Efficient hydroarylation of terminal alkynes with sodium tetraphenylborate performed in water under mild conditions

Article abstract of DOI:10.1016/j.apcata.2019.117243

The hydroarylation of terminal alkynes with sodium tetraphenylborate was performed in high yield within 3 h at room temperature in water, using palladium(II) complexes with imidazole ligands as catalysts. Under these conditions, differently substituted phenylacetylene substrates were converted to arylalkenes and aryl-substituted dienes. High conversion and excellent selectivity were achieved in the hydroarylation of alkynols with sodium tetraphenylborate. Only one product, arylalkene with an OH group, was formed in these reactions with the yield dependent on the kind of alkynol used. A plausible hydroarylation reaction mechanism was proposed on the basis of the palladium species identified in the reaction mixture and H/D exchange studies. The contribution of water as the hydride source was evidenced.

Full text of DOI:10.1016/j.apcata.2019.117243

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Efficient hydroarylation of terminal alkynes with sodium tetraphenylborate
performed in water under mild conditions
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P. Kociecka, A.M. Trzeciak
PII:
S0926-860X(19)30398-9
DOI:
https://doi.org/10.1016/j.apcata.2019.117243
APCATA 117243
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Applied Catalysis A, General
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6 September 2019
8 September 2019
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Please cite this article as: Kociecka P, Trzeciak AM, Efficient hydroarylation of terminal
alkynes with sodium tetraphenylborate performed in water under mild conditions, Applied
Catalysis A, General (2019), doi: https://doi.org/10.1016/j.apcata.2019.117243
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Efficient hydroarylation of terminal alkynes with sodium tetraphenylborate
performed in water under mild conditions
P. Kocięcka, A.M. Trzeciak*
University of Wrocław, Faculty of Chemistry, 14 F. Joliot-Curie St., 50-383 Wrocław, Poland
Graphical abstract
Highights
 Palladium(II) – imidazole complexes efficiently catalyse the hydroarylation of
terminal alkynes and alkynols.
 Hydroarylation proceeds under mild conditions with sodium tetraphenylborate as
phenylating agent and water as the hydride source.
 Differently substituted phenylacetylenes were converted to arylalkenes and aryl-
substituted dienes with high conversion.
 In the hydroarylation of alkynols only one product, arylalkene with an OH group, was
formed selectively.
Abstract
The hydroarylation of terminal alkynes with sodium tetraphenylborate was performed in high
yield within 3 h at room temperature in water, using palladium(II) complexes with imidazole
ligands as catalysts. Under these conditions, differently substituted phenylacetylene substrates
were converted to arylalkenes and aryl-substituted dienes. High conversion and excellent
selectivity were achieved in the hydroarylation of alkynols with sodium tetraphenylborate.
Only one product, arylalkene with an OH group, was formed in these reactions with the yield
dependent on the kind of alkynol used. A plausible hydroarylation reaction mechanism was
proposed on the basis of the palladium species identified in the reaction mixture and H/D
exchange studies. The contribution of water as the hydride source was evidenced.

Keywords: alkynes; hydroarylation; palladium; sodium tetraphenylborate; arylalkenes.
Introduction
The functionalization of alkynes via the addition of an aryl group, namely the arylation of
alkynes, presents a very attractive pathway to organic compounds such as arylalkenes [1-4].
These compounds are intermediates in the synthesis of biologically active species,
pharmaceutical and agrochemical agents (Figure 1) [5-8]. The medical properties of stilbene
derivatives, the products of hydroarylation, have been extensively studied [9, 10]. For
example, trans-resveratrol and its derivatives exhibit significant antioxidant activities [11-15].
Arylalkenes are usually produced by the Heck coupling of aryl halide and olefin [16-18]. This
method is highly efficient in applying to a wide range of substrates. However its main
drawback is the formation of halide-containing wastes, harmful for the environment.
Therefore, other methods, avoiding the application of aryl halides as substrates to obtain
products in a greener way, are needed. An interesting alternative is offered by catalytic
hydroarylation, which involves the activation of the triple bond of alkyne, used instead of
alkene.
The hydroarylation of alkynes can be performed according to different protocols, the
Fujiwara reaction being the most explored one [19-20]. In the Fujiwara hydroarylation,
benzene derivatives bearing alkyl, OR, or OH groups [21-22] or N-heteroarenes [23-25] react
with electron-deficient alkynes in an acidic medium. Advantageously, the products of Z
conformation are preferably formed. Another method used for the production of arylalkenes
from alkynes employs arylboron compounds, as the donors of the aryl group, instead of
benzene derivatives. In this method, the C-H activation step is omitted, making the procedure
more universal. The hydroarylation of alkynes with arylboronic acid has been attained with
Pd [26-29], Rh [30, 31], Ni [32-35], Cu [36-38], Mn [39] and Co [40].
Till now, most of the hydroarylation reactions of alkynes with arylboronic acid have
referred to internal alkynes, and only in a few cases has the successful hydroarylation of
terminal alkynes been reported [19, 20, 41]. Typical palladium catalysts used in these
reactions have been PdCl2(PPh3)2 and Pd(OAc)2. In contrast, the application of sodium
tetraphenylborate as the phenylating agent has been reported only scarcely [41-43].
We have focused our efforts on the hydroarylation of terminal alkynes and alkynols. In this
paper, we present the results obtained in the hydroarylation with sodium tetraphenylborate

using Pd(II) complexes with imidazole ligands as catalysts. Complexes of this type can be
prepared in high yield with different imidazoles, and they are stable in the presence of air and
water. Their ability to activate arylboronic compounds has been confirmed in the Suzuki–
Miyaura cross-coupling and carbonylative coupling under mild conditions [44-47]. Therefore
it was expected that Pd(II) imidazole complexes would also be good catalysts for the
hydroarylation of terminal alkynes in the water medium.
Experimental
Synthesis of PdCl2(imidazole)2 complexes
PdCl2(imidazole)2 complexes (I–V) were obtained according to the reported procedure [47].
General procedure for the hydroarylation of terminal alkynes with NaBPh4
The reactions were carried out in an air atmosphere in Schlenk tubes equipped with a
magnetic stir bar. In a typical run, PdCl2(imidazole)2 (I–V) was loaded into the tube, and
2 mL of the solvent was added. Next, glacial acetic acid (1.0 mmol, 0.12 mL), NaBPh4
(1.0 mmol, 0.34 g), and the corresponding alkyne (1.0 mmol) were introduced to the tube. The
reaction mixture was stirred at room temperature or in a silicon oil bath at 60 °C for 1 h or
3 h. After this time, the reactor was cooled down, and the organic products were extracted
with 10 mL of n-hexane. 0.26 mL of dodecane was added as internal standard. The extracts
were analysed by using a GC-flame ionization detector.
Results and discussion
For the first catalytic tests, we selected Pd(II) complexes with different imidazole
ligands, such as 1-benzylimidazole (I), 1-methylbenzimidazole (II), 1,2-dimethylimidazole
(III), 1-benz-2-methylimidazole (IV), and 1-butyl-2-methylimidazole (V) (Fig. 2).
The reaction of phenylacetylene with sodium tetraphenylborate (NaBPh4) was carried out
at room temperature in water (Fig. 3, Table 1). It was found that addition of AcOH is
necessary to obtain hydroarylation products, similarly as it was described for other systems
[41]. It should be noted, that the catalytic system is multiphasic because the alkynes are
insoluble in water. In addition, the palladium complexes are poorly water-soluble although
they dissolve partly in the reaction medium.
In each reaction, the conversion of phenylacetylene was 100%, and two products were
identified. One, stilbene (2a), was formed by the addition of the aryl group of

tetraphenylborate to phenylacetylene. The other product, (3a), containing three aryl groups,
was formed by two molecules of phenylacetylene and one phenyl group originating from
tetraphenylborate. Both products formed as single isomers (Fig. 3). The compound 3 had
previously been found in a small amount as a side product in the hydroarylation of the internal
alkyne with boronic acids in the presence of Rh/NiZn catalyst [48]. In addition, an analogous
product had been obtained in the Fujiwara hydroarylation of ethyl propiolate with benzene
catalysed by the chelating dicarbene palladium(II) complex in the presence of AgBF4/HBF4
[21]. However, in most of hydroarylation reactions, exclusively stilbene derivatives (2) were
formed [49]. Considering the literature data, the formation of product 3a can be considered as
an original feature of our system.
In all reactions investigated in these studies, product 3a was created with higher yield than
product 2a. The biggest difference between the yield of 2a and that of 3a was observed for
complex I (Table 1, entry 1). In contrast, an excess of 3a over 2a was smaller for complexes
II and IV, which formed ca. 40% of 2a and ca. 60 % of 3a (Table 1, entries 2 and 4). Thus,
the kind of imidazole ligand influenced reaction selectivity. In particular, the introduction of a
methyl group at the C2 carbon of the imidazole ring in the benzimidazole ligand (complex
IV) resulted in a decrease in 3a yield from 71% (catalyst I) to 57% (catalyst IV) (Table 1).
Table 1. Hydroarylation of phenylacetylene in the presence palladium(II) complexes (I–V).[a]
Yield (%)[b]
Entry Complex
Conv. (%)[b]
2a
29
39
31
43
32
3a
71
61
69
57
68
1
2
3
4
5
100
100
100
100
100
I
II
III
IV
V
[a] Reaction conditions: catalyst (1.0 mol%), phenylacetylene (1.0 mmol) NaBPh4 (1.0 mmol), AcOH
(2.0 mmol), 2 ml water, rt, 3h. [b] Determined by GC
The optimization of the reaction conditions was performed for a palladium complex with a
1-benzylimidazole ligand (I) (Table 2). Interestingly, almost the same yield of the products
was obtained at room temperature and at 60 °C (Table 2, entries 1 and 2). Advantageously,
hydroarylation experiments can be carried out under mild conditions. The lowering of catalyst
loading from 1.0 to 0.5 mol% did not affect the results significantly (Table 2, entries 2 and 4),
but, at 0.1 mol% of catalyst, the conversion of phenylacetylene decreased from 98% to 50%

(Table 2, entry 5). The shortening of the reaction time from 3 to 1 h had a negative effect on
the conversion of phenylacetylene, which decreased to 66%. In order to check whether an
amount of NaBPh4 lower than 1 mmol was sufficient to convert phenylacetylene to the
desired products, 0.5 mmol was used. However, only 75% of phenylacetylene reacted in these
conditions, indicating the participation of less than four phenyl groups in the reaction.
Considering the solvent, the reaction can be carried out efficiently in water, while the
products were not formed in organic solvents (2-propanol, THF). However, the addition of
water to 2-propanol made it possible to obtain 33% conversion (Table 2, entry 15).
In is worth to note that hydroarylation with PhB(OH)2 used instead of NaBPh4 was not
successful. The test reactions, performed with phenylacetylene and phenylboronic acid in
water, THF and water/THF mixture did not lead to any products. This is in line with previous
observations that PhB(OH)2 gave good results in hydroarylation of internal alkynes, but not
with terminal ones [20, 29]. For example, only traces of the product were obtained from
phenylacetylene [29].
Attempts to increase the reaction selectivity and influence the ratio of products 2a/3a by
changing the substrate amounts failed. Similarly, the introduction of an excess of
phenylacetylene during the reaction changed the composition of the products only slightly,
although selectivity to product 3a increased. Interestingly, the addition of an excess of free 1-
benzylimidazole resulted in an increase in selectivity to product 3a (decrease in the 2a/3a
ratio) with only a small decrease in conversion (Table 2, entries 4 and 9). On the other hand,
the application of a bigger amount of 1-benzylimidazole (1 mmol) caused the inhibition of the
reaction and a decrease in the conversion of phenylacetylene at room temperature and at
60 °C (Table 2, entries 12 and 13). The highest yield of 3a (76%) was obtained at 0.5 mmol of
1-benzimidazole (Table 2, entry 10).
In order to get deeper understanding of the observed selectivity, changes of conversion and
yield were analyzed for the reaction of 1a with NaBPh4 (Fig. S2). It was found that already at
the beginning of the reaction two products were formed with a predominance of 3a. After ca.
60 min. the amount of 2a stabilized while product 3a was still formed. Thus, products 2a and
3a were formed separately and 2a was not converted to 3a.
Under the same reaction conditions PdCl2(PPh3)2 was tested for comparison (Table 2, entry
17). The conversion of 1a was lower (72%) than that with catalyst I (100%) and he selectivity
was also worse. Two isomers of hydroarylation product 2a were created: isomer Z (25%) and
isomer E (6%). The product 3a was formed in 39% yield and small amount of the
phenylacetylene dimer was observed (1%).

Table 2. Optimization of hydroarylation conditions with catalyst I.[a]
Yield[b]
2a
29
31
28
35
17
24
11
12
21
17
31
-
Catalyst
Entry
Solvent
Conv 1a[b]
[mol%]
1.0
3a
71
69
38
63
33
51
47
56
71
76
62
5
1.[c]
2.
3.[d]
water
100
100
66
98
50
75
61
76
92
93
93
5
1.0
water
1.0
water
4.
0.5
water
5.
0.1
water
6.[e]
7.[f]
8.[g]
9 [h]
0.5
water
0.5
water
0.5
water
0.5
water
10.[i] 0.5
11.[j] 0.5
12.[k] 0.5
13.[l] 0.5
water
water
water
water
14
0
-
14
-
14.
15.[m] 1.0
16. 1.0
17.[n] 1.0
1.0
2-propanol
2-propanol/water
THF
-
33
0
19
-
31[o]
14
-
water
72
39
[a] Reaction conditions: catalyst I (1.0 mol%), phenylacetylene 1a (1.0 mmol, 0.11 mL) NaBPh4
(1.0 mmol), AcOH (2.0 mmol), in 2 mL water, rt, 3h.
[b] Determined by GC
[c] 60oC
[d] 1h
[e] NaBPh4 (0.50 mmol, 0.17 g)
[f] phenylacetylene 1a (1.0 mmol, 0.11 mL) was added after 1h
[g] phenylacetylene 1a (2.0 mmol, 0.22 mL)
[h] 1-benzylimidazole (0.25 mmol, 0.079 g) was added
[i] 1-benzylimidazole (0.5 mmol, 0.079 g) was added
[j] 1-benzylimidazole (0.5 mmol, 0.079 g) was added, 60oC
[k] 1-benzylimidazole (1.0 mmol, 0.16 g) was added
[l] 1-benzylimidazole (1.0 mmol, 0.16 g) was added, 60oC
[m] addition (2.0 mmol, 0.036 g) H2O
[n] PdCl2(PPh3)2 as catalyst
[o] Z (25%) and E (6%) isomers
Promising results obtained in the reaction of phenylacetylene (1a) with NaBPh4 in the
presence of I provided motivation to test the scope of that reaction using a variety of terminal

alkynes (Fig. 4). For this purpose, the optimized conditions indicated as entry 4 in Table 2
were employed.
The obtained results shown an influence of the alkyne structure on the hydroarylation
selectivity.
The phenylacetylene substrate 1e, bearing a methyl group in the 2-position, reacted
similarly to unsubstituted 1a, forming two products (Table 3, entry 5). The presence of a
methyl group at the 4-position (1b) caused a lower conversion, 87% (Table 3, entry 2). In
reactions of 1c and 1d, containing chlorine and fluorine substituents, two products were
formed, although the conversion of 1d was lower than that of 1c. In most reactions leading to
two products, the yield of 3 was higher than that of 2. An exception was noted for 1n, methyl-
2-propynyl ether, which gave 56% of 2n and 37% of 3n.
The steric effect of the substituents was observed in reactions of alkynes with an electron-
donating OMe group at the 4-position of the aryl ring, 1f and 1k. The former one, 1f without
other substituents, was converted in 88%, while conversion decreased to 18% when a methyl
group was present at the 2-position (1k). Interestingly, only one product, namely 2f or 2k, was
formed from both of the substrates. A similar selectivity was noted for the alkyne 1h
containing an electron-withdrawing NO2 substituent at the 4-position. In this case, a relatively
low conversion (58%) was achieved. However, only product 2h was formed selectively.
The internal alkyne 1m gave two products at a low substrate conversion. One product of
the hydroarylation, 2m, was formed with 12% yield. The other compound was formed by the
attachment of two phenyl rings to the triple bond of alkyne (8% yield).
The effect of the kind of alkyne substrate on product composition can be illustrated by the
ratio of products 2/3. Alkynes 1a, 1b, and 1e reacted with NaBPh4 providing products 2 and 3
in the molar ratio of about 1:2; for alkyne 1d, the ratio was about 1:3, and for alkyne 1g, about
1:4. A different trend and a lower yield of 3 were found for 1c and 1n. The 4-
chlorophenylacetylene, 1c, was transformed to the corresponding products 2/3 in a molar ratio
of about 1:1, and the methyl-2-propynyl ether, 1n, in a ratio of about 1.5:1. In seven cases (1f,
1h–1m), product 3 was not created. It is worth noting that the conversion of 1j and 1k
significantly increased at a higher temperature, 60 °C. Fortunately, selectivity remained
unchanged, and only products 2j and 2k were formed.
It can be concluded that product 3 was formed with application of alkynes having only one
small substituent on the phenyl group. The amount of product 2 increased with increase of the
number and bulkiness of substituents. On the other hand, the electron properties of
substituents probably didn’t affect significantly the reaction selectivity. For example, alkynes

containing a donating group (e.g. −CH3) or a withdrawing group (e.g. –Cl) formed both
products.
Table 3. Results of the hydroarylation of different alkynes (1a–l) with sodium tetraphenylborate
catalysed by complex I.[a]
Yield[%][b]
Entry
Alkyne
Conv 1[%][b]
2
3
1.
2.
98
87
35
63
1a
1b
31
53
15
56
47
53
3.
4.
100
68
1c
1d
5.
6.
100
88
30
88
70
-
1e
1f
7.
8.
56
54
12
54
44
-
1g
1h
9.
20
7
20
7
-
-
1i
10.
1j
1j
1k
1k
11.[c]
12.
81
18
51
81
18
51
-
-
13.[c]
-
14.
15.
16.
17
27
93
17
-
1l
12 (8)[d]
56
-
1m
1n
37

[a] Reaction conditions: catalyst I (2.5x10-5 mol, 0,0125 g), alkyne 1 (1.0 mmol) NaBPh4 (1.0 mmol, 0.34 g),
AcOH (2.0 mmol, 0.12 mL), in 2 mL water, rt, 3h.
[b] Determined by GC
[c] 60oC
[d] Two products.
Alkynols presented the next group of substrates tested in hydroarylation with catalyst I
(Table 4). Nickel [50] and rhodium [51] complexes were reported as catalyst of
hydroarylation reactions of alkynes bearing hydroxyl group. In both cases, boronic acid was
used as phenylating agent. Two reactions of terminal alkynols with NaBPh4 were carried out
in the presence PdCl2(PPh3)2 [41].
Under the applied mild conditions, the selected alkynols reacted with moderate to excellent
yield forming exclusively one product of the 2 type. The highest yield of the arylation
product, 94–95%, was obtained for 1u and 1t containing secondary and tertiary alcohols,
respectively (Table 4, entries 5 and 7). The secondary alcohol 1w, an analogue of 1t, gave
slightly lower yield, 89%. Interestingly, with PdCl2(PPh3)2 two products were obtained from
1t with the total yield of only 28% [41].
The primary alkynol, prop-2-yn-1-ol (1o), reacted with NaBPh4 to give the expected
hydroarylation product 2 with a 70% yield (Table 4, entry 1). The presence of two methyl
substituents at the α-carbon of alkynol (1s) increased the yield to 86% indicating the positive
influence of steric hindrance. In contrast, the prolongation of the distance between the triple
bond and the OH group (1p and 1r) resulted in a significant decrease in the yield of the
product compared to 1o.
In two cases, 1s and 1u, apart from the main products, small amounts of side products were
also created. In reaction of the alkynol 1s, the product of m/z = 246, 3s, was identified by GC-
MS. It was formed by two molecules of alkynol and one phenyl group (Fig. 5a). It is worth
noting that our system selectively transformed 1s to the arylated alcohol 2s, while an arylated
diene was obtained with the catalyst PdCl2(PPh3)2. [41] 1u showed similar reactivity to 1s and
formed the product 3u comprising three-molecules, but not having one OH group (m/z = 281)
(Fig. 5b).
Table 4. Results of the hydroarylation of different alkynols (1o–u) with sodium tetraphenylborate
catalysed by complex I.
Entry Alkynol
1.
Conv. 1 [%][b]
Yield 2 [%][b]
72
70
1o

2.
3.
4.
10
12
93
10
12
86
1p
1r
1s
5.
95
95
1t
6.
7.
89
98
89
94
1w
1u
[a] Reaction conditions: catalyst I (2.5x10-5 mol, 0,0125 g), alkynol 1 (1.0 mmol) NaBPh4 (1.0 mmol, 0.34 g),
AcOH (2.0 mmol, 0.12 mL), in 2 mL water, 60oC, 3h.
[b] Determined by GC
Mechanistic studies
In order to get knowledge about the nature of the active catalyst, the Hg(0) poisoning test was
performed [52]. For this purpose, two reactions were carried out with a 500-fold excess of
Hg(0) to catalyst I. In the first reaction, Hg(0) was added together with other substrates at the
beginning, while in the second reaction, Hg(0) was introduced after 20 minutes (Fig. 6). The
conversion of phenylacetylene in these reaction was monitored for 3 h and compared to the
mercury-free system. In the presence of Hg(0), the efficiency of the catalytic system
decreased slightly, but conversion still remained high, 85% or 75%, when Hg(0) was added
after 20 min or at the beginning, respectively. These results indicated the domination of the
soluble palladium species as the catalytically active form.
In an attempt to identify the palladium species involved in the catalytic cycle, ESI-MS
analyses were performed for the reaction mixtures containing phenylacetylene, NaBPh4, and
various imidazole complexes.
The ESI-MS spectra of the reaction mixtures showed that the chloride dissociated from
palladium to form Pd(im)2 (Fig. 7). Such chlorine-free fragments were found for three
different imidazoles. The coordination of AcO to palladium was evidenced by the signal at
m/z = 509 assigned to Pd(1-benz-2-methylimidazole)2(AcO). The two next analogues
containing other imidazoles were also found. Moreover, the ions of composition PdCl(im)3
were identified, indicating the easy exchange of imidazole ligands [47].

The ESI-MS data evidenced the presence of Pd-imidazole intermediates in the reaction
mixture. It can be assumed that imidazole coordinated to palladium-stabilized catalysts and
influenced its reactivity.
After the identification of palladium-imidazole complexes in the reaction mixture, attempts
were undertaken to explain the formation pathway of products 2 and 3. To that end,
deuterated substrates were employed, and first D2O was used in the reaction of
phenylacetylene (1a) with NaBPh4 in the presence of catalyst I. Under these conditions, two
products, 2a-d (m/z = 181) and 3a-d (m/z = 283), containing one D-atom each, were formed.
Additionally, NMR studies of the reaction mixture revealed the presence of D-atoms at the
C=C bond. By comparing the 1H NMR spectra of the non-deuterated and deuterated
compounds 2, changes resulting from deuterium substitution were defined (Fig. 8). The signal
(b), detected at 7.1 ppm (Fig. 8a) and assigned to two protons at the double bond in product
2a, was not observed for the product 2a-d obtained in D2O. Instead, the broad triplet (a) from
one proton coupled with D was observed.
Two signals appeared in the 6.50–6.65 ppm range of the 1H NMR spectrum of product 3a:
a singlet (c) and a doublet (d) (Fig. 9a). The correlation COSY spectrum showed that another
doublet expected for the other proton at the double bond should be present at ca. 7.4 ppm;
however, it overlaps with the aromatic signals. In the spectrum of 3a-d, a doublet at 6.5 ppm
was not observed. This is in agreement with the presence of a C=CD fragment in the molecule
(Fig. 9b). Thus, the experiment with D2O confirmed that water acted as a hydride donor in the
reaction of phenylacetylene with NaBPh4.
Hydroarylation of 1a with sodium tetraphenylborate was also carried out in deuterated
acetic, used as a solvent instead of water. Unfortunately, only 2% conversion was achieved in this
reaction. This is clear evidence that water is essential to create products of hydroarylation.
On the basis of the obtained results, the mechanism of alkyne hydroarylation with NaBPh4
was proposed (Fig. 10). In the first step, NaBPh4 reacted with AcOH, in the presence of
palladium, forming BPh3 and biphenyl [42, 43, 53]. The presence of both the products was
evidenced by GC-MS. An alternative pathway has been proposed in the literature, based on
the formation of BPh3 without the contribution of palladium [42, 54]. However, the formation
of benzene, rather than biphenyl, was expected in this case. Simultaneously with the
formation of BPh3, palladium was reduced and a Pd(0) species (B) was created. Imidazole
ligands play a key role in stabilizing the active forms of Pd(0), preventing their degradation
to inactive palladium black. Next, the oxidative addition of BPh3 led to a Pd(II) intermediate

with Ph and BPh2 ligands (C) [41, 55]. Oxidative addition of BPh3 to Pd(0) was proposed in
other systems employing NaBPh4 as phenylating agent [41-43, 55]. In contrast, oxidative
addition of C-B bond to Pd(0) was ruled out in hydroarylation with 4-acetylphenylboronic
acid [28]. After the coordination of phenylacetylene to palladium, migratory insertion
occurred giving the corresponding palladium intermediate (E) with the structure typical of an
anti-Markovnikov-type product. Imidazole ligand facilitated this type of transformation,
because it occupied less space around palladium than phosphine ligand (in PdCl2(PPh3)2) [41].
At this stage two alternative pathways can be considered. The reaction of (E) with water led
to a palladium hydrido complex (F), which could next be transformed to product 2 or 3.
Product 2 (stilbene) was formed by a reductive elimination process. Alternatively,
protonolysis of the Pd-vinyl intermediate E with AcOH may be considered as the route
leading to product 2 via an intermediate F,. We were not able to identify a hydrido complex F,
which is similar to that proposed in diboron mediated alkyne hydroarylation [28]. On the
other hand, protonolysis of the Pd-vinyl complex E by AcOH present in excess in the
catalytic mixture is very plausible.
Formation of product 3 can be also explained by two different schemes. In the first one,
another molecule of phenylacetylene should coordinate to the palladium and two alternative
pathways can be proposed. intermediate (F). Next, an intermediate (G) was formed as a result
of migratory insertion. The diene product 3 was obtained after reductive elimination, and the
catalytically active form (B) was recovered [55].
In the alternative pathway, second molecule of phenylacetylene coordinated to the
intermediate E forming G, which was next transformed to the species H, by migratory
insertion. The final step involved protonolysis of H, by AcOH to form product 3. Only
phenylacetylenes with one small substituent on phenyl group can create product 3. The more
expanded phenyl group made it difficult coordination of the second molecule of alkyne to
palladium and blocked formation of product 3.
It is important to underline that the reaction of commercially available stilbene with
phenylacetylene under the corresponding catalytic conditions was unsuccessful. This result
suggested that the first hydroarylation product (2) cannot be re-coordinated before the
reaction with the second molecule of phenylacetylene. In fact, the selectivity of
hydroarylation depended on the reactivity of palladium species (F and E) and competition

between the reductive elimination of stilbene (2) and coordination of the second
phenylacetylene to palladium.
Further mechanistic studies will be needed to verify the proposed reaction mechanism. At
this point, we wanted to highlight the molecular reaction pathway based on the activity of
palladium complexes bearing imidazole ligands during the whole catalytic process. Their
presence is an advantage over phosphine (PPh3) which results in higher yield of products.
Moreover, PPh3 provided similar amounts of both products, 2 and 3, while excess of 3 was
obtained in reaction with imidazole palladium catalyst.
Conclusions
In summary, we developed an effective Pd-catalysed method of terminal alkyne
hydroarylation with sodium tetraphenylborate. The present procedure works efficiently in
water under mild conditions. Depending on the alkyne structure, stilbenes and aryl-substituted
dienes were formed. Higher steric hindrance of alkyne substrate promoted formation of
stilbene, while less substituted alkynes were hydroarylated to dienes as the main products. In
contrast, alkynols reacted with sodium tetraphenylborate forming exclusively arylalkenes with
excellent yield. To the best of our knowledge, this is the first effective palladium catalytic
system for alkynols hydroarylation. The reactivity of alkynols depended on the distance
between the OH group and the triple bond with a preference for a shorter one. Mechanistic
studies revealed that water was not only a solvent but also a hydride source in the studied
hydroarylation. A similar conclusion was recently formulated for diboron mediated alkyne
hydroarylation [28]. One limitation of the presented method should be noted. Because sodium
tetraphenylborate was used as an arylating agent, only phenylated derivatives could be
prepared.
Acknowledgements
Project funded by Polish Ministry of Science and Higher Education - research funding of the
development of young scientists and doctoral students at the University of Wroclaw.

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Figure 1. Stilbenes present in pharmaceutical agents and in natural products.
Figure 2. The palladium-imidazole complexes I–V used in these studies.
Figure 3. Palladium catalysed hydroarylation of phenylacetylene with NaBPh4.
Figure 4. Hydroarylation of various alkynes.
Figure 5. Side products 3s and 3u obtained in the hydroarylation of: 2-methyl-3-butyn-2-ol (1s) and 1-
ethynylcyclopentanol (1u).

Figure 6. Effect of mercury on the reaction of phenylacetylene with NaBPh4 catalysed by complex I.
Figure 7. The Pd-imidazole intermediates identified in the reaction mixtures.

Figure 8. 1H NMR spectra in CDCl3 of: (a) 2a; (b) 2a-d.
Figure 9. 1H NMR spectra in CDCl3 of: (a) 3a; (b) 3a-d.
Figure 10. Proposed reaction mechanism for the hydroarylation of alkynes.