Palladium-catalysed Suzuki-Miyaura cross-coupling with imidazolylidene ligands substituted by crowded resorcinarenyl and calixarenyl units

Article abstract of DOI:10.3906/kim-1503-82

Two N-heterocyclic carbene (NHC) palladium complexes of formula [PdBr₂(NHC)(pyridine)] in which the carbenic ring is anked by sterically crowded cavitand substituents were prepared from appropriate imidazolium salts bearing either two resorcina

Full text of DOI:10.3906/kim-1503-82

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 1171 – 1179
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1503-82
Research Article
Palladium-catalysed Suzuki–Miyaura cross-coupling with imidazolylidene ligands
substituted by crowded resorcinarenyl and calixarenyl units
Neslihan S¸AHIN1,2, David SEMERIL1,∗, Eric BRENNER1, Dominique MATT1,
˙
Cemal KAYA2, Lo¨ıc TOUPET3
1Laboratory of Molecular Inorganic Chemistry and Catalysis, UMR-CNRS 7177, Strasbourg University, France
2Department of Chemistry, Faculty of Science, Cumhuriyet University, Sivas, Turkey
3Institute of Physics, UMR-CNRS 6251, Rennes 1 University, France
Received: 24.03.2015
•
Accepted/Published Online: 29.04.2015
•
Printed: 25.12.2015
Abstract: Two N -heterocyclic carbene (NHC) palladium complexes of formula [PdBr2 (NHC)(pyridine)] in which the
carbenic ring is flanked by sterically crowded cavitand substituents were prepared from appropriate imidazolium salts
bearing either two resorcinarene or a combination of resorcinarene and calixarene fragments. Both complexes displayed
high stability and good activities in the cross-coupling of aryl bromides with phenyl boronic acid. One of the imidazolium
salts was characterised by an X-ray diffraction study.
Key words: Resorcinarene, calixarene, cavitands, N -heterocyclic carbene, palladium, PEPPSI complexes, Suzuki–
Miyaura coupling
1. Introduction
Over the last 20 years, N -heterocyclic carbenes have gained real practical importance in numerous catalytic
processes,1−12 the most prominent application for such ligands being their use in palladium-catalysed Suzuki–
Miyaura cross coupling reactions. Current efforts in this latter area focus on the design of sophisticated NHCs
able to exert high steric pressure on the catalytic centre, with the ligand displaying preferentially variable steric
bulk, that is having adaptive steric properties ensuring both high catalyst efficiency and increased stability of
certain catalytic intermediates. The concept of variable bulk was introduced by Glorius for NHCs in which
the N atoms are part of a conformationally flexible ring that may alternately bend towards the metal centre,
thereby favouring the reductive elimination step and stabilising reactive intermediates, and then move away
from it so as to facilitate substrate approach.13−16 A few other NHCs have been considered to show the same
property,17,18 notably NHCs in which the N atoms are connected to aryl or naphthyl groups with strongly
shielding substituents (e.g., –CHPh2),8,9,19,20 and also alkylfluorenyl-substituted imidazolylidenes.12
We have recently reported the synthesis of the PEPPSI-type complex 1 (PEPPSI = Pyridine-Enhanced
Precatalyst Preparation Stabilization and Initiation21), in which the carbenic ring bears a bulky, rotationally
mobile resorcinarenyl substituent (Figure 1).22 Complex 1 showed remarkable activity in Suzuki–Miyaura cross
coupling between phenyl boronic acid and aryl bromides. The performance and stability of the catalytic system
was attributed to the ability of two pentyl groups of the freely rotating resorcinarenyl substituent to temporarily
∗Correspondence: dsemeril@unistra.fr
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interact with the metal’s first coordination sphere, thereby providing side group assistance in the reductive
elimination step.
Wondering whether the use of a NHC bearing an additional cavitand substituent tethered to the carbenic
ring would improve the catalytic performance, we prepared and tested the unsymmetrical PEPPSI-type complex
2 containing two distinct substituents each based on a bulky resorcinarene cavitand skeleton. For comparison
purposes, the related PEPPSI complex 3 was also prepared, this containing a sterically less encumbered
calixarenyl substituent. While calixarene- and resorcinarene-derived ligands, notably phosphines, have been
extensively studied in catalytic chemistry,23−26 to the best of our knowledge there is no literature report on the
use of NHCs with two cavitand substituents. The three ligands tested in this study all contain an imidazolylidene
ring and therefore are of comparable donor strength.
H11C5
H11C5
H11C5
H11C5
O
O
H11C5
O
O
H11C5
H11C5
O
H11C5
Bn
O
O
O
O
Br
O
N
O
O
O
O
O
N
O
N
N
O
N
O
O
Br
O
O
Br
O
O
O
Br
N
Pd
Br
N
Pd
Br
N
N
O
O
O O
O
H11C5
O
C5H11
C5H11
H11C5
O
O
1
OBn
H11C5
BnO
OBn
C5H11
C5H11
BnO
H11C5
3
2
Figure 1. PEPPSI-type palladium complexes tested in the present study.
2. Results and discussion
2.1. Synthesis of two palladium complexes containing cavitand-substituted NHCs
Complex 2, which contains a resorcinarenyl and a resorcinarenylmethyl substituent, was prepared according
to Scheme 1. The first step consisted of alkylating imidazolyl-cavitand 4 with the bromomethyl derivative 5
in refluxing chloroform, this giving imidazolium salt 6 in 96% yield. The structure of 6 was confirmed by a
single-crystal X-ray diffraction study (Figure 2), which revealed that in the solid state the two resorcinarene
˚
bowls adopt a head-to-tail arrangement, while the length of the molecule approaches 30 A. Imidazolium salt
6 was then converted into the PEPPSI-complex 2 (53%) by reaction with [PdCl2 ] in refluxing pyridine in the
presence of K2 CO3 and a large excess of KBr. The related calixarene-resorcinarene complex 3 was prepared
in a similar manner, starting from imidazolyl-calixarene 7 (with an overall yield of 39%) (Scheme 2). Both
complexes as well as their imidazolium precursors, 6 and 8, respectively, were unambiguously characterised by
1 H and 13 C NMR and elemental analyses (see Experimental section). Note that in the 1 H NMR spectrum
of complex 3, as well as in that of its imidazolium precursor 8, the ArCH2 signals, which appear as two AB
systems with a large AB separation (>1 ppm), are consistent with a cone conformation of the calixarene unit
(see Experimental section).27
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Figure 2. Molecular structure of salt 6.3(Et2 O).0.5(CH2 Cl2 ). Only the ether molecules are shown.
N
H11C5
H11C5
N
H11C5
H11C5
H11C5
H11C5
O
O
O
O
O
O
O
O
O
O
O
O
H11C5
H11C5
O
O
O
O
O
O
H11C5
C5H11
C5H11
H11C5
O
O
O
[PdCl2]
K2CO3, KBr, Py
O
O
O
4
CHCl3
reflux, 2 d
96 %
+
N
N
O
80°C, 18 h
53 %
Br
Pd
Br
Br
Br
N
N
O
N
O
O
O
O
O
O
O O
O
O
O O
O
O
O
O
O
O
O
O
O
O
H11C5
H11C5
H11C5
C5H11
C5H11
C5H11
C5H11
C5H11
C5H11
H11C5
H11C5
H11C5
5
6
2
Scheme 1. Synthesis of palladium complex 2.
N
H11C5
H11C5
N
H11C5
H11C5
H11C5
H11C5
O
O
O
O
H11C5
H11C5
O
O
O
O
O
O
O
OBn
O
BnO
OBn
O
BnO
O
O
O
[PdCl2]
K2CO3, KBr, Py
7
CHCl3, reflux
2 d
94 %
N
N
Br
+
Br
Br
80°C, 18 h
41 %
N
N
Pd
N
O
O
O O
O
O
Br
O
O
H11C5
OBn
C5H11
C5H11
OBn
H11C5
BnO
OBn
BnO
OBn
BnO
BnO
5
3
8
Scheme 2. Synthesis of palladium complex 3.
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2.2. Catalytic Suzuki–Miyaura cross-coupling of aryl bromides
The new palladium complexes 2 and 3 were evaluated as catalysts for Suzuki–Miyaura cross-coupling between
three aryl bromides and phenylboronic acid. The runs were performed using an aryl halide/Pd ratio of 10,000:1
with NaH as the base. The conversions were determined after 1 h reaction time at 100 ◦ C in 1,4-dioxane
(Scheme 3). In order to get a better insight into the role played by the substituents, notably for that of the
methylresorcinarenyl group, catalytic runs were also carried out under similar conditions with the reference
complex 1.
[Pd], NaH
+
(HO)2B
Br
dioxane, 100°C, 1 h
R
R
Scheme 3. Suzuki–Miyaura cross-coupling reaction.
As a general trend, the conversions increased with increasing ligand size, that is in the order 1 < 3 <
2 (Table). For example, starting from 2-bromo-6-methoxynaphthalene resulted in the corresponding coupling
product in yields of 59.4%, 63.9%, and 67.1% using complexes 1, 3, and 2, respectively (Table, entry 2).
The only side-product in these reactions was the homocoupling product (Ph–Ph), the yield of which did not
exceed 3%. The highest conversion, 76.5%, was observed for the arylation of 4-bromotoluene with complex 2
(Table, entry 1; TOF = 7650 mol(ArBr) mol(Pd)−1
h
−1). Thus, replacement of the benzyl group of 1 by a
bulkier resorcinarenylmethyl moiety improved the catalytic outcome. This effect, although moderate, reflects
the ability of the resorcinarenylmethyl group of 2 to sterically interact during catalysis with the palladium first
coordination sphere in a better way than does a N-benzyl group. Molecular modelling (SPARTAN)28 revealed
that such interactions, when occurring (that is when the metal points away from the cavity), involve either one
of the two methylenic OCH 2 O or one of the two pentyl groups attached to the NCH2 Ar ring (Figure 3). Note
that for the other N-substituent of 2, namely the resorcinarenyl fragment, only two pentyl groups may come in
contact with the metal centre. The fact that the resorcinarenylmethyl fragment of 2 may freely rotate about
the corresponding N–CH2 (resorc) bond, and thus exert a variable steric pressure on the metal centre, may
explain why the activity increase observed on going from 1 to 2 remains limited to ca. 15%. For complex 3,
the activity increase was less pronounced, this as a result of a sterically less demanding calixarenyl substituent.
Table. Comparison of palladium complexes in the Suzuki–Miyaura cross-coupling of aryl bromides.
Complexes
Entry
ArBr
1
2
3
Br
Conversion (%)
Conversion (%)
1
2
66.5
76.5
72.6
Br
59.4
61.1
67.1
66.5
63.9
66.2
MeO
Br
3
Conversion (%)
Conditions: [PdBr2 (NHC)Py] (5 × 10−5 mmol, 1 × 10−2 mol %), ArBr (0.5 mmol), PhB(OH)2 (0.091 g, 0.75 mmol),
NaH (60% dispersion in mineral oil; 0.030 g, 0.75 mmol), decane (0.05 mL), dioxane (1.5 mL), 100 ◦ C, 1 h. The
conversions were determined by GC, the calibrations being based on decane.
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O
O
O
O
O
O
O
O
N
N
H11C5
O
O
C5H11
C5H11
C5H11
O
N
N
Pd
Pd
O
C5H11
O
O
O
O
O
O
O
Ph
Ar
O
Ar
Ph
O
O
O
O
O
C5H11
O
O
O
O
C5H11
O
O
C5H11
C5H11
O
H11C5
H11C5
Figure 3. Steric interactions (involving pentyl and/or OCH 2 O fragments) that may occur in catalytic intermediates
derived from 2.
3. Experimental
All manipulations were performed in Schlenk-type flasks under dry nitrogen. Solvents were dried by con-
ventional methods and distilled immediately prior to use. CDCl3 was passed down a 5 cm thick alumina
1
column and stored under nitrogen over molecular sieves (4 A). Routine 1 H and 13 C{ H} spectra were recorded
˚
with Bruker FT instruments (AVANCE 300 and 400). 1 H spectra were referenced to residual protonated
solvents (7.26 ppm for CDCl3) and 13 C chemical shifts are reported relative to deuterated solvents (77.16
ppm for CDCl3). Chemical shifts and coupling constants are reported in ppm and in Hz, respectively. El-
emental analyses were performed by the Service de Microanalyse, Institut de Chimie, Universit´e de Stras-
bourg. trans-Dibromo-[2-{4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylresorcin[4]aren-
5-yl}-5-benzyl-imidazol-2-yliden] pyridine palladium(II) (1),22 5-N -imidazolyl-4(24),6(10),12(16),18(22)-tetra-
methylenedioxy-2,8,14,20-tetrapentyl-resorcin[4]arene (4),22 5-bromomethyl-4(24),6(10),12(16),18(22)-tetrame-
thylenedioxy-2,8,14,20-tetrapentyl-resorcin[4]arene (5),29 and 5-N -imidazolyl-25,26,27,28-tetrabenzyloxycalix[4]
arene (7)30 were prepared according to literature procedures.
3.1. General procedure for the preparation of the imidazolium salts 6 and 8
N -Aryl-imidazole (0.25 mmol) and 5-bromomethyl-4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-
tetrapentylresorcin[4]arene (5) (0.25 mmol) were dissolved in CHCl3 (10 mL). The reaction mixture was heated
to reflux for 2 days. After cooling to room temperature, the solvent was removed under vacuum. The solid was
washed with pentane and recrystallised from CH2 Cl2 /isopropyl ether to afford the corresponding imidazolium
salt.
3.1.1. 2-N -[(4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-resor cin[4]are-
ne-5-methyl]-5-N-[4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentylre-
sorcin[4]arene-5-yl]imidazolinium bromide (6)
Yield, 0.429 g, 96%; 1 H NMR (400 MHz, CDCl3): δ = 10.36 (s, 1H, NCHN), 7.35 (s, 1H, arom. CH), 7.22 (s,
1H, arom. CH), 7.20 (s br, 1H, NCH CHN), 7.13 (s, 3H, arom. CH), 7.09 (s, 3H, arom. CH), 7.02 (s br, 1H,
NCHCH N), 6.66 (s, 2H, arom. CH), 6.55 (s, 1H, arom. CH), 6.53 (s, 2H, arom. CH), 6.47 (s, 1H, arom. CH),
2
6.13 and 4.48 (AB spin system, 4H, OCH2 O, J = 7.4 Hz), 5.80 (s, 2H, NCH2), 5.74 and 4.41 (AB spin system,
2
2
4H, OCH2 O, J = 7.2 Hz), 5.73 and 4.67 (AB spin system, 4H, OCH2 O, J = 7.4 Hz), 5.63 and 4.70 (AB spin
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system, 4H, OCH2 O, 2J = 7.6 Hz), 4.78–4.66 (m, 8H, CH CH2), 2.32–2.14 (m, 16H, CHCH2), 1.46–1.32 (m,
48H, CH2 CH2 CH2 CH3), 0.92 (t, 12H, CH2 CH3 , 3J = 7.2 Hz), 0.91 (t, 12H, CH2 CH3 , 3J = 7.2 Hz); 13 C
NMR (100 MHz, CDCl3): δ= 155.70–116.68 (arom. C’s), 137.74 (s, NCHN), 121.36 (s, NC HCHN), 117.80
(s, NCHC HN), 100.02 (s, OCH2 O), 100.54 (s, OCH2 O), 99.71 (s, OCH2 O), 99.52 (s, OCH2 O), 43.84 (s,
NCH2), 36.77 (s, C HCH2), 36.48 (s, C HCH2), 32.16 (s, C H2 CH2 CH3), 32.09 (s, C H2 CH2 CH3), 32.01 (s,
C H2 CH2 CH3), 30.10 (s, CHC H2), 30.00 (s, CHC H2), 29.90 (s, CHC H2), 27.68 (s, CHCH2C H2), 22.81
(s, C H2 CH3), 22.78 (s, C H2 CH3), 14.22 (s, CH2C H3); MS (ESI-TOF): m/z: 1712.93 [M – Br]+ expected
isotopic profiles; elemental analysis calcd (%) for C108 H131 N2 O16 Br (Mr = 1793.10): C 72.34, H 7.36, N 1.56;
found: C 72.39, H 7.43, N 1.67.
Single crystals of 6 · 3 Et2 O · 0.5 CH2 Cl2 suitable for a single crystal X-ray diffraction study were
obtained by slow diffusion of diethyl ether into a dichloromethane solution of the imidazolium salt. M r =
˚
2057.88, monoclinic, space group P 21 /n, a = 18.0840(10), b = 22.4160(10), c = 28.819(2) A, β = 104.932(5),
3
V = 11287.9(11) A , Z = 4, Dx = 1.211 mg m−3 , λ(MoKa) = 0.71073 A, µ = 0.454 mm−1 , F (000) =
˚
˚
4412, T = 120(2) K. The sample (0.408 × 0.309 × 0.182 mm) was studied on an Oxford Diffraction Xcalibur
˚
Sapphire 3 diffractometer (graphite monochromated MoKa radiation, λ = 0.71073 A). The data collection
(2θmax = 27◦ , omega scan frames via 0.7◦ omega rotation and 30 s per frame, range HKL: H –22.23 K –28.28
L–35.36) gave 83,816 reflections. The data led to 24,049 independent reflections with I >2.0σ(I) observed. The
structure was solved with SIR-97,31 which revealed the nonhydrogen atoms of the molecule. After anisotropic
refinement, all the hydrogen atoms were found with a Fourier difference. The whole structure was refined with
SHELXL9732 by the full matrix least-square techniques (use of F square magnitude; x, y, z, β ij for C, Br, Cl,
N, and O atoms, x, y, z in riding mode for H atoms; 1316 variables and 24,049 observations with I >2.0σ(I);
calc w= 1/[σ2(Fo2) + (0.849P)2 ], where P = (Fo2 +2Fc2)/3 with the resulting R= 0.0850, RW = 0.2017, and
SW = 0.849, ∆ρ <0.514 eA−3 . The major difficulty in the structure determination arose from the tendency
˚
of the crystal to desolvate rapidly. Disorder was found for one of the pentyl groups. Owing to the difficulties
caused by this disorder, it was necessary to use DFIX and DANG instructions, this explaining the level A alerts.
Supplementary crystallographic data CCDC 842333 can be obtained free of charge from The Cambridge
Crystallographic Data Centre under www.ccdc.cam.ac.uk/data request/cif.
3.1.2. 2-N -[(4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-resorcin[4]are-
ne-5-methyl]-5-N -[25,26,27,28-tetrabenzyloxy-calix[4]arene-5-yl]imidazolinium bromide (8)
Yield, 0.414 g, 94%; 1 H NMR (400 MHz, CDCl3): δ = 9.51 (s, 1H, NCHN), 7.40–7.33 (m, 8H, arom. CH),
7.30–7.24 (m, 6H, arom. CH and NCH CHN), 7.19–7.02 (m, 14H, arom. CH), 6.98 (d, 2H, arom. CH, calixarene,
3J = 7.4 Hz), 6.92 (t, 2H, arom. CH, calixarene, 3J = 7.4 Hz), 6.67 (s br, 1H, NCHCH N), 6.58 (s, 2H, arom.
CH, resorcinarene), 6.50 (s, 1H, arom. CH, resorcinarene), 6.15 (s, 2H, NCH2), 6.14 (s, 2H, arom. CH), 6.02
and 4.57 (AB spin system, 4H, OCH2 O, 2J = 7.2 Hz), 5.74 and 4.49 (AB spin system, 4H, OCH2 O, 2J = 7.2
Hz), 5.71–5.67 (m, 2H, arom. CH), 5.17 and 5.06 (AB spin system, 4H, CH2 C6 H5 , 2J = 12.0 Hz), 4.76–4.71
3
(m, 2H, CH CH2), 4.73 (s, 2H, CH2 C6 H5), 4.72 (s, 2H, CH2 C6 H5), 4.68 (t, 2H, CH CH2 , J = 8.0 Hz), 4.27
2
2
and 3.00 (AB spin system, 4H, ArCH2 Ar, J = 13.6 Hz), 4.07 and 2.85 (AB spin system, 4H, ArCH2 Ar, J =
14.0 Hz), 2.30–2.17 (m, 6H, CHCH2), 2.10–2.00 (m, 2H, CHCH2), 1.43–1.23 (m, 24H, CH2 CH2 CH2 CH3),
0.91 (t, 6H, CH2 CH3 ,3J = 7.0 Hz), 0.88 (t, 6H, CH2 CH3 , 3J = 7.2 Hz); 13 C NMR (100 MHz, CDCl3):
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δ= 156.50–116.90 (arom. C’s), 134.58 (s, NCHN), 122.45 (s, NC HCHN), 122.15 (s, NCHC HN), 100.57 (s,
OCH2 O), 99.71 (s, OCH2 O), 77.36 (C H2 C6 H5), 75.94 (C H2 C6 H5), 44.32 (s, NCH2), 36.78 (s, C HCH2),
36.47 (s, C HCH2), 32.18 (s, C H2 CH2 CH3), 32.13 (s, C H2 CH2 CH3), 31.49 (ArC H2 Ar), 31.47 (ArC H2 Ar),
30.07 (s, CHC H2), 29.93 (s, CHC H2), 27.70 (s, CHCH2C H2), 22.83 (s, C H2 CH3), 22.79 (s, C H2 CH3),
14.23 (s, CH2C H3); MS (ESI-TOF): m/z: 1680.86 [M – Br]+ expected isotopic profiles; elemental analysis
calcd (%) for C112 H115 N2 O12 Br (Mr = 1761.02): C 76.39, H 6.58, N 1.59; found: C 76.44, H 7.61, N 1.47.
3.2. General procedure for the preparation of the PEPPSI-type complexes 2 and 3
A mixture of K2 CO3 (0.069 g, 0.50 mmol), pyridine (3.5 mL), [PdCl2 ] (0.027 g, 0.15 mmol), imidazolium
salt (0.10 mmol), and KBr (0.237 g, 2.00 mmol) was heated at 80 ◦ C for 17 h. The reaction mixture was
then filtered through Celite. The filtrate was evaporated under vacuum, and the solid residue purified by flash
chromatography (EtOAc/petroleum ether 50:50 v/v) to afford the corresponding palladium complex.
3.2.1. trans-Dibromo-[2-{4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-
resorcin[4]aren-5-yl}-5-{(4(24),6(10),12(16),18(22)-tetramethylenedioxy-2,8,14,20-tetra
pentylresorcin[4]arene-5-methyl}-imidazol-2-yliden] pyridine palladium(II) (2)
3
4
Yield, 0.116 g, 53%; 1 H NMR (300 MHz, CDCl3): δ = 8.95 (dd, 2H, arom. CH, Py, J = 6.3 Hz, J = 1.5 Hz),
7.80 (tt, 1H, arom. CH, Py, 3J = 7.6 Hz, 4J = 1.5 Hz), 7.41 (s, 1H, arom. CH of resorcinarene), 7.38 (s, 2H,
arom. CH, resorcinarene), 7.37–7.34 (m, 2H, arom. CH, Py), 7.31 (s, 2H, arom. CH, resorcinarene), 7.29 (s, 3H,
arom. CH, resorcinarene), 6.92 (d, 1H, NCH CHN, 3J = 1.8 Hz), 6.72 (d, 1H, NCHCH N, 3J = 1.8 Hz), 6.64
(s, 2H, arom. CH, resorcinarene), 6.61 (s, 1H, arom. CH, resorcinarene), 6.66 (s, 1H, arom. CH, resorcinarene),
6.59 (s, 2H, arom. CH, resorcinarene), 6.27 and 4.64 (AB spin system, 4H, OCH2 O, 2J = 7.2 Hz), 5.89 and
4.50 (AB spin system, 4H, OCH2 O, 2J = 7.2 Hz), 5.88 and 4.49 (AB spin system, 4H, OCH2 O, 2J = 7.2 Hz),
5.81 and 4.53 (AB spin system, 4H, OCH2 O, 2J = 7.1 Hz), 5.72 (s, 2H, NCH2), 5.02 (t, 2H, CH CH2 , 3J =
8.1 Hz), 4.96–4.83 (m, 6H, CH CH2), 2.48–2.26 (m, 16H, CHCH2), 1.61–1.32 (m, 48H, CH2 CH2 CH2 CH3),
3
3
1.04 (t, 18H, CH2 CH3 , J = 7.2 Hz), 1.01 (t, 6H, CH2 CH3 , J = 7.2 Hz) ppm; 13 C NMR (75 MHz, CDCl3):
δ = 155.04–115.93 (arom. C), 99.69 (s, OCH2 O), 99.63 (s, OCH2 O), 99.50 (s, OCH2 O), 99.21 (s, OCH2 O),
46.80 (s, NCH2), 36.66 (s, C HCH2), 36.37 (s, C HCH2), 32.04 (s, C H2 CH2 CH3), 31.96 (s, C H2 CH2 CH3),
29.86 (s, CHC H2), 29.79 (s, CHC H2), 29.69 (s, CHC H2), 27.56 (s, CHCH2C H2), 27.48 (s, CHCH2C H2),
22.69 (s, C H2 CH3), 14.10 (s, CH2C H3) ppm; elemental analysis calcd (%) for C113 H135 N3 O16 Br2 Pd (M r
= 2057.52): C 65.96, H 6.61, N 2.04; found: C 66.07, H 6.66, N 1.98.
3.2.2. trans-Dibromo-[2-{25,26,27,28-tetrabenzyloxycalix[4]arene-5-yl}-5-{(4(24),6(10), 12(16),
18(22)-tetramethylenedioxy-2,8,14,20-tetrapentyl-resorcin[4]arene-5-methyl}-imidazol-2-
yliden] pyridine palladium(II) (3)
Yield, 0.101 g, 41%; 1 H NMR (300 MHz, CDCl3): δ = 8.90–8.86 (m, 2H, arom. CH, Py), 7.72–7.65 (m, 1H,
arom. CH, Py), 7.33–7.06 (m, 28H, arom. CH), 6.71–6.53 (m, 7H, arom. CH and NCH CHN), 6.50 (s, 2H,
arom. CH, resorcinarene), 6.47 (s, 1H, arom. CH, resorcinarene), 6.43–6.34 (m, 4H, arom. CH and NCHCH N),
2
2
6.09 and 4.46 (AB spin system, 4H, OCH2 O, J = 7.5 Hz), 5.74 and 4.40 (AB spin system, 4H, OCH2 O, J =
7.5 Hz), 5.46 (s, 2H, NCH2), 5.01 (s, 2H, CH2 C6 H5), 4.97 (s, 2H, CH2 C6 H5), 4.91 and 4.86 (AB spin system,
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4H, CH2 C6 H5 , 2J = 10.5 Hz), 4.80 (t, 2H, CH CH2 , 3J = 7.9 Hz), 4.73 (t, 2H, CH CH2 , 3J = 7.8 Hz), 4.22
2
2
and 2.94 (AB spin system, 4H, ArCH2 Ar, J = 13.8 Hz), 4.17 and 2.94 (AB spin system, 4H, ArCH2 Ar, J =
14.1 Hz), 2.33–2.14 (m, 8H, CHCH2), 1.46–1.31 (m, 24H, CH2 CH2 CH2 CH3), 0.91 (t, 6H, CH2 CH3 , 3J =
3
6.9 Hz), 0.91 (t, 6H, CH2 CH3 , J =6.9 Hz) ppm; 13 C NMR (75 MHz, CDCl3): δ = 155.80–116.59 (arom. C),
99.85 (s, OCH2 O), 99.73 (s, OCH2 O), 76.81 (s, C H2 C6 H5), 76.61 (s, C H2 C6 H5), 76.50 (s, C H2 C6 H5),
45.83 (s, NCH2), 36.87 (s, C HCH2), 36.59 (s, C HCH2), 32.24 (s, C H2 CH2 CH3), 31.56 (ArC H2 Ar), 30.09
(s, CHC H2), 29.91 (ArC H2 Ar), 27.78 (s, CHCH2C H2), 22.90 (s, C H2 CH3), 14.33 (s, CH2C H3) ppm;
elemental analysis calcd (%) for C117 H119 N3 O12 Br2 Pd (M r = 2025.44): C 69.38, H 5.92, N 2.07; found: C
69.47, H 6.01, N 1.98.
3.3. General procedure for palladium-catalysed Suzuki–Miyaura cross-coupling reactions
A mixture of [PdBr2 (NHC)(pyridine)] (5 × 10−5 mmol, 1 × 10−2 mol %), ArBr (0.5 mmol), PhB(OH)2
(0.091 g, 0.75 mmol), NaH (60% dispersion in mineral oil; 0.030 g, 0.75 mmol) dioxane (1.5 mL), and decane
(0.05 mL, internal reference) was heated for 1 h at 100 ◦ C. After cooling to room temperature, a small amount
(0.5 mL) of the resulting solution was passed through a Millipore filter and analysed by GC. All products were
unambiguously identified by NMR after their isolation. The NMR spectra were compared to those reported in
the literature.
4. Conclusion
We have described the first PEPPSI-type palladium complex in which a carbene ligand is N -substituted by two
bulky, freely rotating cavitand subunits. The beneficial role of the resorcinarenylmethyl fragment over that of a
benzyl one was shown in Suzuki–Miyaura cross coupling experiments involving aryl bromides (activity increase
of ca. 15%). The activity increase would probably be enhanced by limiting the degree of rotational freedom of
the resorcinarenylmethyl group, for example by expanding the flat carbenic heterocycle.
Acknowledgments
¨
˙
˙
The authors thank the Scientific and Technological Research Council of Turkey (TUBITAK-BIDEB), the
International Research Fellowship Programme for a grant to N.S., and the French Agence Nationale de la
Recherche (ANR) (ANR-12-BS07-0001-01-RESICAT) for financial support.
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