42
P.K. Mandali, D.K. Chand / Catalysis Communications 47 (2014) 40–44
Table 2
Comparison of the efficiency of PdNPs reported in this work with related Pd catalytic systems reported in the literature for preparation of diarylacetylenes.
S. no.
Catalyst (mol%)
Co-ligand/co-catalyst
Solvent
Base
Temp (°C)
Time (h)
Ref.
1.
2.
3.
4.
5.
PdNPs (2)
CH3CN:CH3OH(l:l)
K2CO3
40 °C
40 °C
Reflux
RT to 80 °C
100 °C
2
48
6
18
1
Present work
Pd(PPh3)4 (10)
Pd(PPh3)4 (5)
PdCl2(PPh3)2 (6)
Pd(OAc)2 (2)
PPh3/CuI(15 mol%)
PPh3
PPh3/CuI (10 mol%)
t-Bu3P (8 mol%)
C6H6
CH3OH
H2O
TEBAC, NaOH
NaOCH3
DBU
18a
19
20
21
DMA
TBAF
a
2-iodothiophene was used as substrate.
diarylacetylene product and Pd(0) is regenerated at the end of a catalytic
cycle [6]. In case of PdNPs as a catalyst, the oxidative addition of aryl
halide (ArX) takes place on the surface of catalyst giving rise to ArPd(II)
X species and that undergoes similar mechanistic steps as described for
Pd complexes regenerating the PdNPs after a catalytic cycle [4]. However,
exact reaction mechanisms of PdNPs catalyzed coupling reactions are
not known. de Vries suggested and proved the participation of anionic
intermediates like ArPdX−2 and PdX−3 during the catalytic cycle in the
event of ligand-free PdNPs as catalyst for C–C coupling reactions [34].
Powder XRD pattern of PdNPs before and after catalytic coupling re-
actions confirmed the fcc lattice of the catalyst. Size of the PdNPs before
and after the catalysis was analyzed by TEM measurements and found
to be 4.5–7 nm. Although, the size of the particles was unaltered, ag-
glomeration was noticed after the second recycle (see SI). The XPS
data was utilized to check the valence state of metal in the catalyst.
The XPS spectrum of catalyst before the catalysis shows the two peaks
at 335.7 eV and 340.9 eV corresponding to Pd3d5/2 and Pd3d3/2 respec-
tively, indicating the zerovalent Pd. The peaks at 337.5 eV and 342.8 eV
represent the PdO. The percentage of Pd(0) and PdO was confirmed as
70.2% and 29.8% by considering relative peak areas. XPS spectrum of
PdNPs after the catalysis revealed major amount of Pd present in zero
oxidation state. However, oxidation of Pd surface during the sample
preparation could not be avoided [35].
To evaluate the activity of catalyst towards the chemoselectivity and
to prepare bis(bromoaryl)acetylenes, we have chosen bromo deriva-
tives of aryl iodides as potential substrates. Further detail of the product
profiles is collected in Table 3. The resultant products contain
derivatizable bromo substituents that can be starting materials for
higher analog of acetylenes under proper conditions. The PdNPs used
in this work is a known catalyst for Suzuki coupling reactions [16].
Thus, Suzuki coupling reaction of bis(3-bromophenyl)acetylene and 4-
methoxyphenylboronic acid was carried out to extend the scope of
the reaction (Scheme 1). Interestingly, coupling of the boronic acid
with the internal acetylene group also happened along with the desired
Suzuki coupling giving a mixture of products (see SI). The crude reac-
tion mixture was characterized by ESI-MS and the products identified
on the basis of a preliminary analysis are shown in Scheme 1. The cou-
pling reaction between 1,4-diiodobenzene and TMSA resulted in a mix-
ture of products (Table 3, entry 1). The heterocycle bearing both iodide
and bromide was also successfully coupled with TMSA resulting in the
bromo-substituted compound in good yield (Table 3, entry 5).
The catalytic activity of PdNPs was further exploited by one-pot syn-
thesis of unsymmetrical diarylacetylenes as shown in Table 4. Initially,
4-iodoanisole was coupled with required amount of TMSA to afford
trimethylsilyl protected 4-ethynylanisole. The in situ formed compound
undergoes deprotection to give the terminal acetylene. The next step
was carried out by adding a fresh batch of aryl iodide, however, without
any further addition of catalyst or base. Thus the desired unsymmetrical
diarylacetylene was formed within 2 h (Table 4, entry 1). When we have
chosen aryl iodide containing electron withdrawing substrates such as
4-iodoacetophenone and 4-iodonitrobenzene for the synthesis of un-
symmetrical moieties, instead of TMS deprotected product we isolated
the symmetrical diarylacetylenes. The substrate scope was examined
by employing various aryl halides to couple with 4-ethynylanisole
(Table 4, entries 2–9). All the coupling reactions conceded unsymmetri-
cal diarylacetylenes in good yields. The sterically hindered substrates like
2-iodotoluene, 2-iodoanisole and 2-iodo-1,3,5-trimethyliodobenzene
were coupled with 4-ethynylanisole to afford corresponding unsymmet-
rical diarylacetylenes in good yields (Table 4, entries 6–8).
In an alternative manner, 4-iodoanisole, 4-iodotoluene and
TMSA were combined at same time to synthesize 4-methoxy-4′-
methyldiphenylacetylene. The desired product was isolated and
the yield was found to be 80% in contrast with the 90% yield when
performed in a step wise manner. Also 5% of bis(p-tolylphenyl)acet-
ylene and 10% of bis(p-methoxyphenyl)acetylene were formed
along with the desired product.
Table 3
One-pot synthesis of bis(bromo)arylacetylenes.a
Entry
X
Y
Time(h)
Yield (%)b
c
1
2
3
4
5
4-I
CH
CH
CH
CH
N
3
4
2.5
3
2
–
4-Br
3-Br
2-Br
5-Br
81
76d
74e
72f
a
Reaction conditions: ArI (1 mmol), TMSA (0.6 mmol), K2CO3 (approx. 2 mmol), CH3CN: CH3OH = 2.5:2.5 mL.
Isolated yields.
Formation of a mixture of products.
5% of 1-bromo-3-iodobenzene is isolated.
9% of 1-bromo-2-iodobenzene is isolated.
b
c
d
e
f
11% of 5-bromo-2-iodopyridine is isolated.