Regioselective C-H arylation of imidazoles employing macrocyclic palladium(II) complex of organoselenium ligand

Article abstract of DOI:10.1016/j.jorganchem.2021.121907

In this report we have presented synthesis of a new air stable bidentate selenium ligand (L1) by the reaction of 1,8-bis(2-(chloromethyl)phenoxy)octane with sodium salt of diphenyl diselenide. The reaction of bidentate selenium ligand (L1) with Pd(CH

Full text of DOI:10.1016/j.jorganchem.2021.121907

Journal of Organometallic Chemistry 946–947 (2021) 121907
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Regioselective C-H arylation of imidazoles employing macrocyclic
palladium(II) complex of organoselenium ligand
1
1
∗
Sunil Kumar , Sohan Singh , Jitendra Gadwal, Parvesh Makar, Hemant Joshi
ISC Laboratory, Department of Chemistry, School of Chemical Sciences and Pharmacy, Central University of Rajasthan, NH-8, Bandarsindri, Ajmer, Rajasthan
05817, India
3
a r t i c l e i n f o
a b s t r a c t
Article history:
In this report we have presented synthesis of a new air stable bidentate selenium ligand (L1) by the re-
action of 1,8-bis(2-(chloromethyl)phenoxy)octane with sodium salt of diphenyl diselenide. The reaction
of bidentate selenium ligand (L1) with Pd(CH3CN)2Cl2 in acetonitrile under reflux conditions resulted
nineteen membered ring macrocyclic palladium(II) complex. The structure of ligand precursors, ligand,
and macrocyclic palladium(II) complex were authenticated by using 1H, 13 C{ H} NMR spectroscopy and
elemental analysis. The repeated attempts to obtain single crystals of macrocyclic palladium(II) complex
were unsuccessful probably due to long flexible aliphatic chain in the molecule. The air and moisture
stable, thermally robust macrocyclic palladium(II) complex was employed as catalyst to catalyze the re-
gioselective arylation reaction of imidazole derivatives. The reaction works efficiently without any need
of inert atmosphere. The current protocol works under mild reaction conditions with exclusive C-5 re-
gioselectivity. Excellent yields (~73–95%) were obtained by using only 1.5 mol% of C1, with wide range of
derivatives and large functional group tolerance. Homogeneous nature of catalysis process was confirmed
with the help of mercury and triphenylphosphine poisoning tests. Further, the catalyst can be reused
with significant loss (22%) in the efficiency.
Received 26 April 2021
Revised 18 May 2021
Accepted 20 May 2021
Available online 25 May 2021
1
Keywords:
Macrocyclic palladium(II) complex
Selenium ligand
Regioselective
Arylation
Imidazole
Homogeneous catalysis
©
2021 Elsevier B.V. All rights reserved.
1
. Introduction
class of bidentate macrocyclic palladium(II) complex of organose-
lenium ligand and explore its catalytic potential for regioselective
C-H arylation of imidazoles with aryl bromide derivations.
In the last two decades organoselenium ligands have made
an appearance as privileged class of ligand moieties due to their
strong soft donor abilities, air and moisture stable nature, low cost
syntheses, higher sigma donation properties, and similar catalytic
properties with respect to carbene and phosphine based ligands
The heterocyclic molecules containing nitrogen atom are among
the elite class of molecules due to their wide range of applications
in natural products, agrochemicals, biologically active molecules,
pharmaceuticals, and in material science [3]. Among the class of
nitrogen containing heterocycles, imidazole and its derivatives has
gained extensive attention due to the plethora of pharmaceutically
active molecules and drugs [4]. The regioselective C-H bond ary-
lation of imidazole is one among the important protocol for the
syntheses of such imidazole derivatives [5]. The arylation reaction
is more favored to C-2 and C-5 positions as compared to C-4 po-
sition of imidazoles [6]. In order to activate C-2 position, strong
base (sodium or potassium tert-butoxide) with copper salts are
used whereas to activate C-5 position, bases like pivalate or ac-
etate with polar solvent were used. The regioselective C-5 arylation
of imidazole reported earlier was mostly catalyzed by palladium
based catalysts with N-heterocyclic carbene or phosphine ligands
[
1]. Several metal complexes of these ligands have been reported
as efficient catalysts for various organic transformations like Heck
coupling, Suzuki-Miyaura coupling, C-H activation, oxidation of al-
cohols, N-alkylation reaction, transfer hydrogenation, and many
more reactions as discussed in the recent review articles [2]. Palla-
dium when coordinates with organoselenium ligands, the selenium
of ligand donates its sigma electrons to palladium center which in
turn increase the sigma electron density on palladium center and
helps palladium center to ease the oxidative addition step in catal-
ysis. This results in greater yield of products in catalysis [1,2]. Con-
sidering these distinctive characteristic of organoselenium coordi-
nated palladium complexes, it was decided to synthesize a new
[
5,7–9]. Recently palladium complexes of organochalcogen ligands
were used as efficient catalyst for regioselective arylation of imi-
dazoles [1b–d]. The advantages of such catalytic systems over the
phosphine and carbene based ligand systems are, (i) the ligand and
∗
Corresponding author.
E-mail address: hemant.joshi@curaj.ac.in (H. Joshi).
1
These authors contributed equally.
https://doi.org/10.1016/j.jorganchem.2021.121907
0
022-328X/© 2021 Elsevier B.V. All rights reserved.

S. Kumar, S. Singh, J. Gadwal et al.
Journal of Organometallic Chemistry 946–947 (2021) 121907
Scheme 1. Synthesis of bidentate organoselenium ligand.
Scheme 2. Synthesis of macrocyclic Pd(II) complex.
complexes are air and moisture stable, (ii) low or comparative cat-
alyst loading, (iii) higher C-5 selectivity of arylated products, (iv)
wide substrate scope with large functional group tolerances, (v)
mild reaction conditions, and (vi) no need of inert atmosphere to
catalyze the reaction.
ppm which was 0.04 ppm shielded with respect to CH OH due to
2
less electronegativity of chlorine as compared to oxygen. The lig-
and precursor B in next step reacts with sodium salt of diphenyl
diselenide in EtOH to give organoselenium ligand (L1) as yellow
liquid in 63% yield (Scheme 1). The 1H NMR spectrum of biden-
tate selenium ligand (L1) shows shielding of 0.52 ppm in SeCH2
peak appearing at 4.16 ppm as singlet, which confirms binding of
less electronegative selenium to the ligand substituting more elec-
tronegative chlorine. The peaks in 13C{ H} NMR spectrum of L1 are
in agreement with its structure proposed in Scheme 1. The ligand
2. Results and discussion
1
The organoselenium ligand and macrocyclic palladium(II) com-
plex were synthesized as shown in Schemes 1 and 2. The new
organoselenium ligand, and macrocyclic palladium(II) complex
showed good solubility in common organic solvents like CH CN,
3
were characterized with the help of 1H, 13C{1H} NMR spectroscopy,
CHCl , CH Cl , THF, MeOH, EtOAc, EtOH, DMF, and DMSO whereas
3 2 2
ꢀ
it is sparingly soluble in non-polar solvents like pentane, hexane
etc.
and elemental analysis. First, the ligand precursor 2,2 -(octane-1,8-
diylbis(oxy))dibenzaldehyde (A) was synthesized by the reaction
Next, to synthesize the macrocyclic palladium(II) complex of
organoselenium ligand, the selenium ligand L1 was reacted with
Pd(CH CN) Cl precursor in acetonitrile. The nineteen membered
of two equivalents of salicylaldehyde with 1,8-dibromooctane in
presence of K CO and DMF solvent. The ligand precursor A was
2
3
isolated as white solid in 48% yield. The formation of ligand pre-
3
2
2
cursor A was confirmed by 1H, and 13C{1H} NMR spectrum. The
1H NMR spectrum of A shows characteristic singlet peak at 10.51
ring containing macrocyclic palladium complex was isolated as
dark red solid in 54% yield. The macrocyclic complex was charac-
terized with the help of 1H and 13C{ H} NMR spectroscopy. The
1H NMR of C1, shows a singlet at 4.69 ppm which was deshielded
by 0.53 ppm with respect to free ligand and confirms coordination
of ligand with palladium precursor. The 13C{ H} NMR spectrum of
C1 is in agreement with its structure proposed in Scheme 2. The
repeated attempts to obtain the single crystals of macrocyclic pal-
ladium(II) complex were unsuccessful probably due to long flexi-
ble aliphatic chain in the molecule. The macrocyclic palladium(II)
complex showed good solubility in common organic solvents like
CH CN, CHCl , CH Cl , THF, DMF, and DMSO whereas it is insolu-
1
ppm due to aldehyde group, whereas the triplet at 4.07 ppm is
due to OCH2 group. The 13C{1H} NMR spectrum shows a charac-
teristic peak at 189.8 ppm due to carbonyl group and other peaks
agree with the structure proposed in Scheme 1. The ligand pre-
cursor A was then taken in MeOH (15 mL) and reacted with 2.2
equivalents of solid NaBH4 to give ligand precursor ((octane-1,8-
diylbis(oxy))bis(2,1-phenylene))dimethanol (Aʹ). The ligand precur-
1
1
sor Aʹ was isolated as white solid in 92% yield. In the H NMR
spectrum of Aʹ, the aldehydic singlet peak which was at 10.51 ppm
in A has disappeared and a new singlet at 4.72 ppm appeared due
3
3
2
2
ble in solvents like pentane, hexane etc.
to conversion of CHO to CH OH. This was further confirmed by
2
13
1
Macrocyclic palladium(II) complex catalyzed regioselective
arylation of imidazoles. The substituted imidazole derivatives
make an important class of privileged scaffolds due to their ex-
tensive applications in industrial synthesis of polymers, functional
materials, pharmaceuticals, natural products, and medicinal chem-
istry [4]. Considering the importance of these compounds, many
researchers reported new catalytic systems for their syntheses [7–
C{ H} NMR spectrum of Aʹ which also shows disappearance of
carbonyl carbon peak and confirm reduction of aldehyde to cor-
responding alcohol. The 1H NMR of Aʹ shows characteristic broad
singlet of OH at 2.51 ppm in the proton NMR. Further, Aʹ was
subjected to chlorination reaction with SOCl2 in dry DCM to give
1
,8-bis(2-(chloromethyl)phenoxy)octane (B) as yellow solid in 91%
yield. In the 1H NMR of B, the CH Cl singlet appeared at 4.68
2
2

S. Kumar, S. Singh, J. Gadwal et al.
Journal of Organometallic Chemistry 946–947 (2021) 121907
Table 1
Optimization of reaction conditions for macrocyclic palladium(II) complex catalyzed regioselec-
tive arylation of imidazolea
Entry No.
Base
Solvent (3 mL)
Additive
Yieldb (%)
1
2
3
4
5
6
7
8
9
-
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMF
-
nd
nd
11
91
46
26
42
38
17
12
26
21
81
nd
13
24
17
93
78
62
53
nd
K2CO3
K2CO3
K2CO3
Na2CO3
Cs2CO3
KOH
-
PhCO2H
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
PivOH
NaOH
t-BuOK
t-BuONa
KOAc
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
NaOAc
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
3
4
DMSO
THF
5
6
1,4-dioxane
Toluene
DMA
DMA
DMA
DMA
DMA
7
8c
9d
0e
1f
2g
a
Reaction conditions: 4-bromobenzonitrile (1.0 mmol), N-methyl imidazole (1.2 mmol), ad-
ditive (0.30 mmol), base (2.0 mmol), solvent (3 mL), temperature (100 °C), time (10 h), under
open air conditions
b
Isolated yield.
c
C1 (2.0 mol%).
d
C1 (1.0 mol%).
e
Reaction time (6 h).
f
Temp (60 °C).
g
Control experiment without catalyst.
9
]. Most of the earlier reported catalysts are based on sterically
The proton NMR spectrum confirms arylation at C-5 position of
imidazole and no C-2 arylated product was observed during the
bulky phosphine, and NHC ligands coordinated metal complexes.
In most cases, these ligand systems are known to be air and mois-
ture sensitive and cannot be used in open air conditions which
limits usage of these catalysts. To overcome this issue, the air
and moisture insensitive organoselenium ligated palladium com-
plexes serve as better alternatives [1b-d]. Intrigued with these ear-
lier reports, we planned to explore the catalytic potential of macro-
cyclic palladium(II) complex of organoselenium ligand for regios-
elective arylation of imidazole derivatives. A reaction between 4-
bromobenzonitrile (2a) and N-methyl imidazole (1a) was selected
as model reaction for optimizing reaction conditions (Table 1). First
we carried out a reaction between 1a and 2a, in presence of C1
reaction. Next, different bases like Na CO , Cs CO , KOH, NaOH, t-
2
3
2
3
BuOK, t-BuONa, KOAc, and NaOAc were screened (Table 1, entries
4-12) for the arylation reaction under similar reaction conditions
as discussed in entry 4. The bases Na CO3 and Cs CO resulted
2
2
3
in 46 and 26% yield of 3a respectively (Table 1, entries 5 and 6).
Moderate yields (38-42%) of 3a were achieved with KOH and NaOH
(Table 1, entries 7 and 8). Other bases like t-BuOK, t-BuONa, KOAc,
and NaOAc were not found suitable for the arylation reaction and
resulted poor yields of 3a (12-26%, Table 1, entries 9-12). With the
best reaction condition in hand (Table 1, entry 4), we screened var-
ious solvents like DMF, DMSO, THF, 1,4-dioxane, and toluene for
the arylation reaction. DMF resulted in 81% yield of 3a (Table 1,
entry 13), whereas reaction did not work with DMSO (Table 1, en-
try 14). Solvents like THF, 1,4-dioxane, and toluene resulted in poor
conversion of 3a (Table 1, entries 15-17). Among all the screened
(
1.5 mol%) in DMA (3 mL) without base and additive, there was
no conversion to desired coupled product 3a and starting mate-
rials recovered as it is (Table 1, entry 1). Next, a similar reaction
was performed as entry 1, in presence of K CO without additive,
2
3
which does not result in coupled product 3a (Table 1, entry 2).
After, when benzoic acid was used as an additive under similar re-
action conditions as entry 2, it resulted in 11% yield of 3a (Table 1,
entry 3). The reaction under the similar conditions as entry 3 but
with pivalic acid (PivOH) as an additive instead of benzoic acid, re-
sulted in 91% yield of coupled product 3a (Table 1, entry 4). This
shows that PivOH as an additive is crucial for the reaction as it
takes part in proton shuttle during C-H bond cleavage [10]. The
high steric hindrance and pKa value of PivOH (5.03) in compari-
son to benzoic acid (4.20) helps in achieving greater yields of 3a.
bases and solvents K CO3 and DMA found best suitable for the
2
arylation reaction (Table 1, entry 4) and hence used for further
catalysis studies. Next, suitable catalyst loading for optimum con-
version was screened (Table 1, entries 18 and 19). Upon increasing
the catalyst loading to 2.0 mol%, the yield of 3a increased slightly
(93%, Table 1, entry 18). A lower yield of 78% was isolated when
1.0 mol% of C1 was used (Table 1, entry 19). When reaction un-
der the conditions as discussed in entry 4 was stopped after 6 h,
the conversion lowered to 62% (Table 1, entry 20). Upon decreasing
the reaction temperature to 60 °C, the conversion of 3a dropped to
3

S. Kumar, S. Singh, J. Gadwal et al.
Journal of Organometallic Chemistry 946–947 (2021) 121907
Table 2
Substrate scope for macrocyclic palladium(II) complex catalyzed arylation of imidazolea
5
3% (Table 1, entry 21). A control experiment without the macro-
(Table 2) as compared to electron withdrawing groups. The ster-
ically bulky deactivated derivative 1-bromonaphthalene resulted in
excellent yield (84%) of arylated product 3e (Table 2). Next, we
performed arylation reactions of 1,2-dimethyl-1H-imidazole with
the aryl bromide derivatives (Table 2, entries 3f-3k). The reac-
tion with deactivated, electron withdrawing, and electron donating
derivatives went smoothly with slightly higher yields as compared
to N-methyl imidazole derivative. The reaction with heterocyclic
derivative (3-bromopyridine) was also proceed smoothly resulting
78% yield of arylated product 3k (Table 2). The arylated products
like 3a and 3e are very important intermediates for useful bioac-
tive molecules [5]. The current protocol suggests an easy one pot
method for their high yield syntheses with greater C-5 regioselec-
tivity. A comparison of current protocol with earlier reports is tab-
ulated in supporting information (Table s1).
cyclic Pd(II) catalyst (C1) resulted no conversion of desired product
3
a (Table 1, entry 22).
The best optimized condition among all the screened condi-
tions is N-methyl imidazole (1.2 mmol), aryl bromide (1.0 mmol),
PivOH (0.3 mmol), K CO (2.0 mmol), with 2.0 mol% C1 for 10 h
2
3
at 100°C to achieve maximum yield of 3a (Table 1, entry 18). A
catalyst loading of 1.5 mol% was chosen for further studies of sub-
strate scope as it gave comparative yield with lower catalyst load-
ing (Table 1, entry 4). We were fascinated to see that no C-2, C-4
arylated and homocoupled products were detected during the ary-
lation reaction.
Next, with the best screened conditions in hand (Table 1, en-
try 4), we explored the substrate scope for structurally diver-
gent aryl bromides and imidazole derivatives (Table 2). The re-
sults suggested that a wide range of structurally divergent aryl
bromide substrates with deactivated, electron withdrawing, and
electron donating functional groups like naphthyl, cyano, nitro,
Imidazole substituted biaryl motifs are considered as an im-
portant intermediate for many functional and biologically active
molecules [11]. Syntheses of such derivative is limited in litera-
ture as only few reports are available to date [8d]. We planned a
sequential arylation and Suzuki-Miyaura coupling reaction for the
COCH , methoxy, and pyridine can be activated using this pro-
3
tocol with excellent yields (73–95%, Table 2, entries 3a-3k) and
greater regioselectivity towards C-5 arylation. The electron with-
drawing groups (cyano and nitro) at para position of aryl bro-
mide upon reaction with N-methyl imidazole resulted in excel-
lent yield (91–92%) of C-5 arylated products 3a and 3b (Table 2).
Whereas the electron withdrawing group (cyano) at ortho posi-
tion of aryl bromide resulted in slight lower yield (86%) of C-5
arylated product 3c (Table 2). When electron donating group on
para position of aryl bromide (-OMe) was tested for the arylation
reaction, it resulted in lower yield (73%) of arylated product 3d
ꢀ
synthesis of 5-([1,1 -biphenyl]-4-yl)-1-methyl-1H-imidazole (3m,
Scheme 3). In first step, a regioselective arylation reaction of 1-
methyl-1H-imidazole with 1-bromo-4-chlorobenzene was carried
out under the optimum reaction conditions (Table 1, entry 4).
The reaction resulted in 84% yield of 3l (Scheme 3) which has a
para chlorine functional group available. The compound 3l in next
step subjected to Suzuki-Miyaura coupling reaction with phenyl-
ꢀ
boronic acid to give 5-([1,1 -biphenyl]-4-yl)-1-methyl-1H-imidazole
(3m, Scheme 3) in 28% yield.
4

S. Kumar, S. Singh, J. Gadwal et al.
Journal of Organometallic Chemistry 946–947 (2021) 121907
Scheme 3. Sequential arylation and Suzuki-Miyaura coupling reactions.
The NMR solvent peaks are referenced as: (δ, ppm): 1H, CHCl3
(7.26); 13C{ H}, CDCl3 (77.00). Elemental analyses were carried out
with a Perkin–Elmer 2400 Series-II C, H, N analyzer.
Further to gain insights about the nature of catalysis process,
mercury and triphenylphosphine poisoning tests were conducted
1
[
12]. First we performed a reaction under standard reaction condi-
tions (Table 1, entry 4) with excess amount of triphenylphosphine
5.0 mol%). The reaction resulted 88% yield of 3a with no signifi-
ꢀ
Synthesis of 2,2 -(octane-1,8-diylbis(oxy))dibenzaldehyde (A).
(
A single neck round bottom flask was charged with salicylaldehyde
(2.44 g, 20.00 mmol), 1,8-dibromooctane (2.72 g, 10.00 mmol), and
K2CO3 (2.79 g, 20.20 mmol) in DMF (20 mL). The mixture was
heated with stirring for 8 h at 75 ˚C. The progress of reaction was
monitored by TLC and after maximum conversion to product, the
reaction mixture was cooled to room temperature and extracted
cant effect on the yield as compared to entry 4 (91%). Next, in a re-
action same as entry 4 but with excess of mercury (Pd/Hg = 1/300)
we observed 83% yield of 3a (see experimental section for detailed
procedure and results). Both these tests suggested that the reac-
tion has negligible effect of mercury and triphenylphosphine poi-
soning which confirms homogeneous nature of arylation reaction.
The reusability experiment gave 76% yield of 3a with 22% loss of
efficiency in the catalyst performance.
∗
with ethyl acetate and cold water (3 20 mL). The ethyl acetate
layer was dried over anhydrous Na SO4 and concentrated using
2
rotary evaporator. The residue was washed with cold hexane and
dried under high vacuum to give A (1.69 g, 4.77 mmol, 48%) as
white solid. Anal. Calcd for C22H26O4 (354.4394): C, 74.55; H, 7.39.
Found: C, 74.49; H, 7.34.
3. Conclusions
NMR (CDCl , δ/ppm): 1H (400 MHz) 10.51 (s, 1H), 7.82 (d,
A new bidentate organoselenium ligand (L1) with long flex-
3
J = 6.0 Hz, 1H), 7.52 (t, J = 7.6 Hz, 1H), 7.02 – 6.96 (m, 2H), 4.07
(t, J = 6.4 Hz, 2H), 1.89 – 1.82 (m, 2H), 1.52 – 1.49 (m, 2H), 1.43
ible aliphatic chain was synthesized by the reaction of 1,8-
−
+
bis(2-(chloromethyl)phenoxy)octane and PhSe Na . Macrocyclic
13
1
–
1.40 (m, 2H); C{ H} (100 MHz) 189.8 (s), 161.5 (s), 135.9 (s),
palladium(II) complex (C1) was synthesized by the reaction of
1
28.1 (s), 124.8 (s), 120.4 (s), 112.5 (s), 68.4 (s), 29.2 (s), 29.0 (s),
5.9 (s).
Synthesis
phenylene))dimethanol (Aʹ). The ligand precursor
.00 mmol) was taken in MeOH (10 mL) and cooled to 0 ˚C. Solid
Pd(CH CN) Cl precursor with L1 in acetonitrile. The air and
3
2
2
2
moisture stable C1 and L1 were characterized with the help of
1H, 13C{1H} NMR spectroscopy, and elemental analysis. The new
of
((octane-1,8-diylbis(oxy))bis(2,1-
(1.42 g,
A
macrocyclic Pd(II) complex was used as catalyst for regioselective
arylation of imidazole with aryl bromide derivatives. The proto-
col resulted in exclusive C-5 arylated product, no C-2, C-4 ary-
lated or homocoupling products were observed during the reac-
tion. The catalyst works efficiently (73–95%) with only 1.5 mol%
loading of C1, covering broad substrate scope. The protocol showed
excellent tolerance towards many functional groups like -CN, -NO2,
4
NaBH4 (0.318 g, 8.4 mmol) was added to the reaction mixture in
small portions over the time of thirty minutes and solution was
allowed to reach room temperature. The mixture was stirred at
room temperature for 8 h. The similar workup as A, gave ligand
precursor Aʹ (1.321 g, 3.68 mmol, 92%) as white solid. Anal. Calcd
for C22H30O4 (358.4712): C, 73.71; H, 8.44. Found: C, 73.76; H,
-
COCH , and -OMe. The protocol was also applied to sequential
3
8
.39.
arylation and Suzuki-Miyaura coupling reaction and showed good
conversions in both the reactions. The mercury and triphenylphos-
phine poisoning tests do not affect the yield of reactions and sug-
gested homogeneous nature of catalysis process. The catalyst can
be reused with significant loss in the efficiency.
NMR (CDCl , δ/ppm): 1H (400 MHz) 7.31 – 7.26 (m, 2H), 6.98
6.89 (m, 2H), 4.72 (s, 2H), 4.05 (t, J = 6.4 Hz, 2H), 2.51 (bs, 1H,
3
–
13
1
OH), 1.89 – 1.82 (m, 2H), 1.54 – 1.43 (m, 4H); C{ H} (100 MHz)
156.9 (s), 129.1 (s), 128.8 (s), 128.6 (s), 120.5 (s), 111.0 (s), 67.8 (s),
6
2.2 (s), 29.2 (2 × s), 26.1 (s).
Synthesis of 1,8-bis(2-(chloromethyl)phenoxy)octane (B). The
4
. Experimental section
ligand precursor Aʹ (1.26 g, 3.50 mmol) was taken in dry DCM
10 mL). The SOCl2 (3.6 mmol) was added dropwise to the reac-
(
General. The reactions for the synthesis of ligand precursors (A
tion mixture and solution was allowed to stir at room tempera-
ture for 10 h. The progress of reaction was monitored by TLC and
after maximum conversion to product, the reaction was neutral-
and B), ligand (L1), and macrocyclic complex (C1) were carried out
using standard Schlenk line techniques. The catalytic arylation of
imidazole with aryl bromide derivatives was carried out in a pres-
sure tube under open air conditions. HPLC grade solvents like DMF,
EtOAc, Hexane, MeOH, DCM, EtOH, DMA, DMSO, THF, 1,4-dioxane,
and toluene were used as received. Salicylaldehyde (Spectrochem),
ized with NaHCO . The similar workup as A, gave ligand precur-
3
sor B (1.27 g, 3.21 mmol, 91%) as yellow solid. Anal. Calcd for
C22H28Cl O (395.3625): C, 66.83; H, 7.14. Found: C, 66.77; H, 7.09.
2
2
NMR (CDCl , δ/ppm): 1H (400 MHz) 7.37 – 7.28 (m, 2H), 6.96 –
3
1,8-dibromooctane (Spectrochem), K CO3 (Spectrochem), NaBH4
2
6
2
.88 (m, 2H), 4.68 (s, 2H), 4.03 (t, J = 6.4 Hz, 2H), 1.88 – 1.81 (m,
(
Spectrochem), SOCl2 (Spectrochem), Diphenyl diselenide (Spec-
13
1
H), 1.55 – 1.50 (m, 2H), 1.45 – 1.41 (m, 2H); C{ H} (100 MHz)
trochem), N-methyl imidazole (Sigma Aldrich), 1,2-dimethyl-1H-
156.8 (s), 129.1 (s), 128.8 (s), 128.6 (s), 120.4 (s), 110.9 (s), 67.8 (s),
imidazole (Sigma Aldrich), CDCl3 (Sigma Aldrich), Na SO4 (EMD),
2
6
2.2 (s), 29.2 (s), 26.0 (s).
and PdCl2 (Sigma Aldrich) were used as received. The reagents for
arylation reaction were purchased from local vendors and used as
received. Bruker 400 MHz instrument was used to record 1H and
Synthesis of 1,8-bis(2-((phenylselanyl)methyl)phenoxy)octane
(L1). A 100 mL two necked round bottom flask was charged with
diphenyl diselenide (0.468 g, 1.5 mmol) in EtOH (25 mL). The
13
1
C{ H} NMR spectrums of compounds at ambient temperature.
5

S. Kumar, S. Singh, J. Gadwal et al.
Journal of Organometallic Chemistry 946–947 (2021) 121907
mixture was stirred at 70 ˚C under N2 atm. Ethanolic solution
of NaBH4 (0.121 g, 3.20 mmol) was added dropwise to the reac-
tion mixture until it become colorless. Ligand precursor B (1.186 g,
Triphenylphosphine poisoning test. Following the general pro-
cedure for arylation reaction, a pressure tube was charged with 4-
bromobenzonitrile (1.0 mmol), 1-methyl-1H-imidazole (1.2 mmol),
K2CO3 (2.0 mmol), PivOH (0.30 mmol), and DMA (3 mL). Catalyst
C1 (1.5 mol%) and triphenylphosphine (5.0 mol%) were added to
this reaction mixture and allowed to stir at 100 ˚C for 10 h. The
analogues workup as general procedure gave 88% yield of 3a.
Mercury poisoning test. Following the general procedure
3
.00 mmol) was dissolved in 2 mL EtOH and added dropwise to
this reaction mixture. The mixture was allowed to stir overnight
under refluxing. The reaction mixture was cooled to room temper-
∗
ature and extracted with CHCl3 and water (3 20 mL). The CHCl3
layer was dried over anhydrous Na SO and concentrated using ro-
2
4
tary evaporator. The residue was washed with hexane and dried
for arylation reaction, a pressure tube was charged with 4-
under high vacuum to give L1 (1.21 g, 1.90 mmol, 63%) as yellow
bromobenzonitrile (1.0 mmol), 1-methyl-1H-imidazole (1.2 mmol),
K2CO3 (2.0 mmol), PivOH (0.30 mmol), and DMA (3 mL). Catalyst
C1 (1.5 mol%) and mercury (Pd/Hg = 1/300) were added to this
reaction mixture and allowed to stir at 100 ˚C for 10 h. The ana-
logues workup as general procedure gave 83% yield of 3a.
liquid. Anal. Calcd for C34H38O Se (636.5843): C, 64.15; H, 6.02.
2
2
Found: C, 64.23; H, 6.08.
NMR (CDCl , δ/ppm): 1H (400 MHz) 7.65 – 7.63 (m, 1H), 7.52
3
–
7.50 (m, 2H), 7.32 – 7.24 (m, 2H), 7.21 – 7.18 (m, 2H), 7.07 –
7
.06 (m, 1H), 6.85 – 6.81 (m, 2H), 4.16 (s, 2H), 3.98 (t, J = 5.2 Hz,
Procedure for Reusability Experiment. In a pressure tube
(10 mL), 4-bromobenzonitrile (0.182 g, 1.0 mmol), 1-methyl-1H-
2
H), 1.85 – 1.79 (m, 2H), 1.55 – 1.50 (m, 2H), 1.45 – 1.41 (m, 2H);
1
3
1
C{ H} (100 MHz) 156.6 (s), 133.6 (s), 130.0 (s), 129.1 (s), 128.7
imidazole (0.098 g, 1.2 equiv.), K CO3 (0.276 g, 2.0 equiv.), PivOH
2
(
(
s), 128.2 (s), 127.7 (s), 127.0 (s), 120.0 (s), 111.3 (s), 67.9 (s), 29.3
s), 29.27 (s), 27.1 (s), 26.1 (s).
(0.031 g, 0.3 equiv.), macrocyclic palladium complex C1 (1.5 mol%),
and DMA (3 mL) were stirred and heated on 100 °C for 10 h.
After 10 h, the reaction mixture was cooled and 150 μL of re-
action mixture was taken out and analyzed using 1H NMR spec-
troscopy. Thereafter a fresh batch of 4-bromobenzonitrile (0.182 g,
Synthesis of macrocyclic palladium(II) complex C1. A round
bottom single necked flask was charged with L1 (0.636 g,
.00 mmol), Pd(CH CN) Cl (0.259 g, 1.00 mmol), and CH CN
1
3
2
2
3
(
10 mL). The mixture was stirred while refluxing overnight. The
1.0 mmol), 1-methyl-1H-imidazole (0.098 g, 1.2 equiv.), K CO
2 3
progress of reaction was monitored by TLC and after maximum
conversion to product, the reaction mixture was cooled to room
temperature and solvent was removed using rotary evaporator. The
remaining residue was washed with hexane and dried under high
vacuum to give C1 (0.44 g, 0.54 mmol, 54%) as dark red solid. Anal.
Calcd for C34H38Cl O PdSe (813.9103): C, 50.17; H, 4.71. Found: C,
(0.276 g, 2.0 equiv.), PivOH (0.031 g, 0.3 equiv.), was added in the
absence of C1 and reaction was allowed to stir at 100 °C for an-
other 10 h. The analysis of aliquot shows 76% conversion of desired
coupled product 3a.
2
2
2
Declaration of Competing Interest
5
0.22; H, 4.76.
NMR (CDCl , δ/ppm): 1H (400 MHz) 7.64 – 7.63 (m, 1H), 7.37
3
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
(
(
d, J = 4.4 Hz 1H), 7.32 – 7.28 (m, 5H), 6.96 – 6.89 (m, 2H), 4.69
s, 2H), 4.05 (t, J = 5.2 Hz, 2H), 1.88 – 1.83 (m, 2H), 1.57 – 1.55 (m,
H), 1.46 – 1.45 (m, 2H); 13C{ H} (100 MHz) 156.8 (s), 131.5 (s),
1
2
130.4 (s), 129.9 (s), 129.1 (s), 127.7 (s), 125.8 (s), 120.3 (s), 111.6 (s),
Acknowledgement
6
8.6 (s), 41.7 (s), 29.2 (s), 29.1 (s), 25.9 (s).
General procedure for macrocyclic palladium complex cat-
SK and SS thank to University Grants Commission, India and
Council of Scientific and Industrial Research, India respectively for
the JRF fellowship. HJ is thankful to Department of Science and
Technology (DST) Inspire (DST/INSPIRE/04/2017/002162). Authors
are also thankful to DST for DST-FIST grant [SR/FST/CSI-270/2015]
and [SR/FST/CSI-257/2014(C)] to BITS Pilani and Central Univer-
sity of Rajasthan. Authors are thankful to Central University of Ra-
jasthan for instrumental and infrastructural support.
alyzed arylation of imidazole. A pressure tube was charged with
aryl bromide (1.0 mmol), imidazole derivative (1.2 equiv.), base (2.0
equiv.), additive (0.30 mmol), and solvent (3 mL). The reaction mix-
ture was stirred at 100 ˚C for 10 h under open air conditions.
The progress of reaction was monitored by TLC and after maxi-
mum conversion to product, the reaction mixture was cooled to
room temperature and extracted with ethyl acetate and cold wa-
∗
ter (3 20 mL). The ethyl acetate layer was dried over anhydrous
Na SO and concentrated using rotary evaporator. The residue was
2
4
Supplementary materials
purified using silica gel column (1 × 12 cm) chromatography with
CH Cl /MeOH (20:1 v/v) solvents. The products as isolated were
2
2
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
then authenticated with the help of 1H and 13C{ H} NMR spec-
1
troscopy and data are presented in SI.
121907.
Experimental procedure for macrocyclic palladium complex
catalyzed arylation of imidazole. A pressure tube was charged
with 4-bromobenzonitrile (0.182 g, 1.0 mmol, 67.0 equiv.), 1-
methyl-1H-imidazole (0.098 g, 1.2 mmol, 80.0 equiv.), K2CO3
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