Hydroxyl-functionalized triazolylidene-based PEPPSI complexes: Metallacycle formation effect on the Suzuki coupling reaction

Article abstract of DOI:10.1039/c8dt04432e

We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction of aryl chlorides/amides with boronic acids. Under basic reaction conditions, complexes 2a-c show a notable increase in their catalytic efficiency compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex (5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c plays an important role in the overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation of metallacycles 6a-b, which are proposed as the active species of cross coupling reactions.

Full text of DOI:10.1039/c8dt04432e

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A. Alvarez-Hernandez, A. L. Rheingold, O. R. Suarez-Castillo and D. Mendoza-Espinosa, Dalton Trans.,
2019, DOI: 10.1039/C8DT04432E.
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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DOI: 10.1039/C8DT04432E
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Hydroxyl-functionalized triazolylidene-based PEPPSI complexes:
Metallacycle formation effect toward the Suzuki coupling reaction
Received 00th January 20xx,
Accepted 00th January 20xx
David Rendón-Nava,a Alejandro Álvarez-Hernández,a Arnold L. Rheingold,b Oscar R. Suarez-
Castillo,a and Daniel Mendoza-Espinosaa*
DOI: 10.1039/x0xx00000x
We report the preparation and full characterization of a series of hydroxyl functionalized 1,2,3-triazolylidene-based PEPPSI
complexes 2a-c and their catalytic application in the Suzuki cross coupling reaction reaction of aryl chlorides/amides with
boronic acids. Under basic reaction conditions, complexes 2a-c display a notable increase in the catalytic efficiency
compared with two ether-wingtip functionalized PEPPSI analogues (3 and 4) and a commercially available NHC-Pd complex
(5). The catalytic results suggest that deprotonation of the hydroxyl group in complexes 2a-c play an important role in the
overall process. Deprotonation of the alcohol moiety of complexes 2a-b with sodium tert-butoxide allows for the isolation
of the metallacycles 6a-b which are proposed as the active species of the cross coupling reactions.
www.rsc.org/
For several years, our group has been interested in the design
and preparation of palladium triazolylidene-based complexes
for synthetic applications, in particular the efficient catalyzed
cross coupling reactions.5 We have anticipated that
Introduction
The discovery of PEPPSI (Pyridine Enhanced Precatalyst
Preparation Stabilization and Initiation) complexes to promote
C-C bond formation is an important breakthrough in
contemporary cross coupling catalytic chemistry.1 In this
subclass of heteroleptic metal complexes, the tightly bonded
N-heterocyclic carbene (NHC) stabilizes the active Pd(0)
species, while the pyridyl group readily departs to generate a
vacant site necessary for the coupling process. This type of
palladium complexes offers among other advantages, a facile
synthesis and air and moisture stability compared to
traditional palladium catalysts.2
Given the increasing popularity of palladium catalysts in both
industry and academia, considerable efforts targeting the
understanding the electronics and steric properties of ligands
in catalyst design are constantly ongoing. For PEPPSI palladium
complexes, it has been found that increasing the steric bulk on
the ortho-position of the N-aryl moiety on the NHC can
significantly improve the catalytic performance.3 However, in
contrast to the extensively studied N-aryl moiety, modification
of the sigma donor ligands or their functionalization has been
scarcely studied.4
triazolylidenes which have demonstrated enhanced sigma-
donor capacities compared to classical NHCs together with the
presence of additional donor atoms at the triazole 4-position,6
could lead to increased stability of the resulting PEPPSI
precatalysts. This hypothesis takes relevance for two main
reasons. First, it has been demonstrated that triazolylidene-
based palladium complexes display enhanced thermal
stabilities (above 100 °C) compared to NHC-based complexes,
and they are also air and moisture stable.5,6 Second, it has
been documented that catalysts bearing both NHCs and
auxiliary ligands such as N-,7 P-8 and other donors,9 display
enhanced catalytic activities in several coupling reactions. For
instance, Ying and coworkers have reported a series of NHC-
palladacycles featuring an oxygen donor group with excellent
activities in the Suzuki-Miyaura coupling.10 Nolan has also
reported similar NHC-palladacycle complexes featuring
enhanced activities toward the Suzuki coupling in the presence
of IPA as solvent at room temperature.11 Palladacycles have
been exhaustively studied in cross coupling reactions and
showed thermally stable properties and high efficiency under
low catalyst loadings.12
With this background, we report herein the preparation and
full characterization of a series of hydroxyl functionalized
triazolylidene-based PEPPSI complexes 2a-c and their
application in the Suzuki coupling of aryl chlorides and amides
with boronic acids. Under the basic reaction conditions
required, complexes 2a-b display a notable increase in the
catalytic activity compared to two ether-wingtip functionalized
PEPPSI analogues (3 and 4) and the commercially available
PEPPSI-IPr complex (5), suggesting that deprotonation of the
a. Área Académica de Química, Universidad Autónoma del Estado de Hidalgo,
Carretera Pachuca-Tulancingo Km. 4.5, Mineral de la Reforma, Hidalgo, 42090,
México. Email: daniel_mendoza@uaeh.edu.mx,
b. Department of Chemistry and Biochemistry, University of California, 9500 Gilman
Drive, La Jolla, San Diego, California 92093, USA.
† Electronic Supplementary Information (ESI) available: General procedures for
catalytic experiments, Sample 1H and 13C NMR spectra of new compounds. CIF file
giving details for the crystal structure determinations of 2a and 2b with the CCDC
numbers 1817461-1817462. For ESI and crystallographic data see
DOI: 10.1039/x0xx00000x
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hydroxyl pendant arm play an important role in the overall were investigated by X-Ray diffraction analysis. TVhieewmArtoiclleecOunllianer
catalytic process.
structures of both complexes are depicteDdOinI: 1F0i.g1u03re9/C1.8DT04432E
Complexes 2a and 2b display relatively close structural
features including a slightly distorted square planar palladium
center with the pyridine and 1,2,3-triazol-5-ylidene ligands
Results and discussion
The 1,2,3-triazolium precursors 1a-c were synthesized located trans to each other. The C(5)-Pd(1)-N(4) and I(1)-Pd(1)-
according to the literature procedure by means of a copper I(2) angles of 177.3(18)° and 178.11(2)° for 2a and 178.8(2)°
catalysed cycloaddition (CuAAC) of mesityl azide and the and 171.7(2)° for 2b, are close to the expected 180° for a
appropriate propargyl alcohol, followed by N-quaternarization square plane geometry. The Pd(1)-C(5) bond distances of
with methyl iodide.5e The preparation of PEPPSI complexes 2a- 1.976(5) Å for 2a and 1.964(6) Å for 2b, and Pd(1)-N(4) bond
c was carried out by the one-pot reaction of the triazolium distances of 2.093(5) and 2.093(5)
Å for 2a and 2b,
precursors with K2CO3 and PdCl2 in pyridine at 100°C. Despite respectively, are similar to analogous 1,2,3-triazolylidene-
that the PEPPSI complexes were obtained using this initial based PEPPSI complexes previously reported in the
methodology, the yields were lower than 35% after literature.5e,14 The 1,2,3-triazolylidene ring in 2a and 2b is
purification by column chromatography. Disappointed by the planar with negligible deviations denoted by the
poor conversions, we performed various structural analyses of C(5)N(1)N(2)N(3) (0.25-0.30) and C(4)N(3)N(2)N(1) (0.47-1.13)
the heteroleptic complexes and observed that the data was torsion angles. In both structures the alcohol arm points out of
not consistent with palladium centers containing chlorine the metal coordination sphere.
atoms. Further product analysis revealed that a Cl/I halogen
exchange took place likely favoured by the presence of in situ
generated potassium iodide salt and the heating process. With
this information and considering the asymmetric
halide/carbene stoichiometry in the PEPPSI complex, we
modified the reaction conditions by adding an excess of KI (2
equiv) to the mixture and prolonged the stirring for 24 h
(Scheme 1). The addition of KI in excess will favour a complete
halogen exchange and thus to the generation of a single
product.13 The optimized conditions allowed for the isolation
of 2a-c in high yields (84-92%) as microcrystalline yellow solids.
Complexes 2a-c are air and moisture stable and can be stored
Figure 1 Molecular structures of complexes 2a (left) and 2b (right). Ellipsoids
are shown at 50% probability.
in air indefinitely.
Palladium catalyzed Suzuki cross coupling has stablished itself
N
N
as a benchmark process for the formation of new C-C bonds.15
The catalyst structure, particularly that of the ligands, defines
electronic and steric properties toward the palladium center
playing a key role in organic transformations. Despite the
significant progress in cross coupling reactions during the last
decade, several drawbacks still exist. For instance, the
reactions typically require of inert conditions, whereas the
high sensitivity of NHC-Pd0 species toward oxygen can
interrupt the catalytic cycle. Additionally, the use of aryl
chlorides for the coupling process remains as a challenging
task. Ever since the seminal work by the groups of Herrmann,16
Beller,12a and Hartwig,17 palladacycles18 have become
mainstream precatalysts19 for cross coupling reactions. More
recently with the observations reported by Yin,10 Nolan,11 and
Buchwald20 that formation of six-membered ring palladacycles
promoted by the presence of additional donor heteroatoms
(N, S, O) lead to a significant improvement in the efficiency of
cross coupling reactions, we decided to test the catalytic
activities of our complexes 2a-c in the formation of a variety of
biaryls. In order to investigate the effect of the hydroxyl
pendant arm featured in complexes 2a-c, their catalytic
efficiencies were compared with two phenoxy-functionalized
complexes containing a rigid mesityl and a flexible benzyl
moiety at the N1- position (3 and 4, respectively), and a non-
oxygen functionalized PEPPSI version (5) (Figure 2).
HO
N
I
n
N
N
PdCl2, 2KI
HO
I
Pd
N
I
N
n
K2CO3, Py, 100oC
H
2a;
2b;
2c
n = 3:
n = 1:
n = 2:
1a;
1b;
1c
n = 3:
n = 1:
n = 2:
Scheme 1 Synthesis of PEPPSI complexes 2a-c.
The formation of the PEPPSI complexes was evident by the loss
of the acidic hydrogen 1H-NMR peak of the cationic precursors
(around 9.1 ppm) and the presence of new aromatic peaks
belonging to the pyridine ligand. The selective deprotonation
of the triazolium proton was confirmed by the presence of the
hydroxyl group in the PEPPSI complexes displayed as a triplet
signal in a range of 2.18-3.27 ppm. The 13C NMR spectra show
the MIC-Pd signals at 134.2, 135.4, and 135.5 ppm for 2a-c
respectively, which are consistent with previously reported
PEPPSI ligands containing analogue triazolylidenes.5c,13 The low
field chemical shift of the MIC-Pd signal has been previously
observed in analogue PEPPSI complexes and is related to the
weak Pd-MIC bonding which may have important contribution
to catalytic applications. Single crystals of complexes 2a and 2b
were obtained by slow diffusion of hexanes into
concentrated THF solution and their solid-state structures
a
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the reaction conditions presented in Table 1 (Entries 11-16). As
illustrated
View Article Online
DOI: 10.1039/C8DT04432E
Cl
N
N
N
N
N
O
N
Dipp
O
N
Dipp
N
in Figure 3, complexes 2a and 2b reach conversions higher
than 60 % after only 30 min of reaction and their maximum
conversions are observed in 2 h. The commercial PEPPSI
complex 5 reaches yields over 67% after 2 h and the phenoxy-
functionalized complexes 3 and 4 display a similar catalytic
behavior reaching maximum conversions at 2.5 h with no
further change after this time. In the case of complex 2c, the
generation of the coupling product is much slower reaching its
Cl Pd Cl
N
I
Pd
N
I
I
Pd
N
I
Cl
3
5
4
Figure 2. PEPPSI complexes 3-5.
We began our investigation by testing precatalysts 2-5 in the higher conversion after 3 h.
Suzuki-Miyaura coupling of phenylboronic acid with p-
chloroanisole as model reaction. Initial reaction conditions
included loading of 3 mol% of the appropriate precatalyst,
K2CO3 as the base, dioxane as solvent and 2 h of reaction.5a As
observed in Table 1, complexes 2a and 2b and the commercial
PEPPSI complex 5 are the most active of the series with yields
over 95%. Complexes 3 and 4 led to moderate cross coupling
reaction yields (< 67%) and interestingly, complex 2c which
also features the hydroxyl pendant arm shows the lowest
conversion (53%). Optimization of the reaction conditions
demonstrate that loading of complexes 2a and 2b can be
Figure 3. Cross coupling of phenylboronic acid with p-chloroanisole
reduced to 1 mol% and using room temperature, without
significant loss on the conversions. Under the same conditions,
the PEPPSI complex 5 reduces its efficiency to 84% yield (Table
1, entry 16).
catalyzed by 2-5. Reactions carried out with dioxane-d8 at 25°C, 1 mol%
catalyst. Conversions determined by 1H NMR spectroscopy based on the
amount of p-chloroanisole remaining in solution.
With the optimized conditions in hand, we next tested
complexes 2a and 2b in the coupling of several aryl chlorides
(activated and unactivated) with phenylboronic acid.
According to Table 2, the corresponding biaryls were obtained
in good to excellent yields.
Table 1. Cross coupling reaction of phenylboronic acid with p-
chloroanisole using precatalysts 2-5.
[Cat]
Cl
+
Ph B(OH)2
NaOtBu, Dioxane
MeO
MeO
Table 2. Suzuki-Miyaura coupling of aryl chlorides with
phenylboronic acid.
Entry
1
2
3
4
5
6
7
8
Cat
2a
2b
2c
3
[cat]-mol%
Temp[C]
60
Yield[%]a
3
3
3
3
3
3
3
3
1
1
1
1
1
1
1
1
97
99
53
77
71
95
97
99
97
99
95
99
43
58
55
84
[Cat, 1 mol%]
Cl
60
60
60
60
60
50
50
40
40
25
25
25
25
25
25
+
B(OH)2
NaOtBu, Dioxane, 25oC
R
R
Entry
Aryl halide
Product
Cat : Yield(%)a
4
5
Cl
1
2a: 96
2b: 98
2a : 92 2b : 95
2a : 93 2b : 96
2a : 95 2b : 98
2a
2b
2a
2b
2a
2b
2c
3
9
Cl
2
10
11
12
13
14
15
16
3
Cl
Cl
4
5
MeO
MeO
4
Reaction conditions: p-chloroanisole (1.0 mmol), phenylboronic acid (1.2
mmol), NaOtBu (1.5 mmol), dioxane 2 mL, 2h. aIsolated yields as the average
of two runs.
OMe
OMe
Cl
5
2a : 90 2b : 93
In order to get more insight into the catalytic behavior of
complexes 2-5, the catalytic profiles for the reaction of
phenylboronic acid with p-chloroanisole were performed with
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With the overall catalytic results is easily noticeable that
complexes containing and hydroxyl group are the most active
View Article Online
DOI: 10.1039/C8DT04432E
NH2
NH2
6
2a : 85 2b : 89
2a : 94 2b : 96
Cl
of the series. Given the presence of hydroxyl protons in 2a-c
and the basic conditions used in the coupling reaction, the
possibility of deprotonation and formation of palladacycles as
active catalysts was present.21 To gain more insight on the
enhanced catalytic activity of complexes 2a-b, deprotonation
tests were carried out by the reaction of 2a-c with excess of
NaOtBu in THF at room temperature. After work-up,
complexes 6a-b were obtained quantitatively (Scheme 2) as
bright yellow solids. Deprotonation was confirmed by the
disappearance of the triplet signals belonging to the initial
O
O
7
Cl
Reaction conditions: p-chloroanisole (1.0 mmol), phenylboronic acid
(1.2 mmol), NaOtBu (1.5 mmol), dioxane 2 mL, 2h. Isolated yields as
a
the average of two runs.
With the mild conditions employed and high performance
demonstrated of complexes 2a and 2b, we decided to further
evaluate their catalytic abilities for the preparation of di- and
tri-ortho-substituted biaryls. According to Table 3, the
reactions proceed smoothly delivering a series of ortho-
substituted biaryls in good yields. As expected, the less
hindered coupling partners render the highest yields (93-96%)
while the more challenging tri-ortho-substituted products
reach maximum yields of 90%.
1
hydroxyl groups in H NMR spectroscopy and the elemental
analysis values indicating the loss of an iodine atom.
Interestingly, isolation of complex 6c was not possible as the
deprotonated species were highly unstable in solution. This
result is may be related to the formation of a large seven-
membered palladacycle which decomposes rapidly. The poor
stability of complex 6c may be also related to the low catalytic
efficiency of precatalyst 2c.
Table 3. Preparation of di- and tri-ortho-substituted biaryls.
N
N
N
N
N
N
N
N
[Cat, 1 mol%]
Cl
OH
Mes
Mes
Mes
N
+
B(OH)2
n
NaOtBu, Dioxane, 25oC
R1
NaOtBu
THF, r.t.
R
R1
O
R
I
Pd
I Pd
O
I
Pd I
N
N
N
Entry
Aryl halide
Boronic acid
Product
Cat : Yield(%)*
6b
n = 1: 2a; n = 2: 2b
6a
1
2a : 94
2b : 95
Cl
B(OH)2
Scheme 2. Preparation of complexes 6a and 6b.
OMe
OMe
Cl
To investigate the catalytic activities of palladacycles 6a and
6b, we tested them in the Suzuki-Miyaura coupling of several
aryl bromides and boronic acids. As observed in Table 4, the
biaryl products are obtained in similar yields to those recorded
for precatalysts 2a-b.
2a : 87
2a : 95
2b : 94
2b : 96
2
3
B(OH)2
Cl
B(OH)2
B(OH)2
Table 4. Preparation of substituted biaryls using complexes 6a-b.
[Cat, 1 mol%]
Cl
+
B(OH)2
NaOtBu, Dioxane, 25oC
4
5
6
2a : 88 2b : 90
R1
Cl
Cl
R
R1
R
Entry
Aryl halide
Boronic acid
Product
Cat : Yield(%)*
B(OH)2
2a : 85
2a : 87
2b : 89
2b : 89
B(OH)2
1
6a : 95
6b : 97
Cl
OMe
Cl
Cl
OMe
Cl
B(OH)2
B(OH)2
B(OH)2
6a : 90
6a : 93
6b : 92
6b : 97
2
3
Cl
7
2a : 88
2b : 90
B(OH)2
Reaction conditions: Arylchloride (1.0 mmol), boronic acid (1.2 mmol),
NaOtBu (1.5 mmol), dioxane 2 mL, 2h. aIsolated yields as the average of two
runs.
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View Article Online
B(OH)2
DOI: 10.1039/C8DT04432E
1
Ph
H
5 : 94 6a : 94
6b : 92
4
5
6a : 89
6a : 86
6b : 91
6b : 93
Cl
Cl
Ph
Ph
Ph
2-CH3
4-CH3
5 : 97
5 : 97
5 : 98
6a : 96 6b : 98
6a : 97 6b : 95
6a : 96 6b : 96
B(OH)2
2
3
Reaction conditions: Arylchloride (1.0 mmol), boronic acid (1.2 mmol),
NaOtBu (1.5 mmol), dioxane 2 mL, 2h. Isolated yields as the average
a
4
5
4-OMe
of two runs.
Additionally, to compare the performances of palladacyles 6a-
b with the former complexes 2a-b, the catalytic profiles for the
reaction of phenylboronic acid with p-chloroanisole under
catalytic 2a-b and 6a-b, were carried out using the reaction
conditions presented in Table 1 (Entries 11-12). As illustrated
in Figure 4, complexes 6a and 6b reach conversions higher
than 75% after only 30 min of reaction while, their maximum
conversions are observed in 90 min. Conversion to products
are slower in case of complexes 2a and 2b, where maximum
conversions after achieved after 120 min of reaction.
Ph
4-CO2Me
5 : 90
6a : 89 6b : 90
6
7
8
3-OMe
3-Me
4-CN
H
H
H
5 : 96
5 : 98
5 : 82
6a : 92 6b : 93
6a : 95 6b : 96
6a : 88 6b : 90
Reaction conditions: Boronic acid (2.0 mmol), amide (1.0 mmol), K2CO3
a
(3.0 mmol), Cat (3 mol%), THF, 5 mL, 15 h. Isolated yields as the
average of two runs.
According to Table 5, the scope of boronic acid and substituted
N-glutarimide is broad and tolerates sterically hindered,
electron rich, and electron-withdrawing substituents.
Interestingly, the yields obtained with the metallacylces 6a-b
are similar to those reported for complex 5, and even higher in
case of the entry 8.
With the similar catalytic activities for observed for complexes
6a-b and the temperature used in the procedure, the
possibility of formation of heterogeneous palladium as the
active catalyst prompted us to perform mercury poisoning
tests23 in the formation of benzophenone (Table 5, entry 1).
Thus, the formation of benzophenone in presence of 6a-b was
disturbed by the addition of excess of Hg(0) at different
reaction times (t = 0-6 h). The conversion percentages in the
case of complex 6a when mercury is added at 0 and 2 h
decreased to 84 and 88%, respectively (Figure 5). These results
indicate that after addition of the metal poison, the catalytic
activity is dominated essentially by molecular species.
Additionally, if the poisoning process is delayed form times
between 4 and 6 h, no significant changes in the conversions
were noticed (94-96%).
Figure 4. Cross coupling of phenylboronic acid with p-chloroanisole
catalyzed by 2a-b and 6a-b. Reactions carried out with dioxane-d8 at 25°C, 1
mol% catalyst. Conversions determined by 1H NMR spectroscopy based on
the amount of p-chloroanisole remaining in solution.
Pd-PEPPSI complexes have been largely used for the coupling
of challenging alkyl and aryl-electrophiles; and there are
several reports on their use as catalysts for the coupling of
amides by C-N bond activation.22 Interested in challenging the
catalytic potential of metallacycles 6a-b, we decided to test
them in this recently reported cross coupling reaction. Thus,
following similar reaction conditions as those reported by
Szostak22c and coworkers, complexes 6a-b where compared to
the commercial Pd-PEPPSI complex 5 for the coupling of
substituted N-glutarimides and various boronic acids.
Table 5. Suzuki-Miyaura cross coupling of amides and boronic
acids using complexes 5 and 6a-b.
O
O
O
B(OH)2
[Cat, 3 mol%]
N
+
K2CO3, THF, 60oC
O
R2
R1
R1
R2
Entry R1
R2
Cat : Yield (%)*
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least squares against F2.25 All non hydrogen atomsVwiewerAerticrleefOinnleinde
DOI: 10.1039/C8DT04432E
anisotropically. The position of the hydrogen atoms were kept
Figure 5. Mercury poisoning tests at different reaction times for the
formation of benzophenone catalyzed for complex 6a.
fixed with common isotropic display parameters. The
crystallographic data and some details of the data collection and
refinement are given in Table 6.
Using
a similar methodology, we applied the mercury
poisoning test to complex 6b in the formation of
benzophenone. The most noticeable decrease in conversion
(85%) is observed when mercury is added at t = 0 (Figure S1).
Addition of mercury after 2 h results in small variations of the
yield (92-96%), suggesting again a molecular nature of the
active catalyst.
General procedure for the synthesis of PEPPSI complexes 2a-c.
The appropriate triazolium salt (1.0 equivalents), potassium
iodide (2.0 equivalents), anhydrous potassium carbonate (5
equivalents), and palladium chloride (0.95 equivalents) were
charged in a 20 mL pressure tube. Dry pyridine was added (7
mL) and the reaction mixture was heated at 100 °C for 65 h.
After reaching room temperature, the solvent was removed
under reduced pressure and the residue was extracted with 15
mL of DCM. After filtration over celite and vacuum drying, the
crude product was purified by column chromatography using a
CH2Cl2:EtOH (97:3 v/v) mixture as eluent.
Complex 2a: Following the general procedure and using
triazolium 1a (100 mg, 0.258 mmol), K2CO3 (193 mg, 1.39
mmol), KI (76 mg, 0.46 mmol) and PdCl2 (42 mg, 0.24 mmol,)
the title product is obtained as orange solid in 89% yield (140
mg, 0.21 mmol). m.p. = 165-167 °C. 1H-NMR (CDCl3, 400 MHz):
δ = 2.32 (s, 6H, CH3), 2.41 (s, 3H, CH3), 3.27 (t, J = 7.6 Hz, 1H,
OH), 4.24 (s, 3H, CH3), 5.15 (d, J = 7.5 Hz, 2H, CH2), 7.04 (s, 2H,
ArH), 7.24-7.27 (m, 2H, ArH), 7.65-7.69 (m, 1H, ArH), 8.81-8.83
(m, 2H, ArH). 13C-NMR (CDCl3, 100 MHz): δ = 21.0, 28.9, 36.9,
55.5, 124.3, 129.5, 132.5, 134.2 (MIC-Pd), 135.2, 135.6, 137.4,
140.4, 143.8, 153.7. FT-IR/ATR ν max cm-1: 3431, 2957, 2925,
2856, 1725, 1602, 1484, 1446, 1372, 1331, 1270, 1213, 1151,
1122, 1071, 1025, 1005, 956, 942, 864. Anal. Calcd. for
C18H22I2N4OPd: C, 32.24; H, 3.31; N, 8.35; Found: C, 32.17, H,
3.22, N, 8.17.
Complex 2b. Following the general procedure and using
triazolium 1b (100 mg, 0.28 mmol), K2CO3 (208 mg, 1.50
mmol), KI (93 mg, 0.56 mmol) and PdCl2 (45 mg, 0.25 mmol),
the title product is obtained as orange solid in 84% yield (145
mg, 0.21 mmol). m.p. = 160-165 °C. 1H-NMR (CDCl3, 400 MHz):
δ = 2.32 (s, 6H, CH3), 2.40 (s, 3H, CH3), 2.63 (bs, 1H, OH), 3.37
(t, J = 5.72 Hz, 2H, CH2), 4.13 (s, 3H, CH3), 4.63 (bs, 2H, CH2),
7.04 (s, 2H, ArH), 7.21-7.21 (m, 2H, ArH), 7.61-7.66 (m, 1H,
ArH), 8.77-8.79 (m, 2H, ArH). 13C-NMR (CDCl3, 100 MHz): δ =
21.2, 29.6, 29.7, 37.0, 59.5, 124.3, 129.5, 135.1, 135.4 (MIC-
Pd), 135.6, 137.3, 140.3, 142.9, 153.7. FT-IR/ATR ν max cm-1:
3380, 2954, 2921, 2853, 1726, 1602, 1445, 1377, 1324, 1286,
1244, 1213, 1146, 1039, 994, 947, 911, 850. Anal. Calcd. for
C19H24I2N4OPd: C, 33.33; H, 3.53; N, 8.18; Found: C, 33.07, H,
3.19, N, 8.31.
Complex 2c. Following the general procedure and using
triazolium 1c (100 mg, 0.258 mmol), K2CO3 (193 mg, 1.39
mmol), KI (86 mg, 0.52 mmol) and PdCl2 (42 mg, 0.24 mmol),
the title product is obtained as orange solid in 92% yield (151
mg, 0.22 mmol). m.p. = 120-126 °C. 1H-NMR (CDCl3, 400 MHz):
δ = 2.18 (bs, 1H, OH), 2.31 (s, 6H, CH3), 2.38 (s, 3H, CH3), 2.50
(q, 2H, CH2), 3.38 (t, J = 6.64 Hz, 2H, CH2), 3.83 (bs, 2H, CH2),
4.11 (s, 3H, CH3), 7.03 (s, 2H, ArH), 7.20-7.23 (m, 2H, ArH),
7.61-7.64 (m, 1H, ArH), 8.77-8.78 (m, 2H, ArH). 13C-NMR
Concluding remarks
We have reported the preparation and full characterization of
a series of hydroxyl functionalized triazolylidene-based PEPPSI
complexes 2a-c and their application in the coupling of aryl
chlorides/amides with boronic acids under mild conditions.
Using standard Suzuki-Miyaura cross coupling conditions,
complexes 2a-b display a notable increase in the catalytic
activity compared to two ether-wingtip functionalized PEPPSI
analogues (3 and 4) and similar activity to the commercially
available PEPPSI-IPr complex 5. This behaviour suggested that
deprotonation of the hydroxyl pendant arm in 2a-b play an
important role in the overall catalytic process. Deprotonation
of the alcohol moiety of complexes 2a-b with sodium tert-
butoxide allowed for the isolation and solution
characterization of the orthometallated species 6a-b which are
proposed as the catalytic active intermediates of the coupling
reactions. Interestingly, the formation of the expected seven-
membered palladacycle 6c after deprotonation of complex 2c
is not possible as it quickly decomposes in solution. This
solution instability may be closely related to the poor catalytic
efficiency observed for 2c. Further investigation of the catalytic
potential of complexes of type 2 is currently underway in our
laboratory.
Experimental section
General Considerations
Commercially available reagents and solvents were used as
received. Triazoliums 1a-c,5e,24 and complexes 3-5,5c were
synthesized as reported in the literature. Synthesis of all metal
complexes was performed under an atmosphere of dry nitrogen
using standard Schlenk techniques. Solvents were dried by
standard methods and distilled under nitrogen. IR spectra were
recorded on a Bruker Alpha FT-IR/ATR spectrometer. Melting
points were determined on a Fisher-Johns apparatus and are
uncorrected. NMR spectra were obtained with a Bruker Ascend
(400 MHz) spectrometer. Elemental analyses were obtained with a
Thermo Finnegan CHNSO-1112 apparatus and a Perkin Elmer
Series II CHNS/O 2400 instruments. GC-MS analyses were
performed in an Agilent GC model HP 5890 coupled with a mass
detector model 5973. X-Ray diffraction analyses were collected in
an Agilent Gemini Diffractometer using Mo K radiation (l =
0.71073 Å). Data were integrated, scaled, sorted, and averaged
using the CrysAlisPro software package. The structures we solved
using direct methods, using SHELX 2014 and refined by full matrix
6 | J. Name., 2012, 00, 1-3
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COMMUNICATION
(CDCl3, 100 MHz): δ = 21.2, 21.3, 23.6, 30.2, 36.8, 61.4, 124.2, c, Å
129.4, 135.4, 135.5 (MIC-Pd), 135.6, 137.3, 140.2, 145.0, 153.6. , deg
Anal. Calcd. for C20H26I2N4OPd: C, 34.38; H, 3.75; N, 8.02; , deg
16.6138(8)
90
17.2512(V3i)ew Article Online
DOI: 10.1039/C8DT04432E
79.583(2)
109.756(5)
90
88.011(2)
88.420(2)
2564.19(10)
2
Found: C, 33.99, H, 3.59, N, 8.31.
, deg
V, Å3
2248.89 (8)
4
1.981
General procedure for the synthesis of PEPPSI complexes 6a-b.
Z
dcalc g.cm-3
1.921
The appropriate PEPPSI precursor (1.0 equivalents), sodium , mm-1
tert-butoxide (2.0 equivalents), and THF (10 mL) were charged refl collected
in a Schlenk tube and the reaction mixture was stirred at room Tmin/Tmax
temperature for 4 h. The solvent was removed under reduced Nmeasd
pressure and the residue was extracted with 15 mL of DCM. [Rint]
After filtration over celite and vacuum drying, the residue was R [I > 2sigma(I)]
washed thrice with diethyl ether (10 mL) and concentrated R (all data)
under vacuum. Pure products as yellow solids were obtained Rw[I > 2sigma(I)]
after precipitation by addition of pentane into a concentrated Rw (all data)
3.584
56197
0.895
3954
3.305
64747
0.923
9012
0.0598
0.0346
0.0320
0.0435
0.0709
0.0814
0.0394
0.0494
0.1080
0.1177
THF solutions of the crude materials.
GOF
1.120
1.068
Complex 6a: Following the general procedure and using
PEPPSI complex 2a (100 mg, 0.15 mmol), NaOtBu (29 mg, 0.30
mmol), the title product is obtained as yellow solid in 96%
Acknowledgements
We are grateful to PRODEP (project UAEH-PTC-792) for
financial support.
1
yield (78 mg, 0.14 mmol). m.p. = 134-136 °C. H-NMR (CDCl3,
400 MHz): δ = 2.18 (s, 6H, CH3), 2.35 (s, 3H, CH3), 4.14 (s, 3H,
CH3), 4.61 (s, 2H, CH2), 6.96 (s, 2H, ArH), 7.22-7.23 (m, 2H,
ArH), 7.61-7.65 (m, 1H, ArH), 8.61-8.64 (m, 2H, ArH). 13C-NMR
(CDCl3, 100 MHz): δ = 19.0, 29.7, 37.0, 62.0, 124.2, 129.5,
134.9, 135.2, 135.4 (MIC-Pd), 137.3, 140.2, 143.0, 153.7. FT-
IR/ATR νmax cm-1: 3029, 2971, 2964, 2856, 1783, 1741, 1684,
1446, 1398, 1377, 1283, 1151, 1100, 1045, 1033, 1005, 992,
974, 878. Anal. Calcd. for C18H21IN4OPd: C, 39.84; H, 3.90; N,
10.32; Found: C, 40.03, H, 3.79, N, 10.53.
Notes and references
1
(a) Selected examples: (a) M. G. Organ, M. Abdel-Hadi, S.
Avola, N. Hadei, J. Nasielski, C. J. O´Brien and C. Valente,
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Complex 6b. Following the general procedure and using
PEPPSI complex 2b (100 mg, 0.15 mmol), NaOtBu (28 mg, 0.29
mmol), the title product is obtained as yellow solid in 93%
1
yield (76 mg, 0.14 mmol). m.p. = 147-149 °C. H-NMR (CDCl3,
400 MHz): δ = 2.19 (s, 6H, CH3), 2.35 (s, 3H, CH3), 3.75 (t, J = 5.7
Hz, 2H, CH2), 4.00 (s, 2H, CH2), 4.61 (t, J = 5.7 Hz, 2H, CH2), 6.96
(s, 2H, ArH), 7.22-7.24 (m, 2H, ArH), 7.61-7.65 (m, 1H, ArH),
8.61-8.63 (m, 2H, ArH). 13C-NMR (CDCl3, 100 MHz): δ = 21.2,
25.6, 29.7, 37.0, 59.5, 64.6, 124.2, 129.5, 134.9, 135.1, 135.4,
135.6 (MIC-Pd), 140.2, 143.0, 149.8. FT-IR/ATR νmax cm-1:
3002, 2974, 2947, 2853, 1726, 1634, 1492, 1389, 1324, 1286,
1231, 1203, 1146, 1039, 997, 934, 923, 892. Anal. Calcd. for
C19H23IN4OPd: C, 40.99; H, 4.16; N, 10.06; Found: C, 41.12, H,
4.34, N, 9.93.
2
3
(a) C. J. O. O´Brien, A. B. Kantchev, C. Valente, N. Hadei, G. A.
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Table 6. Crystallographic Data and Summary of Data Collection
and Structure Refinement.
2a
2b
C18H22I2N4OPd
C39H44Cl3I4N8O2Pd2
Formula
Fw
670.6
1483.57
Triclinic
P-1
cryst syst
Space group
T, K
Monoclinic
P 21/n
293(2)
293(2)
a, Å
11.8996(6)
12.0868(5)
9.8862(2)
15.2996(4)
b, Å
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
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