Novel motif for bidentate P,N ligands. Application to Pd-catalyzed Suzuki cross-coupling reactions

Article abstract of DOI:10.1016/j.jorganchem.2013.01.028

A library of novel C₂-symmetric P,N ligands based on a Tr?ger's base backbone was prepared via a concise two-step synthesis starting from a commercially available aniline derivative. X-ray crystallography and ³¹P NMR studies confirm that a single ligand molecule can accommodate two palladium atoms via coordination to nitrogen and phosphorus. Preliminary results proved the synthetic potential of this species in Suzuki cross-coupling reactions of aryl bromides with arylboronic acids.

Full text of DOI:10.1016/j.jorganchem.2013.01.028

Journal of Organometallic Chemistry 729 (2013) 81e85
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Journal of Organometallic Chemistry
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Novel motif for bidentate P,N ligands. Application to Pd-catalyzed Suzuki
cross-coupling reactions
*
ꢀ
Raul Pereira, Ján Cvengros
Department of Chemistry and Applied Biosciences, HCI H230, Swiss Federal Institute of Technology, ETH Zürich, CH-8093 Zürich, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A library of novel C2-symmetric P,N ligands based on a Tröger’s base backbone was prepared via a concise
two-step synthesis starting from a commercially available aniline derivative. X-ray crystallography and
31P NMR studies confirm that a single ligand molecule can accommodate two palladium atoms via co-
ordination to nitrogen and phosphorus. Preliminary results proved the synthetic potential of this species
in Suzuki cross-coupling reactions of aryl bromides with arylboronic acids.
Received 29 November 2012
Received in revised form
27 January 2013
Accepted 30 January 2013
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Phosphane ligand
Tröger’s base
Palladium
Cross-coupling
1. Introduction
together with a unique V-shape of the Tröger’s base scaffold has
inspired numerous scientists to exploit this motif in various fields
Metal-catalyzed cross-coupling reactions have matured over the
years into an incredibly powerful tool in the hands of synthetic
chemists [1]. The broad spectrum of protocols covers cross-
coupling reactions basically of all types of hybridized carbons [2]
with carbon, nitrogen [3], oxygen [4], sulfur [5] and phosphorus
[6] atoms. Besides countless applications in academic research [7],
these types of transformations have attracted also the attention of
industrial chemists as demonstrated by scientific reports on their
use in the synthesis of pharmaceuticals and agrochemicals [8]. This
galore of cross-coupling reactions was possible due to the detailed
understanding of the reaction mechanism, extensive screening of
reaction conditions and sophisticated design of ligands [9]. As
environmental and economic needs evolve with time, ligand evo-
lution is imperative. Employment of new structural motifs into
rational ligand design appears to be an attractive approach for the
development of new ligands.
such as molecular recognition, bioorganic chemistry and supra-
molecular chemistry [12]. Its implementation in the ligand design
is, however, scarce [13]. Despite the fact that the Tröger’s base
skeleton represents an easily accessible, rigid, chiral moiety, the
design of ligands profiting from the presence of this unit is rather
underdeveloped [14]. We have recently reported a straightforward
synthesis of new S,N and Se,N doubly bidentate ligands based on
Tröger’s base backbone [15]. Their coordination properties in Ag(I)-
complexes were studied by NMR spectroscopy as well as X-ray
diffraction crystallography and it was shown that a single molecule
of the ligand can accommodate two silver atoms via coordination to
chalcogen and nitrogen atoms. Herein, we would like to expand our
approach to a new class of P,N ligands.
2. Results and discussion
Tröger’s base[10] is a textbook example of a molecule containing
stable stereogenic nitrogen atoms [11]. The pyramidal inversion at
nitrogen centers is hindered due to the presence of the methylene
bridge between the nitrogen atoms thereby preventing the race-
mization of enantiomerically pure Tröger’s base derivatives under
non-acidic conditions even at elevated temperature. This feature
2.1. Ligand synthesis
The synthesis starts with the formation of 4,10-dibromo Tröger’s
base (1) by the condensation of 2-bromo-4-methylaniline with
paraformaldehyde in trifluoroacetic acid [16]. Compound 1 was
formed in a highly efficient manner and the crystallization of the
partially concentrated organic extracts after an aqueous work-up
was fully sufficient for the isolation of a pure product (97% yield).
Two strategies were applied for the installment of the phosphane
moieties. Typically, the lithiumebromine exchange followed by the
* Corresponding author. Tel.: þ41 44 633 43 31; fax: þ41 44 632 13 10.
ꢀ
E-mail address: cvengros@inorg.chem.ethz.ch (J. Cvengros).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.01.028

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R. Pereira, J. Cvengros / Journal of Organometallic Chemistry 729 (2013) 81e85
82
quenching with a chlorophosphane gave the corresponding P,N li-
gands 2aec (Scheme 1).
We have observed throughout different projects that the sta-
bility of the dilithiated species (upon lithiumebromine exchange of
1) is somewhat limited and only reactive electrophiles yielded the
corresponding products in high yields [17]. This tendency was also
observed in the present case. The reaction proceeded with sat-
isfactory yields when the substituents on chlorophosphanes were
phenyl or 4-methylphenyl. The increased steric hindrance in the
case of bulkier chlorodialkylphosphanes resulted in prolonged re-
action time during which the dilithiated species deteriorated and
the anticipated phosphanes were obtained only in low (2c) or no
(2d) yields. Furthermore, the dicyclohexylphosphane ligand proved
extremely sensitive towards oxidation rendering its isolation and
purification exceedingly cumbersome. We have thus treated the
crude reaction mixture with an excess of borane tetrahydrofuran
complex (5 equiv) and isolated the ligand as its borane adduct 2c0.
The deprotection was achieved by an action of diethylamine and
the crude phosphane ligand 2c was directly used in catalysis. Dis-
satisfied with the poor outcome of the synthesis of tBu-phosphane
2d, we have attempted an alternative synthesis via palladium-
catalyzed phosphanation. To our delight, 2d was obtained in 26%
yield. In general, the presented approach represents a concise two-
step strategy for the preparation of a library of new P,N ligands. The
introduction of the phosphane moiety in the last step allows for
a high modularity of this protocol.
Scheme 2. Coordination of 2a to palladium.
We were able to grow single crystals of 3a by vapor diffusion of
pentane into a dichloromethane solution. The ORTEP view of 3a is
shown in Fig. 1. Two molecules of dichloromethane are present in
the crystal structure. The chlorine atoms (Cl5 and Cl6) in the
dichloromethane molecule closer to the ligand are disordered over
two sites with site occupancy factors of 0.56 and 0.44. As a con-
sequence, one of the chlorine atoms (Cl1) at Pd(1) is disordered
over two sites with site occupancy factors of 0.58 and 0.42.
The X-ray analysis confirmed that a single molecule of ligand 2a
is able to accommodate two palladium atoms resulting in dipalla-
dium complex 3a. Both palladium atoms coordinate in the same
fashion with square-planar geometry around palladium. The com-
plex 3a thus retains the C2-symmetry of the parent ligand. Nitrogen
and phosphorus atoms replaced COD as ligands in the coordination
sphere of palladium. The bond lengths between palladium and
phosphorus are 2.1868(10) Å and 2.1996(11) Å, respectively, which
corresponds to the bond lengths in other palladium containing
five-membered cycles. The same goes for the bonds between pal-
ladium and nitrogen (2.123(3) Å and 2.122(3) Å). The values for the
PePdeN angles (85.75(9)ꢁ and 86.37(9)ꢁ) are in agreement with
values reported for other five-membered palladacycles with this
ligand set. The phosphorus and nitrogen atoms are located out of
plane of the aromatic rings. The torsion angle defined by P(1)e
C(1)eC(14)eN(1) is ꢀ8.7(5)ꢁ and ꢀ14.3(5)ꢁ in the case of N(2)e
C(7)eC(8)eP(2). The dihedral angle between the aromatic planes
is 102.56(12)ꢁ.
2.2. Coordination properties
We have studied the coordination properties of the new ligands
upon complexation with palladium. The solution of ligand 2a in
DCM was added to the solution of 2 equivalents of [PdCl2(COD)] in
DCM and the resulting solution was stirred for 1 h at RT. The
analysis of the reaction mixture by 31P NMR revealed that the signal
for the free ligand at ꢀ15.1 ppm disappeared and single phosphine
species was detected at 36.1 ppm as a doublet (JHeP ¼ 10.9 Hz)
which was assigned to the dipalladium complex 3a (Scheme 2).
Similarly, the NMR experiment with the cyclohexyl analogue 2c
revealed the complete disappearance of the 31P NMR signal of the
free ligand upon addition of 2 equivalents of [PdCl2(COD)]. The
coordination resulted again in single phosphine species at
62.4 ppm (proton decoupled 31P NMR).
2.3. Catalysis
With a small library of ligands in our hands we have performed
some initial tests to evaluate their catalytic properties in palladium-
Scheme 1. Synthesis of P,N ligands 2aed.

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R. Pereira, J. Cvengros / Journal of Organometallic Chemistry 729 (2013) 81e85
83
Table 2
Suzuki cross-coupling of aryl halides with arylboronic acids.a
Entry
1
Ligand
Aryl halide
Boronic acid
Time (h)
1.5
Yield (%)b
2a
92
79
91
2
3
2a
2a
7
7
4
2b
2a
1.5
22
90
0
Fig. 1. ORTEP representation of complex 3a. Hydrogen atoms and solvent molecules
are omitted for clarity. Thermal ellipsoids are set to 50% probability. Selected bond
lengths (Å) and bond angles (ꢁ): Pd1eN1 2.123(3), Pd2eN2 2.122(3), Pd1eP1
2.1868(10), Pd2eP2 2.1996(11), Pd1eCl1 2.424(3), Pd1eCl1A 2.332(3), Pd1eCl2
2.2584(11), Pd2eCl3 2.3640(11), Pd2eCl4 2.2693(10), N1ePd1eP1 85.75(9), N2ePd2e
P2 86.37(9).
5
a
Reaction conditions: 1.5 mmol aryl halide, 2.25 mmol arylboronic acid, 3 mmol
K3PO4, Pd(OAc)2 (2 mol%), ligand (1 mol%), 3 mL THF, 70 ꢁC.
b
Isolated yields after column chromatography.
catalyzed CeC cross-coupling reactions. A fast screening revealed
that Pd(OAc)2 is superior to Pd2(dba)3 as the palladium source for
the reaction between bromobenzene and phenylboronic acid
(Table 1, Entries 1 and 2). The use of potassium triphosphate
resulted in shorter reaction time compared to cesium carbonate
(Entries 2 and 3). THF proved to be the most suitable solvent
resulting in the lowest temperature and the highest reaction rate
(Entries 2, 4e7). Finally, we evaluated the potential of other syn-
thesized ligands. The p-tolyl analogue 2b gave basically same result
as 2a yielding the product in 99% yield (Entry 8). Interestingly, the
electron-rich bulky ligands 2c and 2d turned out to be completely
inactive even after a prolonged reaction time (Entries 9 and 10).
Applying the most suitable conditions, we checked a few sub-
strates to briefly evaluate the scope and limitation of the catalyst
(Table 2). The presence of the electron-withdrawing substituent
facilitates the oxidative addition of palladium into the carbone
bromine bond resulting in shorter reaction times (Entry 1). Inter-
estingly, one or two substituents in the vicinity of the bromine
atom are tolerated (Entries 2 and 3). Ortho-substituted boronic acid
can be also efficiently applied under these conditions (Entry 4).
Unfortunately, the presented catalytic system completely failed to
involve aryl chlorides in the Suzuki cross-coupling. The above-
mentioned results suggest that a few sophisticated changes have to
be done regarding the catalyst design.
and X-ray crystallographic studies of the corresponding Pd(II)-
complex 3a confirmed that a single ligand acts as doubly biden-
tate and can thus accommodate two palladium atoms via coordi-
nation to nitrogen and phosphorus. Preliminary tests of the
catalytic activity revealed that the diarylphosphane ligands 2a and
2b catalyze the Suzuki cross-coupling of aryl bromides with bor-
onic acids. We are currently trying to modify the design of Tröger’s
base derived ligands in order to improve their catalytic perfor-
mance as well as to test the enantiomerically pure ligands in
asymmetric catalysis.
4. Experimental
4.1. General
The reactions were carried out in oven-dried glassware under
argon using Schlenk techniques. All solvents were freshly distilled
under argon from an appropriate drying agent before use. Flash
chromatography was performed with Fluka silica gel 60. NMR
spectra were measured on Bruker Avance DPX-300, DPX-400, III HD
Nanobay-300 and III HD Nanobay-400 spectrometers. The chemical
shifts are recorded in ppm and are referenced to 85% H3PO4 (for 31P)
and to tetramethylsilane (1H and 13C). The 2D lock frequency of
CDCl3 was used as the internal secondary reference in all cases.
High-resolution mass spectra were measured by the MS-Service of
the “Laboratorium für Organische Chemie der ETH” on a Bruker
Daltonics maXis ESI-QTOF. The X-ray structure was measured on
a Bruker APEX2 diffractometer with CCD area detector and Mo-Ka
radiation. Single crystals were coated at room temperature with
perfluoroalkylether oil and mounted on a 0.1 mm glass capillary.
The structures were solved by direct methods in SHELXTL and
successive interpretation of the different Fourier maps, followed by
full-matrix least-squares refinement (against F2).
3. Conclusion
We have demonstrated that the Tröger’s base scaffold can be
efficiently used as a backbone for new class of P,N ligands. 31P NMR
Table 1
Screening of the reaction conditions (Pd-source, ligand, base, solvent, temperature)
for the Suzuki cross-coupling.
Entry Catalyst
Ligand Base
Solvent
Temperature Time Yield
(ꢁC)
(h)
(%)a
1
2
3
4
5
6
7
8
9
Pd2(dba)3 2a
K3PO4
K3PO4
Cs2CO3 THF
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
THF
THF
70
70
70
90
75
100
90
70
70
70
19
4
22
24
17
5
17
4
18
18
33
99
68
48
88
73
45
99
0
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
2a
2a
2a
2a
2a
2a
2b
2c
2d
4.2. Ligand synthesis
Toluene
EtOH
DMF
Dioxane
THF
THF
THF
4.2.1. Synthesis of rac-4,10-bis(diphenylphosphino)-2,8-dimethyl-
6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine [2a]
A Schlenk flask was charged with 1 (0.82 g, 2 mmol) under
argon and THF (20 mL) was added. Then it was cooled down
to ꢀ78 ꢁC and n-BuLi (1.6 M in hexane, 2.8 mL, 4.4 mmol, 2.2 equiv)
was added dropwise to form a pale yellow suspension. After
10
0
a
Isolated yields after column chromatography.

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84
10 min, chlorodiphenylphosphane (0.78 mL, 4.4 mmol, 2.2 equiv)
was added dropwise and the suspension turned clear again. The
stirring was continued overnight at ambient temperature. Sat.
NH4Cl solution (20 mL) was added and it was extracted with DCM
(3 ꢂ 20 mL). Combined org. layers were washed with brine, dried
over MgSO4 and concentrated under reduced pressure. The crude
product was purified by flash column chromatography (SiO2 (40 g);
DCM/hexane 4:1) to give 2a as a white solid (640 mg, 53%).
(d, JCeP ¼ 13.6 Hz), 26.8 (d, JCeP ¼ 11.2 Hz), 27.0 (d, JCeP ¼ 2.5 Hz), 27.1
(d, JCeP ¼ 1.9 Hz), 27.2 (d, JCeP ¼ 11.2 Hz), 28.0 (d, JCeP ¼ 36.5 Hz), 28.5
(d, JCeP ¼ 35.5 Hz), 33.2 (d, JCeP ¼ 33.9 Hz), 34.1 (d, JCeP ¼ 32.4 Hz),
57.4 (CH2N), 66.0 (NCH2N), 122.9 (d, JCeP ¼ 44.1 Hz), 129.0 (d,
JCeP ¼ 5.7 Hz), 130.6 (d, JCeP ¼ 2.1 Hz), 134.8 (d, JCeP ¼ 11.5 Hz), 137.1
(d, JCeP ¼ 12.7 Hz), 149.3; 31P NMR (162 MHz, CDCl3):
[ppm] ¼ 32.1;
d
HRMS (ESI): m/z [M þ H]þ calcd for C41H67N2P2B2: 671.4974 (100.0%);
found: 671.4977 (100.0%).
1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 2.08 (s, 6H, CH3), 4.18 (s, 2H,
Prior to the catalytic test, the borane protected phosphane 2c0
(50 mg, 0.075 mmol) was placed into a J Young flask under argon.
Dry DCM (3 mL) and diethylamine (5 mL) were added and the flask
was sealed. The reaction mixture was stirred at 50 ꢁC for 24 h. It
was cooled down, silica (2 g) was added and the volatiles were
removed. The residue was placed at the top of a short silica column
(6 g) and eluted with hexane/EtOAc 4:1 þ0.1% Et3N to give a white
solid (30 mg, 64%) which was briefly checked by 1H and 31P NMR
and immediately used in catalysis. 1H NMR (300 MHz, CD2Cl2):
0.75e2.04 (m, 44H, Cy), 2.14 (s, 6H, CH3), 4.09 (s, 2H, NCH2N) 4.25
(d, 3J ¼ 16.7 Hz, 2H, endo CH2N), 4.39 (d, 3J ¼ 16.6 Hz, 2H, exo CH2N),
6.61 (s, 2H, AreH), 6.93 (s, 2H, AreH); 31P NMR (121 MHz, CD2Cl2):
NCH2N), 4.32 (d, 3J ¼ 17.3 Hz, 2H, endo CH2N), 4.55 (d, 3J ¼ 17.3 Hz,
2H, exo CH2N), 6.50 (s, 2H, AreH), 6.60 (s, 2H, AreH), 7.25e1.39 (m,
20H, AreH); 13C NMR (75 MHz, CDCl3):
d
[ppm] ¼ 21.4 (CH3), 57.7
(d, JCeP ¼ 9.7 Hz, CH2N), 67.9 (NCH2N), 128.6, 128.7, 128.8,
128.9, 129.0, 129.1, 133.1 (d, JCeP ¼ 11.7 Hz), 133.5 (d, JCeP ¼ 19.3 Hz),
133.9, 134.4, 134.9 (d, JCeP ¼ 20.4 Hz), 137.8 (d, JCeP ¼ 11.2 Hz), 138.3
(d, JCeP ¼ 11.0 Hz), 149.3 (d, JCeP ¼ 21.1 Hz); 31P NMR (162 MHz,
CDCl3): e15.1; HRMS (ESI): m/z [MH]þ calcd for C41H37N2P2:
619.2420; found: 619.2426.
4.2.2. Synthesis of rac-4,10-bis(di-p-tolylphosphino)-2,8-dimethyl-
6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine [2b]
d
[ppm] ¼ ꢀ14.9.
The title compound was synthesized in analogy to the
abovementioned procedure for 2a starting from
1
(650 mg,
4.2.4. Synthesis of rac-4,10-bis(di-tert-butylphosphino)-2,8-
1.6 mmol). solution of chlorobis(4-methylphenyl)phosphine
A
dimethyl-6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine [2d]
(880 mg, 3.52 mmol) in THF (5 mL) was used to quench the lithi-
ated species. The crude product was purified by flash column
chromatography (SiO2; hexane/ethyl acetate 10:1 to 2:1 containing
0.1% Et3N) to give a yellowish-white solid which was further pu-
rified via re-precipitation from a mixture of DCM/hexane to give 2b
as a white solid (354 mg, 32%). 1H NMR (300 MHz, CDCl3):
In the glove-box, a Schlenk flask was charged with 1
(204 mg mg, 0.5 mmol), Pd(OAc)2 (9.5 mg, 4.2 mmol, 4.2 mol%),
NaOt-Bu (115 mg, 1.2 mmol, 2.4 equiv), (tBu)3P (10.2 mg, 5 mmol,
5 mol%) and (tBu)2PH (146.2 mg, 1.0 mmol, 2 equiv). Schlenk flask
was taken out of the glove-box, dry toluene (2 mL) was added and
the reaction mixture was stirred at 100 ꢁC for 24 h under argon. It
was then cooled down to room temperature and Et2O (10 mL) was
added and the mixture was transferred into a separate funnel. The
reaction vessel was rinsed with diethyl ether (2 ꢂ 20 mL). The
combined ether layers were washed with water (2 ꢂ10 mL) and
brine (2 ꢂ 10 mL). The aqueous layers were back extracted with
ether (20 mL). The combined ether layers were dried over MgSO4
and the organic solvent was removed under reduced pressure. The
crude material was purified via recrystalization (hexane/ethyl ac-
etate, 1:1) to give a white solid (75 mg, 26%).
d
[ppm] ¼ 2.08 (s, 6H, CH3), 2.35 (s, 6H, CH3), 2.40 (s, 6H, CH3), 4.16
(s, 2H, NCH2N) 4.32 (d, 3J ¼ 17.4 Hz, 2H, endo CH2N), 4.52(d,
3J ¼ 17.4 Hz, 2H, exo CH2N), 6.51 (s, 2H, AreH), 6.59 (s, 2H, AreH),
7.12 (m, 8H, AreH), 7.18 (m, 4H, AreH), 7.25 (m, 4H, AreH); 13C
NMR (75 MHz, CDCl3):
d
[ppm] ¼ 21.0 (CH3), 21.3 (CH3), 21.4
(CH3), 57.2 (d, JCeP ¼ 10.5 Hz, CH2N), 67.5 (NCH2N), 128.3, 128.5 (d,
JCeP ¼ 4.9 Hz), 129.0 (d, JCeP ¼ 7.0 Hz), 129.3 (d, JCeP ¼ 7.3 Hz), 133.1
(d, JCeP ¼ 19.5 Hz), 133.0, 133.3, 133.7, 134.0 (d, JCeP ¼ 10.1 Hz),
134.4 (d, JCeP ¼ 20.7 Hz), 134.6 (d, JCeP ¼ 9.8 Hz), 137.9, 138.5, 149.3
(d, JCeP ¼ 21.1 Hz); 31P NMR (121 MHz, CDCl3):
d
[ppm] ¼ ꢀ17.8;
1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 1.17 (d, 3JHeP ¼ 11.4 Hz,18H,
HRMS (ESI): m/z [M þ H]þ calcd for C45H45N2P2: 675.3052; found:
(CH3)3),1.36 (d, 3JHeP ¼ 11.7 Hz,18H, (CH3)3), 2.25 (s, 6H, CH3), 4.16 (s,
2H, NCH2N) 4.36 (d, 3J ¼ 17.1 Hz, 2H, endo CH2N), 4.61 (d, 3J ¼ 17.1 Hz,
2H, exo CH2N), 6.72(s, 2H, AreH), 7.36 (s, 2H, AreH); 13C NMR
675.3042.
4.2.3. Synthesis of rac-4,10-bis(dicyclohexylphosphino)-2,8-
(75 MHz, CDCl3):
d
[ppm] ¼ 21.1 (AreCH3), 29.8 (d, JCeP ¼ 14.3 Hz,
dimethyl-6,12-dihydro-5,11-methanodibenzo[b,f][1,5]diazocine [2c]
The title compound was synthesized in analogy to the above-
mentioned procedure for 2a starting from 1 (200 mg, 0.5 mmol). A
solution of chlorodicyclohexylphosphane (256 mg, 1.1 mmol) in THF
(5 mL) was used to quench the lithiated species. The reaction was
allowed to reach room temperature overnight. It was then cooled to
0 ꢁC and BH3.THF (10 mL, 10 mmol of a 1 M solution in THF) was
added. The reaction mixture was allowed to reach room temperature
and then it was stirred for additional 6 hours. The reaction was then
transferred into a saturated solution of NH4Cl (20 mL) under argon
and it was stirred for 5 min. It was extracted with DCM (3 ꢂ 30 mL).
Combined organic layers were dried over MgSO4 and concentrated.
The crude product was purified by flash column chromatography
(SiO2 (30 g); hexane/ethyl acetate 4:1 containing 0.1% Et3N) to
give a pale yellow solid as the borane protected phosphane 2c0
C(CH3)3), 31.5 (d, JCeP ¼ 24.9 Hz, C(CH3)3), 31.8 (d, JCeP ¼ 16.2 Hz,
C(CH3)3), 33.3 (d, JCeP ¼ 23.9 Hz, C(CH3)3), 58.6 (d, JCeP ¼ 11.7 Hz,
CH2N), 67.4 (NCH2N), 128.4, 128.8 (d, JCeP ¼ 5.9 Hz), 131.8, 132.5 (d,
JCeP ¼ 24.5 Hz), 134.8 (d, JCeP ¼ 4.0 Hz), 151.2; 31P NMR (121.5 MHz,
CDCl3):
d
[ppm] ¼ 15.9; HRMS (ESI): m/z [M þ H]þ calcd for
C33H53N2P2: 539.3678; found: 539.3692.
4.3. Synthesis of (PdCl2)2(2a) [3a]
A Schlenk flask was charged with ligand 2a (31 mg, 0.05 mmol)
and [PdCl2(cod)] (28.6 mg, 0.10 mmol, 2 equiv) under argon. DCM
(5 mL) was added and the resulting mixture was stirred for 4 h at
RT. It was filtered through a pad of celite and pentane was added.
The precipitation was filtered off, washed with pentane and dried
to give 3a as a yellow solid (48 mg, 98%). 1H NMR (400 MHz, CDCl3):
(307 mg, 62%). 1H NMR (400 MHz, CDCl3):
d
[ppm] ¼ 0.23e1.02 (br,
d
[ppm] ¼ 2.26 (s, 6H, CH3), 4.15 (d, 3J ¼ 16.6 Hz, 2H, endo CH2N),
6H, BH3),1.16e1.97 (m, 38H, Cy), 2.07 (d, 2J ¼ 13.3 Hz, 2H, Cy), 2.30 (s,
6H, CH3), 2.36e2.47 (m, 2H, Cy), 2.68e2.78 (m, 2H, Cy), 3.94 (d,
2J ¼ 17.1 Hz, 2H, endo CH2N), 4.20 (s, 2H, NCH2N), 4.69 (d, 2J ¼ 17.1 Hz,
2H, exo CH2N), 6.81(s, 2H, AreH), 7.36 (d, JHeP ¼ 12.9 Hz 2H, Are
5.99 (d, 3J ¼ 16.6 Hz, 2H, exo CH2N), 6.12 (s, 2H, NCH2N), 6.82 (s, 2H,
AreH), 7.12 (d, 3J ¼ 10.5 Hz, 2H, AreH), 7.48e7.53 (m, 4H, AreH),
7.62e7.73 (m, 10H, AreH), 7.78e7.82 (m, 2H, AreH), 8.09e8.14
(m, 4H, AreH); 13C NMR (101 MHz, CDCl3):
d
[ppm] ¼ 20.5 (CH3),
H); 13C NMR (101 MHz, CDCl3):
d
[ppm] ¼ 21.0 (AreCH3), 25.9
63.5 (CH2N), 72.5 (NCH2N), 117.2, 126.2 (d, JCeP ¼ 18.0 Hz), 126.3

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R. Pereira, J. Cvengros / Journal of Organometallic Chemistry 729 (2013) 81e85
85
(d, JCeP ¼ 9.5 Hz), 126.7, 127.0, 128.2, 128.7, 129.0 (d, JCeP ¼ 12.9 Hz),
129.8 (d, JCeP ¼ 11.7 Hz), 131.9, 132.6 (d, JCeP ¼ 2.9 Hz), 132.9 (d,
JCeP ¼ 2.7 Hz), 133.6 (d, JCeP ¼ 10.7 Hz), 134.0 (d, JCeP ¼ 11.8 Hz),
134.7, 141.8 (d, JCeP ¼ 7.2 Hz), 149.5 (d, JCeP ¼ 16.1 Hz); 31P NMR
(162 MHz, CDCl3): 36.1 (d, JHeP ¼ 10.9 Hz); HRMS (ESI): m/z
[M þ NH4]þ calcd for C41H40 Cl4N3P2Pd2: 991.9508; found:
991.9489.
hospitality. Acknowledgment is also made to Rino Schwenk and
Raphael Aardoom for measuring and solving the X-ray crystal
structure.
Appendix A. Supplementary material
CCDC 912274 (Pd-complex 3a) contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from the Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif and are also available in the
supporting information for this article.
4.4. General procedure for Suzuki cross-coupling
A Schlenk flask was charged with aryl halide (1.5 mmol), aryl-
boronic acid (2.25 mmol, 1.5 equiv), K3PO4 (3.0 mmol, 2 equiv),
Pd(OAc)2 (0.03 mmol, 2 mol%), ligand (0.015 mmol, 1 mol%), evac-
uated and back filled with argon (3ꢂ). Dry THF (3.0 mL) was then
added. The resulting mixture was stirred at 70 ꢁC under argon for
a specified time and then cooled down to room temperature. It was
diluted with DCM (50 mL), filtered over a celite plug and evapo-
rated under reduced pressure to give a crude material which was
purified via column chromatography on silica gel.
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4.4.1. 1,10-Biphenyl
White solid; 1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 7.37(m, 2H,
AreH), 7.47 (m, 4H, AreH), 7.62 (m, 4H, AreH), 13C NMR (75 MHz,
CDCl3):
d
[ppm] ¼ 127.2, 127.3, 128.9, 141.3, MS (EI): m/z [M]þ calcd
for C12H10: 154.07; found: 154.0.
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Commun. (1999) 161e162;
4.4.2. 4-Nitro-1,10-biphenyl
White solid; 1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 7.43e7.53 (m,
(b) M. Valík, R.M. Strongin, V. Král, Supramol. Chem. 17 (2005) 347e367;
(c) B. Dolenský, J. Elguero, V. Král, C. Pardo, M. Valík, Advances in Heterocyclic
Chemistry, vol. 93, Elsevier Academic Press Inc, San Diego, 2007, pp. 1e56;
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3H), 7.63 (dd, 3J ¼ 8.1 Hz, 4J ¼ 1.4 Hz, 2H), 7.74 (d, 3J ¼ 8.7 Hz, 2H),
8.30 (d, 3J ¼ 8.7 Hz, 2H); 13C NMR (75 MHz, CDCl3):
[ppm] ¼ 124.1,
d
127.4, 127.8, 128.9, 129.2, 138.8, 147.6; MS (EI): m/z [M]þ calcd for
(e) B. Dolenský, M. Havlík, V. Král, Chem. Soc. Rev. 41 (2012) 3839e3858.
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(b) Y.-M. Jeon, G.S. Armatas, D. Kim, M.G. Kanatzidis, C.A. Mirkin, Small 5
(2009) 46e50;
C12H9NO2: 199.0; found: 199.0.
4.4.3. 2-Methyl-1,10-biphenyl
(c) C. Michon, M.-H. Goncalves-Farbos, J. Lacour, Chirality 21 (2009) 809e817;
(d) E.B. Veale, D.O. Frimannsson, M. Lawler, T. Gunnlaugsson, Org. Lett. 11
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(e) Z. Wu, M. Tang, T. Tian, J. Wu, Y. Deng, X. Dong, Z. Tan, X. Weng, Z. Liu,
C. Wang, X. Zhou, Talanta 87 (2011) 216e221;
White solid; 1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 2.33(s, 3H,
CH3), 7.27e7.33 (m, 4H, AreH), 7.35e7.40 (m, 3H, AreH), 7.42e7.47
(m, 2H, AreH); 13C NMR (75 MHz, CDCl3):
d
[ppm] ¼ 20.5, 125.8,
126.8, 127.3, 128.1, 129.2, 129.8, 130.3, 135.4, 141.9, 142.0; MS (EI):
(f) S. Satishkumar, M. Periasamy, Tetrahedron: Asymmetry 20 (2009)
2257e2262;
m/z [M]þ calcd for C13H12: 168.1; found: 168.1.
(g) B. Bhayana, M.R. Ams, J. Org. Chem. 76 (2011) 3594e3596;
(h) C.L. Ramirez, C. Pegoraro, L. Trupp, A. Bruttomesso, V. Amorebieta,
D.M.A. Vera, A.R. Parise, PCCP 13 (2011) 20076e20080.
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(2000) 12675e12682;
4.4.4. 2,6-Dimethyl-1,10-biphenyl
Colorless oil; 1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 2.07(s, 6H,
CH3-H) 7.11e7.20 (m, 5H, AreH), 7.33e7.38 (m, 1H, AreH), 7.42e
7.47 (m, 2H, AreH); 13C NMR (75 MHz, CDCl3):
d
[ppm] ¼ 20.8,
126.6, 127.0, 127.3, 128.4, 129.0, 136.1, 141.1, 141.9; MS (EI): m/z [M]þ
calcd for C14H14: 182.1; found: 182.1.
(d) D.R. Jensen, J.S. Pugsley, M.S. Sigman, J. Am. Chem. Soc. 123 (2001) 7475e7476;
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8005e8008;
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(i) E. Poli, E. Merino, U. Díaz, D. Brunel, A. Corma, J. Phys. Chem. C 115
(2011) 7573e7585;
4.4.5. 20-Chloro-[1,10-biphenyl]-4-carbonitrile
White solid; 1H NMR (300 MHz, CDCl3):
d
[ppm] ¼ 7.32e7.39 (m,
3H), 7.49e7.54 (m, 1H), 7.56 (d, 3J ¼ 8.2 Hz, 2H, AreH), 7.75 (d,
3J ¼ 8.2 Hz, 2H, AreH); 13C NMR (75 MHz, CDCl3):
d
[ppm] ¼ 111.5,
118.8, 127.2, 129.6,130.3, 130.3, 131.0,131.9, 132.2, 132.7,133.4,138.7,
(j) X. Du, Y.L. Sun, B.E. Tan, Q.F. Teng, X.J. Yao, C.Y. Su, W. Wang, Chem.
Commun. 46 (2010) 970e972.
144.0. MS (EI): m/z [M]þ calcd for C13H8NCl: 213.0; found: 213.0.
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(b) T. Weilandt, U. Kiehne, J. Bunzen, G. Schnakenburg, A. Lützen, Chem. Eur. J.
16 (2010) 2418e2426;
Acknowledgments
(c) M.S. Khoshbin, M.V. Ovchinnikov, C.A. Mirkin, J.A. Golen, A.L. Rheingold,
Inorg. Chem. 45 (2006) 2603e2609.
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J. C. thanks for the financial support from the Swiss National
Science Foundation (Ambizione project PZ00P2_131855). Prof.
Antonio Togni is gratefully acknowledged for generous support and
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