The stability of Pd(0) NPs was evidenced in recycling experiments. Similar Heck coupling results were also obtained with the palladium compounds PdCl₂(cod) and Pd(OAc)₂ under the same conditions.

Full text of DOI:10.1016/j.tet.2017.03.034

Accepted Manuscript
Heck arylation of allyl alcohol catalyzed by Pd(0) nanoparticles
Stanisława Tarnowicz, Waleed Alsalahi, Ewa Mieczyńska, Anna M. Trzeciak
PII:
S0040-4020(17)30270-3
10.1016/j.tet.2017.03.034
TET 28540
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 29 September 2016
Revised Date: 4 March 2017
Accepted Date: 12 March 2017
Please cite this article as: Tarnowicz S, Alsalahi W, Mieczyńska E, Trzeciak AM, Heck arylation of allyl
alcohol catalyzed by Pd(0) nanoparticles, Tetrahedron (2017), doi: 10.1016/j.tet.2017.03.034.
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ACCEPTED MANUSCRIPT
I
O
Pd(0)NPs
base
OH
OH
H
isomerization
+
β-substitution

ACCEPTED MANUSCRIPT
Heck arylation of allyl alcohol catalyzed by Pd(0) nanoparticles.
Stanisława Tarnowicz, Waleed Alsalahi, Ewa Mieczyńska, Anna M. Trzeciak*
Faculty of Chemistry, University of Wrocław, 14 F. Joliot-Curie, 50-383 Wrocław, Poland
*corresponding author (Anna M. Trzeciak): tel. +48 71 375 72 53; fax: +48 71 328 23 48; e-mail:
anna.trzeciak@chem.uni.wroc.pl
Abstract
Pd(0) nanoparticles ca. 2 nm in diameter were obtained by the reduction of PdCl2 and Pd(OAc)2 in
water at 80 °C in the presence of a PVP-stabilizing polymer. Pd(0) NPs were successfully used in the
Heck coupling of allyl alcohol with iodo- and bromobenzenes. Iodobenzenes reacted under solventless
conditions or in DMF solution producing 3-arylpropanals and 2-arylpropanals as the main products.
The same products were obtained in the reaction of bromobenzene in TBAB as the reaction medium.
The stability of Pd(0) NPs was evidenced in recycling experiments. Similar Heck coupling results
were also obtained with the palladium compounds PdCl2(cod) and Pd(OAc)2 under the same
conditions.
Key words: palladium; nanoparticles; allyl alcohol; arylation; Heck coupling
Introduction
The arylation of allylic alcohols, leading to arylated aldehydes or ketones, was reported for the
first time by Heck in 19681. In that stoichiometric reaction, aryl-palladium complexes reacted with
allylic alcohols according to Fig. 1. The aryl group was bonded to the olefinic fragment in position α
or β giving unsaturated arylated alcohols of different structures. Depending on the catalyst ability,
isomerization occurred in the next step with the formation of carbonyl compounds, aldehydes, or
ketones. The arylation of allylic alcohols is a relatively simple and attractive method leading to such
products used as substrates in the pharmaceutical industry2. In the literature, there are only scarce
examples of the arylation of allylic alcohols to arylated alcohols without further isomerization.3, 4
Ar
Ar
R2
R1
R2
isom.
α-subst.
R1
OH
O
α
R1
R2
Pd
β
ArX
OH
R2
Ar
R2
Ar
isom.
β-subst.
R1
OH
R1
O
allyl alcohols
carbonyl
compounds
1

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Fig. 1. Arylation of allylic alcohols leading to carbonyl compounds.
Attempts to perform such processes have been undertaken with additives such as silver salts4, 5 or
cesium carbonate6, or with aryl triflates7 or aryliodonium salts8 as substrates. Jeffery9 demonstrated a
positive effect of ammonium salts on the arylation of allylic alcohols, and the best results were noted
for Bu4NCl. A similar strategy was tested in water for the synthesis of 3-phenylpropanal.10
In the arylation of allylic diols with iodobenzene catalyzed by Pd(OAc)2 and n-Bu3P, different
products have been obtained depending on the kind of base used. With K2CO3, arylated diols have
been formed, whereas the use of Et3N has resulted in the formation of arylated α-hydroxyketones11.
However, in the case of allylic alcohols, mixtures of arylated products have been formed, composed of
unsaturated alcohols and saturated ketones. Further studies12, performed in the molten salt Bu4NBr
with the PdCl2 and NaHCO3 bases, have resulted in the formation of α-arylated carbonyl compounds.
The applicability of the elaborated method to the synthesis of nabumetone from 2-bromo-6-
methoxynaftalene has been evidenced. Not only aryl halides but also aryldiazonic salts have been used
as substrates in the synthesis of α-arylated carbonyl compounds, and 3-phenylpropanal has been
obtained in reactions catalyzed by Pd(OAc)213 or Pd(dba)214 in the absence of a base. Very interesting
results have been obtained in ionic liquids as the reaction medium.15 When benzothiazole palladium
complex was used in TBAB with the NaHCO3 base, arylated ketones and aldehydes were obtained,
and regioselectivity was dependent on the steric hindrance at the double bond. In further studies,
Pd(OAc)2 has been used. In TBAB, arylated aldehydes have been obtained. The exchange of the
bromide anion for acetate in tetrabutylammonium salt (TBAA) has resulted in the total reverse of
reaction selectivity and the formation of unsaturated alcohols as products. In these systems, the role of
Pd(0) NPs, formed in situ from palladium precursors, has been underlined.
To the best of our knowledge, the only example of the application of pre-made Pd(0) NPs in
the arylation of allylic alcohols has recently been reported.16 In this work, COF material was used as a
support for Pd(0) NPs. Interestingly, unsaturated alcohols were obtained selectively.
The studies reported in that paper aimed at the elaboration of an efficient catalytic system for
the arylation of allyl alcohol using Pd(0) NPs as catalysts. We employed Pd(0) NPs obtained from
simple palladium precursors and stabilized with PVP, as well as these precursors not reduced before
the Heck reaction. In this way it was possible to compare the activity of the pre-formed nanoparticles
with the in situ formed ones.
2. Experimental
2.1. Reactants
Iodobenzene, substituted iodobenzenes, bromobenzene, mesitylene, and tetradecane (Sigma
Aldrich) were used without purification or drying. PVP K-30 (N-polyvinylpyrrolidone, Mw = 40,000)
was purchased from Fluka and used without purification.
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2.2 Synthesis of catalysts
2.2.1 Synthesis of PdCl2/PVP-K30
A solution of polyvinylpyrrolidone K-30 (1.185 x 10-5 mol) (Mw = 40,000) dissolved in water (10
cm3) was added to a solution of PdCl2 (3.56 x 10-4 mol) in water (10 cm3). The solution was stirred for
2 h at 80 °C. The solution color changed from brown to dark brown. The obtained mixture was
filtered, and then the solution was dried completely in a rotary vacuum evaporator yielding a film of
PVP-stabilized Pd(0) NPs (palladium nanoparticles).
2.2.2 Synthesis of Pd(OAc)2/PVP-K30
A solution of polyvinylpyrrolidone K-30 (1.185 x 10-5 mol) (Mw = 40,000) dissolved in water (10
cm3) was added to a solution of Pd(OAc)2 (2.4 x 10-4 mol) in water (10 cm3). The solution was stirred
for 2 h at 80 °C. The solution color changed from brown to dark brown. Finally, the obtained mixture
was filtered, and then the solution was dried completely in a rotary vacuum evaporator yielding a film
of PVP-stabilized palladium nanoparticles.
2.2.3 Sample preparation for ICP measurements
A 0.02 g sample of Pd/PVP was placed in a 10 cm3 volumetric flask, and the flask was filled to 10
cm3 with distilled water, and then mixed until complete dissolution.
2.2.4 Sample preparation for TEM analysis
For TEM analysis a droplet of the reaction mixture after Heck reaction was placed on a carbon
coated copper grid and dried in air.
2.3 Catalytic reactions
2.3.1 Heck reaction (method A)
The reactions were carried out in a 50 cm3 Schlenk tube with magnetic stirring. The reagents,
iodobenzene (1 mmol, 0.112 cm3), allyl alcohol (1.5 mmol, 0.102 cm3), triethylamine (1.5 mmol,
0.209 cm3), and Pd-PVP (5×10−6 mol), were introduced to the Schlenk tube under a N2 atmosphere.
The reaction was carried out at 120 °C for 30 min. Afterward, the Schlenk tube cooled down to the
ambient temperature. The organic products were separated by extraction with diethyl ether (two times
with 5 cm3) and analyzed by GC-MS (Hewlett Packard 8454A) with mesitylene (0.1 cm3) as the
internal standard.
2.3.2 Heck reaction (method B)
The reaction was carried out in a 50 cm3 Schlenk tube. The solid substrates, the NaHCO3 base,
NaOAc or Et3N (2 mmol), and the Pd catalyst (0.5–1×10-5 mol), were weighed and placed in the
Schlenk tube under a N2 atmosphere. Next, olefin and iodobenzene (1 mmol) and 5 cm3 of DMF were
added with a pipette. The Schlenk tube was closed with a rubber plug, and the reaction mixture was
stirred at 85–120 °C. After the given reaction time, the Schlenk tube was cooled down, and the organic
products were separated by extraction with diethyl ether (three times with 7, 3, and 2 cm3). For better
3

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phase separation, 5 cm3 of water was added, and the products were GC–MS analyzed (Hewlett
Packard 8454A) with mesitylene (0.1 cm3) as the internal standard.
2.3.3 Heck reaction of different aryl dihalides
The reaction was carried out in a 50 cm3 Schlenk tube. The solid substrates, NaOAc (2 mmol),
and the Pd catalysts (1×10-5 mol) were weighed and placed in the Schlenk tube under a N2 atmosphere.
Next, olefin, aryl dihalides (1 mmol), and 5 cm3 of DMF were added with a pipette. The Schlenk tube
was closed with a rubber plug, and the reaction mixture was stirred at 85 °C. After the given reaction
time, the reactor was cooled down, and the organic products were separated by extraction with diethyl
ether (three times with 7, 7, and 7 cm3). For better phase separation, 5 cm3 of water was added, and the
products were and GC–MS analyzed (Hewlett Packard 8454A) with mesitylene (0.076 cm3) as the
internal standard.
2.3.4 Heck reaction of bromobenzene in molten tetrabutylammonium salt (methods A and B)
The reaction was carried out in a 50 cm3 Schlenk tube. Method A: the TBAB salt (1g) and the
Pd catalysts (1×10-5 mol) were weighed and placed in the Schlenk tube under a N2 atmosphere and
then heated at 130 °C for 20 min. Subsequently, the mixture was cooled to room temperature, allyl
alcohol (1 mmol), bromobenzene (1 mmol), and NaOAc (2 mmol) were added, and the mixture was
heated up to the reaction temperature (130 °C) for 24 h.
Method B: the TBAB salt (1 g), the Pd catalysts (1×10-5 mol), and NaOAc (2 mmol) were
weighed and placed in the Schlenk tube under a N2 atmosphere. Next, allyl alcohol and bromobenzene
(1 mmol) were added with a pipette. The reactor was closed with a rubber plug, and the reaction
mixture was stirred at 130 °C for 24 h.
After the given reaction time, the reactor was cooled down, and the organic products were
separated by extraction with diethyl ether (three times with 7, 3, and 2 cm3). For better phase
separation, 5 cm3 of water was added, and the products were GC–MS analyzed (Hewlett Packard
8454A) with tetradecane (0.1 cm3) as the internal standard.
2.3.5 Hg(0) test
The Heck coupling of allyl alcohol with iodobenzene catalyzed by PdCl2(cod) was carried out
as described above. After the specified reaction time (10 minutes), mercury (Hg/Pd = 500) was added
and reaction was continued.
3. Results and discussion
3.1 Characteristics of supported catalysts, PdCl2/PVP and Pd(OAc)2/PVP
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Pd(0) NPs were prepared from PdCl2 and from Pd(OAc)2 in water in the presence of PVP-K30
as a stabilizing polymer and characterized by TEM and ICP methods (Table 1). It was interesting to
find that just heating simple palladium compounds to 80 °C in water without any extra reducing agent
resulted in the formation of Pd(0) NPs with an average diameter of ca. 2 nm, as evidenced by TEM,
showed well dispersed, non-agglomerated nanoparticles of palladium. The size distribution of Pd(0)
NPs was relatively narrow, and it was similar for both materials (Fig. 2 a, b). In fact, the kind of
palladium source did not significantly influence the properties of the final structure of PVP-stabilized
Pd(0) NPs, although the palladium content was slightly lower in the material obtained from Pd(OAc)2.
While the self-reduction of Pd(OAc)2 to Pd(0) NPs at a higher temperature had been reported
earlier17, an analogous reaction was unknown for PdCl2. Therefore, in the case of our reactions, the
contribution of water as a reducing agent should be considered.
Table 1. Characteristics of Pd(0) NPs obtained from different precursors
Pd
content
(%)b
Number of
Pd NPsc
Short name of
catalyst
Color of Pd(0)
NPs
Particle
size [nm]
Catalyst
Precursor
PdCl2
PdCl2/PVP-K30
PdCl2/PVP
Dark brown
Dark brown
5.6
4.3
2-2.5
2-2.5
200
250
Pd(OAc)2/PVP-K30 Pd(OAc)2/PVP Pd(OAc)2
a 80oC, 2h, water, b Palladium content determined by ICP, c Number of Pd NPs analyzed to determine the average
size.
a
50
40
30
20
10
0
1.5-2
2-2.5
2.5-3
3-3.5
Size [nm]
10 nm
b
5

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60
50
40
30
20
10
0
1.5-2
2-2.5
2.5-3
3-3.5
Size [nm]
20 nm
c
35
30
25
20
15
10
5
0
Size [nm]
50 nm
d
80
70
60
50
40
30
20
10
0
Size [nm]
10 nm
e
6

ACCEPTED MANUSCRIPT
45
40
35
30
25
20
15
10
5
0
5-15 15-25 25-35 35-45 45-55
Size [nm]
50 nm
Fig. 2. TEM micrograph and particle size distribution histograms of Pd NPs, a) PdCl2/PVP, b) Pd(OAc)2/PVP, c)
PdCl2/PVP after reaction and d) Pd(OAc)2/PVP after reaction, e) PdCl2(cod) after reaction
The diameter of Pd NPs stabilized with PVP after the catalytic reaction was also measured and we
observed an increase of their size for PdCl2/PVP (Fig. 2 c), whereas in case of Pd(OAc)2/PVP the
diameter was not significantly changed (Fig. 2 d). Pd NPs formed in situ from PdCl2(cod) presented
unsymmetrical size distribution with one maximum in range 5-15 nm and second around 35-45 nm
(Fig. 2 e).
3.2. Catalytic activity of Pd(0) NPs in the Heck reaction
The obtained Pd(0) NPs were tested in the Heck reaction of allyl alcohol with iodobenzene
(Fig. 3). In this reaction, saturated alcohols, 3-phenylpropanal (1) and 2-phenylpropanal (2), were
expected as the main products, formed by the isomerization of the arylated alcohols produced in the
Heck reaction. Products 3–5 were found in the reaction mixtures in much smaller amounts.
O
O
O
X
H
H
[Pd]
base
OH
H
+
R
R
R
R
+
+
+
X = I, Br
R = H, OH, CH3, CH3CO, OCH3, NO2
1
2
3
O
+
R
R
R
H
R
4
5
Fig. 3. Heck coupling of allyl alcohol with ArX.
The first reactions were conducted without a solvent with NEt3 as the base. In order to
estimate the stability of Pd(0) NPs under the catalytic reaction conditions, the catalyst was recovered
after the first run and used again with a new portion of the reactants. Both catalysts provided very
good conversions, 81–92% after 30–40 min (Table 2 and 3). The main product, linear aldehyde (1),
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was obtained with 53–68% yield, whereas 16–17% of iodobenzene was converted to the branched
aldehyde (2). Thus, high selectivity to carbonyl products was noted as a result of the efficient
isomerization of the first reaction product, unsaturated aryl alcohol, which in fact was not found in the
reaction mixture.
Table 2. Recycling experiments with Pd(OAc)2/PVP.
Yield (%)
Time
(min.)
PhI conversion
(%)
TOF
( h-1)
Run
TON
1
2
5
12
5
6
4
1
2
3
4
5
30
30
30
30
30
81
81
84
77
45
53
60
62
58
34
16
16
16
15
9
106
120
124
116
68
212
240
248
232
136
2
Reaction conditions (method A): iodobenzene (1 mmol), allyl alcohol (1.5 mmol), NEt3 (1.5 mmol), [Pd] =
5×10-6 mol
Table 3. Recycling experiments with PdCl2/PVP.
Yield (%)
Time
(min.)
PhI conversion
(%)
TOF
( h-1)
Run
TON
1
2
5
7
2
4
4
3
1
2
3
4
5
40
30
30
30
30
92
78
83
77
68
68
60
62
57
51
17
15
16
15
13
136
120
124
114
102
204
240
248
228
204
Reaction conditions (method A): iodobenzene (1 mmol), allyl alcohol (1.5 mmol), NEt3 (1.5 mmol), [Pd] =
5×10-6 mol
Under the applied conditions, a side process occurred, the aldol condensation of the aldehyde,
leading to the formation of product 5 (Fig. 4). The yield of this product was at the level of 7%, and it
was slightly lower for the catalyst obtained from PdCl2. In the presence of a catalyst prepared from
Pd(OAc)2, up to 18% of the aldol condensation product was formed.
O
O
OH
- H2O
H
2
H
H
H
1
5
O
Fig. 4. Aldol condensation of 3-phenylpropanal
In recycling experiments, high conversion was noted in four subsequent runs, while
conversion dropped in the fifth one, to 45% for the catalyst prepared from Pd(OAc)2 and to 68% for
the one prepared from PdCl2 (Tables 2 and 3). The latter catalyst showed higher stability in a
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prolonged experiment. It should also be noted that reasonably high TOF values, over 200 h-1, were
obtained in both systems indicating a high reaction rate.
3.3. Catalytic activity of soluble palladium catalysts in the Heck reaction
Under the same conditions, the Heck reactions were performed using palladium complexes as
catalysts. It was expected that simple palladium precursors would be reduced under Heck reaction
conditions to Pd(0) NPs18, and it was interesting to compare the activity of the pre-made Pd(0) NPs
with the in situ formed ones.
The data presented in Table 4 shows that the highest conversions of PhI (87–92%) were
obtained for Pd(0) NPs stabilized with PVP. Interestingly, a Pd2(dba)3 compound which contained
Pd(0) gave worse results, very similar to those obtained with a PdCl2(cod) catalyst (Table 4).
Table 4. Results of the Heck reaction with different palladium catalysts without solvent
PhI
conversion
Yield (%)
TOF
( h-1)
Entry
Catalyst
TON
(%)
1
2
5
1
2
3
4
5
PdCl2(cod)
PdCl2/PVP
Pd(OAc)2
Pd(OAc)2/PVP
Pd2(dba)2
77
92
74
87
54
68
40
50
52
13
17
13
18
13
10
7
21
18
10
108
136
80
100
104
162
204
120
133
156
76
Reaction conditions (method A): PhI (1 mmol), allyl alcohol (1.5 mmol), NEt3(1.5 mmol), [Pd] (5×10-6 mol),
120oC, 40 min.
In further studies, different palladium precursors were used for comparison in the same Heck
reaction; however, a lower temperature, 85 °C, was applied, and DMF was used as a solvent (Table 5).
Fortunately, a high conversion, 85–87%, was achieved for four catalysts in 1.5 h. In general, in this
method, aldol condensation was diminished, and the yield of the desired products was therefore
higher. In the best cases, the yield of 3-phenylpropanal was 76–79%. Interestingly, not only
conversion but also reaction selectivity were very similar in all cases and Pd(0) NPs and palladium
complexes provided similar results. This indicates that the same catalytically active palladium forms,
presumably Pd(0) NPs, were present in all systems. It should also be noted that, under those
conditions, Pd(0) NPs formed in situ from PdCl2 or from Pd(OAc)2 exhibited similar activity as pre-
made nanoparticles stabilized with a polymer. However, the presence of a PVP-stabilizing polymer
advantageously enabled efficient catalyst recycling (Tables 2 and 3). For comparison, the reaction was
carried out with commercially available Pd immobilized on activated charcoal (Pd/C). In this
experiment a very low conversion of iodobenzene (7%) was obtained. It can be therefore argued that
our catalysts are significantly more effective than Pd/C.
In order to estimate a radical contribution to the studied reactions, an experiment with radical
trap (0.5 mmol of 4-hydroxy-TEMPO ) has been performed in the same conditions (Table 5, Entry 4).
9

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The yield of product A (76%) was similar to that without radical trap, while product B was not
formed. Thus, a free-radical mechanism can participate in formation of product B.
Table 5. Results of the Heck reaction with different palladium catalysts in DMF
PhI
conversion
(%)
Yield (%)
Time
(min)
TOF
h-1
Entry
Catalyst
Method T(0C)
TON
1
2
1
2
3
4
5
6
7
8
A
A
B
B
B
B
B
B
85
120
85
85
85
85
85
85
40
40
90
90
90
90
90
90
68
99
95
76
96
91
87
7
56
79
79
76
76
76
71
7
11
17
13
-
16
12
14
-
112
158
79
76
76
76
71
7
168
237
53
51
51
51
47
4.7
PdCl2(cod)
PdCl2(cod) + TEMPO
Pd(OAc)2
PdCl2/PVP
Pd(OAc)2/PVP
Pd/C
Reaction conditions (modified method A): PhI (1 mmol), allyl alcohol (1.5 mmol), NaOAc (1.5 mmol), [Pd]
(5×10-6 mol), solvent DMF (5 cm3)
(method B): PhI (1 mmol), allyl alcohol (1 mmol), NaOAc (2 mmol), [Pd] (1×10-5 mol), solvent DMF (5 cm3)
In order to learn more about the influence of the reaction conditions on the catalytic efficiency
of the systems studied, two methods, A and B, were tested. They differed in two parameters, namely
the palladium amount, which was higher in method B, and in the amount of allyl alcohol. As a result,
an excess of allyl alcohol versus palladium, represented as the ratio [allyl alcohol]/[Pd], was higher in
method A. Under these conditions, the TOF values were higher; however, PhI conversion and the
yield of 3-phenylpropanal were lower at 85 °C. On the other hand, a significant increase of the TOF
value was noted after the temperature was raised to 120 °C.
It should also be pointed out here that reactions performed in DMF gave better selectivity to
desired aldehydes and, in particular, the formation of the aldol condensation product was diminished
to no more than 1%.
3.4. Heck coupling of bromobenzene with allyl alcohol
Attempts to perform the Heck coupling of allyl alcohol with bromobenzene under conditions
used for iodobenzene failed, because the coupling product was not formed. Therefore, in further
experiments, TBAB was used as the reaction medium instead of DMF. Two different procedures were
tested. In the first one, (A), the palladium catalyst was heated in TBAB (130 °C) before the
introduction of the substrates and the base. In the second case, (B), all components, including TBAB
and the palladium catalyst, were placed in the Schlenk tube, and heating started. In both procedures,
the heating continued for 24 h to reach a 67–82% conversion of bromobenzene. In most reactions,
reaction selectivity was quite high. For example, using PdCl2/PVP, 3-phenypropanal was obtained in
74% yield as the only product (Table 6). In general, higher productivity was noted for PdCl2/PVP than
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for PdCl2(cod) and Pd(OAc)2. Moreover, the introduction of all reactants together before heating
started (method B) gave better selectivity.
Table 6. Results of the Heck reaction of bromobenzene with allyl alcohol
PhBr
Yield (%)
Entry
Catalyst
Method conversion
(%)
1
2
5
1
2
3
4
5
6
7
A
B
B
A
B
B
B
80
67
72
82
74
71
70
56
63
65
73
74
54
68
5
-
2
2
-
19
-
6
7
-
PdCl2(cod)
PdCl2/PVP
Pd(OAc)2
Pd(OAc)2/PVP
-
2
17
-
Reaction conditions: PhBr (1 mmol), allyl alcohol (1 mmol), NaOAc (2 mmol), [Pd] (1×10-5 mol), salt TBAB
(1g), t= 24h, T= 130°C
3.5. Heck reaction of allyl alcohol with different iodobenzenes
The Heck coupling of substituted iodobenzenes with allyl alcohol produced 3-arylpropanals as
the main products (Fig. 1). The highest conversions of substrates to desired aldehydes were noted for
4-iodophenol and 4-iodotoluene. In contrast, the presence of an electron-acceptor NO2 group caused
the lower conversion of 4-iodonitrobenzene to 3-arylpropanal. That is because more electron-acceptor
substituents facilitated the formation of biaryls as side products. Thus, for 4-iodoacetophenone and 4-
nitrobenzene, the lowest selectivity was noted (Table 7).
When the kind of catalyst is considered, in most cases PdCl2(cod) gave better results than
Pd(OAc)2, considering conversion and the yield of product 1. Moreover, the results produced by Pd(0)
NPs in reactions of 4-iodophenol and 4-iodotoluene were similar to those with PdCl2(cod).
Table 7. Results of the Heck reaction of allyl alcohol with substituted iodobenzenes
ArX
Conversion
Yield (%)
Entry
ArX
Catalyst
1
2
3
4
(%)
1
2
3
4
5
6
7
8
9
10
11
12
13
Pd(OAc)2
PdCl2(cod)
PdCl2/PVP
Pd(OAc)2
PdCl2(cod)
PdCl2/PVP
Pd(OAc)2
PdCl2(cod)
Pd(OAc)2
PdCl2(cod)
PdCl2(PhCN)2
Pd(OAc)2
91
100
88
40
90
92
100
85
62
99
68
71
63
31
75
77
60
61
50
79
27
30
48
23
29
23
3
1
15
9
-
-
-
2
14
-
8
4
-
1
2
4
7
-
-
-
-
-
4-Iodophenol
4-Iodotoluene
4-Iodoacetophenone
4-Iodoanisole
-
23
11
-
-
-
9
12
19
3
-
-
32
48
79
14
24
4-Iodonitrobenzene
PdCl2(cod)
11

ACCEPTED MANUSCRIPT
Reaction conditions: ArX (1 mmol), alkohol allilowy (1 mmol), [Pd] (1×10-5 mol), NaHCO3 (2 mmol), DMF (5
cm3), 5h, 850C
3.6. Heck reaction of different aryl dihalides
The Heck coupling of diiodobenzenes and iodobromobenzenes with allyl alcohol gave mono-
and disubstituted aryls as the main products (Fig. 5). For 1,4-diiodobenzene, the highest conversion,
93%, was obtained with 1 mmol of alcohol and NEt3 as the base. Under those conditions, 53% of
product 1a and 19% of product 6 were formed (Table 8, run 4). At a twice lower amount of alcohol,
selectivity was reversed, and the yield of product 6 (31%) exceeded that of product 1a (18%) (Table 8,
run 3). Clearly, under those conditions, the activation of the second C-I bond was more efficient.
That result was better than with the NaOAc base, which made it possible to obtain 45% of 1a and 21%
of 6 after 6h (Table 8). Again, the results obtained with PdCl2(cod) and with PdCl2/PVP were very
similar (Table 8, run 8 and 11).
I
O
O
O
[Pd]
H
OH
H
H
H
H
+
+
+
X
+
X
base
X
O
H
O
O
1a
2a
6
7
X= I, Br
+
X
X
8
Fig. 5. Heck coupling of allyl alcohol with 1,4-diiodobenzene or 1-bromo-4-iodobenzene
Table 8. Results of the Heck reaction of allyl alcohol with different aryl dihalides
Yield (%)
ArX2
conversion
(%)
[ꢀꢁꢂꢃꢄꢃꢁ]
[ꢅꢆ]
Time
(h)
Entry
ArX2
Modification
1a
2a/3
6/6a
7
8
-
1
2
3
3
3
3
100
200
100
66
80
62
36
45
18
6
7
3
15
20
31
5
7
10
1
-
-
2 mmol of alcohol
NaHCO3
2 mmol of alcohol,
4
3
200
88
53
9
19
8
-
NaHCO3
DMF:H2O (1:1)
H2O
NEt3 (1.5 ml)
5
6
7
8
9
10
11
12
13
3
3
3
6
6
6
6
3
6
100
100
100
100
100
100
100
100
100
60
0
20
0
4
-
3
7
11
5
6
6
6
27
-
4
19
14
21
18
17
18
9
-
-
-
-
3
6
2
3
2
8
1,4-diiodobenzene
1,3-diiodobenzene
37
80
93
75
73
71
93
29
40
47
40
38
38
38
1
6
9
5
5
6
8
-
Et3N
TBAB (0.5 mmol)
PdCl2/PVP
-
-
12

ACCEPTED MANUSCRIPT
-
-
-
-
14
15
16
17
18
3
6
3
6
3
100
100
100
100
100
54
72
85
95
85
33
34
52
51
55
6
6
8
8
9
9
-
-
3
3
4
-
-
6
13
4
1,2-diiodobenzene
26
13
14
13
1-bromo-4-iodobenzene
PdCl2/PVP
Reaction conditions: ArX (1 mmol), allyl alcohol (1 mmol), PdCl2(cod) (1×10-5 mol), NaOAc (2 mmol), DMF (5
cm3), 850C
1,3-diiodobenzene reacted similarly. It was converted to 1a (38%) and to 6 (17%) in 3 h. The
prolongation of the reaction time to 6 h resulted only in an increase in the yield of other products,
while the amount of 1a did not increase. In contrast, 1,2-diiodobenzene reacted with higher selectivity
forming just small amount of side products (Table 8, run 14). A cyclization product 6a was formed
that was not observed for other substrates (Fig. 9). Positively, when the reaction was prolonged to 6 h,
a significant increase in the yield of product 6a (26%) was achieved.
O
I
O
I
O
H
I
[Pd]
H
H
OH
+
+
+
base
1a
3
6a
Fig. 6. Heck coupling of allyl alcohol with 1,2-diiodobenzene
1-bromo-4-iodobenzene demonstrated quite good reactivity with the 52% yield of 1a and 13%
of 6 at 85% conversion with PdCl2(cod) (Table 8, run 16) and almost the same with PdCl2/PVP (Table
8, run 18). Unfortunately, this result was not improved in a longer time, 6 h, when an additional
amount of the biaryl side product was formed (Table 8, run 17).
It is worth noting that the C-Br bond was activated in 1-bromo-4-iodobenzene under those
conditions in the absence of TBAB, and 16–17% of di-substituted arenes were obtained. Thus, the
reactivity of bromo-iodobenzene towards the Heck coupling appeared higher than that of
bromobenzene.
3.7 Hg(0) test
In order to evidence the role of Pd NPs as catalytically active form of palladium the Hg(0) test
was performed 19-22. As it is demonstrated in Tale 9, strong inhibiting effect of Hg(0) (500-fold excess)
was observed and no further reaction occured after Hg(0) addition.
Table 9 Mercury test in Heck reaction with allyl alcohol and PhI
PhI
Yield (%)
Time
(min)
Entry
Catalyst
Amount of Hg
conversion
(%)
1
2
4
3
1
2
-
10
90
29
28
23
22
PdCl2(cod)
500
13

ACCEPTED MANUSCRIPT
Reaction conditions: PhI (1 mmol), allyl alcohol (1 mmol), NaOAc (2 mmol), [Pd] (1×10-5 mol), T= 85 °C,
Hg(0) was added after 10 minutes
On the basis of the result of Hg(0) poisoning test we propose the heterogeneous mechanism as
the dominating one, with Pd NPs being catalytically active form of the catalyst. However some
contribution of the underligated Pd forms, leached from the Pd NPs, cannot be completely ruled out.
These species could be also have been eliminated by Hg(0).
Conclusions
It was demonstrated that Pd(0) NPs stabilized with a PVP polymer efficiently catalyzed the
Heck coupling of allyl alcohol with iodo- and bromobenzenes. Due to the fast isomerization of
arylated alcohols, 3-arylpropanals and 2-arylpropanals were formed as the main products. In contrast,
Pd(0) NPs supported on the COF material were not active in isomerization and produced unsaturated
alcohols in a similar Heck reaction.16
The high catalytic activity of Pd(0) NPs was noted under solventless conditions as well as in
DMF used as a solvent. In most of reactions studied, employing substituted iodobenzenes or di-
iodobenzenes, similar results were also obtained with the application of phosphine-free palladium
compounds, such as PdCl2(cod) or Pd(OAc)2. Therefore, it can be assumed that Pd(0) NPs play an
important role in this catalytic process. This point has only scarcely been addressed in the literature.
While similar products can be obtained with pre-made Pd(0) NPs and with other palladium
compounds, the advantage of the Pd/PVP consists in simple and efficient recycling, making it possible
to achieve higher TON values and to diminish catalyst costs.
Acknowledgements
Financial support of National Science Centre (NCN, Poland) with grant 2014/15/B/ST5/02101 is
gratefully acknowledged.
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