Highly efficient palladium precatalysts of homoscorpionate bispyrazolyl ligands for the more challenging Suzuki-Miyaura cross-coupling of aryl chlorides

Article abstract of DOI:10.1039/c003174g

Highly efficient palladium precatalysts {[RN{-(CH₂) _n-pz^3,5-Me₂}₂]PdCl₂} _m [m = n = 1; R = 2,6-Me₂C₆H₃ (1), 2,4,6-Me₃C₆H₂ (2), CH₂Ph (3) and m = n = 2; R = CH₂Ph (4)] of a series of homoscorpionate bispyrazolyl ligands for the Suzuki-Miyaura cross-coupling of the more challenging aryl chloride substrates are reported. In particular, the palladium 1-4 precatalysts carried out the Suzuki-Miyaura cross-coupling of a wide variety of aryl chloride substrates bearing electron withdrawing, electron donating and heteroaryl substituents. Remarkably enough, the molecular structure determination of the 1-4 precatalysts by X-ray diffraction studies revealed the presence of anagostic [C-H...Pd] type interactions in the mononuclear 1-3 complexes of methylene bridged bispyrazolyl ligands whereas the ethylene bridged analog 4 yielded an interesting dimeric 20-membered macrometallacyclic complex devoid of any such interaction.

Full text of DOI:10.1039/c003174g

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Highly efficient palladium precatalysts of homoscorpionate bispyrazolyl
ligands for the more challenging Suzuki–Miyaura cross-coupling of aryl
chlorides†
Alex John,a Mobin M. Shaikh,b Ray J. Butcherc and Prasenjit Ghosh*a
Received 15th February 2010, Accepted 19th May 2010
First published as an Advance Article on the web 6th July 2010
DOI: 10.1039/c003174g
Highly efficient palladium precatalysts {[RN{-(CH2)n-pz3,5-Me2}2]PdCl2}m [m = n = 1; R = 2,6-Me2C6H3
(1), 2,4,6-Me3C6H2 (2), CH2Ph (3) and m = n = 2; R = CH2Ph (4)] of a series of homoscorpionate
bispyrazolyl ligands for the Suzuki–Miyaura cross-coupling of the more challenging aryl chloride
substrates are reported. In particular, the palladium 1–4 precatalysts carried out the Suzuki–Miyaura
cross-coupling of a wide variety of aryl chloride substrates bearing electron withdrawing, electron
donating and heteroaryl substituents. Remarkably enough, the molecular structure determination of
the 1–4 precatalysts by X-ray diffraction studies revealed the presence of anagostic [C–H ◊ ◊ ◊ Pd] type
interactions in the mononuclear 1–3 complexes of methylene bridged bispyrazolyl ligands whereas the
ethylene bridged analog 4 yielded an interesting dimeric 20-membered macrometallacyclic complex
devoid of any such interaction.
available in a fewer variety.9 The difficulty in achieving the cross-
coupling of aryl chlorides arises from the stronger C–Cl bond as
Introduction
Of the recent methodologies available for the construction of biaryl
opposed to the other C–X (X = Br, I) bonds [bond dissociation
frameworks in numerous target molecules of pharmaceutical,1
biological,2 speciality chemicals3 and materials interests,4 the
Suzuki–Miyaura cross-coupling of aryl halides with aryl boronic
acids is by far the most preferred method of the day.5 With the other
available methods suffering from disadvantages like the toxicity
issues associated with organotin reagents in the Stille coupling,6 or
the inert nature of the carbon nucleophile in organosilicon reagents
in the case of Hiyama coupling,7 the Suzuki–Miyaura reaction
exhibits many distinct advantages like, the flexible use of organic
solvents with inorganic bases, ready availability of substrates, air
and moisture stability of the reagents, functional group tolerance,
coupling of sterically demanding groups, low toxicity of boronic
acids, facile removal of boron containing by-products, high regio-
and stereo-selectivity and easy application in one-pot procedures
etc. make the reaction more acceptable to the synthetic community
among the existing cross-coupling protocols.8
energy (kcal mol-1) for Ph–X: Cl (95), Br (80) and I (65)]10 and
thus, because of which the Suzuki–Miyaura cross-coupling of aryl
chlorides is a topic of contemporary interest.
Despite its pressing need for broad based applicability, the
first breakthrough in the Suzuki–Miyaura cross-coupling of aryl
chlorides however, appeared as late as 1998,9d several decades after
the first report of the cross-coupling of aryl bromide substrates
appeared.5c Since then a plethora of palladium precatalysts
supported over a variety of ligand scaffolds ranging from the
strongly s-donating and also sterically demanding phosphines11
and N-heterocyclic carbenes12 to less basic N-based ligands13
have been developed for the aryl chloride cross-coupling. Quite
interestingly, a recent report by Li and Hor14 indicated better
prospects for pyrazole based precatalysts over the strongly s-
donating N-heterocyclic carbene ligands in a variety of cross-
coupling reactions under aerobic conditions, as it was demon-
strated that the replacement of a N-heterocyclic carbene moiety
by a pyrazole group in the ligand backbone of a palladium complex
led to significant enhancement in the catalytic activity. Because of
the aforementioned reasons, we became interested in examining
the potential of pyrazole derived systems for the Suzuki–Miyaura
coupling of aryl chlorides.
A formidable challenge lies in utilizing inexpensive aryl chlo-
rides as substrates for the cross-coupling reaction because of their
wide diversity and ready availability that make them commercially
important intermediates as opposed to the aryl bromides and
iodides, which are not only considered expensive but are also
aDepartment of Chemistry, Indian Institute of Technology Bombay, Powai,
Mumbai, 400 076. E-mail: pghosh@chem.iitb.ac.in; Fax: +91-22-2572-
3480; Tel: +91-22-2576-7178
With one of our research aims being in the utility of N-
heterocyclic carbenes (NHCs)15 in a variety of C–C bond forming
reactions like the Suzuki–Miyaura,16 Sonogashira,17 Hiyama17c
and Michael addition reactions,18 and in the course of which we
have explored the effect of electron richness at the metal center
of the precatalysts by varying the number of NHC ligands on the
metal and found that more electron rich metal centers performed
better for the cross-coupling reactions,16b,c,17d quite similar to that
observed in case of the phosphine ligands.10c With regards to the
Suzuki–Miyaura reaction, we have recently reported palladium
bNational Single Crystal X-ray Diffraction Facility, Indian Institute of
Technology Bombay, Powai, Mumbai, 400 076
cDepartment of Chemistry, Howard University, 525 College Street, NW,
Washington DC, 20059, USA
† Electronic supplementary information (ESI) available: X-ray crystallo-
graphic data, B3LYP coordinates of the optimized geometries of 1–4,
computational data and the control experiment data. CCDC reference
numbers 626720, 630628, 636906 and 636907. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c003174g
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precatalysts of N-heterocyclic carbenes for the cross-coupling of
aryl bromide and iodide substrates.16 Progressing further along
this theme of research, we became interested in designing Suzuki–
Miyaura precatalysts for the more challenging aryl chloride sub-
strates. In particular, we looked into the possibility of employing
homoscorpionate bispyrazolyl ligand based catalysts for the same.
Here in this contribution, we report a series of palladium
precatalysts (1–4) supported over homoscorpionate bispyrazolyl
ligands for the Suzuki–Miyaura coupling of a wide range of
aryl chloride substrates bearing electron withdrawing, electron
donating and heteroaryl substituents with phenyl boronic acid in
air in a mixed aqueous medium (Fig. 1 and eqn (1)). A diverse
set of bispyrazolyl derived palladium precatalysts ranging from
discrete monomers (1–3) to a 20-membered macrometallacycle (4)
have been employed in the study.
Results and discussion
Several new palladium complexes, {[RN{-(CH2)n-pz3,5-Me2}2]-
PdCl2}m [m = n = 1; R = 2,6-Me2C6H3 (1), 2,4,6-Me3C6H2 (2),
CH2Ph (3) and m = n = 2; R = CH2Ph (4)] derived from the amine
based homoscorpionate pyrazolyl ligands, namely, [RN{-(CH2)n-
pz3,5-Me2}] [n = 1; R = 2,6-Me2C6H3 (1a), 2,4,6-Me3C6H2 (2a),19
CH2Ph (3a)20 and n = 2; R = CH2Ph (4a)21] were synthesized
by the direct reaction of the ligands 1a–4a with (COD)PdCl2 in
refluxing benzene in 56–90% yields (Scheme 1).
Of particular interest is the 1H NMR spectrum of the methylene
bridged moiety in the palladium 1–3 complexes that appeared
as two sets of doublets at ca. 7.50 ppm and ca. 5.20 ppm, each
exhibiting a geminal coupling (2JHH) of 15 Hz. The observation
of the highly downfield shifted doublet at ca. 7.50 ppm pointed
towards the existence of anagostic [C–H ◊ ◊ ◊ Pd] type interactions22
in these complexes. On the contrary, the ethylene bridged palla-
dium complex 4 did not show any such geminal coupling in its
1H NMR spectrum thus negating the possibility of such anagostic
interaction in the complex. In this context it is worth mentioning
that the commonly observed [C–H ◊ ◊ ◊ M] type interactions usually
belong to any of the following two classes, (i) the agostic ones
exhibiting covalent 3c–2e type interaction and (ii) the anagostic
ones of predominantly electrostatic origin. Notably, concurring
well with the anagostic nature of the [C–H ◊ ◊ ◊ Pd] interaction in the
palladium 1–3 complexes, no significant shifts in the methylene C–
H stretching frequencies were observed in the infrared spectrum
of these complexes compared to its free ligand and, thus, are in
contrary to the cases of agostic [C–H ◊ ◊ ◊ M] type interactions,
where the C–H stretching frequencies have been reported to
shift to significantly lower frequencies at ca. 2700–2350 cm-1.23
Furthermore, the 1JCH (13C, 1H) values calculated for the palladium
1–3 complexes [148 Hz (1), 147 Hz (2) and 149 Hz (3)] compare
well with that for a representative bispyrazolyl ligand 1a (149
Hz), further testifying towards the anagostic nature of interaction
between the palladium center and the hydrogen atoms of the
bridging methylene moiety.
(1)
The compelling evidence in favor of the presence of anagostic
interactions in the palladium 1–3 complexes came from the X-ray
diffraction studies that showed clear [C–H ◊ ◊ ◊ Pd] type interactions
present between bridging methylene hydrogen atoms and the metal
center in these complexes (Fig. 2, Fig. S1 and S2, and Table S1,
Fig. 1 Palladium precatalysts, 1–4, of homoscorpionate bispyrazolyl
ligands.
Scheme 1 Synthesis of the palladium precatalysts of homoscorpionate bispyrazolyl ligands.
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Fig. 2 ORTEP of 1 shown with 50% probability ellipsoids. Selected
bond lengths (A) and angles ( ): Pd1–N5 2.027(3), Pd1–N1 2.026(3),
Fig. 3 ORTEP of 4 shown with 50% probability ellipsoids. Selected bond
lengths (A) and angles ( ): Pd1–N10 2.023(4), Pd1–N1 2.027(4), Pd1–Cl2
◦
◦
˚
˚
Pd1–Cl1 2.2852(8), Pd1–Cl2 2.2932(8), Pd1–H6B 2.8777, Pd1–H15A
2.9194, N1–N2 1.356(4), N5–Pd1–N1 91.78(11), N5–Pd1–Cl1 178.11(8),
N1–Pd1–Cl1 87.61(8), N5–Pd1–Cl2 87.67(8), N1–Pd1–Cl2 179.37(8).
2.2915(13), Pd1–Cl1 2.2945(12), Pd2–N6 1.999(4), Pd2–N5 2.025(4),
Pd2–Cl4 2.2788(12), Pd2–Cl3 2.2916 (12), N10–Pd1–N1 174.74(17),
N10–Pd1–Cl2 90.14 (12).
In order to gain deeper insight into the nature of the pyrazole–
palladium interaction in the 1–4 complexes, detailed density
functional theory (DFT) studies were performed. Specifically,
the geometry optimized structures were computed for the 1–4
complexes at the B3LYP/SDD, 6-31G(d) level of theory using
atomic coordinates adopted from X-ray analysis (see Tables S2–
S5, ESI†), followed by single-point calculations on the geometry
optimized structures using natural bond order (NBO) method
for a greater understanding of the electronic properties of these
complexes. Specifically, the electron donation from the pyrazolyl
ligand moiety to the palladium center in complexes 1–4 is apparent
from the natural and Mulliken charge analyses that revealed
significant enhancement in the electron density on the metal center
in 1–4 as compared to that in the PdCl2 fragment (see Tables S6–S9,
ESI†). Additionally, the NBO analysis indicated that the electron
donation from the bispyrazolyl ligand occurred on to the 5s orbital
of the palladium center in the 1–4 complexes (see Tables S10 and
S11, ESI†).
Further estimate of the strength of pyrazole–palladium interac-
tion in complexes 1–4 was obtained by computing the Npyrazole–Pd
bond dissociation energies, De/(Pd–pyrazole), 1 (41.98 kcal mol-1),
2 (42.05 kcal mol-1), 3 (41.87 kcal mol-1) and 4 (50.20 kcal mol-1), at
the B3LYP/SDD, 6-31G(d) level of theory (see Table S12, ESI†),
which pointed towards a reasonably strong interaction. In this
context it is noteworthy that a strong ancillary ligand to metal
interaction plays a pivotal role in preventing catalyst leaching
thereby enhancing its lifetime.
ESI†). Of interest to the anagostic interaction, are the [C–H ◊ ◊ ◊ Pd]
˚
distances [2.8436–2.9194 A] that are slightly longer than the sum
24
˚
of the van der Waals radii of Pd and H (2.72 A) as expected
while the other distal hydrogen atom of the bridging methylene
˚
moiety showed an even greater separation of 4.189–4.231 A from
the metal center in these complexes. It is worth pointing out at
this juncture that the origin of the observed [C–H ◊ ◊ ◊ Pd] type
anagostic interactions in the palladium 1–3 complexes may also be
attributed to the formation of a stable chelate ring conformation
upon coordination of the bispyrazolyl ligand to palladium which
positions the C–H bond in the vicinity of the Pd2+ center.
Consistent with being a d8 metal center, the palladium center in
the 1–3 complexes was found to be in a square planar geometry
˚
and displayed a Pd–N distances of 2.0217(15)–2.0459(18) A and
˚
a Pd–Cl distances of 2.2810(7)–2.2932(8) A. It must be pointed
out that the palladium complex 3 exhibited a crystallographically
imposed mirror symmetry in the solid state. In contrast to the
methylene bridged complexes 1–3, the ethylene bridged analogue
4 showed an unusual 20-membered macrometallacyclic dimeric
structure, {[PhCH2N{-(CH2)2-pz3,5-Me2}2]PdCl2}2 (4) (Fig. 3 and
Table S1, ESI†). The ethylene bridged complex 4 further differed
from the methylene bridged ones (1–3) by the conspicuous
absence of any anagostic interaction in the complex. Each of the
metal centers in the ethylene bridged complex 4 also displayed
a square planar geometry with the two chlorides and the two
coordinating pyrazolyl-N’s occupying trans positions and were in
sharp contrast to the cis geometry exhibited by the methylene
bridged 1–3 complexes. The Pd–N distances in 4 were found to be
Additional understanding of the pyrazole–Pd interaction in
1–4 was obtained from the molecular orbital (MO) correlation
diagram, constructed from the interaction of the individual
fragment molecular orbitals (FMOs) of the pyrazolyl ligand
˚
1.999(4)–2.027(4) A while the Pd–Cl distances were 2.2788(12)–
˚
2.2945(12) A.
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Fig. 4 Simplified orbital interaction diagram showing major contributions to the pyrazole–palladium bonding orbital HOMO-27 in 1.
Table 1 Suzuki–Miyaura cross-coupling reaction of PhB(OH)2 with p-
fragment with the PdCl2 fragment in these complexes using
AOMix software.25 Of particular relevance is the s-interaction be-
tween the pyrazole moiety and the palladium center in complexes
1–4 as represented by the following molecular orbitals (MOs),
HOMO-27 (1), HOMO-27 (2), HOMO-26 (3) and HOMO-66,
HOMO-71 (4) (Fig. 4 and 5, and Fig. S3–S9, ESI†). It is worth
mentioning that the low-lying nature of the molecular orbitals
(MO) depicting pyrazole–Pd s-interaction in these complexes
indicate a stable ligand–metal interaction.
All palladium 1–4 complexes efficiently catalyzed the Suzuki–
Miyaura cross-coupling reaction of aryl chlorides with phenyl
boronic acid in air in a mixed-aqueous medium (eqn (1)). The
optimized conditions for the coupling reaction were obtained
for a representative substrate, p-chloroacetophenone, by varying
the solvent and base combinations (Table 1). For example, the
catalysis runs performed in both toluene and methanol yielded
insignificant conversion of ca. 2%, while the similar ones in
either DMF or in a mixed medium, DMF–H2O (1 : 1 v/v), gave
though comparatively higher but still sufficiently subdued yields
ca. 45–60%. Quite interestingly, the best results were observed
(>99% conversion) using Cs2CO3 as base in a mixed aqueous
medium (DMF–H2O, 9 : 1 v/v) in presence of added TBAB (1.5
equivalents) at 120 ◦C. The enhanced activity of palladium 1–4
complexes in the mixed aqueous medium can be attributed to the
better solubility of the Cs2CO3 base under aqueous conditions
and also to the stabilization of any unsaturated catalytic species
generated by the coordinating solvents.26
ClC6H4COCH3: solvent and base variation studya
Entry
Solvent
Base
T/◦C
Yieldb
1
2
CH3OH
toluene
Dioxane/H2O (9 : 1)
DMF
DMF
DMF–H2O (1 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
DMF–H2O (19 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
DMF–H2O (9 : 1)
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
CH3COONa
Na2CO3
K2CO3
60
2
2
110
110
120
120
120
120
120
120
120
120
120
120
120
120
3
13
45
50
60
>99
62
>99
20
41
60
63
92
82
4c
5
6
7
8c
9
10
11
12
13
14
15
NaOH
KOH
KOtBu
a Reaction conditions: 1.00 mmol of aryl chloride, 1.20 mmol of boronic
acid, 1.50 mmol of base, 1.50 mmol of TBAB, 2 mol% of 3 in 8 mL of
solvent at T (◦C) for 5 h. b The yields (%) were determined by GC using
diethylene glycol di-n-butyl ether as an internal standard. c In the absence
of added TBAB.
Specifically, when a mixture of the aryl chloride and phenyl
boronic acid in the presence of 2 mol% of precatalysts 1–4 were
7356 | Dalton Trans., 2010, 39, 7353–7363
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Fig. 5 Simplified orbital interaction diagram showing major contributions to the pyrazole–palladium bonding orbital HOMO-66 in 4.
heated at 120 ◦C in air in a DMF–H2O (9 : 1 v/v) mixed medium,
clean conversion to the cross-coupled product was obtained in a
relatively short period of the reaction time of 5 h (Table 2). A
wide variety of aryl chlorides bearing electron withdrawing (NO2,
CN, COPh, CHO, CF3 and COCH3), electron donating (CH3)
and with heteroatom (C5H4N) substituents were all converted to
the respective cross-coupled products in moderate to excellent
yields (23–>99%). In addition to the frequently encountered
(sp2–sp2) couplings, the palladium 1–4 complexes also catalyzed
the cross-coupling of the more challenging benzyl and cinnamyl
chlorides with phenyl boronic acid (sp2–sp3 centers) in moderate
to excellent yields (19–>99%). Furthermore, in order to highlight
the practical utility of Suzuki–Miyaura cross-coupling by the
palladium precatalysts, the isolated yields are provided for a
representative precatalyst 3 (Table 2). The temperature dependence
profile obtained for the coupling of p-nitrochlorobenzene with
phenyl boronic acid for the catalysis runs of 5 h showed that
higher conversions of >90% could only be achieved above 100 ◦C
(see Fig. S10, ESI†).
The homogeneous nature of the Suzuki–Miyaura coupling by
the palladium 1–4 precatalysts was verified by performing the
“classical Hg-drop test”27 that showed no significant effect on the
catalysis results (see Table S13, ESI†).
For substrates where quantitative conversions were observed
(entries 1–7 and 12, Table 2), additional catalysis runs were
performed at lower precatalyst loadings for a representative
precatalyst 3 in order to gauge the lower limit of the catalyst
efficiency. Significantly enough, precatalyst 3 continued to exhibit
low to high conversions (6–95%) even at a lower precatalyst
loading of 0.05 mol% (entries 1–2, Table 3). In this regard it is
worth mentioning that remarkably high turnover numbers (TON)
of up to 4100 (TOF = 820 h-1) could be observed for the activated
p-nitrochlorobenzene substrate at 0.01 mol% of precatalyst 3
loading in 5 h. Furthermore, a high conversion of 91% exhibiting a
turnover number (TON) of 910 was seen for the more challenging
(sp2–sp3) coupling of benzyl chloride with phenyl boronic acid at
0.1 mol% of the precatalyst 3 loading.
The high turnover number (TON) of 4100 observed in the case
of 3 for an activated p-nitochlorobenzene substrate is comparable
to that of the [8-(di-tert-butylphosphinooxy)quinoline]PdCl2
Significant “ligand-influence” of the bispyrazolyl ligands in
precatalysts 1–4 was very much evident when compared to the
control experiments performed using the simple metal precursors
like, PdCl2 and (COD)PdCl2, that showed significant enhance-
ments of up to 84% in the reaction yields for 1–4 precatalysts.
precatalyst, which too exhibited
a similar high turnover
number of 3750, also for an activated aryl chloride
substrate, p-chloroacetophenone.28 Another N-based ligand
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Table 2 Selected results for Suzuki–Miyaura cross-coupling reaction of aryl chlorides catalyzed by 1–4
Yieldb
Entry
1
Reagenta
Reagenta
Cross-coupled product
1
2
3
4
>99
>99
>99
>99
>99
>99
>99 (78)
>99 (87)
>99 (93)
>99 (90)
>99 (84)
>99
>99
>99
>99
>99
2
3
4
5
>99
>99
>99
>99
6
7
8
9
>99
83
>99
81
>99 (61)
>99 (78)
66
>99
87
62
89
41
58
46
35
43
10
11
28
23
28
19
28
21
26
22
12
13
>99
>99
>99 (92)
>99
7
19
17
16
a Reaction conditions: 1.00 mmol of aryl chloride, 1.20 mmol of boronic acid, 1.50 mmol of Cs2CO3, 1.50 mmol of TBAB, 2 mol% of catalyst 1–4 in 8 mL
of DMF–H2O (9 : 1), at 120 ◦C for 5 h. b The yields (%) were determined by GC using diethylene glycol di-n-butyl ether as an internal standard. Isolated
yields for representative runs are given in the parentheses.
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Table 3 Selected results for Suzuki–Miyaura cross-coupling reaction of chlorides catalyzed by 3: catalyst loading variation
Entry
1
Reagenta
Reagenta
Cross-coupled product
Catalyst loading (mol%) Yieldb (%)
TON
0.5
>99
>99
78
200
0.1
1000
1560
4100
0.05
0.01
41
2
3
4
5
0.5
98
97
95
196
0.1
970
0.05
1900
0.5
0.1
0.05
>99
93
6
200
930
120
0.5
0.1
>99
68
200
680
0.5
0.1
>99
74
200
740
6
0.5
0.1
93
91
186
910
7
8
0.5
0.1
80
31
160
310
0.5
16
32
a Reaction conditions: 1.00 mmol of aryl chloride, 1.20 mmol of boronic acid, 1.50 mmol of Cs2CO3, 1.50 mmol of TBAB and 3 in 8 mL of DMF–H2O
(9 : 1), at 120 ◦C for 5 h. b The yields (%) were determined by GC using diethylene glycol di-n-butyl ether as an internal standard.
chlorides substrates at a lower catalyst loading of 0.1 mol% at room
supported palladium precatalyst, {N-cyclohexyl-N¢di-(2-
temperature in 20–24 h.34 Among, the phosphine counterparts,
pyridyl)methylurea}PdCl2, is notable for its high turnover number
of 6500 for the coupling of p-chloroacetophenone at 100 ◦C in
a palladium 2,6-bis-(trimethylsilyl) substituted phosphabarrelene
72 h.29 The precatalyst showed excellent activity for the coupling
of a wide variety of aryl, heteroaryl, benzylic, allyl chlorides,
with aryl and alkyl boronic acids in neat H2O as well as in
mixed aqueous mediums namely, acetone–H2O (3 : 2 v/v) and
DMF–H2O (95 : 5 v/v).28
complex carried out the coupling of several electronically modu-
lated aryl chlorides at a low catalyst loading of 0.2 mol% in low
◦
to excellent yields (5–99%) at 80 C in 1–20 h.35 Also notable
is a palladium meta-terphenyl diphosphinite [PCP] pincer type
precatalyst, [2,6-(2-iPr2POC6H4)2C6H3]PdBr, that exhibited good
to excellent yields (52–99%) in the coupling of aryl chlorides at
Important is the comparison of the bispyrazolyl based 1–
4 precatalysts with other reported N-heterocyclic carbene,30
phosphine31 and the N-donor ligand13,14,32 based ones for the
Suzuki–Miyaura cross-coupling of aryl chlorides with aryl
boronic acids. For example, the sterically demanding N-
heterocyclic carbene based PEPPSI themed trans-[1,3-bis(2,6-
diisopentylphenyl)imidazol-2-ylidene]PdCl2 (3-chloropyridine)
complex carried out the challenging coupling of o-disubstituted
aryl and heteroaryl chlorides with o-disubstituted phenyl boronic
acids in good to excellent yields (49–95%) at 2 mol% of the
catalyst loading at 65 ◦C in 24 h, thereby opening up a new
synthetic route to crowded tetra ortho-substituted biaryls.33 An-
other highly active N-heterocyclic carbene based palladium {[1,3-
bis(2,6-diisopropylphenyl)imidazol-2-ylidene]PdCl(m-Cl)}2 com-
plex displayed clean conversions (36–99%) for aryl and heteroaryl
◦
0.5–2 mol% palladium loading at 75–100 C in 20 h.36 Against
this backdrop, significant conversions of 23–>99% were observed
for the bispyrazolyl derived 1–4 precatalysts at 2 mol% of catalyst
◦
loading at 120 C and thus their activity compares well with the
other reported ones.
Conclusions
In summary, several new highly efficient palladium precatalysts
1–4 of homoscorpionate bispyrazolyl ligands have been designed
for the more challenging Suzuki–Miyaura coupling of aryl chlo-
rides with phenyl boronic acids. A wide variety of aryl chlorides
containing electron withdrawing (NO2, CN, COPh, CHO, CF3
and COCH3), electron donating (CH3) as well as with heteroatom
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(C5H4N) substituents could be converted to the respective cross-
coupled products in moderate to high yields. High turnover
numbers (TON’s) of up to ca. 4100 could be obtained for an
activated aryl chloride substrate namely, p-nitrochlorobenzene.
More challenging (sp2–sp3) coupling of benzyl and cinnamyl
chlorides with phenyl boronic acids were also achieved by these 1–
4 precatalysts. Quite interestingly, the structural characterization
of these precatalysts showed that while the discrete monomeric
palladium 1–3 complexes, supported over the methylene bridged
bispyrazolyl ligands displayed [C–H ◊ ◊ ◊ Pd] anagostic interactions,
the ethylene bridged counterpart 4 in contrast was a dimeric
20-membered macrometallacycle without the [C–H ◊ ◊ ◊ Pd] anagos-
tic interaction.
Synthesis of {N,N(bis(3,5-dimethylpyrazolyl)methyl)-2,6-
dimethylaniline}PdCl2 (1)
To
a
stirred suspension of (COD)PdCl2 (0.195 g,
0.683 mmol) in benzene (ca. 30 mL) was added N,N(bis(3,5-
dimethylpyrazolyl)methyl)-2,6-dimethylaniline (0.230 g,
0.682 mmol) and the reaction mixture was stirred at reflux
for 12 h. The reaction mixture was filtered and the filtrate was
pumped under vacuum to yield product 1 as a yellow solid
(0.316 g, 90%). 1H NMR (CDCl3, 400 MHz, 25 ◦C): d 7.51 (d, 2H,
3
2JHH = 15 Hz, CH2), 7.21 (d, 1H, JHH = 8 Hz, m-C6H3(CH3)2),
7.10 (t, 1H, 3JHH = 8 Hz, p-C6H3(CH3)2), 6.87 (d, 1H, 3JHH = 8 Hz,
m-C6H3(CH3)2), 5.81 (s, 2H, C3N2H), 5.23 (d, 2H, 2JHH = 15 Hz,
CH2), 2.85 (s, 6H, CH3), 2.50 (s, 3H, C6H3(CH3)2), 1.74 (s, 6H,
CH3), 0.73 (s, 3H, C6H3(CH3)2). 13C NMR (CDCl3, 100 MHz,
25 ◦C): d 151.1 (C3N2H), 142.9 (C3N2H), 142.8 (ipso-C6H3), 138.5
(o-C6H3), 137.0 (o-C6H3), 129.8 (p-C6H3), 129.0 (m-C6H3), 128.6
(m-C6H3), 107.7 (C3N2H), 69.9 (CH2), 18.6 (C6H3(CH3)2), 15.4
(CH3), 14.6 (C6H3(CH3)2), 11.2 (CH3). IR data (KBr pellet cm-1):
3210 (w), 3131 (w), 2921 (m), 1625 (w), 1552 (s), 1466 (m), 1397
(m), 1295 (m), 1264 (m), 1214 (m), 1192 (s), 1162 (m), 1130 (m),
1103 (w), 1060 (m), 1035 (m), 952 (m), 811 (s), 793 (s), 698 (s), 626
(w), 582 (w). Anal. Calc. for C20H27N5PdCl2: C, 46.66; H, 5.29; N,
13.60. Found: C, 47.04; H, 5.01; N, 12.65%.
Experimental
General procedures
All manipulations were carried out using standard Schlenk
techniques. Solvents were purified and degassed by stan-
dard procedures. 2,6-dimethylaniline and 2,4,6-trimethylaniline
were purchased from Sigma Aldrich, Germany and palla-
dium chloride was purchased from Souvenier Chemicals (In-
dia) and used without any further purification. N-(methylol)-
3,5- dimethylpyrazole,37 N,N(bis(3,5-dimethylpyrazolyl)methyl)-
2,4,6-trimethylaniline,19 N,N(bis(3,5-dimethylpyrazolyl)methyl)-
benzylamine20 and benzyl-bis-[2-(3,5-dimethylpyrazolyl)ethyl]-
amine21 were prepared according to literature procedures. 1H and
Synthesis of {N,N(bis(3,5-dimethylpyrazolyl)methyl)-2,4,6-
trimethylaniline}PdCl2 (2)
To a stirred suspension of (COD)PdCl2 (0.203 g, 0.711 mmol)
in benzene (ca. 30 mL) was added the N,N(bis(3,5-
1
13C{ H} NMR spectra were recorded on a Varian 400 MHz NMR
spectrometer. 1H NMR peaks are labeled as singlet (s), doublet (d),
triplet (t) and multiplet (m). Mass spectrometry measurements
were done on a Micromass Q-Tof spectrometer. GC spectra were
obtained on a PerkinElmer Clarus 600 equipped with a FID.
GCMS spectra were obtained on a PerkinElmer Clarus 600 T
equipped with an EI source. Elemental Analysis was carried out
on Thermo Quest FLASH 1112 SERIES (CHNS) Elemental
Analyzer.
dimethylpyrazolyl)methyl)-2,4,6-trimethylaniline
(0.253
g,
0.721 mmol) and the reaction mixture was stirred at reflux for
16 h. The reaction mixture was filtered and the filtrate was pumped
under vacuum to yield the product 2 as◦a yellow powder (0.211 g,
56%). 1H NMR (CDCl3, 400 MHz, 25 C): d 7.48 (d, 2H, 2JHH
=
15 Hz, CH2), 7.02 (s, 1H, C6H2(CH3)3), 6.69 (s, 1H, C6H2(CH3)3),
5.81 (s, 2H, C3N2H), 5.19 (d, 2H, 2JHH = 15 Hz, CH2), 2.85 (s, 6H,
CH3), 2.43 (s, 3H, C6H2(CH3)3), 2.24 (s, 3H, C6H2(CH3)3), 1.74 (s,
6H, CH3), 0.71 (s, 3H, C6H2(CH3)3). 13C NMR (CDCl3, 100 MHz,
25 ◦C): d 150.8 (C3N2H), 142.9 (C3N2H), 140.1 (ipso-C6H2), 138.2
(o-C6H2), 137.9 (o-C6H2), 136.5 (p-C6H2), 130.5 (m-C6H2), 129.5
(m-C6H2), 107.6 (C3N2H), 70.0 (CH2), 20.9 (p-C6H2(CH3)3), 18.4
(o-C6H2(CH3)3), 15.3 (CH3), 14.5 (o-C6H2(CH3)3), 11.2 (CH3).
IR data (KBr pellet cm-1): 3213 (w), 3123 (w), 2920 (m), 1609 (w),
1554 (m), 1465 (s), 1405 (s), 1297 (m), 1260 (m), 1220 (m), 1194
(m), 1156 (w), 1130 (w), 1060 (w), 1035 (w), 988 (w), 959 (w), 942
(w), 852 (w), 816 (m), 685 (m), 595 (w), 514 (w). Anal. Calc. for
C21H29N5PdCl2: C, 47.70; H, 5.53; N, 13.24. Found: C, 47.73; H,
4.96; N, 12.60%.
Synthesis of N,N(bis(3,5-dimethylpyrazolyl)methyl)-2,6-
dimethylaniline (1a)
A mixture of 2,6-dimethylaniline (1.36 g, 11.2 mmol) and N-
(methylol)-3,5-dimethylpyrazole (2.87 g, 22.8 mmol) was taken
in CH3CN (ca. 50 mL) and heated at 80 ◦C for 14 h. The solvent
was removed under vacuum to obtain the crude product 1a as a
white solid, which was purified by recrystallizati◦on from hexane
1
(2.81 g, 74%). H NMR (CDCl3, 400 MHz, 25 C): d 7.01-6.91
(m, 3H, C6H3(CH3)2), 5.67 (s, 2H, C3N2H), 5.29 (s, 4H, CH2),
2.22 (s, 6H, C6H3(CH3)2), 1.81 (s, 6H◦, CH3), 1.72 (s, 6H, CH3). 13
C
Synthesis of {bis[2-(3,5-dimethylpyrazolyl)methyl]-
benzylamine}PdCl2 (3)
1
{ H} NMR (CDCl3, 100 MHz, 25 C): d 147.8 (C3N2H), 143.4
(ipso-C6H3N), 139.0 (C3N2H), 137.5 (o-C6H3), 128.8 (m-C6H3),
126.6 (p-C6H3), 105.5 (C3N2H), 64.8 (CH2), 17.5 (C6H3(CH3)2),
13.6 (CH3), 10.2 (CH3). IR data (KBr pellet cm-1): 2949 (s), 2918
(s), 2861 (s), 1667 (w), 1552 (s), 1479 (m), 1459 (m), 1422 (m), 1378
(w), 1301 (m), 1269 (w), 1238 (w), 1196 (s), 1159 (w), 1110 (m),
1029 (m), 986 (w), 959 (w), 797 (s), 784 (s), 770 (s), 709 (w), 631
(w), 582 (w). Anal. Calc. for C20H27N5: C, 71.18; H, 8.06; N, 20.75.
Found: C, 70.52; H, 8.49; N, 19.90.
To
a
stirred suspension of (COD)PdCl2 (0.281 g,
0.985 mmol) in benzene (ca. 40 mL) was added bis[2-(3,5-
dimethylpyrazolyl)methyl]benzylamine (0.321 g, 0.994 mmol) and
the reaction mixture was stirred at reflux for 12 h. The reaction
mixture was filtered and the filtrate was pumped under vacuum to
yield the product 3 as a yellow powder (0.378 g, 77%). 1H NMR
(CDCl3, 400 MHz, 25 ◦C): d 7.33–7.23 (m, 5H, C6H5 and CH2),
7360 | Dalton Trans., 2010, 39, 7353–7363
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7.06 (d, 2H, 3JHH = 8 Hz, o-C6H5), 5.84 (s, 2H, C3N2H), 5.35 (d,
2H, 2JHH = 16 Hz, CH2), 3.92 (s, 2H, CH2Ph), 2.83 (s, 3H, CH3),
(i) s-donation from the [bis-pyrazole → PdCl2] fragment
(ii) p-back donation from [bis-pyrazole ← PdCl2] fragment and
(iii) repulsive polarization (r)
◦
2.00 (s, 3H, CH3). 13C NMR (CDCl3, 100 MHz, 25 C): d 151.2
(C3N2H), 143.8 (C3N2H), 136.4 (ipso-C6H5), 129.1 (m-C6H5),
128.0 (p-C6H5), 126.5 (o-C6H5), 108.0 (C3N2H), 68.9 (CH2), 53.2
(CH2Ph), 15.4 (CH3), 11.9 (CH3). IR data (KBr pellet cm-1): 3131
(w), 3008 (w), 2928 (w), 1636 (w), 1557 (m), 1493 (w), 1460 (m),
1444 (m), 1421 (s), 1403 (s), 1346 (m), 1278 (w), 1252 (s), 1218
(w), 1170 (s), 1154 (m), 1138 (m), 1061 (w), 1003 (w), 973 (w), 896
(w), 821 (m), 805 (m), 732 (w), 702 (m), 620 (w), 473 (w). Anal.
Calc. for C26H24N2PdCl2: C, 45.57; H, 5.03; N, 13.99. Found: C,
45.86; H, 5.01; N, 14.39%.
The CDA calculations are performed using the AOMix,25 using
the B3LYP/SDD, 6-31G(d) wave function. Molecular orbital
(MO) compositions and the overlap populations were calculated
using the AOMix program. The analysis of the MO compositions
in terms of occupied and unoccupied fragment orbitals (OFOs and
UFOs, respectively), construction of the orbital interaction dia-
grams, the charge decomposition analysis (CDA) was performed
using the AOMix-CDA.45
General procedure for the Suzuki–Miyaura cross-coupling reaction
of aryl chlorides
Synthesis of {(benzyl-bis[2-(3,5-dimethylpyrazolyl)ethyl]amine)-
PdCl2}2 (4)
In a typical catalysis run, performed in air, a 25 mL vial was
charged with a mixture of the aryl chloride (1.00 mmol), phenyl
boronic acid (0.146 g, 1.20 mmol), Cs2CO3 (0.487 g, 1.50 mmol),
TBAB (0.484 g, 1.5 mmol) and diethyleneglycol-di-n-butyl ether
(internal standard) (0.218 g, 1.00 mmol). Palladium complexes
1–4 (2 mol%) were added to the mixture followed by the solvent
(DMF–H2O, 9 : 1 v/v, 8 mL) and the reaction mixture was heated
at 120 ◦C for an appropriate period of time, after which an aliquot
was filtered and the product analyzed by gas chromatography
using diethyleneglycol-di-n-butyl ether as an internal standard.
To a stirred suspension of (COD)PdCl2 (0.278 g, 0.974 mmol)
in benzene (ca. 30 mL) was added benzyl-bis-[2-(3,5-
dimethylpyrazolyl)ethyl]amine (0.344 g, 0.980 mmol) and the
reaction mixture was allowed to stir at reflux for 12 h. The
reaction mixture was cooled to room temperature, filtered and then
concentrated under vacuum to yield the product 4 as an orange-
yellow solid (0.369 g, 72%). 1H NMR (CDCl3, 400 MHz, 25 ◦C):
d 7.45 (d, 2H, JHH = 8 Hz, o-C6H5), 7.35 (t, 2H, JHH = 8 Hz,
m-C6H5), 7.31 (t, 1H, 3JHH = 8 Hz, p-C6H5), 5.86 (s, 4H, C3N2H),
5.39 (br, 8H, CH2), 4.18 (s, 4H, CH2Ph), 3.38 (br, 8H, CH2), 2.88
(s, 12H, CH3), 2.19 (s, 12H, CH3). 13C NMR (CDCl3, 100 MHz,
25 ◦C): d 152.7 (C3N2H), 144.1 (C3N2H), 134.0 (ipso-C6H5), 131.6
(m-C6H5), 129.4 (p-C6H5), 128.9 (o-C6H5), 108.3 (C3N2H), 68.7
(CH2), 61.1 (CH2), 48.4 (CH2Ph), 15.2 (CH3), 11.8 (CH3). IR data
(KBr pellet cm-1): 2925 (m), 1634 (w), 1555 (s), 1495 (m), 1424 (s),
1398 (s), 1320 (w), 1232 (w), 1159 (w), 1054 (w), 912 (w), 878
(w), 791 (m), 746 (m), 704 (m). Anal. Calc. for C42H58N10Pd2Cl4:
C, 47.70; H, 5.53; N, 13.24. Found: C, 47.31; H, 5.45; N,
12.38%.
3
3
General procedure for the Hg(0) drop test
A 25 mL vial was charged with a mixture of the aryl chlo-
ride (1.00 mmol), phenyl boronic acid (0.146 g, 1.20 mmol),
Cs2CO3 (0.487 g, 1.50 mmol), TBAB (0.484 g, 1.5 mmol) and
diethyleneglycol-di-n-butyl ether (internal standard) (0.218 g,
1.00 mmol). Palladium complex 3 (2 mol%) was added to the
mixture followed by the solvent (DMF–H2O, 9 : 1 v/v, 8 mL) and
excess Hg(0) (~ 100 times). The reaction mixture was heated at
◦
120 C for an appropriate period of time, after which an aliquot
was filtered and the product analyzed by gas chromatography
using diethyleneglycol-di-n-butyl ether as an internal standard.
Computational methods
Density functional theory calculations were performed on the
palladium 1–4 complexes including their fragments using GAUS-
SIAN 0338 suite of quantum chemical programs. The Becke
three parameter exchange functional in conjunction with Lee-
Yang-Parr correlation functional (B3LYP) has been employed
in the study.39 Stuttgart-Dresden effective core potential (ECP),
representing 19 core electrons along with the valence basis sets
(SDD) is used for palladium atom.40 All other atoms are treated at
the 6-31G(d) basis set.41 All stationary points are characterized as
minima by evaluating Hessian indices on the respective potential
energy surfaces. Tight SCF convergence (10-8 a.u.) was used for all
calculations. Natural bond orbital (NBO) analysis was performed
using the NBO 3.142 program implemented in the GAUSSIAN 03
package.
X-Ray structure determination
X-Ray diffraction data for compounds 1–4 were collected on
a Bruker P4 diffractometer equipped with a SMART CCD
detector and crystal data collection and refinement parameters are
summarized in Table S1.† The structures were solved using direct
methods and standard difference map techniques, and were refined
by full-matrix least-squares procedures on F2 with SHELXTL
(Version 6.10).46
Acknowledgements
Inspection of the metal–ligand donor–acceptor interactions was
carried out using the charge decomposition analysis (CDA).43
CDA is a valuable tool in analyzing the interactions between
molecular fragments on a quantitative basis, with an emphasis
on electron donation.44 The orbital contributions in the geometry
optimized palladium(II) (bispyrazole)PdCl2 1–4 complexes can be
divided into three parts:
We thank DST, New Delhi, for financial support of this research.
We are grateful to the National Single Crystal X-ray Diffraction
Facility and Sophisticated Analytical Instrument Facility at IIT
Bombay, India, for the crystallographic and other characterization
data. Computational facilities from the IIT Bombay Computer
Center are gratefully acknowledged. AJ thanks CSIR, New Delhi
for research fellowship.
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The Royal Society of Chemistry 2010
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