Sterically hindered selenoether ligands: Palladium(ii) complexes as catalytic activators for Suzuki-Miyaura coupling

Article abstract of DOI:10.1039/c4ra06313a

2-Hydroxy/(benzyloxy)-3,5-di-tert-butyl benzaldehyde reacts with PhSeCH₂CH₂NH₂resulting in a sterically hindered selenoether ligand (Schiff base) [2-HO-3,5-(C(CH₃)₃)₂-C₆H₂

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Sterically hindered selenoether ligands: palladium(II) complexes as
catalytic activators for SuzukiꢀMiyaura coupling
Umesh Kumar, Pooja Dubey, Ved Vati Singh, Om Prakash and Ajai K. Singh*
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
2ꢀHydroxy/(benzyloxy)ꢀ3,5ꢀditertbutyl benzaldehyde reacts with PhSeCH2CH2NH2 resulting in sterically
hindered selenoether ligand (Schiff base) [2−HO−3,5−(C(CH3)3)2−C6H2−C=N−(CH2)2SePh] (L1)/
[2−PhCH2O−3,5−(C(CH3)3)2−C6H2−CH2−NH−(CH2)2SePh] (L2). The reactions of L1 and L2 with
Na2PdCl4 in methanol and acetoneꢀwater mixture at room temperature have resulted in complexes, [PdCl
10 (L1ꢀH)] (1) and [PdCl2(L2)] (2)], respectively. Both the complexes and their ligands have been
characterized with 1H, 13C{1H} and 77Se{1H} NMR spectroscopy. The molecular structures of complexes
1 and 2 have been determined with single crystal Xꢀray diffraction. The Pd−Se bond lengths in 1 and 2
are 2.370(1) and 2.366(1) Å, respectively. The geometry around palladium in both the complexes is
nearly square planar. Complexes 1 and 2 (0.1 mol % Pd) have been found efficient as catalyst for Suzukiꢀ
15 Miyaura C−C coupling reactions in the presence of K2CO3 in ethanol. The catalysis in water with
complex 1 in the presence of K2CO3 was found feasible but with low conversion (up to 40%). The
efficiency of 1 in carrying out the coupling is marginally better than that of 2.
reactions. However no report on sterically hindered
organoselenium ligands is in our knowledge inspite of several
reports on activation of SuzukiꢀMiyaura coupling with palladium
complexes and palladacycles10 of several organochalcogen donors
50 including Se ones,4a,cꢀe,11 and chalcogenated carbenes,12 which
have high efficiency and can be modified with ease.
Introduction
The strong electronꢀdonor ability of selenium has led to the
20 synthesis of many transition metal complexes of organoselenium
ligands, which have been found promising as catalysts for various
organic transformations.1 This has made organoselenium ligands
currently important for designing catalysts for a variety of
organic reactions. A wide range of such ligands is known e.g.
25 selenocarbonyl,2 pincer type,3 and Schiff bases4 promising for
catalyst designing. One notable feature of transition metal
complexes of organoselenium ligands is that they are much less
insensitive to air and moisture in comparison to those which have
traditional phosphorous donors. At present palladium complexes
30 of organoselenium ligands in terms of catalytic activity are
considered not only rivals of their respective phosphorus and
sulphur analogues but have been found in many cases to
outperform them.3a
The steric properties of ligands significantly influence the rate
35 and selectivity of the reactions catalyzed by their transition metal
complexes. The steric bulk of a ligand can increase the stability
of the catalyst in its coordinatively unsaturated intermediate form
or accelerate some steps, such as reductive elimination in crossꢀ
coupling reactions which are sterically sensitive.5 Thus steric
40 bulk of a ligand may play a role in tailoring efficiency of its
We report herein the synthesis and structural characterization
of
sterically
hindered
selenoether
ligands,
[2−HO−3,5−(C(CH3)3)2−C6H2−C=N−(CH2)2SePh] (L1) and
55 [2−PhCH2O−3,5−(C(CH3)3)2−C6H2−CH2−NH−(CH2)2SePh] (L2)
and their palladium(II) complexes of the type , [PdCl (L1ꢀH)] (1)
and [PdCl2(L2)] (2)], respectively. Two complexes 1 and 2 have
been found suitable activators for Suzuki and Miyaura
C(sp2)−C(sp2) coupling reactions, which are powerful synthetic
60 tools in organic synthesis,13,14 along with other transition metalꢀ
catalyzed carbonꢀcarbon bond forming reactions such as Heck,
Sonogashira, Hiyama, Stille and Kumada coupling reactions.
Suzuki and Miyaura coupling has made a significantly higher
impact than others in the laboratory and the chemical industries
65 due to three key reasons: (i) The reaction is feasible with a wide
range of substrates and many functional groups are tolerated due
to mild reaction conditions. This is very helpful in the total
synthesis of complex molecules including drugs. (ii)
Phenylboronic acid, starting material is readily available, stable
70 and sustainable.15 (iii) The product biaryl is a very important core
component of various biologically and pharmaceutically
important compounds (viz. antiꢀhypertensive, antiꢀcancer, antiꢀ
biotic, antiꢀinflammatory, and antifungal) and in nonlinear optical
materials.16
complex as
a catalyst for Suzuki–Miyaura crossꢀcoupling
reactions.6 Palladium complexes derived from electronꢀrich and
sterically demanding ligands, of the type monophosphanes,7
Nꢀheterocyclic carbenes,8 and C2ꢀsymmetric bisꢀhydrazone,9 have
45 been found effective catalyst for Suzuki–Miyaura crossꢀcoupling
75
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Experimental Section
48.7 (NHCH2), 48.8(CH2NH), 75.3 (OCH2), 123.2, 125.2, 126.4,
60 126.6, 127.3, 128.3, 128.8, 129.8, 132.5, 132.8, 138.0, 141.8,
145.8 and 154.1 (Ar–C). (77Se{1H} NMR, CDCl3): δ(ppm) =
267.8.
Materials and methods: Diphenyl diselenide, 2ꢀhydroxyꢀ3,5ꢀ
ditertꢀbutyl benzaldehyde, 2ꢀchloroethyl amine, sodium
tetrachloropalladate, phenylboronic acid, potassium carbonate,
and aryl bromides were procured from SigmaꢀAldrich (USA).
Reagents (commercially available from local sources) were used
as received without further purification. PhSe(CH2)2NH2 and
2ꢀ(benzyloxy)ꢀ3,5ꢀdiꢀtertꢀbutylbenzaldehyde were prepared by a
following the methods reported in the literature.17,18 The progress
5
Synthesis of complex 1: The Na2PdCl4 (0.147 g, 0.5 mmol) was
65 dissolved in 20 mL of methanol. The homogeneous methanolic
solution of ligand L1 (0.208 g, 0.5 mmol dissolved in 10 mL of
methanol) was added with stirring. The mixture was further
stirred for 6 h. The orange red coloured precipitate was obtained
which was filtered and dried under vacuo. The single crystals of 1
70 were grown from CH3OH−CH3CN mixture (1:1) by slow
evaporation of its solution for one week. Yield: 88% (0.245 g,
0.44 mmol). Anal. Calcd for C23H30ClNOPdSe (Mw: 557.29): C,
49.57; H, 5.43; N, 2.51%. Found: C, 49.60; H, 5.24; N, 2.64.1H
NMR (DMSOꢀd6): δ (ppm) = 1.23 (s, 9 H, C(CH3)3),1.37 (s, 9 H,
75 C(CH3)3), 3.10–3.20 (m, 2 H, CH2), 3.72 (m, 1 H, CH2), 4.46–
4.50 (m, 1 H, CH2), 7.17–8.18 (broad m, 5 H, C6H5), 8.09 (s, 1 H,
N=CH), 7.35 (s, 1 H, C5H2), 8.19 (s, 1 H,C5H2). 13C{1H}
(CDCl3): δ(ppm) = 29.9 and 31.7 (C(CH3)3), 32.9 (CH2Se), 34.0
and 35.8 (C(CH3)3), 65.7 (N=CH2), 119.3, 125.6,
80 128.7,129.9,130.4, 130.5, 133.5, 136.1, 138.8, 161.3 (Ar–C),
162.4 (N=CH). (77Se{1H} NMR, CDCl3): δ(ppm) = 432.1.
10 of every coupling reaction was monitored with NMR
spectroscopy. The products of Suzuki reactions were
authenticated by matching their spectroscopic data with the
reported literature values. 1H, 13C{1H} and 77Se{1H} NMR
spectra were recorded on a Bruker Spectrospin DPX 300 NMR
15 spectrometer at 300.13, 75.47 and 57.24 MHz respectively. The
chemical shifts are reported in ppm relative to internal standard
(tetramethylsilane in case of H, and 13C{1H} NMR and Me2Se
1
for 77Se{1H} NMR). Elemental analyses were carried out with a
Perkin–Elmer 2400 Series II C, H, N analyzer.
20 Xꢀray diffraction data of crystals of 1 were collected on a Bruker
AXS SMARTꢀAPEX diffractometer with a CCD area detector.19
Similar data of 2 were collected on an Oxford Xcalibur S
diffractometer with Sapphireꢀ3 CCD detector.20 Mo Kα
radiations were used in both the cases. Both Crys Alis Pro
25 software suite21 and SADABS22 software were used as per
requirement. The structures were refined using the SHELXꢀ97
program package and SHELXL97 (within the Win GX program
package).23ꢀ26 Nonꢀhydrogen atoms were refined anisotropically.
The molecular structures were created with the Diamond
30 program.27 Crystallographic data are given in Table 1.
Synthesis of 2: The Na2PdCl4 (0.147 g, 0.5 mmol) was dissolved
in 5.0 mL of water. A solution of ligand L2 (0.254 g, 0.5 mmol)
85 made in 10 mL of acetone was added to homogenized aqueous
solution of Na2PdCl4 drop wise with vigorous stirring. The
mixture was further stirred for
2 h resulting orangeꢀred
precipitate. It was poured into cold water (40 mL) and extracted
with chloroform (4 x 30 mL). The combined extract was dried
90 over anhydrous sodium sulphate. Its solvent was reduced to 5 mL
and complex was precipitated using nꢀhexane. The precipitate
was filtered and dried under vacuo. The single crystals of 2, as
2ꢁCH3CN were grown from CH3OH−CH3CN mixture (1:1) by
slow evaporation of its solution for one week. Yield: 84% (0.306
95 g, 0.42mmol). Anal. Calcd for C30H38Cl2NOPdSeꢁCH3CN (Mw:
725.93): C, 52.94; H, 5.69; N, 3.86%. Found: C, 52.82; H, 5.67;
N, 3.78.1H NMR (CDCl3): δ (ppm) = 1.35 (s, 9 H, C(CH3)3),1.39
(s, 9 H, C(CH3)3), 2.63–2.65 (m, 2 H, CH2), 3.55–3.58 (broad m,
2 H, CH2), 4.19–4.27 (m, 1 H, CH2), 4.69–4.73 (m, 1 H, CH2),
100 4.80–4.90 (m, 2 H, OCH2), 7.20–7.87 (broad m, 12 H, Ar–H).
13C{1H} (CDCl3): δ (ppm) = 29.3 and 29.7 (C(CH3)3), 29.6 and
33.4 (C(CH3)3), 33.0 (CH2Se), 33.5 (NHCH2), 49.0
(NH(CH2)Ar), 51.0 (OCH2Ph), 123.4, 124.5, 125.4,125.6,125.9,
126.6, 126.8, 128.1, 128.2, 131.2, 135.1, 140.0, 145.9, 152.9 (Ar–
105 C). (77Se{1H} NMR, CDCl3): δ(ppm) = 489.3.
Synthesis of L1: The selenated amine C6H5Se−(CH2)2−NH2
(0.412 g, 2.06 mmol) and 2ꢀhydroxyꢀ3,5ꢀditertꢀbutyl
benzaldehyde (0.469 g, 2.00 mmol) were reacted in absolute
35 ethanol (20 mL) at room temperature for 12 h. The volatiles from
resulting reaction mixture were removed using rotary evaporator
which resulted ligand L1 as yellow viscous liquid in 93% yield
1
(0.776 g, 1.86 mmol). H NMR (CDCl3): δ (ppm) = 1.36 (s, 9 H,
C(CH3)3), 1.51 (s, 9 H, C(CH3)3), 3.24 (t, 2 H, SeCH2), 3.91 (t, 2
40 H, NCH2), 7.12–7.58 ( 7 H, Ar–H), 8.35(S, 1 H, CH=N), 13.55
(broads, 1 H, OH). 13C{1H} (CDCl3): δ(ppm) = 28.3 (SeCH2),
29.5 and 31.5 (C(CH3)3), 34.1 and 35.0 (C(CH3)3), 59.6 (NCH2),
117.7, 126.0, 127.1, 129.2, 129.6, 132.9, 136.7, 140.1, 158.0 (Ar–
C) 166.9 (N=CH). (77Se {1H} NMR, CDCl3): δ(ppm) = δ 281.1.
45
Synthesis of L2: The selenated amine C6H5Se−(CH2)2−NH2
(0.412 g, 2.06 mmol) and 2ꢀ(benzyloxy)ꢀ3,5ꢀdiꢀtertꢀ
butylbenzaldehyde (0.650 g, 2.00 mmol) were reacted in absolute
ethanol (30 mL) at room temperature for 12 h. The volatiles were
50 removed using rotary evaporator which resulted in yellow liquid.
The yellow liquid formed was reacted with NaBH4 (0.080 g, 2.11
mmol) in ethanol to obtain ligand L2 as colourless viscous liquid
in 91% yield (0.925 g, 1.82 mmol). 1H NMR (CDCl3): δ (ppm) =
1.32 (s, 9 H, C(CH3)3), 1.43 (s, 9 H, C(CH3)3), 1.74 (broad s, 1
55 H, NH), 2.86 (t, JHH = 7.05 Hz, 2 H, SeCH2), 2.97 (t, JHH = 6.45
Hz, 2 H, NCH2CH2), 3.82 (s, 2 H, CH2N), 5.00 (s, 2 H, OCH2),
7.16–7.49 (broad m, 12 H, Ar–H). 13C{1H} (CDCl3): δ(ppm) =
28.3 (CH2Se), 31.2 and 31.5 (C(CH3)3), 34.4 and 35.3 (C(CH3)3),
Procedure for Suzuki reaction of aryl bromides with
phenylboronic acid: An oven dried flask was charged with aryl
bromide (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0
110 mmol) and ethanol (4.0 mL). The flask was placed on an oil bath
at 80 ºC under aerobic condition and the reaction mixture stirred
until maximum conversion of aryl bromide to coupled product
occurred, as revealed with NMR spectroscopy. The mixture was
extracted with diethyl ether (100 mL). The extract was washed
115 with water (100 mL) and dried over anhydrous Na2SO4.
2
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Results and discussion
Synthesis: The synthetic details of ligands L1 and L2, and their
corresponding palladium metal complexes and are
1
2
summarized in Scheme 1. The L1, L2, 1 and 2ꢁCH3CN are stable
under ambient conditions. The complexes can be stored for six
month without noticeable decomposition. The ligands and their
complexes 1 and 2 have been characterized by their 1H and
13C{1H} and 77Se{1H}NMR spectra (See ESI for the spectra).
These data are consistent with the structures depicted for them in
5
10 Scheme 1.
Crystal structures: Molecular structures of 1 and 2 were
determined with single crystal Xꢀray diffraction. The crystal and
structure refinement data are given in Table 1. The Fig. 1 depicts
15 molecular structure of 1 (Ellipsoid at 30% probability level;
hydrogen atoms have been omitted for clarity). The palladium in
1 is surrounded by O1, N1, Se1 and Cl1 resulting in almost
square planner geometry as shown in Fig. 1. The ligand in
complex 1, coordinates with Pd in a mono anionic tridentate (Se,
20 N, Oꢀ) mode forming a six and five membered chelate ring. The
six membered ring is formed via Oꢀ and N whereas five
membered via Se and N. The Pd–N bond length 1.996(4) Å in 1
is comparable to the reported values of 1.985(4) Å for
60
Scheme 1 Synthesis of ligands L1 and L2 and their Pd(II) complexes.
Table 1 Crystallographic data and structure refinement summary for 1
and 2.
1
2·CH3CN
Formula
Formula weight
T/K
C23H30NOPdClSe C32H41N2OPdCl2Se
557.29
725.93
298(2)
298(2)
[PdCl{2−O−3−CH(CH2CH3)2−C6H3−C=N−(CH2)2SeMe}]
(I)
λ/Å
0.71073
Orthorhombic
Pbca
10.9228(4)
10.0631(4)
43.0466(17)
90.00
0.71073
Monoclinic
P21/c
12.3315(5)
10.4554(4)
26.1643(10)
90.00
25 and 2.003(7)Å for [PdCl{2−O−C6H4C(CH3)=N(CH2)2SePh}]
(II).4a,4b However, it is slightly shorter than those reported for
[PdCl{2−O−C6H4C(C6H5)=N(CH2)2SePh}] (III) 2.010(4) Å and
[PdCl{2−O−C10H6C(CH3)=N(CH2)2SePh}] (IV) 2.010(3) Å.4c,4d
The Pd–O distance 2.001(3) Å in 1 is comparable with values
30 reported for (I) 2.017(4) and (III) 1.993(3) Å and longer than
those of (II) 1.977(6) and (IV) 1.973(2)Å. Similarly Pd–Se bond
distance 2.370(1) Å in 1 is comparable with values for (I)
2.365(1) and (II) 2.367(1) Å and longer than those of (III)
2.358(1) and (IV) 2.360(1) Å. The Pd–Cl distance 2.302(1) Å in
35 1 is comparable with the value reported for (II) 2.305(2) Å and
little shorter than those of (I) 2.323(2), (III) 2.315(2) and (IV)
2.316(1) Å. The molecular structure of 2 is shown in Fig. 2
(Ellipsoid at 30% probability level; hydrogen atoms have been
omitted for clarity). The palladium in 2 has nearly square planner
40 geometry constituted by N1, Se1, Cl1 and Cl2. The bond
distances: Pd–Se 2.366(1) and Pd–Cl1 2.306(1) Å in 2 are
comparable with the values mentioned above, 2.370(1) Å and
2.302(1) Å in case of 1, respectively. However, the Pd–N
distance 1.996(4) Å in 1 is shorter than 2.066(3) Å found in case
45 of 2⋅CH3CN. Significant nonꢀcovalent interactions observed in 1
and 2 are listed in Table 2. In the crystal of complex 1
intermolecular C−H⋅⋅⋅Cl nonꢀcovalent interactions exist as shown
in Fig. 3. The Cl1 atom acts as a bifurcated hydrogen bond
acceptor with H2 (SePh) and H8A (methylene group of adjacent
50 molecule; C8). In crystal of 2⋅CH3CN C−H⋅⋅⋅N hydrogen bonding
interactions are present in addition to intermolecular C−H⋅⋅⋅Cl
nonꢀcovalent interactions as shown in Fig. 4. In this crystal, the
Cl2 atom acts as a bifurcated hydrogen bond acceptor with H8B
(methylene group; C8) and H20 (CH2Ph) of another adjacent
55 molecule. The Cl1 atom is hydrogen bonded with H31B
(CH3CN) of adjacent molecule. In addition, the N2 atom of
CH3CN acts as hydrogen bond acceptor with H16A (C16 of
CH2Ph group) of adjacent molecule.
Crystal system
Space group
a/Å
b/Å
c/Å
α/deg
β/deg
90.00
90.00
4731.5 (3)
8
1.565
97.856(4)
90.00
3341.7 (2)
4
1.443
1476
γ/deg
Vol/Å3
Z
Dcalcd/ g.cmꢀ3
F(000)
2240
2.91−25.00
4159
3702
259
3.06−25.00
5885
5337
359
θ
range/deg
Reflections measured
Reflections used
Parameters
ꢀ(Mo Kα) (cmꢀ1)
R1, wR2[I > 2σ(I)]a
R1, wR2 (all data)b
GooFc
2.449
1.830
0.0488, 0.0558
0.1014, 0.1040
1.302
0.0357, 0.0407
0.0818, 0.0840
1.216
2
65
aR1 = ΣFo – Fc/ΣFo; bwR2 = {Σ[w(Fo2 – Fc2)2]/Σ[w(Fo )2]}1/2
;
cS = {Σ[w(Fo2 – Fc2)2]/(n–p)2]}1/2
Fig. 1 ORTEP representation of 1. Selected bond lengths (Å) and angles
(°): Pd1–N1 1.996(4), Pd1–O1 2.001(3), Pd1–Se1 2.370(1), Pd1–Cl1
70 2.302(1); N1–Pd1–O1 92.5(2), N1–Pd1–Cl1 178.3(1), O1–Pd1–Cl1
88.3(1), N1–Pd1–Se1 89.1(1), O1– Pd1–Se1 177.2(1), Cl1–Pd1–Se1
90.1(1).
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Fig. 4 Packing diagram of 2·CH3CN illustrating intermolecular C−H⋅⋅⋅Cl
and C−H⋅⋅⋅N hydrogen bonding in the crystal lattice.
consistent with the value reported in literature (δ 281.5 ppm for
Fig. 2 ORTEP representation of 2. Selected bond lengths (Å) and angles
(°): Pd1–N1 2.066(3), Pd1–Se1 2.366(1), Pd1–Cl1 2.306(1), Pd1–Cl2
2.331(1); N1–Pd1–Cl1 176.0(1), N1–Pd1–Cl2 88.0(1), Cl1–Pd1–Cl2
95.1(1), N1–Pd1–Se1 88.9(2), Cl1– Pd1–Se1 87.9(4), Cl2–Pd1–Se1
176.8(1).
1
25 selenoether4d). In H NMR spectrum of ligand L2, the signal of
NH proton appears as a broad singlet at δ 1.74 ppm. The signals
for CH2N and OCH2 protons in proton NMR spectrum of ligand
L2 appear at δ 3.82 and 5.00 ppm, respectively. All expected
signals were observed in 13C{1H} NMR spectra of ligands L1 and
30 L2. 77Se{1H}NMR spectrum L2 shows a signal at δ 268.9 ppm.
In complex 1, the ligand L1 coordinates to Pd in a mono anionic
tridentate (Se, N, Oꢀ) mode which is corroborated by its single
crystal structure. The signal of phenolic proton present at δ =
13.55 ppm in the NMR spectrum of ligand L1 has been found
35 disappeared on complexation. This indicates the deprotonation of
phenolic OH on formation of complex 1. In 1H NMR spectrum of
complex 1, each proton of both CH2 groups becomes
diastereotopic and this results in three multiplets at δ, 3.10–
3.20, 3.72 and 4.46–4.50 ppm (2H, 1H and 1H, respectively; see
40 Fig. S7 in ESI), supporting Pd–N and Pd–Se bond formation.
The77Se{1H}NMR spectrum of complex 1 has a signal at δ 432.1
ppm which is highly deshieded (151 ppm) compared to that of
free ligand. This may be ascribed due to coordination of selenium
to palladium. The large deshielding may be due to formation of
45 five membered chelate ring4d with Pd. In complex 2 the ligand L2
coordinates to Pd in a neutral bidentate (Se, N) mode as
5
1
corroborated by its single crystal structure. In H NMR spectrum
Fig.
3 Packing diagram of 1 illustrating intermolecular C−H⋅⋅⋅Cl
10 hydrogen bonding in the crystal lattice.
of complex 2 also, protons of each CH2 group become
diastereotopic and result in five multiplets at δ2.63–2.65, 3.55–
50 3.58, 4.19–4.27, 4.69–4.73 and 4.80–4.90 ppm (2H, 2H, 1H, 1H,
and 2H, respectively see Fig. S10 in ESI), consistent with Pd–N
and Pd–Se bond formation, which makes the NCH2 and CH2Se
protons rigid. The 77Se{1H}NMR spectrum of complex 2 has a
signal at δ 489.3 ppm, highly deshieded (221.5 ppm) compared to
55 that of free ligand, indicating the coordination of selenium with
palladium.
Table 2 Selected nonꢀcovalent interactions of 1 and 2 (Inter atomic
distances in Å and bond angles in deg).
D–H
D–H⋅⋅⋅⋅A
H⋅⋅⋅⋅A
D⋅⋅⋅⋅A
D–H⋅⋅⋅⋅A
1
C8–H8A⋅⋅⋅⋅Cl1a
C2–H2⋅⋅⋅⋅Cl1b
0.97
0.93
2.68
2.85
2
2.69
2.79
2.90
2.90
3.45
3.69
140
151
C16–H16A⋅⋅⋅⋅N2c
C31–H31B⋅⋅⋅⋅Cl1d
C20–H20⋅⋅⋅⋅Cl2e
C8–H8B⋅⋅⋅⋅Cl2f
0.97
0.96
0.93
0.97
3.55
3.71
3.61
3.68
148
162
135
139
SuzukiꢀMiyaura C−C coupling catalyzed with 1 and 2:
SuzukiꢀMiyaura C−C coupling reactions were carried out in the
60 presence of complexes 1 and 2 as catalyst and results obtained are
summarized in Table 3 and 4. The optimization of reaction
conditions with present catalyst 1 was carried out by coupling
4ꢀbromotoulene and phenylboronic acid under aerobic conditions
at 80 ºC for 5 h using different bases and solvents. The best
65 results were obtained with K2CO3 and ethanol (Table 3, Entry 6).
The conversion was also observed in water (up to 40%) with
K2CO3 (Table 3, Entry 13). The coupling reaction of 4ꢀbromoꢀ
a = ½ꢀx,ꢀ1/2+y,z; b=1ꢀx,1ꢀy,ꢀz; c= 1ꢀx,1ꢀy,1ꢀz; d= x,y,1+z; e= ꢀ1+x, y, z;
f = 1ꢀx,ꢀy,2ꢀz.
15
Spectroscopic studies: The 1H and 13C{1H} NMR spectra of
ligand L1 and L2 have been found consistent with their structures
depicted in Scheme 1. The signal of OH proton in 1H NMR
spectrum of L1 has been observed at δ 13.55 ppm. The
20 77Se{1H}NMR spectrum of L1 has a signal at δ 281.1 ppm
4
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DOI: 10.1039/C4RA06313A
Table 3 Optimization of reaction conditions for SuzukiꢀMiyaura C−C
group on the aryl ring, the reactivity increases in the order of NO2
> H > OMe. The catalytic efficiency of 1 appears to be slightly
higher than that of 2. In comparison to palladium complexes4a,cꢀ
e,9,11 other than the palladacycles and pincer ligand based ones the
35 performance of 1 and 2 is comparable or better in comparison to
those containing N/S donors. Similarly the present complexes are
comparable (in some cases favourably) with palladium
nanoparticle based catalytic systems.28 Air and moisture
insensitivity of 1 and 2 are their additional advantages. However
40 they cannot be recycled for catalytic applications
coupling reactions of 4ꢀbromotoluene with phenylbronic acida
Entry
No
1
2
3
4
5
6
7
8
9
10
11
12
13
Solvent
Base
Yieldb (%)
1,4ꢀdioxane (4 mL)
1,4ꢀdioxane (4 mL)
DMF (4 mL)
DMF (4 mL)
EtOH (4 mL)
CH3ONa
K2CO3
CH3ONa
K2CO3
CH3ONa
K2CO3
CH3ONa
K2CO3
CH3ONa
K2CO3
CH3ONa
K2CO3
42
50
41
47
82
88
41
42
33
39
52
68
40
EtOH (4 mL)
1,4ꢀdioxane: water (3:1 mL)
1,4ꢀdioxane: water (3:1 mL)
DMF: water (3:1 mL)
DMF: water (3:1 mL)
EtOH: water (3:1 mL)
EtOH: water (3:1 mL)
Water (4 mL)
Conclusions
Two sterically hindered selenated Schiff bases and their
palladium(II) complexes have been synthesized and structurally
1
45 characterized with H, 13C{1H} and 77Se{1H} NMR spectra. In
K2CO3
77Se{1H} NMR spectra deshielding of signal on complexation
was up to 221.5 ppm compared to that of corresponding free
ligand. The ligand L1 coordinates as tridentate (N, Se, O−) mode
in complex 1 whereas L2 coordinates as a bidentate (N, Se)
50 ligand in 2 as revealed by single crystal structure. Complexes 1
and 2 are efficient catalysts for SuzukiꢀMiyaura C−C coupling
reactions for aryl bromide with phenylboronic acid in presence of
K2CO3 in ethanol at 80 ºC in 5 h. The reactivity increases on the
nature of the substituent on the aryl ring and follows the order
55 NO2 > H > OMe. The catalytic conversion was also observed up
to 40% with K2CO3 in water with catalyst 1. The efficiency of 1
in carrying out the coupling is slightly higher than that of 2.
aReaction conditions: 1.0 equiv of aryl halide, 1.2 equiv of phenylboronic
acid, and 2 equiv of base and temperature of bath 80 °C. Catalyst 1: 0.1
mol% Pd. Time: 5 h; bNMR(%) yield.
5
Table 4 Suzuki−Miyaura Coupling Reaction Catalyzed by Catalysts 1
and 2.a
Pd-Catalyst
Br
(HO)2B
+
Base
R
10
R
Entry
No
Aryl halide
Catalyst Yieldb (%)
(TON)
TOF(hꢀ1)
1
2
3
4
5
6
7
8
4ꢀBromonitrobenzene
4ꢀBromonitrobenzene
4ꢀBromobenzonitrile
4ꢀBromobenzonitrile
4ꢀBromoacetophenone
4ꢀBromoacetophenone
4ꢀBromotoluene
1
2
1
2
1
2
1
2
1
2
1
2
1
2
100(1000)
100(1000)
100(1000)
100(1000)
100(1000)
100(1000)
88(880)
82(820)
75(750)
71(710)
35(350)
200
200
200
200
200
200
176
164
150
142
70
Acknowledgements
60 The authors acknowledge the Department of Science and
Technology, New Delhi (SR/FT/CSꢀ79/2011) (UK) and
(SR/WOSꢀA/CSꢀ57/2012) (VVS) for financial support. Authors
thank IIT Delhi for providing HRꢀTEM and powder XRD
facility.
4ꢀBromotoluene
9
4ꢀBromobenzaldehyde
4ꢀBromobenzaldehyde
4ꢀBromoanisol
4ꢀBromoanisol
4ꢀBromobenzoic acid
4ꢀBromobenzoic acid
65 Notes and references
10
11
12
13
14
Department of Chemistry, Indian Institute of Technology Delhi, New
Delhi, 110016, India. Eꢁmail: aksingh@chemistry.iitd.ac.in,
ajai57@hotmail.com; Fax: +91 11 26581102; Tel: +91 11 26591379
32(320)
70(700)
61(610)
64
140
122
70 † Electronic Supplementary Information (ESI) available: 1H, 13C{1H} and
77Se{1H} NMR spectra of L1, L2, 1 and 2; Crystallographic data in CIF
format for complexes 1 and 2 (CCDC1009785 and 1009786). For ESI and
crystallographic data in CIF or other electronic format. See
DOI: 10.1039/b000000x/
aReaction conditions: 1.0 equiv of aryl halide, 1.2 equiv of phenylboronic
acid, and 2 equiv of base (K2CO3), solvent: EtOH (4 mL) and temperature
of bath 80 °C. Catalyst: 0.1 mol% Pd; Time: 5 h. bNMR(%) yield.
15
nitrobenzene/4ꢀbromobenzoꢀnitrile/4ꢀbromoacetophenone with
phenylboronic acid in the presence of 0.1 mol% of 1 or 2 in 5 h at
80 °C, resulted in corresponding biaryl in 100% yield (Table 4,
75
(1) A. Kumar, G. K. Rao, F. Saleem and A. K. Singh, Dalton Trans.,
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between 4ꢀbromotoluene and
20 phenylboronic acid in the presence of 0.1 mol% of 1 for 5 h at 80
°C, resulted in corresponding biaryl in 88% yield (Table 4, Entry
7). However, when catalyst 2 was used for the same coupling
under similar reaction conditions yield of corresponding coupling
product, biaryl reduced to 82% (Table 4, Entry 8). Similar trends
25 were observed in the case of 4ꢀbromobenzaldehyde,
4ꢀbromoanisole and 4ꢀbromobenzoic acid under similar reaction
condition which gave 75, 35 and 70% conversion to
corresponding biaryl with catalyst 1 and 71, 32 and 61% with
catalyst 2. These trends are due to the fact that the catalytic
30 activity is dependent on the nature of the electron withdrawing
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DOI: 10.1039/C4RA06313A
Graphical Abstract
Sterically hindered selenoether ligands: palladium(II) complexes
catalytic activators for Suzuki-Miyaura coupling
Umesh Kumar, Pooja Dubey, Ved Vati Singh, Om Prakash and Ajai K. Singh*
[PdCl(L-H)](1)/[PdCl2L](2)](L: Schiff base) and catalyze (at 0.1 mol% loading of Pd) Suzuki
coupling in ethanol at 80 °C. TON/TOF: 100/200