Chelating Bis-N-heterocyclic Carbene-Palladium(II) Complexes for Oxidative Arene C-H Functionalization

Article abstract of DOI:10.1021/om501163m

Bis-N-heterocyclic carbene (NHC)-chelated palladium(II) complexes have been synthesized, characterized fully including single-crystal X-ray structural analyses, and utilized for the first time toward catalytic oxidative C-H functionalization of arenes with PhI(OAc)₂ and N-bromosuccinimide. (Figure Presented).

Full text of DOI:10.1021/om501163m

Article
pubs.acs.org/Organometallics
Chelating Bis-N-heterocyclic Carbene−Palladium(II) Complexes for
Oxidative Arene C−H Functionalization
Sai Puneet Desai,† Moumita Mondal, and Joyanta Choudhury*
Organometallics & Smart Materials Laboratory, Department of Chemistry, Indian Institute of Science Education and Research
Bhopal, Bhauri, Bhopal By-pass Road, Bhopal 462 066, India
S
* Supporting Information
ABSTRACT: Bis-N-heterocyclic carbene (NHC)-chelated palladium(II) complexes have
been synthesized, characterized fully including single-crystal X-ray structural analyses, and
utilized for the first time toward catalytic oxidative C−H functionalization of arenes with
PhI(OAc)2 and N-bromosuccinimide.
INTRODUCTION
■
The selective functionalization of unactivated arene and alkane
C−H bonds to valuable organic derivatives has been a
challenging research area in organometallic catalysis for a
long time.1 In this context, more recently, the oxidative
functionalization approach, involving high-valent palladium
intermediates (PdIV and PdIII) in the catalytic cycles, has
attracted considerable attention owing to its successful
demonstration in the halogenation, acetoxylation, and other
functionalization reactions of alkanes and arenes.2,3 The key to
the success of the above oxidative catalysis is the stability of
high-valent palladium species (e.g., PdIV) in the catalytic cycles
under oxidizing (and acidic) conditions. The stereoelectronic
properties of the ancillary ligands play an important role in
stabilizing such high oxidation states of the metal center.
Palladium complexes with a chelating bis-N-heterocyclic
carbene (NHC) ligand framework4 seem to be ideal candidates
with regard to the design of desirable catalysts. This is because
Figure 1. Application of chelated bis-NHC scaffold for oxidative
of the following factors: (i) due to strong σ-donating ability,
PdII/IV-based stoichiometric and catalytic C−H bond functionalization.
NHCs would facilitate oxidative addition at palladium(II) with
the oxidant and also stabilize the octahedral d6-PdIV center via
efficient σ(sp2)carbene → σ(eg)Pd donation; (ii) the bis-NHC
scaffold would impart extra stability to PdIV via chelation; (iii)
NHC ligands possess high stability under strong oxidizing and
acidic conditions (as required for oxidative transformations);
and (iv) metalated NHCs are less prone to competitive
reductive elimination from the PdIV intermediates and would
thus favor desired functionalization reaction.5 Strassner et al.
effectively utilized the above features in designing a family of
chelated palladium(II)-bis-NHC complexes and demonstrated
elegant examples of oxidative trifluoroacetoxylation of methane
and propane with a K2S2O8/CF3COOH reagent system (Figure
1).6 Hou et al. succeeded in achieving a regioselective
trifluoroacetoxylation of n-propyl and n-octyl esters by utilizing
similar palladium(II)-bis-NHC catalysts (Figure 1).7 Recently,
Kraft et al. advanced one more step and actually synthesized
(bis-NHC)PdIVCl4 complexes for further direct chlorination of
aromatic and aliphatic C−H bonds, albeit in a stoichiometric
manner (Figure 1).8 In a similar study, Arnold and Sanford also
implemented a chelating anionic alkoxy-tethered NHC ligand
to achieve PdII/PdIV-based aromatic C−H halogenations.5 In
parallel to the alkane activation, oxidative functionalization of
arene C−H bonds is another highly demanding transformation.
Although palladium(II) complexes with other ligand systems
Received: November 18, 2014
© XXXX American Chemical Society
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(mainly pyridine-based) including mono-NHCs have been
previously used for direct acetoxylation of arenes,9 the
application of the above-described bis-NHC strategy in this
transformation is yet to be executed. Motivated by this
background, herein we report the first efforts in the exploration
of the strategy of utilizing bis-NHC-chelated palladium(II)
complexes for catalytic oxidative functionalization of arene C−
H bonds (Figure 1).
Ccarbene−PdII−Ccarbene bite angles of 83.0(7)° (for 1) and
83.38(16)° (for 2), which resulted in I−PdII−I bond angles of
94.01(6)° (for 1) and 93.105(14)° (for 2). The PdII−Ccarbene
bond distances in 1 and 2 were 1.988(12) and ∼1.987(4) Å,
respectively. The other bond lengths and bond angles were
typical of PdII−bis NHC complexes.11
As NHCs are known to exert strong σ-donating influence to
the metal centers in their complexes, it is expected that these
palladium complexes would undergo an easy oxidation to
generate highly electrophilic PdIV species, conducive for facile
C−H activation, under oxidative catalysis conditions. First we
selected acetoxylation of arene C−H bonds to explore the
above opportunity using reported catalytic conditions.5,9a The
model acetoxylation reaction of toluene (10 equiv) with
PhI(OAc)2 (1 equiv) at 95 °C in a AcOH/Ac2O (9:1) solvent
mixture revealed that 3 mol % of complex 1 or 2 afforded ∼52−
58% yield of the product after 24 h (Scheme 2). With the use of
RESULTS AND DISCUSSION
■
The chelated bis-NHC palladium(II) complexes 1 and 2 were
synthesized from the corresponding imidazolium salts (L1 and
L2) in good yields via the reaction with Pd(OAc)2 in dimethyl
sulfoxide solvent in a controlled manner, as shown in Scheme 1.
Scheme 1. Synthesis of the Pd(II) Complexes 1 and 2
Scheme 2. Catalytic C−H Acetoxylation of Toluene by
Complexes 1 and 2
1
a 1:1 molar ratio of toluene and PhI(OAc)2, the reaction did
not proceed to yield any acetoxylated product, whereas the
other molar ratios of toluene/PhI(OAc)2 (5:1 and 15:1) led to
a decreased yield of the product (see Supporting Information).
However, the regioisomeric distribution of acetoxylated toluene
was found to be unchanged in all these cases. The acetoxylation
reaction did not proceed in solvent like acetonitrile. Similarly,
the yield of the acetoxylated product was found to be reduced
when the reaction was run for less than 24 h (Figure S28,
Supporting Information). A comparative catalytic acetoxylation
of toluene performed with complex 2, a model Strassner’s
complex [(NHC-CH2-NHC)PdI2], and Pd(OAc)2 as catalysts
indicated that complex 2 was a marginally better catalyst (see
Supporting Information for details). Hence, with 3 mol %
loading of catalyst 1 and catalyst 2, we explored other arene
molecules for catalytic C−H functionalization with PhI(OAc)2
as oxidant (Table 1). There was a marginal increase in the
yields with anisole as substrate. The regioisomeric ratio (ortho/
meta/para) of the acetoxylated products was clearly regulated
by the inductive and mesomeric effects of the functional groups
present in the substrate. In order to verify the effect of a
directing group on the regioselectivity of the products, 2-
phenylpyridine and acetophenone were employed. As expected,
a single regioisomer (ortho) was obtained in these cases, which
indicated that the pyridine and carbonyl functionalities must
have coordinated to the metal center, thus making it difficult for
the formation of multiple isomers. The catalysts showed poor
reactivity with an unactivated substrate (benzene). We further
tested this oxidative C−H functionalization protocol with N-
bromosuccinimide (NBS) as oxidant, and some preliminary
results showed that NBS could also be utilized for the same
(Table 1). Both complexes 1 and 2 were found to be active in
bromination of only the substrates with directing groups. The
yields were found to be moderate to good.
Characterization of the complexes was achieved by H and
13C{1H} NMR as well as 2D NMR spectroscopy, high-
resolution electron-spray ionization mass spectrometric
(ESIMS) methods, and elemental analyses (see Experimental
Section). The 1H NMR spectra of complexes 1 and 2 disclosed
the absence of the imidazolium proton signals indicating
metalation. The 13C{1H} NMR spectra of the complexes
exhibited the carbenic carbon (Pd−Ccarbene) signals at 166.3
ppm (for 1) and 162.1 ppm (for 2), respectively.10 Both the
complexes were further unambiguously characterized by single-
crystal X-ray diffraction analyses. As depicted in Figure 2, the
molecular structures of 1 and 2 confirmed the usual square
planar geometry around the d8-PdII center with a slight
distortion. The bidentate coordination mode of the bis-NHC
ligand to the palladium center generated a six-membered
metallacycle with a boat conformation. The chelates exhibited
Figure 2. ORTEP diagrams of complexes 1 (left) and 2 (right) (30%
ellipsoid probability level). Selected bond lengths (Å) and bond angles
(deg): Complex 1: C1−Pd1 = 1.988(12); Pd1−C1i = 1.988(12);
Pd1−I1 = 2.6637(13); C1i−Pd1−C1 = 83.0(7); I1−Pd1−I1i =
94.01(6). Complex 2: C1−Pd1 = 1.987(4); C15−Pd1 = 1.982(4);
Pd1−I1 = 2.6474(4); Pd1−I2 = 2.6528(4); C15−Pd1−C1 =
83.38(16); I1−Pd1−I2 = 93.105(14).
To gain some insight into the mechanistic steps, we
conducted a series of control experiments (Scheme 3). It was
B
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Table 1. (Bis-NHC)Pd(II)-Complex-Catalyzed
Acetoxylation and Bromination of Arenes
Scheme 3. Control Experiments
a
a
Reaction conditions A: ArH (10 mmol), PhI(OAc)2 (1 mmol), bis-
NHC-Pd(II) catalyst (complex 1 or complex 2) (0.03 mmol), AcOH/
Ac2O (1 mL; 9:1, v/v), 95 °C, 24 h. Reaction conditions B: ArH (10
mmol), NBS (1 mmol), bis-NHC-Pd(II) catalyst (complex 1 or
complex 2) (0.03 mmol), CH3CN (1 mL), 95 °C, 24 h. Yields were
calculated by GCMS using chlorobenzene as internal standard added
b
c
after the reaction was over. Ortho/meta/para: 1.2/1.0/1.3. Ortho/
d
e
meta/para: 1.16/1.0/1.44. Ortho/meta/para: 1.1/1.0/1.9. 2-/3-
Isomeric mixture.
observed that in the absence of oxidant the reaction did not
yield any functionalized product (Scheme 3a), indicating the
oxidative nature of the catalysis. An important observation of
the formation of iodotoluene (trace amount based on toluene,
but 78% based on catalyst 1) along with the desired
acetoxylated product, in the catalytic reaction of toluene with
PhI(OAc)2 by complex 1, provided a strong indication in favor
of the formation of an “Ar−PdIV−I” intermediate during the
catalysis (Scheme 3b). This PdIV intermediate was possible to
generate via oxidation of (NHC)2PdII(I)2 by PhI(OAc)2
followed by acetate-assisted aryl C−H activation with the
resulting (NHC)2PdIV(OAc)2(I)2 intermediate. A similar
observation was also reported by Car
́
denas et al.9a The
involvement of the C−H bond-breaking reaction in the rate-
determining step of the catalysis was indicated by the
intermolecular kinetic isotope effect (KIE) value of 4.25
C
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(Scheme 3c, Figure S31, Supporting Information). Next, a
stoichiometric reaction using equimolar amounts of complex 2
and the oxidant PhI(OAc)2 in AcOH/Ac2O (9:1, v/v) at 95 °C
was performed to check the integrity and robustness of the
“(NHC)2Pd” coordination backbone as well as to evaluate the
nature of the active catalyst during the catalysis (Scheme 3d).
Analysis of the reaction mixture by GCMS indicated the
presence of only iodobenzene as the organic product, which
should have been generated from the oxidant PhI(OAc)2 after
the oxidation of palladium(II) in 2. There was no evidence for
the presence of acetoxylated-NHC ligands. The above results
suggested that the “(NHC)2Pd” backbone of the catalyst
remained intact during catalysis under an oxidizing environ-
ment. It is noteworthy to mention here that ESIMS studies
have been proved very useful in investigating the mechanistic
details of palladium-catalyzed C−H acetoxylation reactions as
recently demonstrated by Xu and co-workers.12 Along the same
line, the ESIMS studies of the above reaction mixture suggested
the generation of an (NHC)2Pd(OAc)2 species plausibly via
oxidation of the palladium(II) precursor followed by a
reductive elimination of I2 from the oxidized
(NHC)2PdIV(OAc)2(I)2 intermediate (Scheme 3d). Reductive
elimination of I2 was confirmed by extraction of I2 from the
above reaction mixture with CCl4, which showed the
characteristic violet color (see Supporting Information for
details). It was further confirmed that the liberation of I2 was
not observed without oxidant under similar reaction conditions.
No C−H acetoxylation was observed from the reaction of
toluene with KOAc (instead of PhI(OAc)2) in the presence of a
catalytic amount of complex 2 under similar reaction conditions
(Scheme 3e), suggesting that the present acetoxylation reaction
is not a simple C−H substitution with acetate of the active
species, (NHC)2Pd(OAc)2, which was found to be generated
from the reaction of complex 2 with KOAc as well via anion
exchange (Scheme 3f). The proposed active species,
(NHC)2Pd(OAc)2, was independently synthesized and charac-
terized by NMR spectroscopic and ESI-MS techniques
(Scheme 3g; see Supporting Information for details). Control
catalytic experiments by using KOAc and PhI(OAc)2 as
acetoxylating agents suggested again that in the presence of
oxidizing agent (PhI(OAc)2) only, C−H acetoxylation catalysis
was successful (Scheme 3h).
The activation of the catalyst precursor (NHC)2Pd(I)2 to
generate active (NHC)2Pd(OAc)2 species could occur via
oxidation by PhI(OAc)2 to (NHC)2Pd(I)2(OAc)2 followed by
a reductive elimination of I2 (Scheme 4a).13 It is rational to
hypothesize that the metal center in (NHC)2Pd(OAc)2, being
electron-rich through coordination by strong σ-donor NHC
ligands, undergoes oxidation prior to C−H activation, to result
in the PdIV intermediate (NHC)2Pd(OAc)4 (Scheme 4b). Next,
an acetate-assisted electrophilic C−H activation of arene by the
PdIV center generated a PdIV−aryl intermediate, well-poised for
the reductive elimination of the product ArOAc. The
generation of the trace amount of iodoarene side product
during catalysis could be explained by a similar acetate-assisted
electrophilic C−H activation of arene by the PdIV center in
(NHC)2Pd(I)2(OAc)2 followed by reductive elimination of ArI
(Scheme 4c). Although no PdIV intermediate complex could be
isolated from the reaction and a PdII/Pd0 cycle cannot be ruled
out completely, all the experimental results of the control
studies suggested the involvement of a PdII/PdIV cycle under
the present catalytic conditions.
CONCLUSIONS
■
In summary, we have reported the synthesis of two bis-NHC-
chelated palladium(II) complexes, which were characterized
fully including single-crystal X-ray structures. These bis-NHC-
PdII complexes have been demonstrated for the first time in
oxidative arene C−H bond functionalization catalysis with
PhI(OAc)2 and NBS as oxidants. Plausible catalytic steps have
been proposed based on some key control experiments, which
suggested the involvement of a PdII/PdIV cycle during catalysis.
Improvement of the catalytic performance through efficient
ligand design will be the subject of future work.
EXPERIMENTAL SECTION
■
I. General Methods and Materials. 1H and 13C{1H} NMR
spectra were recorded on Bruker AVANCE III 400 and 700 MHz
NMR spectrometers at room temperature unless mentioned otherwise.
Chemical shifts (δ) are expressed in ppm using the residual proton
resonance of the solvent as an internal standard (DMSO: δ = 2.50
1
ppm for H spectra, 39.5 ppm for 13C{1H} spectra; acetone: δ = 2.05
ppm for 1H spectra, 206.3 and 29.8 ppm for 13C{1H} spectra;
CH3CN: δ = 1.94 ppm for 1H spectra, 118.3 and 1.3 ppm for 13C{1H}
spectra). All coupling constants (J) are expressed in hertz (Hz) and
only given for 1H−1H couplings unless mentioned otherwise. The
following abbreviations were used to indicate multiplicity: s (singlet), d
(doublet), t (triplet), and m (multiplet). ESI mass spectrometry was
performed on a Bruker microTOF QII spectrometer. Dry solvents and
reagents were obtained from commercial suppliers and used without
further purification. Deuterated solvents were purchased from Aldrich.
[(4-N,N′ Dimethylaminobenzene)bis(imidazaloyl)methane] and [(4-
methoxybenzene)bis(imidazaloyl)methane] were prepared by follow-
ing a reported procedure (see Supporting Information).14
Based on the above control studies and available literature
reports,3,5,9,12 a plausible catalytic cycle is shown in Scheme 4.
Scheme 4. Plausible Catalytic Cycle
II. Synthetic Procedures. Ligand Precursor L1. A round-bottom
flask was charged with a mixture of [(4-N,N′-dimethylaminobenzene)-
bis(imidazaloyl)methane] (80 mg, 0.3 mmol) and iodomethane (55.9
μL, 0.9 mmol) under a nitrogen atmosphere. Acetonitrile (1 mL) was
added to the flask, and the reaction mixture was stirred at room
temperature for 24 h. The progress of the reaction was monitored by
TLC (CH2Cl2/CH3OH, 9:1 (v/v)). After the completion of the
reaction, the solvent was removed by suction filtration, and the
product was washed several times with diethyl ether. The solid was
dried under vacuum. Yield: 190 mg (92%). 1H NMR (400 MHz,
[D6]DMSO): δ 9.42 (s, 2H, CHimz), 8.57 (s, 1H, NCHN), 8.18 (d,
3JH−H = 9.1 Hz, 2H, CHarom), 8.00 (s, 2H, CHimi), 7.96 (s, 2H, CHimi),
7.74 (d, 3JH−H = 9.0 Hz, 2H, CHarom), 3.92 (s, 6H, CHMe‑imi), 3.67 ppm
(s, 9H, CHNMe3). 13C{1H} NMR (100 MHz, [D6]DMSO): δ 148.9,
D
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138.2, 132.8, 129.7, 125.3, 121.9, 120.9, 70.6, 56.6, 36.5 ppm. Anal.
Calcd for C18H26I3N5: C, 31.19; H, 3.78; N, 10.10. Found: C, 31.10;
H, 3.75; N, 10.02.
V. General Procedure for the Catalytic Oxidative Acetox-
ylation and Bromination of Arenes with Complexes 1 and 2.
Acetoxylation: Diacetoxyiodobenzene (40.4 mg, 0.125 mmol),
complex 1 (2.96 mg, 0.00375 mmol), or complex 2 (2.40 mg,
0.00375 mmol) and the arene (1.25 mmol) were placed in a sealed
tube that was equipped with a magnetic bar. One milliliter of freshly
prepared acetic acid/acetic anhydride (9:1 v/v) was added to the
mixture. The reaction mixture was stirred at 95 °C for 24 h. It was
then cooled to room temperature, and the yield was calculated by
GCMS analysis using PhCl as an internal standard added after the
reaction was over.
Bromination: N-Bromosuccinimide (22.2 mg, 0.125 mmol),
complex 1 (2.96 mg, 0.00375 mmol) or complex 2 (2.40 mg,
0.00375 mmol), and the arene (1.25 mmol) were placed in a sealed
tube, which was equipped with a magnetic bead. One milliliter of
acetonitrile was added to the mixture. The reaction mixture was stirred
at 95 °C for 24 h. It was then cooled to room temperature, and the
yield was calculated by GCMS analysis using PhCl as an internal
standard added after the reaction was over.
Ligand Precursor L2. A round-bottom flask was charged with a
mixture of [(4-methoxybenzene)bis(imidazaloyl)methane] (44 mg,
0.17 mmol) and iodomethane (107.8 μL, 1.73 mmol) under a nitrogen
atmosphere. Acetonitrile (0.5 mL) was added to the flask, and the
reaction mixture was stirred at room temperature for 24 h. The
progress of the reaction was monitored by TLC (CH2Cl2/CH3OH,
9:1 (v/v)). After the completion of the reaction, the mixture was
added in a dropwise manner to a round-bottom flask containing
diethyl ether. The precipitate thus formed was collected by suction
filtration and was washed quickly with 1 mL of ice-cold dichloro-
methane. Finally, the solid was dried under vacuum. Yield: 67 mg
(70%). 1H NMR (400 MHz, CD3CN): δ 9.24 (s, 2H, CHimi), 8.60 (s,
1H, NCHN), 7.76 (t, JH−H = 1.9 Hz, 2H, CHimi), 7.57 (t, JH−H = 1.9
Hz, 2H, CHimi), 7.48 (d, 3JH−H = 8.8 Hz, 2H, CHarom), 7.13 (d, 3JH−H
=
8.8 Hz, 2H, CHarom), 3.93 (s, 6H, CHMe‑imi), 3.88 ppm (s, 3H,
CHOMe). 13C{1H} NMR (100 MHz, [D6]DMSO): δ 163.0, 138.4,
130.6, 126.2, 122.7, 122.2, 116.1, 72.8, 56.5, 37.8 ppm. Anal. Calcd for
C16H20I2N4O.H2O: C, 34.55; H, 3.99; N, 10.07. Found: C, 34.82; H,
3.94; N, 10.01.
ASSOCIATED CONTENT
■
S
* Supporting Information
1
1
1H, 13C{1H}, H−1H COSY, H−13C HSQC, and HMBC
NMR spectra; ESI mass spectra; additional text; CIF files of
complexes 1 (CCDC 1034491) and 2 (CCDC 1034492). The
Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/om501163m.
Complexes 1 and 2. Pd(OAc)2 (23.3 mg, 0.1 mmol) and ligand
precursor L1 (68 mg, 0.1 mmol) or L2 (56 mg, 0.1 mmol) were
charged into a Schlenk flask. The flask was evacuated and subsequently
filled with argon gas three times. Under this condition, 5 mL of
degassed DMSO was added to the flask. The mixture was initially
stirred at room temperature for 1 h and then at 60 °C for 12 h (in the
case of 1) or at 80 °C for 12 h (in the case of 2) and at 120 °C for 1 h
(in the case of 1) or at 120 °C for 2 h (in the case of 2) more. The
reaction mixture was cooled, and the solvent was removed through
vacuum distillation (at 60 °C). The solid thus obtained was collected
by suction filtration and washed with water, ether, and ice-cold
dichloromethane. Then the product was dried under vacuum.
AUTHOR INFORMATION
Corresponding Author
*E-mail: joyanta@iiserb.ac.in.
■
Present Address
†Department of Chemistry, University of Minnesota, Minne-
apolis−St. Paul, Minnesota, USA.
1
Complex 1: yield 40 mg (60%). H NMR (400 MHz, CD3CN): δ
3
8.46 (s, 1H, NCHN), 8.06 (d, JH−H = 1.9 Hz, 2H, CHimi), 8.02 (d,
3
3JH−H = 8.8 Hz, 2H, CHarom), 7.68 (d, JH−H = 8.8 Hz, 2H, CHarom),
Notes
7.38 (d, 3JH−H = 1.9 Hz, 2H, CHimi) 3.96 (s, 6H, CHMe‑imi), 3.88 ppm
(s, 9H, CHNMe3). 13C{1H} NMR (100 MHz, [D6]acetone): δ 166.3,
141.5, 130.8, 123.9, 123.0, 121.6, 75.4, 58.1, 30.6 ppm (one peak was
not observed). HRMS (ESI, positive ion): [M]+ = 669.9179
(calculated as 669.9157 for [C18H24I2N5Pd]+). Anal. Calcd for
C18H24I3N5Pd·CH3SOCH3: C, 27.43; H, 3.45; N, 8.00. Found: C,
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
J.C. sincerely thanks DAE-BRNS, DST, and IISER Bhopal for
generous financial support. S.P.D. thanks the DST and IISER
Bhopal for an INSPIRE fellowship during the BSMS program.
M.M. thanks the CSIR for a doctoral fellowship. The authors
thank Dr. Debasish Ghorai for help.
1
27.25; H, 3.47; N, 8.03. Complex 2: yield 35 mg (53%). H NMR
(400 MHz, [D6]DMSO): δ 7.95−7.80 (m, 3H, CHimi+NCHN), 7.53−
3
7.24 (m, 4H, CHimi+CHarom), 6.98 (d, JH−H = 8.8 Hz, 2H, CHarom),
3.89 (s, 6H, CHMe‑imi), 3.77 ppm (s, 3H, CHOMe). 13C{1H} NMR (100
MHz [D6]DMSO): δ 162.1, 159.5, 129.2, 128.1, 123.3, 122.2, 113.7,
74.3, 55.3, 40.7 ppm. HRMS (ESI, positive ion): [M]+ = 514.9506
(calculated as 514.9561 for [C16H18IN4OPd]+). Anal. Calcd for
C18H24I3N5Pd·0.25CH3SOCH3: C, 29.93; H, 2.97; N, 8.46. Found: C,
29.69; H, 3.00; N, 8.48.
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ray diffraction data were collected using a Bruker SMART APEX II
CCD diffractometer with graphite-monochromated Mo Kα (λ =
0.710 73 Å) radiation at different low temperatures for each crystal.
Structures were solved with direct methods using SHELXS-97 and
refined with full-matrix least-squares on F2 using SHELXL-97.15 Full
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IV. General Procedure for the Catalytic Oxidative Acetox-
ylation of Toluene with Complexes 1 and 2. Diacetoxyiodo-
benzene (40.4 mg, 0.125 mmol), catalyst (varying amounts), and
toluene (1.25 mmol) were placed in a sealed tube that was equipped
with a magnetic bar. One milliliter of freshly prepared acetic acid/
acetic anhydride (9:1 v/v) mixed solvent was added to the mixture.
The reaction mixture was stirred at 95 °C for 24 h. It was then cooled
to room temperature, and the yield was calculated by GCMS analysis
using PhCl as an internal standard added after the reaction was over.
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DOI: 10.1021/om501163m
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