Synthesis and structure of 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene palladium complexes and application in catalytic hydroarylation of alkynes

Article abstract of DOI:10.1021/om1011984

An abnormal or mesoionic N-heterocyclic carbene (NHC), namely 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene (Tz), formed from 1,4-diphenyl-3-methyl-1,2,3-triazolium iodide and silver oxide, gave the corresponding silver carbene complex. The transmetalatio

Full text of DOI:10.1021/om1011984

ARTICLE
pubs.acs.org/Organometallics
Synthesis and Structure of 1,4-Diphenyl-3-methyl-1,2,3-triazol-
5-ylidene Palladium Complexes and Application in Catalytic
Hydroarylation of Alkynes
Rajendran Saravanakumar, Venkatachalam Ramkumar, and Sethuraman Sankararaman*
Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India
S Supporting Information
b
ABSTRACT: An abnormal or mesoionic N-heterocyclic car-
bene (NHC), namely 1,4-diphenyl-3-methyl-1,2,3-triazol-5-yli-
dene (Tz), formed from 1,4-diphenyl-3-methyl-1,2,3-
triazolium iodide and silver oxide, gave the corresponding silver
carbene complex. The transmetalation reaction of the silver
carbene complex with PdCl2(CH3CN)2 gave a trans mono-
nuclear complex, [(Tz)2PdCl2]. Reaction of the silver carbene
complex with Pd(OAc)2 gave an acetate-bridged binuclear
complex in which the palladium had undergone a C-H
insertion at the ortho position of the phenyl group attached
to the triazole nitrogen (1-position). The structures of these
two complexes were established by single-crystal XRD data and spectroscopic data. The acetate complex was used as a catalyst for
the stereoselective hydroarylation of alkynes.
’ INTRODUCTION
the synthesis of metal complexes of 1,2,3-triazol-5-ylidenes. The
synthesis and structural characterization of 1,2,3-triazol-5-yli-
dene-metal (Pd, Rh, and Ir) complexes were first reported by
Albrecht.6a We have reported the first examples of chiral and
chelate type palladium-1,2,3-triazol-5-ylidene complexes and
their catalytic activity in Suzuki-Miyaura coupling reactions.6b
Recently Bertrand has isolated and structurally characterized a
free 1,2,3-triazol-5-ylidene and established the donor properties
to be superior to those of imidazol-2-ylidenes and 1,2,4-triazol-5-
ylidenes but inferior to those of imidazol-4-ylidenes (another
example of an abnormal NHC).7 In the present study we have
used 1,4-diphenyl-3-methyl-1,2,3-triazol-5-ylidene(Tz) asa ligand
for the synthesis of two types of palladium complexes. The first is a
mononuclear chloro derivative, and the second is a dinuclear
acetate-bridged derivative. The synthesis and structure of these
two complexes are reported herein. The acetate complex has been
used for the catalytic hydroarylation of alkynes. During the course
of the review process of this paper Albrecht has reported similar
complexes.6c
N-heterocyclic carbenes (NHCs) have emerged as versatile
ligands for metal-NHC complexes of a variety of metal ions.1
NHCs have been shown to be useful nucleophilic organo-
catalysts,2 and several transition-metal-NHC complexes and
in particular Pd-NHC complexes are gradually replacing con-
ventional phosphine-metal complexes in many catalytic proce-
sses.3 NHCs have been proven to be excellent σ donors and poor
π acceptors and are often rated as stronger and better ligands
than the traditional phosphine ligands.4 A vast majority of reports
on NHCs are based on 1,3-disubstituted imidazolylidenes and
1,4-disubstituted 1,2,4-triazolylidenes.5 NHCs can be classified
into two broad categories: namely, normal NHCs, whose struc-
ture can be represented in neutral canonical form, and abnormal
NHCs, whose structure can only be represented as a zwitterion.
Imidazolylidenes and 1,2,4-triazolylidenes belong to the normal
NHCs. Recently NHCs derived from substituted 1,2,3-triazol-5-
ylidenes have been reported, and they belong to the class of
abnormal NHCs.6 The term “mesoionic carbene” has been
introduced to describe abnormal NHCs in the recent literature.7
Substituted 1,2,3-triazoles are relatively easy to synthesize by the
1,3-dipolar cycloaddition of an azide to an alkyne (click reaction).
Subsequent N-alkylation results in the formation of substituted
1,2,3-triazolium ions which are precursors for the synthesis of
1,2,3-triazol-5-ylidenes. In comparison to normal NHCs, abnor-
mal NHCs are stronger σ donors according to the Tolman
electronic parameter.8 1,2,3-Triazolylidenes are emerging as
popular ligands in the area of organometallic chemistry of
transition metals. To date only a few reports have appeared on
’ RESULTS AND DISCUSSION
Synthesis and Characterization. 1,4-Diphenyl-3-methyl-
1,2,3-triazolium iodide (2) was obtained in 70% yield by
methylation of 1,4-diphenyl-1,2,3-triazole (1), which was ob-
tained by the click reaction between phenylacetylene and phenyl
Received: December 26, 2010
Published: February 28, 2011
r
2011 American Chemical Society
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Scheme 1. Synthesis of 1,4-Diphenyl-3-methyl-1,2,3-triazo-
lium Iodide (2)
Figure 1. ORTEP drawing of 4 with thermal ellipsoids at the 30%
ꢀ
probability level. Selected bond distances (Å) and angles (deg): Pd1-
C7 = 2.035(2), C7-N1 = 1.373(3), C7-C8 = 1.392(3); N1-C7-
C8 = 102.7(2), C7-Pd1-Cl1 = 91.05(7), N1-C7-Pd1 = 126.09(18),
C8-C7-Pd1 = 130.94(18).
Scheme 2. Synthesis of Silver and Palladium Chloride
Complexes 3 and 4
Complex 4 was found to be stable toward air and moisture for
a prolonged period. 1H and 13C NMR spectra of 4 showed two
sets of signals in the aromatic and N-methyl regions, indicating
that 4 could exist as a pair of isomers in solution. For example, a
pair of singlet resonances appeared for the N-methyl protons at δ
3.98 and 3.96 ppm, respectively, in a nearly 1:1 ratio. When the
spectrum was measured at 55 °C, the two methyl resonances
merged and gave a sharp singlet at δ 3.97 ppm. On the basis of
the NMR study we propose that the “as synthesized” complex 4
exists as two rotational isomers, namely the syn and the anti
conformers with respect to the C-Pd-C bond axis (Scheme 2).
At 55 °C rapid interconversion of 4-syn and 4-anti occurred,
1
resulting in the averaging of the H NMR spectrum. Upon
cooling to room temperature, the original NMR spectrum
reappeared with a pair of methyl signals at δ 3.98 and 3.96
ppm, respectively, in a nearly 1:1 ratio. A systematic study of the
dynamics using VT-NMR in the temperature range of 25-56 °C
revealed the coalescence temperature to be 54 °C. From the
NMR data the free energy of activation for the interconversion of
4-syn and 4-anti at the coalescence temperature is estimated to be
72.0 ( 0.1 kJ mol-1 (Supporting Information). The structure of
4-anti was unambiguously established by single-crystal XRD
data. Single crystals suitable for XRD studies were grown from
a chloroform-acetone (1:1) mixture by slow evaporation. The
X-ray structure revealed square-planar geometry around palla-
dium with two triazolylidene ligands and two chlorines in a trans
geometry (Figure 1). The molecular center of symmetry lies on
the palladium atom. The Pd-C7 bond length is 2.035 Å, which is
slightly longer than the Pd-C bond length reported in another
Pd-triazolylidene complex.6a The N-Ccarbene-C (N1-C7-
C8) bond angle is 102.7°, which is slightly smaller than that
reported earlier.6a This structure is reminiscent of the corre-
sponding structure of an imidazolylidene palladium complex.9
The in situ generated silver carbene complex 3 was treated
with Pd(OAc)2 to give the palladium acetate complex 5 as a
bright yellow solid in 89% yield (Scheme 3).
azide in 90% yield (Scheme 1). The triazole proton in 1 appeared
as a singlet at δ 8.16 ppm and in 2 at 9.69 ppm in NMR
measurements in CDCl3.
The palladium complexes were conveniently synthesized by
transmetalation of a silver carbene complex precursor.6a Triazo-
lium iodide 2 was treated with freshly prepared silver oxide in
dichloromethane at room temperature (35 °C) for 8 h in the dark
(Scheme 2). The silver carbene complex 3 was sensitive to
moisture and room light. It was isolated as a colorless solid and
1
characterized by H and 13C NMR spectroscopy, ESI-MS, and
elemental analysis. The carbene carbon appeared at δ 166.3 ppm,
consistent with earlier reports of metal-NHC complexes.6a ESI-
MS showed the molecular ion corresponding to [(Tz)2Ag] with
the expected isotope distribution for the mononuclear silver
complex. The crude silver carbene complex was immediately
used for the synthesis of palladium complexes. Complex 3 pre-
pared in situ in dichloromethane was treated with [Pd(Cl)2-
(CH3CN)2] (Scheme 2). The dichloropalladium complex 4 was
isolated as a pale yellow solid in 88% yield.
The structure of the acetate complex 5 was unequivocally
established by single-crystal XRD data. The X-ray structure of 5
clearly revealed that it is a dinucelar complex with a Pd-Pd
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of palladium across the ortho C-H bond followed by the
reductive elimination of a molecule of acetic acid. Thus, the
triazole ring, the five-membered palladacycle, and the phenyl ring
attached to the nitrogen form a fused tricyclic ring system. The
two tricyclic rings thus formed are within a π-stacking distance of
3.4-3.5 Å. A view along the axis passing through the Pd atoms
shows the π-stacking interaction between the triazole ring of
one ligand with the phenyl ring of the other ligand (Figure 2).
The molecule possesses a C2 axis of symmetry passing through
the center of the two palladium atoms. The triazolylidene ligands
are C2 symmetry related with respect to each other, and so are the
two acetate ligands.
Catalytic Reactions. Transition-metal complexes of NHCs
have been shown to catalyze a wide variety of reactions.11 The
catalytic activity of the metal center can be fine-tuned by the
nature of the NHC ligand. Palladium-NHC complexes have
been shown to catalyze a variety of reactions, including the C-C
cross coupling reaction,12 C-H activation,13 oxidation,14 hydro-
carboxylation,15 to name a few. The catalytic activity of complex
5 was tested for hydroarylation of alkynes.
Hydroarylation is the addition of an electron-rich aryl group
and a hydrogen atom across a C-C multiple bond, typically an
acetylenic bond resulting in the formation of a vinylarene.
Fujiwara13 performed a hydroarylation reaction, also commonly
termed as an aryl C-H activation reaction, using Pd(OAc)2 in
trifluoroacetic acid (TFA). Nolan16 reported the hydroarylation
of alkynes using IPr-Pd(OCOCF3)2 and IPr-Pd(OAC)2
(IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene) comple-
xes as catalyst with TFA as the solvent. Hydroarylation reactions
using 1,2,3-triazolylidene-Pd complexes have not been reported
earlier. Initially mesitylene (6) and ethyl propiolate (7) were
used as substrates for the hydroarylation reaction in the presence
of 0.5 mol % of 5 and an excess of TFA in dichloromethane. The
reaction yielded a mixture of ethyl (Z)-2,4,6-trimethylphenyla-
crylate (8) and the corresponding bis addition product (9) in 44
and 13% yields, respectively (Scheme 4). The reaction was highly
stereoselective, and only the Z isomer of 8 was formed. When the
reaction was carried out with excess mesitylene, 8 and 9 were
formed in 79 and 19% yields, respectively (Table 1). Formation
of the bis addition product 9 could not be suppressed even with
an excess of arene. In the absence of TFA only the polymerization
of ethyl propiolate was observed. When the reaction was perfor-
med in the absence of the catalyst (5), no reaction was observed
and the starting materials were recovered. Hydroarylation with
other substrates was carried out using 0.5 mol % of 5, and the
results are summarized in Table 2.
Figure 2. ORTEP drawing of 5 with thermal ellipsoids at the 30%
probability level (top) and view along the Pd-Pd axis (bottom).
ꢀ
Selected bond distances (Å) and angles (deg): Pd1-C1 = 1.957(5),
Pd2-C16 = 1.960(5), Pd1-C14 = 1.988(5), Pd2-C29 = 1.982(4),
Pd2-O2 = 2.123(3), Pd2-O4 = 2.085(3), C16-C17 = 1.387(6),
C16-N6 = 1.376(6); N3-C1-C2 = 101.8(4), N6-C16-C17 =
102.3(4). Distance between the Pd atoms Pd1 and Pd2: 2.8705(5) Å.
Scheme 3. Synthesis of Palladium Acetate Complex 5
The scope of substrate for hydroarylation was investigated
with a variety of arenes and acetylenes. The results are summar-
ized in Table 2. Electron-rich arenes gave a good to moderate
yield of the hydroarylation product with ethyl propiolate (7).
Reaction with 4-tert-butylphenol gave a single product that was
identified as 6-tert-butylcoumarin (Table 2, entry 5). When
phenylacetylene (10) was used as the acetylenic component,
hydroarylation with pentamethylbenzene (11) gave the corre-
sponding hydroarylated product in 54% yield. When the reaction
was carried out with mesitylene, multiple substitution occurred,
resulting in the formation of mono- and disubstituted products
(Table 2, entry 7). Reaction with phenylpropiolic acid (12) gave
only mono addition product with mesitylene as well as penta-
methylbenzene (Table 2, entries 8 and 9). Partial hydrolysis was
observed in the hydroarylated product when methyl phenylpro-
piolate (13) was used as the substrate. Overall the reactivity and
distance of 2.870 Å (Figure 2). This distance is much greater than
the Pd-Pd bond distance of 2.56 Å reported earlier for a similar
binuclear Pd cyclic complex with acetate bridges.10 The geome-
try around each palladium is square planar. The stereochemistry
is cis with respect to the two triazolylidene and two acetate units.
Two acetates act as bridging ligands for the two palladium atoms.
Each palladium atom is attached to the carbene carbon as well as
to the carbon at the ortho position of the phenyl ring attached to
the nitrogen atom of the triazole ring, forming a five-membered
palladacycle. The palladacycle is formed by the oxidative addition
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Scheme 4. Hydroarylation of Ethyl Propiolate with Mesitylene
4.43 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 144.3, 134.7, 132.1,
132.1, 130.5, 130.2, 129.7, 127.4, 122.1, 121.4, 40.1. IR (KBr, cm-1):
3020, 1607, 1589, 1488, 1466. ESI-MS: m/z 236 (Mþ), 237 (M þ 1).
HRMS: m/z calcd for C15H14N3 (Mþ) 236.1188, found 236.1198; calcd
for C15H15N3 (M þ 1) 237.1266, found 237.1274.
Table 1. Hydroarylation of Ethyl Propiolate using 5
amt of 6
amt of 7
amt of 5
time
(h)
product
(equiv)
(equiv)
(mol %)
(yield (%))
1
1
2
1
1
2
1
1
1.0
1.0
1.0
0.5
48
48
24
48
8 (43), 9 (13)
8 (43), 9 (20)
8 (79), 9 (19)
8 (44), 9 (13)
Synthesis of Silver Carbene Complex 3. To a solution of 2
(250 mg, 0.69 mmol) in CH2Cl2 (10 mL) was added freshly prepared
Ag2O (96 mg, 0.41 mmol). The mixture was stirred at room temperature
for 8 h in the dark, and then an aliquot (3 mL) of the solution was taken
and filtered through a sintered-glass crucible. Solvent was removed
under vacuum at room temperature to yield 3 as a white solid. 1H NMR
(400 MHz, CDCl3): δ 7.87 (d, 2H, J = 8.0 Hz), 7.62 (d, 2H, J = 6.8 Hz),
7.54-7.40 (m, 6H), 4.24 (s, 3H). 13C NMR (100 MHz, CDCl3): δ
166.3 (C-Ag), 149.4, 139.8, 130.5, 130.4, 129.7, 129.6, 129.4, 127.2,
123.4, 38.0. ESI-MS: m/z (with expected isotope pattern) 578 (Mþ).
HRMS: m/z calcd for C30H26N6Ag 577.1270, found 577.1262. Anal.
Calcd for C30H26N6Ag2I2: C, 38.29; H, 2.76; N, 8.93. Found: C, 38.50;
H, 2.96; N, 8.99.
selectivity of complex 5 for hydroarylation of alkynes is moderate
in comparison to those for Pd complexes of normal NHC
ligands.16,17
’ CONCLUSION
Palladium complexes of an abnormal NHC, namely 1,4-
diphenyl-3-methyl-1,2,3-triazol-5-ylidene, have been synthesized
via the transmetalation of the corresponding silver NHC com-
plex 3 and characterized by various spectroscopic techniques and
single-crystal X-ray crystallography. The dichloro derivative 4 is a
trans mononuclear complex existing as interconvertible syn and
anti isomers. The free energy of activation for the interconversion
of the syn and anti isomers is estimated to be 72.0 ( 0.1 kJ mol-1
Synthesis of Chloro Complex 4. To a solution of 2 (250 mg,
0.69 mmol) in CH2Cl2 (10 mL) was added Ag2O (96 mg, 0.41 mmol).
The mixture was stirred at room temperature for 8 h in the dark, and
then Pd(CH3CN)2Cl2 (106 mg, 0.41 mmol) was added. The mixture
was stirred at room temperature for 12 h and then filtered through
Celite. The filtrate was concentrated to 2 mL, upon which 4 precipitated
as a yellow solid when excess ether was added. The solid was dissolved in
CH2Cl2 and precipitated by addition of ether. The solid was dried under
vacuum to obtain pure 4 (196 mg, 88%). 1H NMR (400 MHz, CDCl3,
25 °C): δ 8.31 (d, 2H, J = 8.0 Hz), 8.26 (d, 2H, J = 7.6 Hz), 7.85 (d, 4H,
J = 7.6 Hz), 7.57-7.49 (m, 2H), 7.44-7.38 (m, 6H), 7.24 (d, 2H, J = 6.4
Hz), 7.16 (t, 2H, J = 8.0 Hz), 3.98 (s, 3H), 3.96 (s, 3H). 1H NMR (400
MHz, CDCl3, 55 °C): δ 8.31 (broad s, 4H), 7.85 (d, 4H, J = 7.2 Hz),
7.43-7.17 (m, 12H), 3.95 (s, 6H). 13C NMR (100 MHz, CDCl3,
25 °C): δ 158.4, 158.2 (C-Pd), 145.1, 139.8, 139.6, 131.5, 130.8, 130.5,
129.6, 129.2, 129.1, 129.0, 128.9, 128.9, 128.6, 127.9, 127.6, 125.3, 124.9,
124.5, 37.2, 37.1. ESI-MS: m/z 647 (M - Cl þ CH3CN) (showed the
expected isotope pattern for the mononuclear complex). IR (KBr, cm-1):
3443, 1593, 1484. Mp: 235-236 °C. Anal. Calcd for C30H26N6Cl2Pd
(647.87): C, 55.56; H, 4.01; N, 12.96. Found: C, 55.48; H, 3.85; N,
12.81. HRMS: m/z calcd for C30H26N6ClPd 611.0942, found 611.0945.
Synthesis of Acetate Complex 5. To a solution of 2 (250 mg,
0.69 mmol) in CH2Cl2 (10 mL) was added Ag2O (96 mg, 0.41 mmol).
The mixture was stirred at room temperature for 8 h in the dark, and
then Pd(OAc)2 (93 mg, 0.41 mmol) was added. After the mixture was
stirred at room temperature for 12 h, it was filtered through Celite. The
filtrate was concentrated to 2 mL, and ether was added to yield 5 as a
bright yellow solid. The solid was dissolved in CH2Cl2 and precipitated
1
from the VT H NMR data. The diacetate derivative 5 is a
binuclear palladacycle complex with bridging acetate ligands.
The acetate derivative showed moderate catalytic activity toward
hydroarylation reactions of alkynes in the presence of TFA.
Several electron-rich arenes underwent hydroarylation of phe-
nylacetylene, ethyl propiolate, and phenylpropiolic acid and its
methyl ester, resulting in the formation of the corresponding
vinyl derivatives in a stereoselective manner.
’ EXPERIMENTAL SECTION
Synthesis of Triazole 118. Phenylacetylene (5.0 g, 49.0 mmol)
was added to a stirred mixture of copper iodide (1.87 g, 9.8 mmol) and
phenyl azide (6.4 g, 53.9 mmol) in DMSO/H2O (90 mL/10 mL). The
mixture was stirred at room temperature for 36 h, during which time the
progress of the reaction was monitored by TLC. The crude product was
precipitated by addition of cold water to the reaction mixture. The solid
was separated by filtration and washed (10 ꢀ 100 mL) with water and
dried under vacuum to give a light brown solid in 90% yield (9.75 g). Mp:
160-162 °C. 1H NMR (400 MHz, CDCl3): δ 8.16 (s, 1H), 7.90 (d, 2H,
J = 7.2 Hz), 7.78 (d, 2H, J = 8.0 Hz), 7.55-7.34 (m, 6H). 13C NMR (100
MHz, CDCl3): δ 148.5, 137.3, 130.4, 129.9, 129.0, 128.9, 128.6, 126.7,
126.0, 120.7, 117.7. IR (KBr, cm-1): 1596, 1502, 1231. ESI-MS: m/z
222 (M þ 1).
1
once again by addition of ether (245 mg, 89%). H NMR (400 MHz,
CDCl3): δ 7.58-7.47 (m, 5H), 7.17-7.09 (m, 2H), 7.00-6.95 (m,
2H), 3.83 (s, 3H), 1.28 (s, 3H). 13C NMR (100 MHz, CDCl3): δ 180.0,
154.2 (C-Pd), 145.3, 143.5, 143.0, 135.1, 130.5, 129.5, 128.5, 126.4,
125.1, 123.2, 112.8, 36.6, 23.2. ESI-MS: m/z (showed the expected
isotope pattern) 422 [(M/2) þ Na]. IR (KBr, cm-1): 3460, 3051, 2926,
1570, 1416. Mp: 240 °C dec. Anal. Calcd for C34H30N6O4Pd2 (798): C,
51.12; H, 3.75; N, 10.52. Found: C, 51.05; H, 3.30; N, 10.17. HRMS:
m/z calcd for C17H15N3O2Pd 422.0097, found 422.0091.
Synthesis of Triazolium Iodide 2. Triazole 1 (1.0 g, 4.5 mmol)
was added to acetonitrile (8 mL) and methyl iodide (3.86 g, 27.2 mmol),
and the reaction mixture was heated in a Teflon-lined sealed steel vessel
at 80 °C for 48 h. Solvent was removed under vacuum. The solid residue
was washed with ethyl acetate (5 ꢀ 10 mL) to give an off-white solid in
70% yield (1.14 g). Mp: 158-160 °C. 1H NMR (400 MHz, CDCl3): δ
9.69 (s, 1H), 8.15 (m, 2H), 7.96 (m, 2H), 7.58 (m, 3H), 7.49 (m, 3H),
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Table 2. Hydroarylation of Alkynes with Various Arenesa
a Conditions: arene (1 mmol), alkyne (1 mmol), TFA (1 mL), 5 (0.5 mol %), CH2Cl2 (0.25 mL), room temperature.
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General Procedure for C-H Activation Reaction. TFA
(1 mL) was added to arene (1 mmol) dissolved in dichloromethane
(0.25 mL). 5 (4 mg, 0.5 mol %) and alkyne (1 mmol) were added, and
the mixture was stirred at room temperature for the required time (see
Table 2). The reaction was followed by TLC. After completion of the
reaction the mixture was quenched with water and extracted with dichlo-
romethane. The organic layer was dried over sodium sulfate, and solvent
was removed under vacuum. The crude product was subjected to column
chromatography on silica gel.
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’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, tables, and CIF
b
files giving characterization data for all the hydroarylation pro-
ducts in Table 2, 1H and 13C NMR spectra of 2-5, VT NMR data
for 4, and crystallographic data for 4 and 5. This material is available
free of charge via the Internet at http://pubs.acs.org.
€
€
’ AUTHOR INFORMATION
Corresponding Author
*Fax: þ91 44 2257 0545. E-mail: sanka@iitm.ac.in.
’ ACKNOWLEDGMENT
We thank the CSIR, New Delhi, India, for financial assistance,
the UGC, New Delhi, India, for a fellowship (R.S.), and the SAIF
and Department of Chemistry, IIT Madras, for spectral and
X-ray data.
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