Oxazoline chemistry. Part VIII. Synthesis and characterization of a new class of pincer ligands derived from the 2-(o-anilinyl)-2-oxazoline skeleton - Applications to the synthesis of group X transition metal catalysts

Article abstract of DOI:10.1139/v05-163

The synthesis and characterization of a new and readily synthesized class of potentially anionic pincer ligands with C₁ point group symmetries is described. These materials can be made via amide coupling of a 2-(2′-anilinyl)-2-oxazoline unit wi

Full text of DOI:10.1139/v05-163

1185
Oxazoline chemistry. Part VIII. Synthesis and
characterization of a new class of pincer ligands
derived from the 2-(o-anilinyl)-2-oxazoline
skeleton — Applications to the synthesis of group
X transition metal catalysts1
Andreas Decken, Robert A. Gossage, and Paras N. Yadav
Abstract: The synthesis and characterization of a new and readily synthesized class of potentially anionic pincer lig-
ands with C1 point group symmetries is described. These materials can be made via amide coupling of a 2-(2′-
anilinyl)-2-oxazoline unit with picolinic acid; the incorporation of enantiopure oxazoline fragments facilitates the con-
struction of chiral C1 pincers. Treatment of the free ligands with Pd metal sources leads to the formation of amido–Pd
pincer complexes in good yield. One of these Pd complexes has been characterized by single crystal X-ray diffraction
methods, which confirms the proposed tridentate binding mode of the ligand and the formation of an amido N—Pd
bond. The metal complexes have been shown to be suitable precusors for catalytically active Pd species that are useful
for C—C bond forming reactions, notably the Heck reaction under standard conditions.
Key words: oxazoline, 4,5-dihydro-2-oxazole, palladium, pincer ligand, amido, Heck reaction.
Résumé : On décrit la synthèse facile et la caractérisation d’une nouvelle classe de ligands anioniques à pinces ayant
des groupes de symétrie ponctuelle C1. Ces produits peuvent être obtenus par couplage d’amide d’une unité 2-(2′-anili-
nyl)-2-oxazoline avec de l’acide picolinique; l’incorporation de fragments oxazolines énantiopurs facilite la formation
de ligands à pince de symétrie C1. Le traitement des ligands libres avec des sources de palladium métallique conduit à
la formation, avec de bons rendements, de complexes amido–Pd pincer. On a caractérisé un de ces complexes du Pd
par diffraction des rayons X par un cristal unique et on a ainsi pu confirmer le mode de liaison tridentate du ligand et
la formation d’une liaison N—Pd amido. On a montré que les complexes métalliques sont des précurseurs appropriés
pour des espèces catalytiquement actives de Pd qui sont utiles dans les réactions de formation de liaisons C—C, parti-
culièrement dans la réaction de Heck dans des conditions standards.
Mots clés : oxazoline, 4,5-dihydro-2-oxazole, palladium, ligand à pince, amido, réaction de Heck.
[Traduit par la Rédaction] Decken et al. 1189
Introduction
dendrimer chemistry, molecular sensor technology, the study
of unusual bonding motifs or metal oxidation states (e.g.,
Ni3+), and metal-mediated catalysis (4). Despite these ad-
vances, there has been increasing interest in ligands of lower
symmetry; this is due to the fact that in many cases superior
enantioselective catalysis can be obtained from species con-
taining such ligands (1a, 1c, 5). Herein, we report the facile
syntheses of C1-symmetric amide pincer ligands containing
a chiral (or achiral) oxazoline group in addition to a second-
ary binding (pyridine) functionality. In addition, these
amides (Fig. 1) are shown to readily deprotonate (to form
anionic pincers) on reaction with Pd sources to form Pd–
In recent years, enantiopure (C2 symmetric) bisoxazolines
have become a popular and useful class of ligands for appli-
cations in asymmetric synthesis promoted by Lewis acids
(1). Oxazolines have been shown to possess a rich transition-
metal chemistry (2), and recently the oxazoline fragment has
been incorporated into the design of the so-called “pincer”
ligand framework. This has resulted in the accessibility of
potentially anionic and C2-symmetric pincers (3). Pincer lig-
ands have been at the forefront of a number of areas of in-
vestigation within organometallic chemistry including
Received 3 March 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 30 September 2005.
A. Decken.2 Department of Chemistry, University of New Brunswick, Fredericton, NB E3B 6E2, Canada.
R.A. Gossage3 and P.N. Yadav. The David Upton Hill Laboratories of Inorganic Chemistry, Department of Chemistry, Acadia
University, 6 University Avenue, Wolfville, NS B4P 2R6, Canada.
1Part VII: A. Decken and R.A. Gossage. J. Inorg. Biochem. 99, 644 (2005). This manuscript is dedicated to Professor Stephen R.
Stobart on the occasion of his retirement.
2Corresponding author for crystallography (e-mail: adecken@unb.ca).
3Corresponding author (e-mail: rgossage@acadiau.ca).
Can. J. Chem. 83: 1185–1189 (2005)
doi: 10.1139/V05-163
© 2005 NRC Canada

1186
Can. J. Chem. Vol. 83, 2005
Fig. 1. Synthetic methodologies for the production of 3–6.
NH2
NH2
N
O
N
O
1
2
COOH
N
N
O
O
[1]
+ 1 or 2
4
3
N
NH
NH
or
coupling
N
O
N
O
[2]
5
6
N
Li2PdCl4
base
O
N
O
3 or 4
or
Pd Cl
N
N
Cl
Pd
N
N
O
O
amido complexes; these metal derivatives are shown to be
active catalysts for the Heck reaction under standard proto-
cols.
To confirm the mononuclear nature of these species, a single
crystal X-ray diffraction study was carried out on a sample
of complex 5. A molecular respresentation of this material is
found in Fig. 2. Crystal data can be found in Table 1; Ta-
ble 2 gives a list of selected bond lengths and angles for
complex 5.
Results and discussion
The X-ray data confirm that 5 is a monopalladium com-
plex incorporating deprotonated 3; bond lengths and angles
are typical for a formal Pd2+ metal centre (1, 9). Of note is
the formal amido–Pd interaction in addition to the coordina-
tion of both the pyridine and the oxazoline groups forming
five- and six-membered rings, respectively; the latter func-
tionality binds through the N atom, as expected (2). Thus, a
tridenate (κN, κN′, κN′′ ) bonding motif is presented. The
coordination geometry around the Pd atom is best described
as distorted square-planar in nature. Therefore, ligands such
as 3 do form pincer complexes and in excellent yields.
Several Pd pincer compounds are known to be active pro-
moters of selective C—C bond formation (10). We have
therefore performed some preliminary tests on complexes 5
and 6 as promoters of the Heck reaction under typical reac-
tion conditions (9). Both complexes catalyse the regio-
selective coupling of iodobenzene with styrene to give high
yields of trans-stilbene under standard protocols in the pres-
ence of base (NaOAc, see the Experimental section). Nota-
bly, these reactions were preformed in open air and hence
these pincer catalysts appear to be highly robust (11) to the
high temperature oxidizing conditions employed.
The synthesis of amide 3 (Fig. 1) was accomplished using
the in situ produced (SOCl2) acid chloride (6) derivative of
picolinic acid (PA), followed by the low temperature addi-
tion of readily available 4,4-dimethyl-2-(o-anilinyl)-2-
oxazoline (1, Fig. 1, eq. [1]) (7). Unfortunately, yields of
chiral analogue 4 were found to be variable by the above
methodology, but the desired product could be reproducibly
synthesized using PA and 2 under DCC–DMAP protocols
(Fig. 1, eq. [1], also see Experimental section). Both 3 and 4
can thus be easily produced in large quantities (>1 g). The
air-stable nature of these materials makes them easier to
handle than many of the sensitive “PCP” or “pincer carbene”
ligands (1). Facile ligand “tuning” (8) can be readily envi-
sioned because of the availability of a number of analogues
of 1 and 2 (7).
Palladium complexes of deprotonated 3 or 4 are formed in
good yield upon treatment of (MeOH) solutions of the lig-
ands with stoichiometric Li2PdCl4. These reactions lead to
the isolation of solid and air-stable brown-coloured complex
5 (82%) and yellow 6 (91%), respectively (Fig. 1, eq. [2]).
1H NMR data on both materials, in combination with ele-
mental analysis, IR data, and the diamagnetic nature of both
complexes, strongly suggest the formation of 1:1 adducts of
the ligand fragments with Pd2+. Specifically, the disappear-
Conclusions
1
ance of the H NMR signal assigned to the amide N-H pro-
ton and loss of the corresponding IR stretching frequency
assigned to this group, imply the formation of complexes of
the general formula [Pd(L-H)Cl]x (L = 3 or 4, respectively).
In conclusion, access to a new and readily synthesized
class of potentially anionic pincer ligands has been realised.
These compounds can be used to make catalytically active
© 2005 NRC Canada

Decken et al.
1187
Fig. 2. Image of the molecular structure of complex 5 showing the atomic numbering scheme. Thermal ellipsoids are at the 30% prob-
ability level.
Pd species for selective C—C bond forming reactions that
can be used in the absence of inert atmosphere conditions.
The investigation of other transition-metal complexes of
these and related ligands, and their catalytic potentials, will
be disclosed in due course.
and Et3N (7.6 g, 75 mmol) in 35 mL of CH2Cl2 (6). Stirring
was continued for 30 min and then a suspension of 1 (7a)
(9.5 g, 50 mmol) in 20 mL of CH2Cl2 was added (at 0 °C).
The stirring was continued for a further 3 h as the mixture
warmed to room temperature (RT); the reaction vessel was
then left standing overnight. Neutralization by the addition
of satd. aq. Na2CO3 was followed by isolation of the organic
layer, which was subsequently washed with water (3 ×
15 mL) and then evaporated in vacuo. The crude product
thus obtained was purified by (flash) chromatography (SiO2,
230–400 mesh) using hexanes – ethyl acetate (3:1, v/v) as
the eluent (Rf = 0.50). The yellow solid product was thereaf-
ter isolated following solvent evaporation. Yield: 6.63 g
(45%); mp 110 °C. IR (cm–1): 3054 (w), 1670 (s), 1645 (s),
Experimental section
General
All reactions were carried out using standard bench-top
laboratory techniques using commercially available, reagent-
1
grade solvents. H NMR spectra were recorded from chloro-
form-d solutions at 300 MHz using a Bruker Avance 300
NMR spectrometer. Microanalyses were recorded at the ana-
lytical services department of the University of Windsor
(Windsor, Ontario). IR spectra were obtained as Nujol muls
using a PerkinElmer 683 IR spectrometer. Melting point data
were obtained using a Mel-Temp II apparatus and reported
values are uncorrected. Optical rotation measurements were
made on a Bellingham + Stanley Ltd. model ADP220 polari-
meter using a 1 dm path length optical cell.
1
1583 (s), 1540 (s). H NMR (300 MHz, CDCl3) δ: 1.51 (s,
6H), 4.12 (s, 2H), 7.15 (t, 1H, J = 7.9 Hz), 7.47–7.58 (m,
2H), 7.87–7.95 (m, 2H), 8.30 (d, 1H, J = 7.9 Hz), 8.69 (d,
1H, J = 4.5 Hz), 9.03 (d, 1H, J = 8.3 Hz). Anal. calcd. for
C17H17N3O2 (%): C 69.14, H 5.80, N 14.23; found: C 70.03,
H 5.82, N 14.57.
Synthesis of 4
In a round-bottomed flask containing chilled (10 °C)
CH2Cl2 (160 mL) was added 2 (7a) (2.136 g, 11.2 mmol)
and PA (1.674 g, 13.6 mmol). The mixture was stirred under
an inert atmosphere for 25 min at 0 °C. To this mixture was
added DMAP (0.412 g, 3.2 mmol) and DCC (3.492 g,
Syntheses
Synthesis of 3
Thionyl chloride (5.6 g, 50 mmol) was added dropwise to
a stirred and cold (0 °C) mixture of PA (6.15 g, 50 mmol)
© 2005 NRC Canada

1188
Can. J. Chem. Vol. 83, 2005
Table 2. Selected bond lengths (Å) and angles
Table 1. X-ray diffraction data for complex 5.
(°) for complex 5 (esds in brackets).
Formula weight
Formula
436.18
C17H16ClN3O2Pd
Bond lengths (Å)
Pd—N1
Pd—Cl
Pd—N12
Pd—N15
2.0254(14)
Temperature (K)
λ (Å)
Crystal system
198(1)
2.3248(5)
2.0077(14)
2.0236(15)
0.710 73
Monoclinic
P2(1)/c
Space group
Bond angles (°)
N1-Pd-N15
Cl-Pd-N12
N12-Pd-N1
N12-Pd-N15
Unit cell dimensions:
a (Å)
b (Å)
169.07(6)
171.55(4)
90.01(6)
81.76(6)
11.231 8(6)
17.399 1(10)
95.515(1)
8.421 0(5)
1 638.04(16)
4
1.769
1.311
872
0.40 × 0.25 × 0.20
1.82–27.50
–13 ≤ h ≤ 14,
–20 ≤ k ≤ 22,
–10 ≤ l ≤ 10
11 291
β (°)
c (Å)
Volume (Å3)
Z
Density (calcd., Mg/m3)
Absorption coefficient (mm–1)
F(000)
7.15 (t, 1H, J = 8.0 Hz), 7.27 (m, 3H), 7.49–7.57 (m, 2H),
7.67 (t, 1H, J = 4.1 Hz), 7.90–7.95 (m, 2H), 8.31 (d, 1H, J =
7.8 Hz), 8.90 (d, 1H, J = 4.3 Hz), 9.01 (d, 1H, J = 8.3 Hz).
Anal. calcd. for C22H17N3O2 (%): C 74.35, H 4.82, N 11.82;
found: C 74.91, H 4.63, N 12.26.
Crystal size (mm3)
θ Range for data collection (°)
Index ranges
Synthesis of 5
To a 10 mL solution (MeOH) of in situ formed Li2PdCl4
(0.5 mmol, prepared by boiling 0.088 g (0.5 mmol) of PdCl2
and 0.127 g of LiCl (3 mmol)) was added 3 (0.145 g,
0.5 mmol). The mixture was stirred at RT for 48 h and the
resulting brown complex isolated by filtration, washed with
MeOH and ether, and then dried at 60 °C overnight. Yield:
0.179 g (82%), mp 251 to 252 °C. IR (cm–1): 1646 (s), 1614
Reflections
Independent reflections
R(int)
Completeness to θ (°)
Absorption correction
Refinement method
3 679
0.020 6
97.6% (27.50)
SADABS
Full-matrix least-
squares on F2
3 679 / 0 / 281
1.066
1
(s), 1485 (s), 520 (w), 340 (w). H NMR (300 MHz, CDCl3)
δ: 1.79 (s, 6H), 4.30 (s, 2H), 7.02 (t, 1H, J = 7.7 Hz), 7.41–
7.53 (m, 2H), 7.82 (t, 1H, J = 8.1 Hz), 8.04 (m, 2H), 8.49 (d,
1H, J = 8.6 Hz), 9.27 (d, 1H, J = 5.5 Hz). Anal. calcd. for
C17H16N3O2ClPd·1/2H2O (%): C 45.86, H 3.85, N 9.44;
found: C 45.53, H 3.64, N 9.68.
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices (I > 2σ(I))
R1
0.020 0
0.052 3
wR2
R indices (all data)
R1
Synthesis of 6
0.023 6
As described above for complex 5 using Li2PdCl4
(0.5 mmol) and 4 (0.145 g, 0.5 mmol). Yield: 0.225 g (91%)
of a yellow powder; mp > 300 °C. [α]D (300 K, c 0.49,
CH2Cl2) –385°. IR (cm–1): 1623 (s), 1598 (s), 1482 (s), 530
(w), 335 (w). 1H NMR (300 MHz, CDCl3) δ: 3.50 (t, 1H, J =
5.1 Hz), 3.57 (s, 1H), 5.49 (m, 1H), 6.43 (d, 1H, J = 7.1 Hz),
6.98 (t, 1H, J = 8.1 Hz), 7.25–7.37 (m, 3H), 7.43–7.47 (m,
1H), 7.50–7.55 (m, 1H), 7.86 (dd, 1H, J = 8.1 Hz), 8.03–
8.05 (m, 2H), 8.43 (d, 1H, J = 8.4 Hz), 8.53 (t, 1H, J =
5.3 Hz), 9.23 (d, 1H, J = 5.6 Hz). Anal. calcd. for
C22H16N3O2ClPd (%): C 53.25, H 3.25, N 8.47; found: C
53.68, H 3.17, N 9.25.
wR2
0.054 3
0.612 and –0.185
Largest diff. peak and hole (e Å–3)
Note: where wR2 = {Σ[w(Fo – Fc2)2]/Σ[Fo ]}1/2, R1 = Σ||Fo| –
2
4
|Fc||/Σ|Fo2|. Weight = 1/[σ2(Fo ) + (0.0303P)2 + (0.6142P)], where P =
2
(max (Fo , 0) + 2Fc2)/3.
16.8 mmol) and the mixture was then stirred for an addi-
tional 25 min at 0 °C. The cooling bath was removed and
the solution warmed up to RT. After being stirred for 48 h,
the mixture was filtered (thrice) to yield a clear filtrate. The
filtrate was then washed with water (4 × 300 mL), 5% aq.
HOAc (160 mL), and again with water (4 × 200 mL). The
organic layer was then dried (MgSO4), filtered, and then all
volatile components were removed on a rotary evaporator.
The crude product thus obtained was purified by flash silica
gel chromatography (230–400 mesh) using hexanes – ethyl
acetate (75:25, Rf = 0.61) as the eluent, following by solvent
evaporation. Yield: 1.79 g (45%); mp 189 °C. [α]D (300 K, c
0.5, CH2Cl2) –74°. IR (cm–1): 1680 (s), 1636 (s), 1579 (s),
Catalysis
Heck coupling reactions of iodobenzene and styrene
using 5 or 64
Iodobenzene (25 mmol), styrene (30 mmol), N,N-
dimethylformamide (25 mL), sodium acetate (30 mmol), and
0.5 mg (~1 µmol) of 5 or 6 were combined in a round-
bottomed flask equipped with a reflux condenser (9). The
mixture was heated, in open air, to 140–150 °C for 24 h. Af-
ter cooling to RT, 0.1 L of dil. aq. HCl was added and the
1
1520 (s). H NMR (300 MHz, CDCl3) δ: 3.45 (s, 1H), 3.54
(d, 1H, J = 6.7 Hz), 5.49 (m, 1H), 5.97 (d, 1H, J = 7.9 Hz),
4 Nota bene: no attempt was made to optimize the reaction conditions.
© 2005 NRC Canada

Decken et al.
1189
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organic components were extracted with ethyl acetate (3 ×
100 mL). The organic layer was washed (brine) and dried
(Na2SO4). After filtration, volatiles were removed (vacuo)
and the product trans-stilbene (correct mp and H NMR)
was purified by flash column chromatography (hexanes–
1
EtOAc, 9:1). Yield: 95% (using 5); 89% (using 6).
X-ray crystallography of complex 55
Single crystals were coated with Paratone-N oil, mounted
using a glass fibre, and frozen in the cold nitrogen stream of
the goniometer. A hemisphere of data was collected on a
Bruker AXS P4/SMART 1000 diffractometer using ω and θ
scans with a scan width of 0.3° and 10 s exposure times. The
detector distance was 5 cm. The data were reduced (SAINT)
(12) and corrected for absorption (SADABS) (13). The
structure was solved by direct methods and refined by full-
matrix least-squares on F2 (SHELXTL) (14). Hydrogen at-
oms were included in calculated positions and refined using
a riding model.
Acknowledgements
This work has been supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC). Sepra-
cor Canada Ltd. is thanked for a generous donation of chiral
indanols and Johnson-Matthey, Inc. is thanked for a loan of
Pd salts.
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5 Supplementary data for this article are available on the Web site or may be purchased from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0S2, Canada. DUD 4011. For more information on obtaining mate-
rial refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml. CCDC 263208 contains the crystallographic data for this manuscript. These
data can be obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (Or from the Cambridge Crystallographic Data Centre,
12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or deposit@ccdc.cam.ac.uk).
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