Dibenzofuran and dibenzothiophene based palladium(ii)/NHC catalysts-synthesis and applications in C-C bond formation

Article abstract of DOI:10.1039/C8NJ02989J

In the quest for a new ligand system for Pd(ii)/NHCs, we developed new dibenzofuran and dibenzothiophene based palladium N-heterocyclic carbene catalysts D1-D6 in good yields. All the catalysts were characterized by multinuclear NMR spectroscopy and HRMS. The X-ray crystal structure of the representative dibenzothiophene based Pd(ii)/NHC D4 was determined. Among the precatalysts, D1 was shown to be highly effective in the Suzuki-Miyaura cross-coupling reaction of heterocyclic bromides with boronic acids. Besides, D1 affords diverse arylated benzoxazoles via direct C-H bond functionalization with substituted bromo derivatives.

Full text of DOI:10.1039/C8NJ02989J

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Dibenzofuran and Dibenzothiophene based Palladium(II)/NHC
Catalysts – Synthesis and Applications in C-C bond formation
Shanmugam Karthika and Thirumanavelan Gandhi*a
Received 00th January 20xx,
Accepted 00th January 20xx
In the quest of new ligand system for Pd(II)/NHCs, we developed new dibenzofuran and dibenzothiophene based
Palladium N-heterocyclic carbene catalysts D1-D6 in good yields. All the catalysts were characterized by multinuclear NMR
and HRMS. X-ray crystal structure of representative dibenzothiophene based Pd(II)/NHC D4 was determined. Among the
precatalysts, D1 has been shown to be highly effective in the Suzuki-Miyaura cross-coupling reaction of heterocyclic
bromides with boronic acids. Besides, D1 affords diverse arylated benzoxazoles via direct C-H bond functionalization with
substituted bromo derivatives.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Introduction
Dibenzofuran (DBF) and dibenzothiophene (DBTH) derivatives
are valuable targets for synthetic, medicinal and material
chemist owing to their splendid applications in biological,1
N
N
N
N
N
N
N
N
R
O
O
O
R1
S
O
Pt
O
Au
Pt
O
4
pharmaceuticals,2 polymers3 and functional materials.2d,
R2
R
Undeniably, considerable efforts have been devoted to
developing synthetic methodologies for functionalized
dibenzofurans and dibenzothiophenes and conjugated them
with various materials to realize the above-mentioned
applications.5 DBF and DBTH serve as the polyheterocyclic
framework to construct multidentate ligand whose
applications range from chemosensor,6 extractant of f-block
metal cations,7 high-triplet energy host materials,8 and organic
field-effect transistors9 and in organic electronics.10 With
suitable donor substituents on DBF and DBTH scaffold forms
highly stable transition metal complexes viz. zinc,11
N
O
N
N
O
O
N
Pt
O
N
O
N
Ir
O
N
N
chemistry.16a Among various
Fig.1 Representative examples of dibenzofuran and
dibenzothiophene based metal-NHC complexes.
ruthenium,12 magnesium,13 zirconium
&
hafnium14 and
rhodium15 complexes which find application as a catalyst in
lactide polymerization, transfer hydrogenation, ring-opening
polymerization, propylene polymerization and desulfurization
respectively. N-heterocylic carbenes (NHC) have garnered
sufficient accolade both in academic and industry as a versatile
ligand in view of its distinctive σ-donor ability with metal
ions.16 Besides, metal-NHC promotes facile oxidative-addition
of unreactive aryl halides and sterics around the metal center
favoring the reductive-elimination in cross-coupling
azolium salts, imidazolium-based metal-NHCs were prominent
due to its robustness, high thermal stability, and
affordability.16d, 17 Strassner and co-workers have reported the
cycloplatinated N-heterocyclic carbene complexes involving
the DBF and DBTH framework, which exhibited interesting
photophysical properties as phosphorescent emitters.18 Kang
and co-workers have shown the application of isomeric-
homoleptic cycloiridated N-heterocyclic carbene complexes
bearing dibenzofuran in deep-blue phosphorescent OLEDs.19
Along the same line, Venkatesan and co-workers have
reported the NHC-cycloaurated Au(III) complexes arising out of
dibenzofuranyl unit showing deep blue to blue-green
phosphorescence emission.20 The above examples clearly
show the rich photophysical properties originating from both
metal and NHC ligand (Fig. 1). However, to the best of our
knowledge, synthesis and catalytic studies on similar
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Metal/NHC complex (M = Pd, Rh, Ru etc) bearing DBF and
DBTH scaffold have not been reported so far. Palladium-NHC
complexes have been
Scheme 1 Synthesis of dibenzofuran and dibenzothiophene based imidazolium salts (C1-C6).
Scheme 2 Synthesis of dibenzofuran and dibenzothiophene based palladium NHC catalysts D1-D6. Yields are isolated.
profoundly studied in homogeneous catalysis especially in PEPPSI catalysts and its application in Suzuki-Miyaura coupling
cross-coupling reactions and proven to be an ideal alternative and direct C-H arylation reactions of benzoxazoles.
to Pd/phosphine combination.21 In carbon-carbon bond
chemistry, Suzuki-Miyaura coupling22 and direct C-H arylation
of azoles23 catalyzed by transition metal salts have significant
contribution in the field of fine chemical synthesis and
pharmaceuticals. Our group has recently reported the
Pd/NHC-PEPPSI type catalysts decorated with naphthalimide
and pyrene moiety in various catalytic organic
transformations.24 Thus, knowing the importance of DBF and
DBTH core units, herein, we report 4-substituted dibenzofuran
and dibenzothiophene based Pd(II)/N-heterocyclic carbene
Results and discussion
Synthesis of DBF/DBTH functionalized imidazolium salts C1-
C6
On implementing modest change in literature report,18e DBF
and DBTH functionalized imidazoles B1
& B2 were synthesized
by reacting corresponding bromides with imidazole with the
catalytic amount of Cu2O-dipivaloylmethane, Cs2CO3 as a base
in DMSO solvent at 130 °C for 16 h. Expectedly, the
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synthesized imidazoles were obtained in relatively good yields
as bench stable-yellow crystalline solids. Further, 4-
substituted DBF and DBTH based imidazolium salts C1 C6 were
-
obtained in 30-80% of yield, by reacting various ar-alkyl and
alkyl halides with imidazole in dioxane solvent at 110 °C for 16
h (Scheme 1). Apparently, the products are purified by
filtration followed by reprecipitation with diethyl ether and
hexane to furnish as off-white to grey color solids in high
purity. All these compounds
Fig. 2 (a) ORTEP representation of D4 in the solid state with thermal ellipsoids depicted at 35% probability level. Hydrogen
atoms are omitted for clarity (b) Crystal packing of D4 showing CH···π interaction and CH···Br interaction. Selected interatomic
distances (Å) and angles (deg) of D4 are as follows: Pd(1)-C(10), 1.957(4); Pd(1)-N(3), 2.105(3); Pd(1)-Br(1), 2.4181(9); Pd(1)-Br(2),
2.4216(9); N(1)-C(10), 1.337(4); N(2)-C(10),1.360(5); N(3)-C(11),1.333(5); N(3)-C(12), 1.328(5); Br(1)-Pd(1)-N(3), 91.12(8); Br(2)-
Pd(1)-N(3), 92.26(8); Br(2)-Pd(1)-C(10), 89.0(1); Br(1)-Pd(1)-C(10), 87.5(1); Br(1)-Pd-Br(2), 175.86(2); C10-Pd-N3, 177.7(1).
C1-C6 were characterized by multinuclear NMR and HRMS. The molecular structure of D4 was unambiguously confirmed
Comprehensibly, the peak resonates at δ 9.97-10.29 for single by single crystal X-ray analysis. Suitable crystals of D4 were
proton is congruous with -NCHN segment of imidazolium salts grown by slow evaporation of concentrated dichloromethane-
which in turn confirms its formation. The singlet peak diethyl ether solution at low temperature. The molecular
resonates at δ 5.62-6.18 agrees with -CH2 portion of benzyl structure ascertained by this analysis depicted in (Fig. 2a),
and naphthyl substituents of C1, C2, C4, C5 but this is split into shows the presence of triclinic crystal system (Table 1 and ESI,
a triplet for C3 and C6 and resonates at δ 4.36-4.43.
S4). The two trans anti oriented bromide ligands coordinate
the Pd(II) centre along with carbene and pyridine ligands in a
distorted-square planar geometry with shorter Pd-Ccarbene bond
Synthesis of Pd-NHC catalysts D1-D6
Straightforwardly, a series of PEPPSI (Pyridine Enhanced length of 1.957(4) Å. Yet the distance between Pd(1)-Br(1) and
Precatalyst Preparation Stabilization and Initiation) type DBF Pd(1)-Br(2) bond length is 2.4181(9) and 2.4216(9)
and DBTH appended Pd-NHC catalysts D1 D6 were synthesised respectively, which is much larger than the Pd(1)-N(3) bond
Å
-
by deprotonative metalation of imidazolium salts by reacting distance of 2.105(3) Å. The dihedral angle between carbene
with PdCl2 in the presence of K2CO3 as a base, KBr as an carbon of imidazole unit and the two trans bromide ligands as
additive, in pyridine solvent at 85 °C for 16 h yielding moderate in Br(2)-Pd(1)-C(10) and Br(1)-Pd(1)-C(10) are 89.0(1) and
to good yields (48-57%) as yellow crystalline solids (Scheme 2 87.5(1) Å respectively (Fig. 2b). But the angle observed in
and ESI, S3). Notably, the synthesized D1-D6 derivatives are Br(1)-Pd(1)-N(3) and Br(2)-Pd(1)-N(3) are 91.12(8) and 92.26(8)
freely soluble in dichloromethane and chloroform, yet having Å respectively.
poor solubility in polar-protic solvents such as methanol,
ethanol and non-polar solvents such as hexane and toluene.
Table 1 Crystallographic data for Pd-NHC D4
Prominently, the palladation of imidazolium salts is
corroborated by multinuclear NMR spectrum and HRMS. As
Complex
D4
CCDC no
1845489
C27 H21 Br2 N3 Pd S
685.75
1
evident by the disappearance of downfield H NMR signal for
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
the acidic -NCHN proton of imidazolium group and the
appearance of Pd-Ccarbene peak at δ 151-156 ppm in 13C NMR.
298K
0.71073 Å
triclinic
X-ray crystallographic studies
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Space group
P -1
were tested by choosing Suzuki-Miyaura coupling reaction.
Aryl pyridines are important building blocks and ubiquitous in
nature prevailing in biological, medicinal (altinicline,
Nevirapine, atazanavir) and polymers and functional materials.
The augmentation of effective methodologies providing easy
access to those heteroaryl derivatives is very appreciable in
synthetic organic chemistry.25 Thus, the coupling of 2-
a
8.903(5)
9.681(5)
16.055(5)
104.731(5)
95.807(5)
102.598(5)
1288.0(11)
2, 1.768
3.925
b
c
α
β
γ
Volume
bromopyridine with phenylboronic acid was taken as
a
Z, calculated density
Absorption coefficient
F (000)
standard substrate, K2CO3 as a base, Pd-NHC complex (3 mol%)
as the catalyst, in toluene-water at 95 °C for 16 h. The solvent
combination (toluene/water) is a common and effective
solvent mixture for Suzuki-Miyaura reaction.26 Among the
catalysts studied, D1 gave the coupled product 2-
phenylpyridine 3a with 76% of yield. Intriguingly, DBF and
DBTH units having naphthyl and butyl wingtip substituents
moderated the yield of 3a to 72% and 60% respectively in
correlation with benzyl surrogate. This is probably due to the
non-covalent interaction of benzyl unit with the substrate,
which is less favorable in case of naphthyl, where steric
supersedes the non-covalent interaction.
672
Limiting indices
-11 ≤ h ≤ 11
-12 ≤ k ≤ 12
-21 ≤ l ≤ 21
Reflections collected /unique
33029/6395
[R(int) = 0.0447]
Data/restraint/parameters
Goodness- of-fit on F2
6395/0/307
1.032
Final R indices [ I> 2 sigma(I)]
R1=0.0375
wR2= 0.0694
R indices (all data)
R1 = 0.0743
wR2 = 0.0694
The crystals are arranged in head to tail pattern of
dibenzofuran and pyridine ligand to effect CH···π interaction
with the length of 2.898 Å. Besides, the presence of CH···Br
interaction between dibenzofuran unit and twisted bromide
ligands of Pd center is observed with their typical length of
3.001 Å. Both these interactions stabilized D4 in its crystal
lattice.
Photophysical properties
Fig. 3 Normalized UV-Vis absorption spectra of D1-D6 in
dichloromethane at room temperature.
The electronic spectra of D1-D6 were recorded in
dichloromethane at room temperature in the wavelength
range of 200-800 nm are shown in Fig. 3. The complex nature
of the spectra is due to the various possible transitions.
Among these, the presence of intense absorption band at λmax
200-300 nm and 300-350 nm responsible for ligand centered
π–π* and n–π* transitions respectively. The weak metal-
ligand charge transfer (MLCT) band shows an absorbance
maximum in the range 350-400 nm.
Fig. 4 (a) Reactivities of catalysts in Suzuki-Miyaura coupling of
heteroaryl bromides (b) Effect of D1 mol%.
Notably, DBF based Pd-NHC complex D1 gave the maximum
yield as compared with DBTH based Pd-NHC D4. We also
compared our catalysts with other palladium catalysts such as
Catalytic properties
The catalytic applicability of synthesized DBF and DBTH based PdCl2, Pd(PPh3)Cl2 and Pd-PEPPSI-IPr, and observed the
N-heterocyclic carbene catalysts D1-D6 in C-C bond formation
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1Reaction conditions:
1
(0.31 mmol),
2
(0.47 mmol), K2CO3
following order of reactivity: PdCl2< Pd(PPh3)Cl2
PEPPSI-IPr (Fig. 4a).
< D1 ≈ Pd-
(0.63 mmol), D1 (5 mol%), toluene-water, 95 °C, 16 h. Isolated
yields.
These results prompted us to evaluate mol% of added DBF-
Pd-NHC complex D1 in Suzuki-Miyaura coupling. Amazingly
the loading of DBF-NHC to 5 mol% improves the yield of 3a to
85%. No affordable changes in 3a yield were observed in
further increase in mol% of D1 (Fig. 4b). Thus, the optimized
3-bromopyridine 3j
,
3-bromoquinoline 3k
,
6-bromo
quinoline 3l and 8-bromoquinoline 3m reacts smoothly with 4-
methoxyphenylboronic acid to afford the coupled products in
the range of 55-70% yields. Besides, benzofuro[3,2-b]pyridine
3n and dibenzo[b,d]furan 3o derivatives were isolated with
50% and 68% of yields respectively. The applicability of this
optimized condition was also verified with non-pyridine
substrate such as bromobenzene with 4-methoxyphenyl
boronic acid to afford 4-methoxy biphenyl 3p in 90% yield.
(Table 2 and ESI, S7). After successful accomplishment of
Suzuki-Miyaura cross-coupling reaction using our catalyst
inspired us to further explore its applicability in direct C-H
bond functionalization of benzoxazole. The suitable reaction
condition to carry out the reaction was benzoxazole (0.31
mmol), bromobenzene (0.47 mmol), LiOt-Bu (0.63 mmol) as a
base and D1 (5 mol%) as the catalyst, at 120 °C for 16 h.
Methyl group at ortho 6b, meta 6c, and para 6d arylated
derivatives were obtained at the high yield of 41, 55 and 61%
respectively. Highly electron-donating methoxy substituents in
para position 6e improve the yield of product to 64%.
Acceptingly electron- withdrawing substituent fluoro at para
position diminished the yield of 6f to 55%. Heterocyclic 6g
(48%) and polycylicaromatic
conditions for this reaction is
1 (0.31 mmol), 2 (0.47 mmol),
K2CO3 (0.63 mmol), D1 (5 mol%), toluene-water, 95°C, 16 h.
Having an optimum reaction protocol in hand, an investigation
of the substrate scope for the Suzuki-Miyaura cross-coupling
reaction was executed. Interestingly, boronic acids with
methyl substituent in meta and para positions gave high yields
of coupled products 3b and 3c in 81% and 86% respectively.
Compelling results were obtained when a highly electron-
donating group -OCH3 were substituted in 3d (86%) and 3e
(88%). Expectedly, electron-withdrawing substituents such as
chloro and acetyl in 3f and 3g dwindle the yield of product to
66 and 60% respectively. 4-Biphenyl 3h and 2-naphthylboronic
acid 3i particularly moderate the yield of coupling product to
71 and 69% respectively.
Table 2 Suzuki-Miyaura coupling of heteroaryl bromides with
arylboronic acids using D11
Table 3 Direct C-H arylation of benzoxazoles with aryl
HO
OH
B
D1, K2CO3
toluene-water
95 oC, 16 h
N
Br
N
R
R
1
2
3
OCH3
N
N
N
N
3c: 86%
3a: 85%
3d: 86%
3b: 81%
N
N
N
N
OCH3
Cl
O
3e: 88%
3f: 66%
3g: 60%
3h: 71%
O
OCH3
H3CO
N
N
N
N
3j: 63%
3k: 55%
3l: 70%
3i: 69%
bromides using D11
1Reaction conditions:
OCH3
O
4
(0.42 mmol), 5 (0.58 mmol), LiOt-Bu
N
N
O
(1.26 mmol), D1 (5 mol%), DMF, 120 °C, 16 h. Isolated yields.
H3CO
OCH3
OCH3
hydrocarbon 6h (52%) unit impressively afford direct C-H
functionalized products. Variation in the benzoxazole unit
such as 5-methylbenzo[d]oxazole unit yield 6i in a lower yield
3p: 90%
3n: 50%
3o: 44%
3m: 65%
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of 56% (Table 3 and ESI, S13). Based on previous reports,22c
Journal Name
a
These precatalysts were characterized by multinuclear NMR,
HRMS and single crystal X-ray crystallography. The presence
plausible mechanism is depicted in (scheme 3). Initially, an
active palladium species
aromatic bromides to give
A
undergoes oxidative addition with of CH···π and CH···Br interaction was observed and their
photophysical properties were studied. The synthesised
precatalysts were efficiently explored in Suzuki-Miyaura cross-
coupling reaction with electron-donating, electron-
withdrawing substitutents, isoquinolines, benzofuropyridines
and dibenzofuran derivatives. The direct C-H functinalization
of benzoxazoles using D1 were examined with various aryl
bromide including heterocyclic and polycyclic units at
affordable conditions. These studies revealed that such
precatalysts can be adequately applied in various cross-
coupling reactions.
B
and B’. Further, C’ is formed by
sequential deprotonation and palladation of
was then transformed to arylated product
4
6
with
B, which
via reductive
elimination
followed by transmetalation of
organopalladium intermediate . Eventually,
and regenerates Pd0 species
A
(C-H arylation). Likewise, reaction B’ with base
furnishes the
is obtained by
2
D
3
the reductive elimination of
(Suzuki-Miyaura coupling).
D
Conclusions
In summary, dibenzofuran and dibenzothiophene based Pd-
NHC catalysts D1 D6 were synthesized in appreciable yields.
-
Scheme 3 Proposed reaction mechanism of Suzuki-Miyaura coupling and direct C-H arylation of benzoxazole using D1.
Experimental
General methods and materials
ppm relative to deuterated chloroform (77.23 ppm) and DMSO
(39.52 ppm) with tetramethylsilane (TMS) as an internal
standard. Coupling constants (J) are reported in Hz; splitting
patterns are assigned s = singlet, d = doublet, t = triplet, q =
quartet, br = broad signal (ESI, S2)
Unless otherwise mentioned all the reactions were carried out
in screw capped reaction tubes, under an inert atmosphere of
nitrogen. The reagents and solvents were purchased from
commercial sources (Sigma-Aldrich, Alfa-Aesar, AVRA) and
used
General procedure for synthesis of dibenzofuran and
dibenzothiophene imidazole derivatives (B1-B2)
A 50 mL round bottom flask equipped with a stir bar was
charged with 4-bromodibenzo[b,d]furan (1000 mg, 4.08
mmol),
without further purification. 4-bromodibenzo[b,d]furan and 4-
bromodibenzo[b,d]thiophene was synthesised following
a
reported literature.18a Column chromatography purification
was performed by using silica gel (100-200 mesh) as a
stationary phase and chloroform or n-hexane-ethyl acetate
mobile phase. 1H NMR spectra were recorded on Bruker
spectrometer (400 MHz) and are reported in units ppm (parts
per million) relative to the signals for residual chloroform (7.26
ppm) and DMSO in (2.54 ppm) in the deuterated solvent. 13C
NMR spectra were recorded on Bruker spectrometer (100
MHz) and are reported
imidazole (555 mg, 8.16 mmol), Cs2CO3 (3316 mg, 10.20
mmol), Cu2O (58 mg, 0.40 mmol), 2,2,6,6-Tetramethyl-3,5-
heptanedione (Dipivaloylmethane) (150 mg, 0.81 mmol),
DMSO (15 mL) was added with vigorous stirring at room
temperature,
and then the reaction mixture was placed in a preheated oil
bath at 130 °C and stirred for 16 h. After the indicated time,
the
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reaction mixture was diluted with water (500 mL), extracted
ARTICLE
with ethyl acetate (200 mL x 2) and concentrated. The crude 3-benzyl-1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazol-3-ium
material was purified by column chromatography on silica gel bromide C4
with CHCl3-MeOH as eluent and used for further reactions.
Procedure for synthesis of dibenzofuran
dibenzothiophene imidazolium derivatives (C1-C6)
C4 was synthesised by a similar procedure to that used for C1
and but using benzyl bromide (1.28 mmol) to give off-white solid;
(117 mg with 76% yield); 1H NMR (400 MHz, DMSO-d6): δ
An oven-dried vial equipped with a stir bar was charged with 10.08 (s, 1H), 8.68 (d, J = 7.6 Hz, 1H), 8.55-8.53 (m, 1H), 8.43-
1-(dibenzo[b,d]furan-4-yl)-1H-imidazole B1 (100 mg, 0.42 8.42 (m, 1H), 8.21-8.20 (m, 1H), 8.16-8.13 (m, 1H), 7.91 (d, J =
mmol) and the corresponding benzyl bromide (219 mg, 1.28 7.2 Hz, 1H), 7.82 (t, J = 8.0 Hz, 1H), 7.65-7.56 (m, 4H), 7.52-7.50
mmol), were dissolved in dry Dioxane (3 mL) and then the (m, 3H), 5.62 (s, 2H); 13C NMR (100 MHz, DMSO-d6) δ 138.20,
reaction mixture was placed in a preheated oil bath at 110 °C 137.69, 135.02, 134.26, 129.57, 129.42, 128.99, 128.85,
and stirred for 18 h. After the indicated time, the precipitated 126.71, 126.18, 124.54, 124.02, 123.78, 123.39, 53.01;
solid was filtered and washed with Dioxane (3 mL) followed by HRMS(EI) calculated for C22H17N2S [M-Br]+ 341.1107, found
diethyl ether and n-hexane (10 mL each) and dried under 341.1105.
nitrogen.
1-(dibenzo[b,d]thiophen-4-yl-3(napthalen-1-ylmethyl)-1H-
3-benzyl-1-(dibenzo[b,d]furan-4-yl)-1H-imidazol-3-ium
imidazol-3-ium chloride C5
bromide C1
C5 was synthesised by a similar procedure to that used for C1
1
Off-white solid; (137 mg with 80% yield); H NMR (400 MHz, but using 1-chloromethyl naphthalene (1.28 mmol) to give
DMSO-d6): δ 10.20 (s, 1H), 8.51-8.50 (m, 1H), 8.41 (d, J = 7.6 grey solid; (105 mg with 62% yield); 1H NMR (400 MHz, DMSO-
Hz, 1H), 8.31 (d, J = 7.6 Hz, 1H), 8.18-8.17 (m, 1H), 7.97 (d, J = d6): δ 10.20 (s, 1H), 8.66 (d, J = 7.6 Hz, 1H), 8.54-8.52 (m, 1H),
8.0 Hz, 1H), 7.82 (d, J = 8.4 Hz, 1H), 7.69-7.59 (m, 4H), 7.55- 8.42-8.41 (m, 1H), 8.29 (d, J = 8.4 Hz, 1H), 8.18-8.17 (m, 1H),
7.44 (m, 4H), 5.65 (s, 2H); 13C NMR (100 MHz, DMSO-d6) δ 8.14-8.12 (m, 1H), 8.07 (d, J = 8.0 Hz, 2H), 7.89 (d, J = 7.6 Hz,
155.65, 146.83, 136.95, 134.42, 129.03, 128.91, 128.57, 1H), 7.80 (t, J = 8.0 Hz, 1H), 7.72-7.61 (m, 6H), 6.16 (s, 2H); 13C
126.20, 124.17, 124.07, 123.22, 123.05, 122.76, 121.88, NMR (100 MHz, DMSO-d6) δ 138.16, 138.00, 135.01, 134.34,
119.69, 112.11, 52.48;
133.95, 130.96, 130.30, 129.42, 128.83, 128.45, 127.82,
HRMS(EI) for calculated for C22H17N2O [M-Br]+ 325.1335, 126.69, 126.16, 124.60, 124.49, 124.27, 123.75, 123.39, 50.97;
found 325.1333.
HRMS (EI) for C26H19N2S [M-Br]+ 391.1263, found 391.1261.
1-(dibenzo[b,d]furan-4-yl)-3-(naphthalen-1-ylmethyl)-1H-
3-butyl-1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazol-3-ium
imidazol-3-iumchloride C2
bromide C6
C2 was synthesised by a similar procedure to that used for C1 C6 was synthesised by a similar procedure to that used for C1
but using 1-chloromethyl naphthalene (1.28 mmol) to give but using 1-bromo butane (4.2 mmol) to give grey solid; (46
1
grey solid; (131 mg with 75% yield); 1H NMR (400 MHz, DMSO- mg with 30% yield); H NMR (400 MHz, DMSO-d6) δ 9.97 (s,
d6): δ 10.29 (s, 1H), 8.50-8.49 (m, 1H), 8.39 (d, J = 7.6 Hz, 1H), 1H), 8.66 (dd, J = 8.0, 0.8 Hz, 1H), 8.55-8.53 (m, 1H), 8.42-8.41
8.34-8.28 (m, 2H), 8.14-8.13 (m, 1H), 8.06 (d, J = 8.4 Hz, 2H), (m, 1H), 8.26-8.25 (m, 1H), 8.16-8.14 (m, 1H), 7.91 (d, J = 7.6
7.96 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.4 Hz, 1H), 7.73-7.60 (m, Hz, 1H), 7.81 (t, J = 7.6 Hz, 1H), 7.64-7.62 (m, 2H), 4.38 (t, J =
6H), 7.52 (t, J = 7.6 Hz, 1H), 6.18 (s, 2H); 13C NMR (100 MHz, 7.2 Hz, 2H), 1.97-1.90 (m, 2H), 1.42-1.33 (m, 2H), 0.96 (t, J =
DMSO-d6) δ 156.12, 147.35, 137.76, 133.94, 130.98, 130.26, 7.2 Hz, 3H); 13C NMR (100 MHz, DMSO-d6) δ 138.23, 138.08,
129.40, 128.45, 127.77, 126.95, 126.18, 124.65, 124.53, 137.49, 135.02, 130.34, 128.83, 126.72, 126.15, 124.53,
123.81, 123.51, 123.26, 122.37, 120.18, 112.55, 50.90; HRMS 123.79, 123.39, 49.74, 31.63, 19.34, 13.83; HRMS(EI)
(EI) calculated for C26H19N2O [M-Br]+
375.1490.
375.1492, found calculated for C19H19N2S [M-Br]+ 307.1263, found 307.1260.
3-butyl-1-(dibenzo[b,d]furan-4-yl)-1H-imidazol-3-ium
bromide C3
General procedure for synthesis of dibenzofuran and
C3 was synthesised by a similar procedure to that used for C1 dibenzothiophene based palladium-NHC catalysts (D1-D6)
but using 1-bromo butane (4.27 mmol) to give Grey solid; (60 An oven-dried 20 mL reaction tube equipped with a stir bar
1
mg with 38% yield); H NMR (400 MHz, DMSO-d6): δ 10.09 (s, was charged with 3-benzyl-1-(dibenzo[b,d]furan-4-yl)-1H-
1H), 8.52-8.51 (m, 1H), 8.39 (d, J = 7.6 Hz, 1H), 8.30 (d, J = 7.6 imidazol-3-ium bromide C1 (200 mg, 0.49 mmol) potassium
Hz, 1H), ), 8.24-8.23 (m, 1H), 7.97 (d, J = 8.0 Hz, 1H), 7.82 (d, J = carbonate (204 mg, 1.48 mmol), potassium bromide (589 mg,
8.4 Hz, 1H), 7.68-7.62 (m, 2H), 7.51 (t, J = 7.6 Hz, 1H), 4.41 (t, J 4.95 mmol), palladium(II)chloride (115 mg, 0.54 mmol)
= 7.2 Hz, 2H), 1.98-1.90 (m, 2H), 1.43-1.33 (m, 2H), 0.96 (t, J = followed by pyridine 5mL. The reaction mixture was stirred at
7.2 Hz, 3H); 13C NMR (100 MHz, DMSO-d6) δ 156.14, 147.27, 85 °C for 16 h. After the indicated time, the solution was
137.23, 129.31, 126.68, 124.56, 123.65, 123.23, 120.18, cooled to room temperature and 25 mL of dichloromethane
112.62, 49.70, 31.71, 19.36, 13.84; HRMS(EI) calculated for was added and filtered through a short pad of celite and the
C19H19N2O [M-Br]+ 291.1492, found 291.1490.
filtrate was concentrated. The crude material was purified by
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column chromatography on silica gel with CHCl3 as eluent to ylmethyl)-1H-imidazol-3-ium chloride C5 (0.46 mmol) to give
1
yellow solid (185 mg, 53 % yield); H NMR (400 MHz, CDCl3) δ
yield D1 as yellow solid.
1
(190 mg, 57% yield); H NMR (400 MHz, CDCl3) δ 8.66 (d, J = 8.80-8.78 (m, 2H), 8.56 (d, J = 7.6 Hz, 1H), 8.34 (d, J = 8.8 Hz,
7.6 Hz, 1H), 8.78 (d, J = 5.6 Hz, 2H), 8.01-7.92 (m, 2H), 7.62- 1H), 8.22 (d, J = 8.0 Hz, 1H), 8.14-8.12 (m, 1H), 7.88 (t, J = 9.2
7.46 (m, 6H),7.42-7.29 (m, 5H), 7.16 (d, J = 6.4 Hz, 2H), 6.87 (d, Hz, 2H), 7.75-7.47 (m, 7H), 7.43-7.38 (m, 2H), 7.21 (d, J = 2.0
J = 2.0 Hz, 1H), 5.91 (s, 2H); 13C NMR (100 MHz, CDCl3) δ Hz, 1H), 7.17-7.14 (m, 2H), 6.59 (d, J = 2.4 Hz, 1H), 6.29 (s, 2H);
156.41, 152.56, 150.87, 149.33, 137.73, 134.83, 129.52, 13C NMR (100 MHz, CDCl3) δ 152.57, 139.36, 137.69, 136.37,
129.07, 128.74, 127.81, 126.38, 125.85, 124.45, 123.35, 133.97, 131.84, 130.05, 129.50, 128.70, 127.48, 126.87,
122.97, 121.49, 121.19, 120.95, 112.23,55.60; HRMS(EI) 126.44, 125.49, 125.07, 124.76, 124.40, 122.91, 122.57,
calculated for C27H21Br2N3OPd [M]+ 666.9086, found 666.9085. 121.97, 121.76, 53.81; HRMS(EI) calculated for C31H23Br2N3PdS
[M]+ 732.9014, found 732.9012.
Synthesis of D2
D2 was prepared by a similar procedure to that used for D1
,
Synthesis of D6
but using 1-(dibenzo[b,d]furan-4-yl)-3-(naphthalen-1- D6 was prepared by a similar procedure to that used for D1
,
ylmethyl)-1H-imidazol-3-iumchloride C2 (0.48 mmol) to give but using 3-butyl-1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazol-3-
yellow solid (180 mg, 51 % yield); 1H NMR (400 MHz, CDCl3) δ ium bromide C6 (0.51 mmol) to give yellow solid (169 mg, 50
1
8.88 (d, J = 8.0 Hz, 1H), 8.82 (d, J = 5.2 Hz, 2H), 8.35 (d, J = 8.4 % yield); H NMR (400 MHz, CDCl3) δ 8.73-8.71 (m, 2H), 8.49
Hz, 1H), 8.00 (d, J = 7.6 Hz, 1H), 7.93-7.85 (m, 3H), 7.68-7.47 (d, J = 7.6 Hz, 1H), 8.20 (d, J = 8.0 Hz, 1H), 8.13-8.11 (m, 1H),
(m, 8H), 7.41-7.28 (m, 3H), 7.17 (d, J = 6.8 Hz, 1H),6.61 (d, J = 7.75-7.33 (m, 1H), 7.65 (t, J = 7.6 Hz, 1H), 7.60-7.55 (m, 1H),
2.0 Hz, 1H), 6.31 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 156.39, 7.42-7.36 (m, 3H), 7.15-7.12 (m, 3H), 4.62 (t, J = 7.6 Hz, 2H),
152.60, 150.35, 149.28, 137.77, 133.96, 131.84, 130.03, 2.16-2.08 (m, 2H), 1.56-1.47 (m, 2H), 1.02 (t, J = 7.6 Hz, 3H); 13C
129.50, 128.71, 127.80, 127.47, 126.44, 125.50, 124.49, NMR (100 MHz, CDCl3) δ 152.52, 150.58, 139.35, 137.63,
123.91, 123.34, 122.98, 121.19, 120.94, 112.22, 53.59; 136.36, 135.41, 134.33, 127.32, 126.88, 125.02, 124.75,
HRMS(EI) calculated for C31H23Br2N3OPd [M]+ 716.9243, found 124.35, 122.89, 122.22, 121.97, 51.54, 32.04, 20.10, 13.85;
716.9244.
HRMS(EI) calculated for C24H23Br2N3PdS [M]+ 648.9014, found
648.9016.
Synthesis of D3
D3 was prepared by a similar procedure to that used for D1
,
General procedure for Suzuki-Miyaura coupling using D1
but using 3-butyl-1-(dibenzo[b,d]furan-4-yl)-1H-imidazol-3-ium An oven-dried vial equipped with a stir bar was charged with
bromide C3 (0.54 mmol) to give yellow solid (165 mg, 48 % 2-bromo pyridine (0.31 mmol) and the corresponding phenyl
yield); 1H NMR (400 MHz, CDCl3) δ 8.79-8.76 (m, 3H), 7.99-7.91 boronic acid (0.47 mmol), potassium carbonate (0.63 mmol),
(m, 2H), 7.60 (t, J = 7.6 Hz, 1H), 7.53-7.48 (m, 3H), 7.40 (t, J = D1 (5 mol%), followed by toluene-water (4:1 mL). Then the
7.2 Hz, 1H), 7.30 (t, J = 7.6 Hz, 1H), 7.16-7.12 (m, 3H), 4.63 (t, J reaction mixture was placed in a preheated oil bath at 95 °C
= 7.6 Hz, 2H), 2.16-2.08 (m, 2H), 1.57-1.50 (m, 2H), 1.02 (t, J = and stirred for 16 h. After the indicated time, the reaction
7.2 Hz, 3H), 13C NMR (100 MHz, CDCl3) δ 156.39, 152.53, mixture was diluted with water (5 mL), extracted with ethyl
149.81, 149.35, 137.71, 127.78, 125.92, 124.43, 123.95, acetate (5 mL x 2) and concentrated. The crude material was
123.32, 122.92, 121.85, 121.12, 120.95, 112.21, 51.61, 32.12, purified by column chromatography on silica gel with n-hexane
20.11, 13.88. HRMS(EI) calculated for C24H23Br2N3OPd [M]+ - ethyl acetate as eluent, to yield the title compounds and their
632.9243, found 632.9241.
spectra was authenticated with reported literatures (ESI, S18).
Synthesis of D4
General procedure direct C-H arylation of benzoxazoles with
D1
D4 was prepared by a similar procedure to that used for D1
,
but using 3-benzyl-1-(dibenzo[b,d]thiophen-4-yl)-1H-imidazol- An oven-dried vial equipped with a stir bar was charged with
3- ium bromide C4 (0.47 mmol) to give yellow solid (188 mg, benzoxazole (0.42 mmol) and the corresponding
57 % yield); 1H NMR (400 MHz, CDCl3) δ 8.75-8.73 (m, 2H), 8.53 bromobenzene (0.58 mmol), lithium tertiary butoxide (1.26
(d, J = 7.6 Hz, 1H), 8.22 (d, J = 8.0 Hz, 1H), 8.15-8.12 (m, 1H), mmol), D1 (5 mol%), followed by DMF (0.5 mL). Then the
7.79-7.74 (m, 1H), 7.67 (t, J = 7.6 Hz, 1H), 7.59-7.56 (m, 3H), reaction mixture was placed in a preheated oil bath at 120 °C
7.43-7.35 (m, 6H), 7.16-7.13 (m, 2H), 6.86 (d, J = 2.0 Hz, 1H), and stirred for 16 h. After the indicated time, the solvent was
5.90 (s, 2H); 13C NMR (100 MHz, CDCl3) δ 152.54, 151.58, removed and purification by flash column chromatography on
139.35, 137.67, 136.33, 135.41, 134.77, 134.18, 129.48, silica gel with n-hexane - ethyl acetate as eluent, to yield the
129.09, 128.74, 127.35, 126.87, 125.07, 124.78, 124.38, title compound and their spectra was authenticated with
123.17, 122.91, 122.19, 121.88, 55.48, HRMS(EI) calculated for reported literatures (ESI, S18).
C27H21Br2N3PdS [M]+ 682.8858, found 682.8856.
Acknowledgements
Synthesis of D5
D5 was prepared by a similar procedure to that used for D1
,
This work is supported by the DST-SERB (EMR/2016/005461),
and Seed Grant, VIT University, Vellore. S.K. acknowledges
but using 1-(dibenzo[b,d]thiophen-4-yl-3(napthalen-1-
8 | J. Name., 2012, 00, 1-3
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