Factors controlling the reactivity of heteroarenes in direct arylation with arylpalladium acetate complexes

Article abstract of DOI:10.1021/om400636r

The palladium-catalyzed direct arylation of heteroarenes with aryl halides has emerged as a viable alternative to conventional cross-coupling reactions. This paper reports a detailed mechanistic study on factors controlling the reactivity of heteroarenes in direct arylation with well-defined models of the presumed intermediate [PdAr(O₂CMe-κ²O)L] (1). Although recent theoretical studies have provided a reasonable description of the mechanism of C-H bond cleavage by 1, its model compounds so far tested have been evidently less reactive than that expected. We found that [PdPh(O ₂CMe-κ²O)(PPh₃)] (1a) and [Pd(2,6-Me ₂C₆H₃)(O₂CMe-κ²O) (PPh₃)] (1c), generated in situ from isolated [PdPh(μ-O ₂CMe)(PPh₃)]₂ (4a) and [Pd(2,6-Me ₂C₆H₃)(μ-O₂CMe)(PPh ₃)]₄ (4c), respectively, react with a variety of heteroarenes in almost quantitative yields. The reactivity order of heteroarenes was evaluated by competitive reactions, showing that benzothiazole (8) is significantly less reactive than 2-methylthiophene (6), despite the acidity of 8 (pK_a = 27) being much higher than that of 6 (pK_a = 42). This reason was examined by kinetic experiments using 1c as well as DFT calculations using the model compound [PdPh(O₂CMe- κ²O)(PH₃)] (1d). Both heteroarenes reacted with 1 via a sequence of three elementary processes (i.e., substrate coordination, C-H bond cleavage, and C-C reductive elimination), but their energy profiles were significantly different from each other. The reaction of 6 obeyed simple second-order kinetics, and the deuterium-labeling experiments and DFT calculations indicated the occurrence of rate-determining reductive elimination. On the other hand, the reaction of 8 displayed saturation kinetics due to the occurrence of relatively stable coordination of 8 prior to C-H bond cleavage. This coordination stability enhances the activation barrier for C-H bond cleavage, thereby causing the modest reactivity of 8. Thus, although the previous mechanistic studies on direct arylation have been focused largely on the C-H bond cleavage process, not only the C-H bond cleavage but also the substrate coordination and C-C reductive elimination must be considered.

Full text of DOI:10.1021/om400636r

Article
pubs.acs.org/Organometallics
Factors Controlling the Reactivity of Heteroarenes in Direct Arylation
with Arylpalladium Acetate Complexes
Masayuki Wakioka,† Yuki Nakamura,† Yoshihiro Hihara,† Fumiyuki Ozawa,*,†,‡ and Shigeyoshi Sakaki§
†International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan
‡JST ACT-C, Uji, Kyoto 611-0011, Japan
§Fukui Institute for Fundamental Chemistry, Kyoto University, Sakyo-ku, Kyoto 610-8103, Japan
S
* Supporting Information
ABSTRACT: The palladium-catalyzed direct arylation of
heteroarenes with aryl halides has emerged as a viable
alternative to conventional cross-coupling reactions. This
paper reports a detailed mechanistic study on factors
controlling the reactivity of heteroarenes in direct arylation
with well-defined models of the presumed intermediate
[PdAr(O2CMe-κ2O)L] (1). Although recent theoretical studies
have provided a reasonable description of the mechanism of
C−H bond cleavage by 1, its model compounds so far tested
have been evidently less reactive than that expected. We found
that [PdPh(O2CMe-κ2O)(PPh3)] (1a) and [Pd(2,6-Me2C6H3)(O2CMe-κ2O)(PPh3)] (1c), generated in situ from isolated
[PdPh(μ-O2CMe)(PPh3)]2 (4a) and [Pd(2,6-Me2C6H3)(μ-O2CMe)(PPh3)]4 (4c), respectively, react with a variety of
heteroarenes in almost quantitative yields. The reactivity order of heteroarenes was evaluated by competitive reactions, showing
that benzothiazole (8) is significantly less reactive than 2-methylthiophene (6), despite the acidity of 8 (pKa = 27) being much
higher than that of 6 (pKa = 42). This reason was examined by kinetic experiments using 1c as well as DFT calculations using the
model compound [PdPh(O2CMe-κ2O)(PH3)] (1d). Both heteroarenes reacted with 1 via a sequence of three elementary
processes (i.e., substrate coordination, C−H bond cleavage, and C−C reductive elimination), but their energy profiles were
significantly different from each other. The reaction of 6 obeyed simple second-order kinetics, and the deuterium-labeling
experiments and DFT calculations indicated the occurrence of rate-determining reductive elimination. On the other hand, the
reaction of 8 displayed saturation kinetics due to the occurrence of relatively stable coordination of 8 prior to C−H bond
cleavage. This coordination stability enhances the activation barrier for C−H bond cleavage, thereby causing the modest
reactivity of 8. Thus, although the previous mechanistic studies on direct arylation have been focused largely on the C−H bond
cleavage process, not only the C−H bond cleavage but also the substrate coordination and C−C reductive elimination must be
considered.
of constitution units is of particular importance for gaining high
device performance.10c Although such polymers have often
been prepared by Migita−Stille type cross-coupling polymer-
ization, this method uses highly toxic organotin reagents.11 The
direct arylation polymerization most likely serves as a safe
alternative to this method.5 To develop such a catalytic
polymerization process that enables precise control of
constitution units, it is crucial to gain a deep understanding
of the factors governing the reactivity of each heteroarene.
Scheme 1 outlines a generally accepted catalytic cycle for
direct arylation, which is basically similar to that for cross-
coupling reactions. A distinct difference between the two
catalytic cycles exists in the conversion of arylpalladium
complex 1 into diarylpalladium complex 3 (steps b and c).
This process has been examined in detail by DFT calculations,
INTRODUCTION
■
The palladium-catalyzed dehydrohalogenative coupling of
heteroarenes with aryl halides (so-called direct arylation),
which does not need prepreparation of organometallic reagents,
has emerged as a viable alternative to conventional cross-
coupling reactions.1 Although this catalysis should be widely
applicable, its utilization for constructing functional molecules
has been poorly developed until very recently.2 In 2010, we
documented the first successful example of direct arylation
polymerization, in which 2-bromo-3-hexylthiophene is con-
verted to highly regioregular poly(3-hexylthiophene) with high
molecular weight in quantitative yield.3,4 Thereafter, several
research groups have succeeded in the synthesis of alternating
copolymers of heteroarenes via direct arylation.5−9 The
resulting products are π-conjugated polymers that have found
wide application in optoelectronic devices such as photovoltaic
cells and field-effect transistors.10 It has been pointed out that
rational design of polymers with donor−acceptor combinations
Received: June 30, 2013
Published: July 12, 2013
© 2013 American Chemical Society
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Scheme 1. Plausible Catalytic Cycle for Direct Arylation of
Heteroarenes (Ar′H) with Aryl Halides (ArX)
Scheme 3. Reactivity Ratios of Heteroarenes in Direct
Arylation with 4a in 1,4-Dioxane at 90 °C
are reactivity ratios evaluated by competitive reactions of 6 and
each heteroarene (each 0.4 mmol) with 4a (0.01 mmol) in 1,4-
dioxane (0.5 mL) at 90 °C. It is seen that benzothiazole (8) is
clearly less reactive than 2-methylthiophene (6), though 8 (pKa
= 27) is much more acidic than 6 (pKa = 42).19 It is also seen
that, despite their structural similarity, 5-methyl-2,2′-bithio-
phene (10) is more reactive than 2-methylthiophene (6). To
clarify the reasons for the difference in reactivity of these
heteroarenes, the reaction mechanisms of 6 and 8 were
investigated in detail by kinetic experiments and DFT
calculations.
Kinetic Study on Direct Arylation of 2-Methylthio-
phene (6). In a previous study,18 we carried out kinetic
examination of the conversion of 4a and 6 into 7a, but this
system involved a side reaction giving biphenyl. In this study,
we investigated a more selective system using 2,6-dimethyl-
phenyl complex 4c in place of phenyl complex 4a. Complex 4c
has a marked tendency to dissociate into 1c as the reactive
species in solution and thereby cleanly reacts with 6 to afford
the direct arylation product 7c and bis-PPh3 complex 5c in
quantitative yields.
and a concerted metalation−deprotonation (CMD) pathway
involving the arylpalladium carboxylate intermediate [PdAr-
(O2CR-κ2O)L] (1) has been postulated.12−14 Although a
reasonable theoretical description has been provided for the
C−H bond cleavage process in step c, isolated models of 1
tested so far have been much less reactive than that expected
from catalytic reactions.15,16 Therefore, an alternative pathway
with the aid of a cyclometalated complex has been proposed.17
On the other hand, recently we have found that aryl acetate
complexes of the formula [PdAr(μ-O2CMe)(PPh3)]n (4a−c)
smoothly react with 2-methylthiophene (6) to afford 5-aryl-2-
methylthiophenes (7a−c) in high yields, where half of 4a−c is
converted to 7a−c and Pd black and the other half is converted
to bis-PPh3 complexes 5a−c (Scheme 2).18 It has been
Scheme 2. Direct Arylation of 2-Methylthiophene (6) with
Arylpalladium Acetate Complexes (4)
Complex 4c (9.1 mM) was treated with an excess amount of
6 (0.182−1.09 M) at 65 °C in THF in the presence of i-
Pr2EtN.20 When the reaction was followed by 31P{1H} NMR
spectroscopy, a broad singlet at δ 31.0, which is assignable to an
equilibrium mixture of 4c and 1c, gradually decreased to be
replaced by a sharp singlet of 5c at δ 20.9. The reaction obeyed
pseudo-first-order kinetics over 3−4 half-lives (see Figure S1 in
the Supporting Information). The observed rate constants
(kobsd) exhibited a good linear correlation with the concen-
trations of 6: d[5c]/dt = kobsd[PdAr] = k2[6][PdAr] (k2 =
[3.42(8)] × 10−4 s−1 M−1) (Figure 1a).21
These kinetic observations are consistent with the reaction
mechanism presented in Scheme 4, which is basically identical
with that previously proposed for 4a.18 Complex 4c is in
equilibrium with 1c in solution. Coordination of 6 to 1c,
followed by C−H bond cleavage on 2c{6}, forms the diaryl
intermediate 3c{6}, which reductively eliminates the direct
confirmed that, while 4a−c adopt a dimeric or tetrameric
structure in the solid state, they are in rapid equilibrium with
the monomeric species [PdAr(O2CMe-κ2O)(PPh3)] (1a−c) in
solution, and the monomeric species react with 6 to afford
direct arylation products 7a−c.
Having highly selective models of catalytic intermediate 1 in
hand, in this study we attempted a detailed description of the
mechanism of direct arylation using kinetic techniques and
DFT calculations, focusing particularly on the factors
controlling the reactivity of heteroarenes. So far, most of the
mechanistic studies on direct arylation have been focused on
the C−H bond cleavage process.12,13 On the other hand, this
study has clearly indicated that direct arylation is a multistep
process, and its reaction rate is controlled not only by C−H
bond cleavage but also by substrate coordination and C−C
reductive elimination.
RESULTS AND DISCUSSION
■
Relative Reactivity of Heteroarenes. Complex 4a was
sufficiently reactive to compare the reactivity of the series of
heteroarenes given in Scheme 3. The numbers in parentheses
Figure 1. Plots of pseudo-first-order rate constants (kobsd) against the
concentrations of (a) 6 and (b) 8 for the reactions with 4c in THF at
65 °C.
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Scheme 4. Proposed Mechanism of Direct Arylation of 6
with 4c
Figure 2. ORTEP drawing of 2c{8} with 50% probability ellipsoids.
Hydrogen atoms and a solvent molecule (THF) are omitted for clarity.
Selective bond distances (Å) and angles (deg): Pd−C1 = 2.004(3),
Pd−O1 = 2.139(2), Pd−N = 2.115(2), Pd−P = 2.2555(8); C1−Pd−
N = 89.03(11), O1−Pd−N = 88.79(8), C1−Pd−P = 91.29(9), O1−
Pd−P = 91.26(6), C1−Pd−O1 = 174.14(10), N−Pd−P = 176.03(7).
arylation product 7c, along with the formation of [Pd(PPh3)]
and acetic acid. The highly coordinatively unsaturated Pd(0)
species thus generated rapidly decomposes to release free PPh3,
which combines with a [Pd(2,6-Me2C6H3)(μ-O2CMe)(PPh3)]
unit in 4c to form 5c.
Kinetic Study on Direct Arylation of Benzothiazole
(8). The reaction of 4c (9.1 mM) with 8 (0.182−1.45 M) in
THF in the presence of i-Pr2EtN20 at 65 °C was followed by
31P{1H} NMR spectroscopy. Upon addition of 8 to the system,
a broad signal at δ 31.0 arising from an equilibrium mixture of
1c and 4c instantly disappeared and a sharp singlet at δ 28.5
appeared. As direct arylation proceeded, this signal gradually
decreased, to be replaced by the signal of bis-PPh3 complex 5c
at δ 20.9. After the signal at δ 28.5 disappeared, the reaction
solution was examined by HPLC, showing the formation of 2-
(2,6-dimethylphenyl)benzothiazole (9c) in quantitative yield
(Scheme 5).
Scheme 6. Proposed Mechanism of Direct Arylation of 8
with 4c
Scheme 5. Direct Arylation of Benzothiazole (8) with 4c
and 1c undergoes coordination of 8 to form 2c{8}. In this case,
the equilibrium for the resulting mixture lies far to the side of
2c{8}, and this situation is consistent with the saturation kinetics
observed in Figure 1b. Then, 2c{8} undergoes C−H bond
cleavage to form the diaryl intermediate 3c{8}, and finally, the
direct arylation product 9c is reductively eliminated from 3c{8}
,
along with the formation of [Pd(PPh3)] and acetic acid. The
subsequent process is the same as that for 6.
Deuterium Kinetic Isotope Effects. Deuterium kinetic
isotope effects on direct arylation of 6 (or 8) with 4a (or 4c)
were examined by two experimental methods. One is based on
competitive experiments using a common reference compound
(method A), and the other is based on the rate constants
evaluated by individual kinetic runs for deuterated and
nondeuterated substrates, respectively (method B). Table 1
summarizes the results. 5-Deuterio-2-methylthiophene (6-d)
exhibited no notable kinetic isotope effects, irrespective of the
starting complexes and reaction conditions (entries 1−3). On
the other hand, 2-deuteriobenzothiazole (8-d) provided kH/kD
values ranging from 3.3 to 5.5 (entries 4−6). This difference in
kinetic isotope effects suggests that C−H bond cleavage serves
as the rate-determining step for 8 but not for 6.
DFT Calculations on Direct Arylation of 2-Methyl-
thiophene (6). The reaction mechanisms and kinetic
observations describe above were examined by DFT calcu-
lations using the model compound [PdPh(O2CMe-κ2O)(PH3)]
(1d) bearing PH3 instead of PPh3, and the energy diagrams in
Figures 3 and 4 emerged. All geometries were optimized with
the B3LYP functional. The energy changes were evaluated with
the M06-2X functional and triple-ζ-quality basis sets. The
The reaction obeyed pseudo-first-order kinetics over 3−4
half-lives (see Figure S2 in the Supporting Information). Unlike
the reaction of 6, the reaction rate was saturated at high
concentrations of 8 (Figure 1b). This phenomenon indicates
the occurrence of coordination of 8 in a stable form prior to
direct arylation. In fact, treatment of 4c with 8 in THF resulted
in the selective formation of [Pd(2,6-Me2C6H3)(O2CMe)(8)-
(PPh3)] (2c{8}), which was isolated in 77% yield.
Figure 2 shows the X-ray structure of 2c{8}. While two
molecules of 2c{8} were associated with each other by hydrogen
bonding in the crystal (see Figure S3 in the Supporting
Information), one of the molecules is shown in Figure 2 for
clarity. The complex adopts a typical square-planar config-
uration; the sum of the four bond angles around palladium is
360.3°. Benzothiazole is linked to palladium via the nitrogen
atom (dPd−N = 2.115(2) Å) at the cis position of the acetate
ligand. This geometry is suitable for subsequent C−H bond
cleavage via the CMD pathway.
Thus, the mechanism for direct arylation of 8 with 4c may be
depicted as shown in Scheme 6. An equilibrium mixture of 4c
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As can be seen from Figure 3, the coordination of 6 is much
easier than for the others. On the other hand, the transition
state for reductive elimination (TSRE) is clearly located higher
in energy (4.0 kcal/mol) than that for C−H bond cleavage
(TSCH). Hence, the reductive elimination can be assigned to
the rate-determining step, and this situation is consistent with
the deuterium-labeling experiments showing no kinetic isotope
effects (entries 1−3 in Table 1).
5-Methyl-2,2′-bithiophene (10) followed almost the same
reaction course, involving the rate-determining reductive
elimination (see Table S2 in the Supporting Information).
TSRE for 10 was located slightly lower in energy (0.6 kcal/mol)
than that for 6. This tendency is in accord with the
experimental reactivity order in Scheme 3 (10 > 6), though
the calculated energy difference is small.
DFT Calculations on Direct Arylation of Benzothiazole
(8). Direct arylation of 8 with 1d to give 9a was similarly
examined by DFT calculations. As is shown in Figure 4,
benzothiazole (8) is coordinated to 1d via the nitrogen atom
(dPd−N = 2.17 Å) without a notable activation barrier, giving
2d{8}, in which the C−H group at the 2-position of 8 is linked
to the acetate ligand by a hydrogen bond (dO···H = 2.02 Å).
Complex 2d{8} then undergoes C−H bond cleavage via TSCH
to give 3d{8}. The benzothiazol-2-yl ligand is oriented parallel to
the coordination plane owing to the occurrence of a hydrogen
bond between the nitrogen atom and the acetic acid ligand
(dO···H···N = 2.61 Å). On the other hand, for reductive
elimination to take place, the benzothiazol-2-yl ligand is must
be oriented perpendicular to the coordination plane so as to
interact with the phenyl ligand. Thus, the benzothiazol-2-yl
ligand rotates around the Pd−C bond via TSRot, and the
resulting 3d′{8} undergoes reductive elimination via TSRE to
afford a Pd(0) complex coordinated with 9a and acetic acid.
The three transition states TSCH, TSRot, and TSRE are located
at 23.8, 12.6, and 21.4 kcal/mol, respectively, against the
substrates (1d + 8). Thus, unlike the reaction of 6, the rate-
determining step is assigned to the C−H bond cleavage
process. TSCH for 8-d is located at 24.7 kcal/mol and is 0.9
kcal/mol higher than that for 8. The energy difference
Table 1. Deuterium Kinetic Isotope Effects on Direct
Arylation of Heteroarenes with Arylpalladium Acetate
Complexes
a
entry
complex
substrate
method
kH/kD
1.0
1
2
3
4
5
6
4a
4c
4c
4a
4c
4c
6, 6-d
6, 6-d
6, 6-d
8, 8-d
8, 8-d
8, 8-d
A
A
B
A
A
B
1.0
1.00(2)
3.3
5.5
4.2(1)
a
Method A: the kH/kD values were estimated from reactivity ratios of
deuterated and nondeuterated substrates to 2-ethylthiophene as a
common reference compound. Competitive reactions were conducted
in 1,4-dioxane at 90 °C (entries 1 and 4) or in THF at 65 °C (entries 2
and 5), using 4a (18.2 mM) or 4c (9.1 mM) and a substrate (0.727
M). Method B: the kH and kD values were evaluated by individual
kinetic runs for 4c (9.1 mM) and a substrate (0.727 M) in THF in the
presence of i-Pr2EtN (0.182 M) at 65 °C.
solvent effect (1,4-dioxane) was included in PCM calculations:
see the Supporting Information for details of the computation.
Figure 3 shows the schematic geometry changes and Gibbs
energy changes in the direct arylation of 6 with 1d to give 7a,
where the values for 6-d are given in parentheses. The whole
reaction consists of three elementary processes. First, substrate
6 is coordinated with 1d via precursor complex P (ΔG = 3.8
kcal/mol) and transition state TSCO (ΔG = 5.8 kcal/mol). The
thiophene unit in 6 is initially associated with the bidentate
acetate ligand in 1d by two sets of hydrogen bonds (dO···H
=
2.58 and 2.66 Å in P) and then coordinated to palladium with
keeping one of the hydrogen bonds being kept (dO···H = 2.30 Å
in TSCO). The hydrogen bonding is further strengthened in the
resulting complex 2d{6} (dO···H = 2.18 Å), as if to compensate
for loose coordination of the thiophene unit (dPd···C = 2.43 and
2.54 Å). Complex 2d{6} then undergoes C−H bond cleavage
via TSCH, and finally, C−C reductive elimination from 3d{6} via
TSRE forms a Pd(0) complex coordinated with direct arylation
product 7a, together with acetic acid.
Figure 3. Free energy change (ΔG, kcal/mol) in direct arylation of 2-methylthiophene (6) with [PdPh(O2CMe-κ2O)(PH3)] (1d). The values for 6-
d are given in parentheses.
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Figure 4. Free energy change (ΔG, kcal/mol) in direct arylation of benzothiazole (8) with [PdPh(O2CMe-κ2O)(PH3)] (1d). The values for 8-d are
given in parentheses.
corresponds to kH/kD = 4.6, and this value is in agreement with
as a high-energy species on the reaction coordinate,13b,15b and
in this situation, it is difficult to conclude that the reactivity is
governed by the C−H bond cleavage process. Actually, the
present study has clearly demonstrated that direct arylation is a
multistep process, and the energy profile varies significantly
with substrates. Therefore, it is concluded that the reactivity of
heteroarenes is controlled by multiple factors in elementary
processes, including substrate coordination, C−H bond
cleavage, and C−C reductive elimination. Kinetic studies with
well-defined models of key intermediates in conjunction with
theoretical support provide a deep insight into the mechanisms
of reactivity control.
the experimental data (kH/kD = 3.3−5.5; entries 4−6 in Table
1).
The transition state of the rate-determining step for 8 (TSCH
,
23.8 kcal/mol) is apparently lower in energy than that for 6
(TSRE, 25.2 kcal/mol). However, because the thermodynamic
stability of precursor complex 2d{8}, which is caused by
coordination of 8 to palladium via the nitrogen atom, leads to
an additional activation barrier for 8 (23.8 + 5.0 = 28.8 kcal/
mol), the reactivity of 8 becomes lower than that of 6, in
accordance with the experimental observation in Scheme 3.
CONCLUSION
■
EXPERIMENTAL SECTION
■
We have described detailed mechanistic investigations into the
factors controlling the reactivity of heteroarenes (6 and 8) in
direct arylation with well-defined models of the presumed
intermediate [PdAr(O2CMe-κ2O)L] (1). The experimental
data have been reproduced consistently by theoretical treat-
ments.
General Considerations. All manipulations were carried out
under a nitrogen atmosphere using standard Schlenk techniques.
Nitrogen was dried by passing through P2O5 (Merck, SICAPENT).
NMR spectra were recorded on a Bruker Avance 400 spectrometer
(1H NMR, 400.13 MHz; 13C NMR, 100.62 MHz; 31P NMR, 161.97
1
MHz). Chemical shifts are reported in δ (ppm), referenced to H
(residual) and 13C signals of deuterated solvents as internal standards
or to the 31P signal of 85% H3PO4 (δ 0.0) as an external standard.
Elemental analysis was performed by ICR Analytical Laboratory,
Kyoto University. Analytical HPLC was carried out on a JASCO
HPLC assembly consisting of a PU-2080 Plus pump, a Model RI-1530
refractive index detector, and an Intersil ODS-P column (2/1 v/v
CH3CN/H2O). Mass spectra were measured on a Shimadzu GC-MS
QP2010 spectrometer (EI, 70 eV). GLC analysis was performed on a
Shimadzu GC-2025 instrument equipped with a FID detector and a
CBP-1 capillary column (25 m × 0.25 mm).
Pentane (Kanto, dehydrated) was used as received. THF and 1,4-
dioxane were dried over Na/Ph2CO, distilled, and stored over
activated MS4A. CD2Cl2 was dried over CaH2, distilled, and stored
over activated MS4A. 2-Methylthiophene (6), benzothiazole (8),
pentafluorobenzene, and 2-ethylthiophene were distilled after passing
through a short pad of activated alumina. [PdPh(μ-O2CMe)(PPh3)]2
(4a),22 [Pd(2,6-Me2C6H3)(μ-O2CMe)(PPh3)]2 (4c),18 5-methyl-2,2′-
bithiophene (10),23 2-phenylthiazole,24 [Pd(η5-C5H5)(η3-C3H5)],25 5-
deuterio-2-methylthiophene (6-d),26 and 2-deuteriobenzothiazole (8-
Both substrates react with 1 via three elementary processes
(i.e., coordination, C−H bond cleavage, and reductive
elimination) but display significantly different kinetic aspects
from each other. The reaction of 6 obeys simple second-order
kinetics. The deuterium-labeling experiments and DFT
calculations have revealed the occurrence of rate-determining
reductive elimination. On the other hand, substrate 8 displays
saturation kinetics because of its relatively stable coordination
through the nitrogen atom. This coordination stability
enhances the activation barrier for the subsequent C−H bond
cleavage process and thus results in the modest reactivity of 8.
So far, mechanistic studies on palladium-catalyzed direct
arylation have been focused largely on the C−H bond cleavage
process, and there has been a tendency for the difference in
reactivity of heteroarenes to be discussed only with theoretical
values of activation energy for this process.13 However, the
resulting diarylpalladium complex 3 has often been illustrated
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d)27 were prepared according to the literature procedures. All other
chemicals were obtained from commercial suppliers and used without
further purification.
(3H, s), 2.38 (6H, s), 6.30 (2H, d, J = 7.4 Hz), 6.49 (1H, t, J = 7.3
Hz), 7.10−50 (15H, m), 7.50 (1H, dd, J = 8.1, 7.6 Hz), 7.68 (1H, dd, J
= 8.4, 7.5 Hz), 7.88 (1H, d, J = 8.1 Hz), 8.72 (1H, s), 8.82 (1H, d, J =
8.4 Hz). 31P{1H} NMR (CD2Cl2, −80 °C): δ 28.6 (s). 13C{1H} NMR
(CD2Cl2, 20 °C): δ 24.6 (s), 25.8 (d, J = 4.4 Hz), 122.6 (s), 124.4 (s),
124.9 (s), 126.4 (s), 126.5 (s), 127.0 (s), 128.4 (br s) 128.7 (d, J = 11
Hz), 130.9 (d, J = 54 Hz), 131.2 (s), 133.8 (s), 134.8 (d, J = 12 Hz),
141.2 (s), 151.2 (br s), 153.0 (br s), 156.3 (br s). Anal. Calcd for
C35H32NO2PPdS: C, 62.92; H, 4.83; N, 2.10. Found: C, 62.80; H,
4.81; N, 2.23.
Relative Reactivity of Heteroarenes. Reactivity ratios of
heteroarenes were estimated by competitive reactions. A typical
procedure is as follows. Complex 4a (10.1 mg, 0.010 mmol) and
C6Me6 (4.1 mg, 0.025 mmol; an internal standard for HPLC analysis)
were placed in an NMR sample tube equipped with a Teflon screw
cock (J. Young). 1,4-Dioxane (0.50 mL), 2-methylthiophene (6; 39
μL, 0.40 mmol), and benzothiazole (8; 44 μL, 0.40 mmol) were added
at room temperature. The mixture was degassed three times by
freeze−pump−thaw cycles and then heated for 35 min to 90 °C in an
oil bath. 31P{1H} NMR analysis of the resulting solution at room
temperature revealed a 55% conversion of the starting complex, and
HPLC analysis showed the formation of 5-phenyl-2-methylthiophene
(7a) and 2-phenylbenzothiazole in 38 and 15% yields, respectively.
Thus, the reactivity ratio of 8 to 6 was estimated to be 0.39 (15/38).
Kinetic Study. A typical procedure is as follows. Complex 4c (8.5
mg, 0.0040 mmol) and C6Me6 (3.2 mg, 0.020 mmol; an internal
standard for HPLC analysis) were placed in an NMR sample tube
equipped with a Teflon screw cock (J. Young). THF (0.30 mL), i-
Pr2EtN (10 mg, 0.080 mmol), and 2-methylthiophene (6; 31 μL, 0.32
mmol) were added, and the total volume of the solution was adjusted
to 0.44 mL by adding THF at room temperature. The mixture was
degassed three times by freeze−pump−thaw cycles and heated to 40
°C to dissolve 4c. A sealed capillary containing a solution of
PPh2(C6H4-2-OMe) (40 mM) in toluene-d8 was loaded into the NMR
sample tube as an internal standard. The system was degassed three
times by freeze−pump−thaw cycles again and placed in an NMR
sample probe controlled to 65.0 0.1 °C. The reaction progress was
monitored at intervals by 31P{1H} NMR spectroscopy using marker
signals at δ 31.0 (4c + 1c), 20.9 (5c), and −13.9 (PPh2(C6H4-2-
OMe)). After the signal at δ 31.0 disappeared, the amount of direct
arylation product was analyzed by HPLC. The solution was
concentrated to dryness, dissolved in CD2Cl2 (0.5 mL), and examined
X-ray Structural Analysis of 2c{8}. Single crystals of 2c{8} suitable
for X-ray diffraction study were grown by slow diffusion of pentane
into THF solutions at −20 °C. The intensity data were collected on a
RIGAKU Saturn70 CCD instrument with VariMax Mo Optic using
Mo Kα radiation (λ = 0.71070 Å). The intensity data were collected at
103 K and corrected for Lorentz and polarization effects and for
absorption (numerical). The structures were solved by direct methods
(SHELXS-97)28 and refined by least-squares calculations on F2 for all
reflections (SHELXL-97)29 using Yadokari-XG 2009 (software for
crystal structure analyses).30 Non-hydrogen atoms, except for the
carbon and oxygen atoms of THF of crystallization, were refined
anisotropically. Hydrogen atoms were placed at calculated positions
using AFIX instructions. The crystal data and a summary of data
collection and refinement details are summarized in Table S3
(Supporting Information).
DFT Calculations. All calculations were carried out with the
Gaussian 09 program package.31 Geometry optimization was
performed by the DFT method without any symmetry constraints,
where the B3LYP functional was used.32 The Pd atom was described
with the LANL2DZ basis set including a double-ζ basis set and
effective core potentials (ECPs) proposed by Hay and Wadt.33 The 6-
31G(d) basis set was applied to other atoms.34 The Cartesian
coordinates of optimized structures are reported in Table S4
(Supporting Information). Each equilibrium geometry exhibited no
imaginary frequency, and each transition state exhibited one imaginary
frequency (see Figure S4 in the Supporting Information). IRC
calculations35 were carried out to confirm that the transition state
connects the reactant and product. Energy changes were evaluated by
the DFT method with the M06-2X functional36 using the geometries
optimized above. For these calculations, the Pd atom was described
with a triple-ζ basis set and ECPs proposed by the Stuttgart−
Dresden−Bonn group.37 Two f-polarization functions were also added
for Pd (ζf = 0.621 and 2.203).38 The 6-311G(d) basis sets were
employed for other atoms, where one set of anion functions was added
to the O atom.39 Solvent effects (1,4-dioxane) were taken into
consideration using the polarized continuum model (PCM) at 298.15
K.40 The Gibbs free energy in solution was employed for discussion,
where the translational entropy was evaluated by the method
developed by Whitesides et al.41
1
by H and 31P{1H} NMR spectroscopy, showing the formation of 5c
in quantitative yield. The reaction of 4c with 8 was similarly examined
using marker signals at δ 28.5 (2c{8}), 20.9 (5c), and −13.9
(PPh2(C6H4-2-OMe)). The first-order plots are shown in Figures S1
and S2 in the Supporting Information. The observed rate constants are
given in Table S1 in the Supporting Information.
Deuterium Kinetic Isotope Effects. Method A. A typical
procedure is as follows. Complex 4c (8.5 mg, 0.0040 mmol) and
C6Me6 (3.2 mg, 0.020 mmol; an internal standard for HPLC analysis)
were placed in an NMR sample tube equipped with a Teflon screw
cock (J. Young). THF (0.30 mL), i-Pr2EtN (10 mg, 0.080 mmol), 2-
methylthiophene (6; 31 μL, 0.32 mmol), and 2-ethylthiophene (36
μL, 0.32 mmol; reference compound) were added, and the total
volume of the solution was adjusted to 0.44 mL by adding a small
amount of THF at room temperature. The mixture was degassed three
times by freeze−pump−thaw cycles and heated for 1 h to 65 °C in an
oil bath. GLC analysis of the resulting solution revealed the formation
of a 38/41 ratio of 5-phenyl-2-methylthiophene (7a) and 5-phenyl-2-
ethylthiophene. Similarly, a competitive reaction of 5-deuterio-2-
methylthiophene (6-d) and 2-ethylthiophene was conducted, and a
41/46 ratio of 5-phenyl-2-methylthiophene (7a) and 5-phenyl-2-
ethylthiophene were obtained. On the basis of these product ratios, the
reactivity ratio of 6 to 6-d (kH/kD) was estimated as 1.0.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, crystallographic and computational
details, and crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for F.O.: ozawa@scl.kyoto-u.ac.jp.
■
Method B. The pseudo-first-order rate constants for 6 and 6-d (or 8
and 8-d; kH and kD, respectively) were evaluated by individual kinetic
runs for 4c (9.1 mM) with substrates (0.727 M) in THF in the
presence of i-Pr2EtN (0.182 M) at 65 °C. The observed rate constants
are reported in Table S1 (entries 3, 4, 8, and 9).
Notes
Notes. The authors declare no competing financial interest.
The authors declare no competing financial interest.
Isolation of [Pd(2,6-Me2C6H3)(O2CMe)(8)(PPh3)] (2c{8}). To a
suspension of 4c (42.6 mg, 0.0400 mmol) in THF (2.0 mL) was added
benzothiazole (8; 433 mg, 1.60 mmol). The mixture was stirred at 45
°C to dissolve 4c. The solution was concentrated (ca. 0.3 mL), and
pentane (4.0 mL) was added to precipitate a white solid, which was
collected by filtration, washed with pentane (2 × 0.5 mL), and dried
ACKNOWLEDGMENTS
■
We are grateful to Dr. Y. Nakajima (AIST, Tsukuba, Japan) for
X-ray crystallographic assistance. This work was supported by
KAKENHI (23350042, 24750088, 22000009) from Japan
1
under vacuum (41.3 mg, 77%). H NMR (CD2Cl2, −80 °C): δ 1.11
4428
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Organometallics
Article
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(20) The amine i-Pr2EtN (0.182 M) was added to avoid side
reactions caused by free acetic acid generated in the system. It has
been confirmed that the reaction rate is little affected by i-Pr2EtN.18
(21) The kinetic treatment was based on the reaction stoichiometry
given in Scheme 2; i.e., the [PdAr] value corresponds to the amount of
two of the four [Pd(2,6-Me2C6H3)(O2CMe)(PPh3)] units at time t.
The consumption of a mixture of 1c and 4c was in good agreement
with the amount of 5c formed in the reaction system, as confirmed by
31P{1H} NMR spectroscopy. The quantitative formation of 7c was also
confirmed for each run by HPLC analysis.
Society for the Promotion of Science, and by the ACT-C
program of Japan Science and Technology Agency.
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