Cyclopalladation of N-phenylbenzamides: Synthesis and structure of bimetallic palladium(II)-complexes

Article abstract of DOI:10.1016/j.ica.2011.01.059

Three bimetallic palladium(II) complexes were generated by cyclopalladation of N-methyl-N-phenylbenzamide derivatives, substrates known to undergo oxidative intramolecular cross-coupling via palladium catalysis. These isolable Pd-complexes were characterized by X-ray crystallography. Stoichiometric and catalytic experiments with [(3-methoxy-N-methyl-N-phenylbenzamide)Pd(μ-TFA)] ₂ were investigated, and this palladium complex was found to be an effective precatalyst for oxidative cross-coupling.

Full text of DOI:10.1016/j.ica.2011.01.059

Inorganica Chimica Acta 369 (2011) 247–252
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Cyclopalladation of N-phenylbenzamides: Synthesis and structure
of bimetallic palladium(II)-complexes
⇑
Nadine Borduas, Alan J. Lough, Vy M. Dong
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 2 March 2011
Three bimetallic palladium(II) complexes were generated by cyclopalladation of N-methyl-N-phenylb-
enzamide derivatives, substrates known to undergo oxidative intramolecular cross-coupling via palla-
dium catalysis. These isolable Pd-complexes were characterized by X-ray crystallography.
Dedicated to Prof. Robert G. Bergman.
Stoichiometric and catalytic experiments with [(3-methoxy-N-methyl-N-phenylbenzamide)Pd(l-
TFA)]2 were investigated, and this palladium complex was found to be an effective precatalyst for oxi-
dative cross-coupling.
Keywords:
Palladium(II) dimer
C–H activation
Ó 2011 Elsevier B.V. All rights reserved.
Oxidative cross-coupling
Intramolecular reaction
1. Introduction
2. Experimental
Intramolecular oxidative cross-coupling has emerged as an
attractive strategy for generating cyclic architectures from rela-
tively simple precursors [1]. This transition metal-catalyzed
ring-closure involves C–C bond formation by the oxidation of
two C–H bonds [2]. Our group reported the palladium-catalyzed
lactam synthesis by oxidative cyclization of N-phenylbenzamides
2.1. Methods and materials
1H, 13C{1H}, and 19F NMR spectra were recorded on a Varian Mer-
cury 400 or a Bruker AV-III 400 spectrometer at ambient tempera-
ture. All NMR spectra are referenced to TMS or the residual solvent
signal. Data for NMR are reported as follows: chemical shift (d
ppm), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, br = broad), integration, coupling constant (Hz). High
resolution mass spectra (HRMS) were recorded on a micromass 70S-
250 spectrometer (EI) or an ABI/Sciex Qstar Mass Spectrometer (ESI).
Column chromatography was carried out on Silicycle Silica-P Flash
using sodium persulfate (Na2S2O8) as
a convenient oxidant
(Fig. 1) [3]. To further study this process, we sought to isolate a
Pd(II)-complex from the analogous stoichiometric reaction. We
imagined that N-methyl-N-phenylbenzamide (1a) could undergo
initial ortho C–H bond activation by cyclopalladation to generate
either Pd(II) six-membered palladacycle 2a or Pd(II) five-mem-
bered palladacycle 3a (Fig. 2).
Silica Gel (40–63
performed on EMD Silica Gel 60 F254 plates (254
l
m). Preparative thin layer chromatography was
m). Chemical re-
l
We report herein the isolation and characterization of dimeric
palladium-complexes [(3-methoxy-N-methyl-N-phenylbenzamide)
agents were purchased from Aldrich and Pd(OAc)2 was purchased
from Strem. Solvents were purchased from Sigma Aldrich and
Caledon and used without further purification.
Pd(
-TFA)]2 (3b) and [(3,4-dimethoxy-N-methyl-N-phenylbenza-
mide)Pd( -TFA)]2 (3c) derived from cyclopalladation of the corre-
l-TFA)]2 (3a), [(3-methoxy-N-methyl-N-(o-tolyl)benzamide)Pd
(l
2.2. X-ray data collection and crystal structure determination
l
sponding N-methyl-N-phenylbenzamide. The molecular structures
of complexes 3a, 3b and 3c were established by single-crystal X-
ray structure analysis. These palladacycles were tested to their fea-
sibility as intermediates in catalytic oxidative cross-coupling
reactions.
Data were collected on a Bruker-Nonius Kappa-CCD diffractom-
eter using monochromated Mo K
using a combination of / scans and
a
radiation and were measured
scans with offsets, to fill
x
j
the Ewald sphere. The data were processed using the Denzo-SMN
package [4]. Absorption corrections were carried out using SORTAV
[5]. The structure was solved and refined using SHELXTL V6.1 [6] for
full-matrix least-squares refinement that was based on F2. All H
atoms were included in calculated positions and allowed to refine
in riding-motion approximation with Uiso tied to the carrier atom.
⇑
Corresponding author. Tel.: +1 416 978 6484.
E-mail address: vdong@chem.utoronto.ca (V.M. Dong).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.01.059

248
N. Borduas et al. / Inorganica Chimica Acta 369 (2011) 247–252
131.1, 129.9, 128.2, 112.7, 112.6, 112.5, 55.3, 54.8, 40.5 (one C peak
overlapping). 19F NMR (376 MHz, DMF-d7) d À75.04.
2.4. Synthesis of phenylbenzamides
2.4.1. 3-Methoxy-N-methyl-N-(o-tolyl)benzamide (1b)
Prepared according to a literature procedure [3], 1b was synthe-
sized from N,2-dimethylaniline (0.62 mL, 5.0 mmol) and 3-
methoxybenzoyl chloride (0.82 mL, 6.0 mmol) to afford a yellow
oil after purification(1.27 g, 99%). 1H NMR (400 MHz, CDCl3) d
7.13–7.07 (m, 3H), 7.04–7.00 (m, 2H), 6.85–6.83 (m, 2H), 6.75
(dd, 1H, J = 2.1 Hz, J = 8.2 Hz), 3.63 (s, 3H), 3.38 (s, 3H), 2.22 (s,
3H). 13C{1H} NMR (100 MHz, CDCl3) d 170.4, 158.7, 143.6, 137.0,
134.8, 131.3, 128.6, 128.5, 127.7, 127.0, 120.8, 116.2, 113.3, 55.1,
37.6, 17.8. MS (ESI) m/z 256 (M+H), 278 (M+Na); HRMS (ESI) m/z
Calc. for C16H17NO2 [M+H]+: 256.1332. Found: 256.1320.
Fig. 1. Reported Pd(II)-catalyzed intramolecular oxidative cross-coupling.
2.3. Synthesis of palladium complexes
General procedure A: Palladium complexes 3a–3c were prepared
based on a modified literature procedure [3]. To a one-dram vial
was
added
3-methoxy-N-methyl-N-phenylbenzamide
(1a)
(48 mg, 0.20 mmol), Pd(OAc)2 (45 mg, 0.20 mmol) and 1,2-dichlo-
roethane (1 mL). The reaction mixture was stirred for 2 min and
trifluoroacetic acid (16 lL, 0.21 mmol) was subsequently added.
2.5. Catalytic reactions
The resulting solution was stirred on a heating block at 40 °C for
2.5 h under air. After cooling to ambient temperature, the reaction
mixture was left to stand for several minutes until a yellow precip-
itate was observed. The mixture was filtered through a cotton-
plugged pipette and the resulting yellow residue was washed
(4 Â 1 mL) with dichloromethane. The filtrate was then discarded.
The yellow residue was suspended in dichloromethane and subse-
quently transferred into a tared vial and concentrated in vacuo to
afford the bimetallic palladacycle 3a as a yellow solid (53 mg, 58%).
General procedure B: Phenanthridinones 4a–4d were prepared
based on a modified literature procedure [3]. To a one-dram vial
was added 3-methoxy-N-methyl-N-phenylbenzamide (1a) (48 mg,
0.20 mmol),
[(3-methoxy-N-methyl-N-phenylbenzamide)Pd(l-
TFA)] (3a) (9.2 mg, 0.01 mmol) or Pd(OAc)2 (4.5 mg, 0.02 mmol),
Na2S2O8 (190 mg, 0.80 mmol), and 1,2-dichloroethane (1 mL). The
mixture was stirred at room temperature for 2 min and subse-
quently, tetradecane (52
lL, 0.20 mmol) and trifluoroacetic acid
(77 L, 1.0 mmol) were added. The vial was sealed under air with a
Teflon cap and stirred on a heating block at 70 °C for the indicated
amount of time. The reactions were monitored by GC–FID.
l
2.3.1. [(3-Methoxy-N-methyl-N-phenylbenzamide)Pd(l-TFA)]2 (3a)
1H NMR (400 MHz, DMF-d7) d 7.68–7.60 (m, 5H), 6.73 (d, 1H,
J = 7.3 Hz), 6.62 (d, 1H, J = 8.1 Hz), 5.31 (d, 1H, J = 1.9 Hz), 3.51 (s,
3H), 3.24 (s, 3H). 13C{1H} NMR (100 MHz, DMF-d7) d 178.4, 156.2,
143.0, 139.1, 135.8, 132.2, 131.1, 130.0, 127.9, 118.2, 114.2, 54.7,
41.0. 19F NMR (376 MHz, DMF-d7) d À74.78.
3. Crystallography
Recrystallization of 3a–3c from N,N-dimethylformamide and
ether by slow diffusion gave single crystals suitable for X-ray anal-
ysis. A perspective view of each complex 3a, 3b and 3c with the
pertinent atom-labeling scheme is shown in Figs. 3–5, respectively.
Crystal data and refinement information for each structure is given
in Table 1. Selected bond distances and angles are given in Table 2
for 3a, 3b and 3c.
In all cases, the palladium atom is coordinated in a slightly dis-
torted square plane (i.e. clamshell geometry) [7]. This geometry is
also apparent through the difference in bond angles between the
2.3.2. [(3-Methoxy-N-methyl-N-(o-tolyl)benzamide)Pd(l-TFA)]2 (3b)
Prepared according to general procedure A, using 3-methoxy-N-
methyl-N-(o-tolyl)benzamide (51 mg, 0.20 mmol) to afford the
bimetallic palladacycle 3b as a yellow solid (62 mg, 65%). 1H
NMR (400 MHz, DMF-d7) d 7.55–7.48 (m, 4H), 6.75 (dd, 1H,
J = 2.2 Hz, J = 8.4 Hz), 6.64 (d, 1H, J = 7.8 Hz), 5.29 (d, 1H,
J = 2.8 Hz), 3.46 (s, 3H), 3.23 (s, 3H), 2.23 (s, 3H). 13C{1H} NMR
(100 MHz, DMF-d7) d 178.4, 156.5, 141.5, 139.1, 135.8, 135.6,
132.5, 132.2, 130.4, 128.9, 128.2, 118.5, 113.0, 54.8, 39.8, 16.8.
19F NMR (376 MHz, DMF-d7) d À74.96.
Obenzamide–Pd(1)–Pd(2) and OTFA–Pd(1)–Pd(2) where an increase
in 14–22° is observed for the open-end of the clamshell. Of note,
the two arenes of the ligand are perpendicular to each other and
the two ligands are oriented in a head to tail fashion. Furthermore,
we observe that each palladacycle is a Pd(II)-bimetallic complex.
The internuclear distance between the two Pd atoms in 3a, 3b
and 3c were determined to be 2.8945(6), 2.8859(6) and
2.8959(6) Å, respectively. In comparison to other Pd-complexes
bearing bridging trifluoroacetate ligands, bimetallic Pd complex
3a exhibits slightly shorter Pd–Pd bonds [8].
2.3.3. [(3,4-Dimethoxy-N-methyl-N-phenylbenzamide)Pd(l-TFA)]2
(3c)
Prepared according to general procedure A, using 3,4-dime-
thoxy-N-methyl-N-phenylbenzamide (54 mg, 0.20 mmol) to afford
the bimetallic palladacycle 3c as a yellow solid (46 mg, 47%). 1H
NMR (400 MHz, DMF-d7) d 7.68–7.64 (m, 2H), 7.60–7.56 (m, 3H),
6.24 (s, 1H), 5.28 (s, 1H), 3.71 (s, 3H), 3.48 (s, 3H), 3.06 (s, 3H).
13C{1H} NMR (100 MHz, DMF-d7) d 178.2, 150.6, 145.7, 143.1,
Fig. 2. Possible palladacycles from N-phenylbenzamide.

N. Borduas et al. / Inorganica Chimica Acta 369 (2011) 247–252
249
H bond functionalizations [7,9]. For C–H bond activation of N-phe-
nylbenzamides, we reasoned that either the formation of the six-
or five-membered palladacycle could be possible (Fig. 2). To inves-
tigate the regioselectivity of the ortho-palladation, we submitted 3-
methoxy-N-methyl-N-phenylbenzamide (1a) to stoichiometric
Pd(OAc)2 and trifluoroacetic acid (TFA) (Table 3, entry 1).1 Recrys-
tallization of the yellow solid followed by X-ray analysis of a single
crystal allowed confirmation of a TFA-bridged dimeric Pd-complex
(3a). Complexes 3b and 3c were obtained via an analogous method
(Table 3, entries 2 and 3)2. In all three cases, we observed clean con-
version to the thermodynamically and kinetically favored five-mem-
bered palladacycle, presumably by C–H bond activation on the
benzamide ring (Fig. 6). In previous studies on intermolecular oxida-
tive cross-couplings, six-membered palladacycles from the C–H
bond activation of anilides were isolated [3]. In these studies, at-
tempts to isolate related five-membered palladacycles derived from
N-alkyl-benzamides had been unsuccessful.
Fig. 3. ORTEP plot of [(3-methoxy-N-methyl-N-phenylbenzamide)Pd(l-TFA)]2 (3a).
All H atoms have been omitted for clarity. Anisotropic displacement ellipsoids are
shown at the 50% probability level.
By 1H NMR analysis of 3a, 3b, and 3c, the ortho-benzamide
protons have chemical shifts ca.5.30 ppm, which is significantly
more upfield than the benzamide protons of their respective
starting material (chemical shifts between approximately 6.75–
7.0 ppm). This observation is in agreement with the X-ray crystal
structure of the complexes which show a PdÀC single bond in the
solid state. 19F NMR studies show that each complex possesses a
single peak at a slightly different chemical shift (Table 3). We
found that the Pd-complexes are unstable in solution in the pres-
ence of electrolytes.3 The low solubility of 3a, 3b and 3c in the
majority of solvents prevented us from acquiring reliable photo-
physical data. These Pd(II)-dimers exhibit some solubility in N,N-
dimethylformamide and dimethylsulfoxide. All three palladacycles
are bench-stable for months provided that they are kept away from
light.
Fig. 4. ORTEP plot of [(3-methoxy-N-methyl-N-(o-tolyl)benzamide)Pd(l-TFA)]2
(3b). All H atoms have been omitted for clarity. Anisotropic displacement ellipsoids
4.2. Stoichiometric experiments
are shown at the 50% probability level.
With the dimeric palladium complex 3a in hand, we sought to
investigate its viability as an intermediate in the catalytic cycle.
Previous work in our group has demonstrated that the mechanism
of oxidative cross-coupling depends on the directing group and the
oxidant [2k,3]. For instance, in the ortho-arylation of O-phenylcar-
bamates, reductive elimination from the Pd(II) dimers was ob-
served at 70 °C in benzene without additives, while for anilides,
the isolated palladacycles underwent reductive elimination solely
in the presence of TFA and Na2S2O8. In the case of our intramolec-
ular reaction, the coupling partner is part of the ligand. Therefore,
for the anilide portion of the ligand to undergo C–H activation fol-
lowed by reductive elimination, a rearrangement of the ligand pos-
sibly involving dissociation of the TFA or of the carbonyl of the
amide, is required.
In an attempt to observe stoichiometric reductive elimination
directly from 3a, we heated a stoichiometric amount of this Pd-
1
Pd-complex 3a could also be obtained from the reaction of one equivalent of
Pd(TFA)2 and 3-methoxy-N-methyl-N-phenylbenzamide (1a) in 67% yield. This Pd(II)-
dimer gave identical NMR data as a greenish yellow solid.
2
Other substrates known to undergo oxidative cross-coupling under Pd(II)
Fig. 5. ORTEP plot of [(3,4-dimethoxy-N-methyl-N-phenylbenzamide)Pd(l-TFA)]2
(3c). All H atoms have been omitted for clarity. Anisotropic displacement ellipsoids
are shown at the 50% probability level.
catalysis [3] were tested, but failed to generate an isolable Pd-complex. Further,
these complexes were not observed to form in a significant amount based on NMR
studies. The substitution pattern on the arene rings of the benzamide appears to
impact the stability of the corresponding palladacycle.
3
4. Results and discussion
Attempts to acquire cyclic voltammetry data were unsuccessful due to the
decomposition of the complexes in coordinating solvents in the presence of the
electrolyte tetrabutylammonium hexafluorophosphate (TBAPF6). Dissolving the
complexes in acetonitrile, for example, followed by the addition of TBAPF6 resulted
in decomposition to Pd black. The 1H NMR of this solution indicated a complex
mixture of products. The complexes in acetonitrile-d3 are stable as confirmed by
repeated 1H NMR experiments.
4.1. Synthesis of the palladium complexes
Pd(II)-dimers have attracted interest due to their proposed role
as precursors to Pd(III)–Pd(III) or Pd(II)–Pd(IV) intermediates in C–

250
N. Borduas et al. / Inorganica Chimica Acta 369 (2011) 247–252
Table 1
Crystal data and structure refinement for 3a, 3b and 3c.
3a
3b
3c
Empirical formula
Formula weight
T (K)
C
34H28F6N2O8Pd2
C36H32F6N2O10Pd2
979.44
150(1)
C36H32F6N2O8Pd2
947.44
150(1)
919.38
150(1)
k (Å)
0.71073
0.7107
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
C2/c
20.7156(8)
13.0752(5)
15.1485(4)
90
monoclinic
P21/n
12.4030(5)
14.8379(3)
20.6358(8)
90
monoclinic
C2/c
19.6309(5)
12.8287(5)
17.0792(6)
90
a
(°)
b (°)
124.5460(17)
90
3379.6(2)
4
1.807
1.152
97.2140(14)
90
3767.6(2)
4
1.727
1.043
1952
124.0120(15)
90
3565.4(2)
4
1.765
1.095
c
(°)
V (Å3)
Z
Density (calculated) (Mg/m3)
Absorption coefficient (mmÀ1
F(0 0 0)
)
1824
1888
Crystal size (mm)
h range for data collection (°)
Index ranges
0.18 Â 0.06 Â 0.06
2.74–27.50
À26 6 h 6 26, À16 6 k 6 16,
À16 6 l 6 19
11 009
0.22 Â 0.12 Â 0.08
0.16 Â 0.16 Â 0.10
2.86–27.53
À25 6 h 6 23, À16 6 k 6 16,
À21 6 l 6 22
15 737
2.75–27.55
À16 6 h 6 16, À16 6 k 6 19,
À21 6 l 6 26
Reflections collected
25 472
Independent reflections (Rint
Completeness to h = 27.50°
Absorption correction
Maximum and minimum transmission
Refinement method
)
3847 (0.0710)
99.2%
8586 (0.0777)
98.8%
semi-empirical from equivalents
0.925 and 0.831
full-matrix least-squares on F2
8586/0/511
4067 (0.0490)
98.9%
0.937 and 0.750
0.901 and 0.827
Data/restraints/parameters
3847/0/237
1.061
4067/0/247
1.075
Goodness-of-fit (GOF) on F2
1.048
Final R indices [I >
R indices (all data)
r
(I)]
R1 = 0.0471, wR2 = 0.1021
R1 = 0.0954, wR2 = 0.1252
2.065 and À1.092
R1 = 0.0604, wR2 = 0.1246
R1 = 0.1407, wR2 = 0.1621
2.132 and À0.973
R1 = 0.0459, wR2 = 0.1104
R1 = 0.0754, wR2 = 0.1281
1.876 and À0.911
Largest difference in peak and hole
(e ÅÀ3
)
Table 3
Table 2
Selected bond lengths (Å) and bond angles (°) for 3a, 3b and 3c.
Synthesis and characterization of the Pd-complexes.
Entry Substrate R1, R2 Yield 1H NMR (d)
Pd-
complex (%)
19F NMR
(d)
(ppm)
Bond lengths
3a
3b
3c
for Hortho
(ppm)
Pd(1)–Pd
Pd(1)–C
2.8945(6)
1.949(5)
2.017(3)
2.053(3)
2.181(3)
1.288(5)
1.481(6)
1.396(6)
2.8859(6)
1.931(6)
2.008(4)
2.040(4)
2.179(4)
1.294(7)
1.450(8)
1.411(8)
2.8959(6)
1.945(4)
2.024(3)
2.047(3)
2.192(3)
1.272(5)
1.487(6)
1.408(6)
1
2
3
1a
1b
1c
R1 = H,
R2 = H
3a
3b
58
65
47
5.31
5.29
5.28
À74.78
À74.96
À75.04
Pd(1)–Obenzamide
Pd(1)–OTFA,
trans to carbonyl
R
1 = H,
Pd(1)–OTFA,
trans to arene
R2 = Me
O
C
C
carbonyl–Ccarbonyl
carbonyl–Carene
arene–CPd
R
1 = OMe, 3c
R2 = H
C–Pd(1)–Pd
carbonyl–Pd(1)–Pd
100.80(12) 102.37(17) 102.78(12)
O
102.55(8)
80.83(8)
79.92(8)
99.57(12)
83.35(12)
78.12(11)
98.41(9)
84.16(8)
77.51(8)
OTFA,
OTFA,
carbonyl–Pd(1)–Pd
arene–Pd(1)–Pd
trans to
is a viable process from complex 3a and the efficiency varies
depending on the reaction conditions.
trans to
O
O
carbonyl–Pd(1)–OTFA,
carbonyl–Pd(1)–OTFA,
175.16(12) 175.86(16) 175.73(11)
trans to carbonyl
92.81(12)
82.06(16)
130.3(4)
93.95(16)
91.18(12)
90.38(17)
81.9(2)
130.6(6)
94.6(2)
93.34(11)
81.78(15)
130.5(4)
94.35(15)
90.54(12)
trans to arene
C–Pd(1)–Ocarbonyl
OTFA, carbonyl–CTFA–OTFA
C–Pd(1)–OTFA,
trans to
4.3. Catalytic reactions
trans to carbonyl
OTFA,
carbonyl–Pd(1)–OTFA,
93.10(17)
trans to
trans
to arene
Next, Pd-complex 3a was tested as a catalyst in oxidative cross-
coupling reactions (Fig. 8) [10]. When 3a is used to catalyze the
cyclization of substrate 1a, (i.e. the substrate and ligand are of
the same source), the reaction is complete within 24 h, whereas
with Pd(OAc)2, the reaction progresses to only 54% yield (Table 5,
entries 1 and 2). Complex 3a also catalyzed the oxidative cross-
coupling of other N-methyl-N-phenylbenzamide substrates. Of
note, substrate 1c yields 64% of the desired phenanthridinone in
the presence of catalytic Pd(OAc)2, in contrast to yielding 99% with
Pd-complex 3a (Table 5, entries 3 and 4). Pd-complex 3a also cat-
alyzed the reaction with substrate 1d faster than Pd(OAc)2 (Table 5,
entries 5 and 6). These results suggest that Pd-complex 3a is a
more reactive precatalyst than Pd(OAc)2 and that Pd(OAc)2 appears
to require a longer induction period than Pd-complex 3a. Pallada-
cycles 3b and 3c gave similar results in the catalytic reactions; they
Carene–CPd–Pd(1)
114.1(3)
114.5(4)
114.7(3)
complex in 1,2-dichloroethane at 70 °C (Fig. 7). The experiment
generated no desired cross-coupled product within the time frame
of the corresponding catalytic process (24 h) (Table 4, entry 1). On
the other hand, if the reaction was stirred for 4 days at the same
temperature, phenanthridinone 4a could be detected by GC–FID
(Table 4, entry 3). When both TFA and Na2S2O8 were added to
the reaction mixture, the reductive elimination product was de-
tected within 24 h, but in low yield (Table 4, entry 2). Product 4a
appears to be unstable under the reaction conditions over pro-
longed times (Table 4, entry 4). Therefore, reductive elimination

N. Borduas et al. / Inorganica Chimica Acta 369 (2011) 247–252
251
Fig. 6. Synthesis and characterization of the Pd-complexes.
Table 4
Table 5
Stoichiometric reductive elimination reactions.
Comparison of Pd(II) catalysts in the oxidative intramolecular cross-coupling reaction.
Entry
Additive
Time
Yielda
Entry
R1, R2
Substrate
Catalyst
Time (h)
Yielda (%)
1
2
3
4
None
TFA, Na2S2O8
None
24 h
24 h
4 days
4 days
–
1
2
3
4
5
6
R1 = H, R2 = OMe
1a
1a
1c
1c
1d
1d
3a
Pd(OAc)2
3a
Pd(OAc)2
3a
Pd(OAc)2
24
24
24
24
24
24
90
54
99
64
60
33
21%
17%
9%
R
1 = H, R2 = OMe
R1, R2 = OMe
R1, R2 = OMe
R1, R2 = H
TFA, Na2S2O8
a
R1, R2 = H
The yield was determined by GC–FID analysis with C14H30 as an internal
standard.
a
The yield was determined by GC–FID analysis with C14H30 as an internal
standard.
Fig. 7. Stoichiometric reductive elimination reactions.
Fig. 8. Comparison of Pd(II) catalysts in the oxidative intramolecular cross-coupling reaction.
catalyze the intramolecular oxidative arylation more efficiently
than Pd(OAc)2.
Acknowledgments
Funding was provided by the University of Toronto, Canada
Foundation for Innovation, Ontario Research Fund, Boehringer
Ingelheim Ltd. (Canada), and the Natural Sciences and Engineering
Research Council (NSERC) of Canada. V.M.D. is grateful for an
Alfred P. Sloan fellowship and N.B. is grateful for the NSERC André
Hamer Postgraduate Prize and the Julie Payette Research Scholar-
ship. The authors thank Paul DiCarmine and Professor Dwight
Seferos for helpful discussions.
5. Conclusion
In summary, three novel dimeric TFA-bridged palladium(II)
complexes have been isolated from an ortho-palladation reaction.
Single crystal X-ray structures were obtained for Pd-complexes
3a, 3b and 3c and these structures establish a regioselective CÀH
bond activation to form five-membered palladacycles. Pd-complex
3a undergoes reductive elimination at 70 °C to give phenanthridi-
none with low efficiency. Nonetheless, Pd-complex 3a is an effec-
tive precatalyst for oxidative cross-coupling reactions. Further
investigation into the mechanism of this transformation is
warranted.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.01.059.

252
N. Borduas et al. / Inorganica Chimica Acta 369 (2011) 247–252
(d) R.B. Bedford, M. Betham, J.P.H. Charmant, M.F. Haddow, A.G. Orpen, L.T.
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