Pyridine-assisted chlorinations and oxidations by palladium(IV)

Article abstract of DOI:10.1021/om300007w

The reactivity of the bis-NHC complex LPd^IVCl₄ (L = κ²-[R-NHCCH₂NHC-R] with R = C₁₄H ₂₉) in chlorinations and oxidations of organic substrates was considerably increased in the presence of pyridine. For alkene chlorinations, this effect was due to the in situ formation of highly reactive LPd ^IVCl₃(py)⁺, which was able to transfer Cl ⁺ to the C=C bond in a ligand-mediated process (devoid of π complexation), which did not require py dissociation. The enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild conditions in a reaction where py served as a base, broadening the known scope of reactivity for Pd^IV complexes. LPd^IVCl₃(py)⁺ could be formed from Cl ^-/py exchange or from the oxidation of LPd^IICl(py) ⁺ by Cl₂. Taking advantage of the enhanced reactivities that pyridine coordination imparted on both Pd^II and Pd^IV complexes allowed for the catalytic chlorination of styrene with LPd ^IVCl₄ as a sacrificial oxidant, thereby establishing the principal feasibility of Pd^II/Pd^IV catalyses that obviates Pd^II activations of the substrate.

Full text of DOI:10.1021/om300007w

Article
pubs.acs.org/Organometallics
Pyridine-Assisted Chlorinations and Oxidations by Palladium(IV)
A. Scott McCall† and Stefan Kraft*
Department of Chemistry, Kansas State University, Manhattan, Kansas 66506-0401, United States
S
* Supporting Information
ABSTRACT: The reactivity of the bis-NHC complex
LPdIVCl4 (L = κ2-[R-NHCCH2NHC-R] with R = C14H29) in
chlorinations and oxidations of organic substrates was
considerably increased in the presence of pyridine. For alkene
chlorinations, this effect was due to the in situ formation of
highly reactive LPdIVCl3(py)+, which was able to transfer Cl+
to the CC bond in a ligand-mediated process (devoid of π complexation), which did not require py dissociation. The
enhanced reactivity in the presence of pyridine also extended to the oxidation of secondary and benzylic alcohols under mild
conditions in a reaction where py served as a base, broadening the known scope of reactivity for PdIV complexes. LPdIVCl3(py)+
could be formed from Cl−/py exchange or from the oxidation of LPdIICl(py)+ by Cl2. Taking advantage of the enhanced
reactivities that pyridine coordination imparted on both PdII and PdIV complexes allowed for the catalytic chlorination of styrene
with LPdIVCl4 as a sacrificial oxidant, thereby establishing the principal feasibility of PdII/PdIV catalyses that obviates PdII
activations of the substrate.
Scheme 1
INTRODUCTION
■
Over the past 10 years the realm of palladium catalysis has been
greatly expanded through the introduction of PdII/PdIV cycles in
oxidative functionalizations of C−H and CC bonds.1 Despite
some impressive advances, shortcomings in this field include
(a) substrate activations by PdII that are sluggish and often
require Lewis basic handles on the substrate to help overcome
low intermolecular reactivity through chelation effects1d,e,2−5
and (b) a narrowly defined role of PdIV intermediates that
merely serve as a vehicle for reductive eliminations. A
conceivably rich high-oxidation-state palladium chemistry that
bypasses PdII activations and instead relies on sole interactions
between organic substrates and PdIV is only in its infancy;6
reports on intermolecular, non-chelation-controlled C−H bond
activations by PdIV are sparse,7 and interactions between alkenes
and PdIV have not been studied before. Additionally, there have
been no prior studies on oxidations of alcohols by PdIV
complexes in any context. With these substrates in mind,
a successful catalytic cycle would require that two key steps be
established: (I) PdII → PdIV oxidation that does not affect the
substrates present and (II) PdIV-mediated substrate functional-
ization with concomitant regeneration of PdII.
(X− = Cl−) and 2b (X− = TfO−), which have reactivities
significantly higher than that of 1. We will also expound on the
surprisingly expeditious formation of 2a,b by way of PdII/PdIV
oxidation. Furthermore, a facile PdII-catalyzed interconversion
of 1 and pyridine into 2a will be elucidated. Using these three
effects in concert, we will demonstrate the principal feasibility
of a PdII/PdIV catalysis that circumvents substrate activations
by PdII altogether. In addition, we will expand the scope of
PdIV chemistry into the arena of previously unexplored alcohol
oxidations.
Previously, we have documented that isolated LPdIVCl4
(1; L = R-NHCCH2NHC-R, R = C14H29) is capable of
dichlorinating alkenes and alkynes as well as monochlorinating
arenes on aromatic and benzylic C−H bonds (Scheme 1),8 thus
conforming to the requirements of step II. Mechanistic studies
on that system supported a stepwise ionic mechanism with the
initial formation of cation LPdIVCl3 (A) in the slow step; the
+
latter subsequently transferred Cl+ to the substrates in a ligand-
mediated process devoid of π coordination.
In here we will explore avenues that allow for catalytic
chlorinations of organic substrates by PdIV complexes. This
includes the use of novel LPdIVCl3(py)+X− structures 2a
Received: January 5, 2012
Published: April 30, 2012
© 2012 American Chemical Society
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relationships across the ligand backbone was established for
H4, H5, Hexo, H5′, H4′ all across to Hα′/Hβ′. No NOEs were
observed between H4/H4′ and Hendo, consistent with a boat
conformation.
RESULTS AND DISCUSSION
■
Synthesis of LPdIVCl3(py)+TfO−. Efforts to cleanly
generate an isolable LPdIVCl3(py)+ salt were commenced by
reacting tetrachloride 1 with AgOTf; the desired complex
LPdIVCl3(py)+TfO− (2b) was produced; however, it was
obtained only in 80% purity and the crude material could not
be further refined. In contrast, we were more successful with
a two-step procedure (Scheme 2). Cl−/py substitution was
A comparison of one-dimensional 1H NMR spectra of LPdIICl2
(3) and LPdIICl(py)+TfO− (4b) shows downfield shifts Δδ3,4b
for all NHC aromatic protons and bridge hydrogens in the range
of 0.08−0.18 ppm (Table 1), consistent with a reduction of
Table 1
Scheme 2. Synthesis of 2b
a
c
d
,
proton
δ(3)
6.83
δ(4b)
6.98
δ(2b)
7.37
Δδ3,4b
Δδ2b,4b
,
H4
+0.10
+0.19
+0.11
+0.08
+0.15
+0.18
−1.14
+0.04
+0.39
+0.35
+0.35
+0.35
+0.75
+0.51
H5
7.61
6.83
7.61
6.22
6.56
7.80
8.15
b
b
H4′
6.94
7.29
H5′
7.69
8.04
Hendo
Hexo
Hα/Hβ
Hα′/Hβ′
Ho
6.37
7.12
6.74
7.25
e
f
e
4.87, 4.09
4.87, 4.09
3.46, 3.18
4.83, 4.21
8.71
3.45, 3.17
4.95, 4.71
9.22
−0.01
+0.31
+0.51
+0.21
+0.18
b
f
accomplished from PdII complex 3 and a tightly controlled
Hm
Hp
7.47
7.68
amount of AgOTf (1.05 equiv), which afforded LPdIICl-
7.92
8.10
1
(py)+TfO− (4b) in ≥98% purity by H NMR spectroscopy; a
a
Chemical shifts are reported for a CDCl3 solution that contains 4b
larger excess of silver triflate and pyridine gave rise to the
formation of LPdII(py)22+(TfO−)2 (4c). Treatment of a CH2Cl2
solution of 4b with a stream of Cl2 gas quantitatively produced
an orange-yellow solution of LPdIVCl3(py)+TfO− (2b), which
was isolated in 97% purity by removing the solvent with a
stream of N2. In contrast, chlorination of dipyridine complex 4c
produced a complex product mixture with 2b as the main
component, while [LPdIV(py)2Cl2]2+(TfO−)2 could not be
unequivocally identified.
(0.1 M) in order to account for salt effects on 3 (when its chemical
b
shifts are compared to those of 4b). In compound 3 chemical shifts
δ(H4′), δ(H5′), and Hα′/Hβ′ are equivalent by symmetry with chemical
c
d
shifts δ(H4), δ(H5), and Hα/Hβ. Δδ3,4b = δ(4b) − δ(3). Δδ2b,4b
=
e
δ(2b) − δ(4b). Difference Δδ of averaged values of Hα/Hβ.
f
Difference Δδ of average values of Hα′/Hβ′.
electron density on the NHC ligands in the LPdIICl(py)+ cation.
Pyridine resonances for Ho, Hm, and Hp were in the typical range
for a cationic PdII-py complex.9 Whereas “remote” side-chain
protons Hα′/Hβ′ also experienced a 0.04 ppm downfield shift in
4b, the signals for Hα/Hβ were shifted upfield by −1.14 ppm
caused by the diatropic ring current of the nearby py ligand.10
By further decreasing the electron density on palladium in 4c,
resonances for Hexo and Hendo were shifted downfield by 0.81 and
0.64 ppm relative to those for 4b.
NMR Chemical Shift Analysis of NHC Complexes. One-
1
and two-dimensional H NMR of the octahedral complex 2b
unveiled a κ2(C,C),N-mer configuration with the py ligand
trans to a carbene carbon, while no other geometric isomers
were present. A ROESY experiment established a correlation
between the side-chain protons Hα/Hβ and pyridine protons Ho
and Hm, while Hα′/Hβ′ did not show any cross peaks (Figure 1).
Thus, starting from Hα/Hβ a contiguous chain of vicinal
Equally, effects from reduced NHC electron density were
manifested in the 13C NMR spectrum of 4b, which features
small average downfield shifts for the remote aromatic carbons
C4, C4′, C5, and C5′ together with an upfield shift on C2′ relative
to those for 3 (Table 2).11 Huynh and co-workers have
correlated such upfield shifts for carbene carbons in NHC-PdII
complexes with a reduction of donor strengths of ligands trans
to the NHC moiety, thus causing a decrease of electron density
on the carbene.12,13 A side by side comparison of 3 and
−
Herrmann’s bis-NHC complex LPdII(NCCH3)22+(BF4 )2
(L = CH3−NHC−CH2−NHC−CH3)14 further underscores
these trends with a −9.77 ppm upfield shift for C2 and 2.3 ppm
(averaged) downfield shifts for C4 and C5 on the electron-
deficient dication.
Figure 1. ROESY correlations in 2b.
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isolable NHC complexes that also report significant oxidation-
induced upfield shifts on carbene carbons.20−22
Table 2
Alkene Chlorinations with PdIV and Mechanistic
Investigations. The reaction of olefin with PdIV chloride
complexes produced alkene chlorination products at 25 °C
(Table 3). We found that the yield in the reaction between 1
and styrene (87.4% after 2 h, entry 1) was improved to 93.2%
at a dramatically reduced reaction time (4 min) when 10 equiv
Table 3. Chlorinations and Oxidations by 1 and 2a,b
a
From ref 8.
The oxidation of 4b to 2b caused 0.35−0.75 ppm downfield
shifts for NHC backbone, bridge, and side-chain protons
(Table 2). Downfield shifts (0.18−0.51 ppm) were also seen for
Ho, Hm, and Hp, as expected for a PdIV pyridine complex.15,16 A
comparison of 13C NMR chemical shifts (Table 2) for 1 and 2b
(entries 1 and 2) and for 3 and 4b (entries 3 and 4) revealed
PdIV upfield shifts for C2 and C2′ in the range of −17 to −23 ppm
relative to the respective PdII complex. At the same time,
small downfield shifts (1.9−2.3 ppm) were observed for the
remote ring carbons C4, C5, C4′, and C5′ in 1 and 2b in
comparison to 3 and 4b. This reinforces the idea that the
pattern of significant upfield shifts on carbene carbons paired
with small downfield shifts on “remote” aromatic carbons
expresses the degree of electron deficiency on NHC ligands.
Notably, these 13C NMR chemical shift tendencies followed
trends reported in imidazolium cations, with upfield shifts on
C2 and downfield shifts on C4,5 in the presence of electron-
withdrawing groups on C2.17,18 In fact, 13C NMR chemical
shifts of C2 in both 1 and 2b were very similar to those in the
free imidazolium salt 5; the signal for C2′ in the NHC ring in 2b
trans to pyridine was shifted even further upfield from 5 (by
−8.07 ppm), underscoring the electron-deficient character of
the PdIVCl3(py)+ moiety.
On the basis of these trends, we suggest that that the NHC
portion in LPdIV complexes 1 and 2b is best described with
resonance structure I (Scheme 3), while PdII structures 3 and
Scheme 3
4b may be described with partial contributions from resonance
structure II as well (“back bonding”).19 The absence of an
organometallic double-bond character in PdIV complexes is
consistent with the previously observed bond lengthening of
C2−PdII/IV by 0.047 Å in 1 relative to 3.8 Of note are existing
literature accounts on MII/MIV oxidations (M = Pd, Pt) of
a
b
c
From ref 8. Formed in situ from 1 and py. [substrate] = 10−
e
d
20 mM, 1.05 equiv of PdIV reagent. Added Bu4NCl (5 equiv). Added
Bu4NBF4 (5 equiv).
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1
pyridine was added (entry 2). In situ H NMR monitoring
of the reaction (vide infra) revealed the presence of
LPdIVCl3(py)+Cl− (2a). Furthermore, the isolated triflate
analogue LPdIVCl3(py)+TfO− (2b) also produced higher yields
and conversions than did 1 (entries 3, 4, 6, and 8). Cyclohexene
produced a 3.1:1 mixture of 1,2-dichlorocyclohexane (II, trans
only) and 3-chlorocyclohexene (III) (entry 8) following the
second-order rate law d[2b]/dt = −k2[2b][cy] (k2 = (7.1
0.1) × 10−3 M−1s−1). Analogously, the reaction of 2b
and styrene produced a bimolecular rate constant of 3.35 ×
10−3 M−1 s−1. In combination, the exclusive trans geometry in II,
the particular 3.1:1 ratio of II to III in entry 8, and the absence
of 4-chlorocyclohexene in the reaction mixture were indicative
of an ionic mechanism23 that proceeds via the formation of
an organic cyclic chloronium species of type B in the stereo-
determining step (Scheme 4). The steric and electronic
involve the intermediacy of LPdIVCl3(η2-alkene)+ π com-
plexes.24 The analogous 1-hexene/trans-3-hexene competition
experiment in the presence of complex 1 (acting as a precursor
for the active chlorinator A) was also biased toward the internal
alkene; however, to a significantly lesser extent (6:1 ratio).
We conclude from this divergence in chemoselectivity that the
two species 2b and A react with alkenes in similar but not
identical transition states. In other words, pathway a does not
pertain to the reactivity of 2b. The fact that the addition of
10 equiv of pyridine did not slow down the reaction between
2b and styrene (and even caused a rate acceleration; vide infra)
further discredits the possibility of a pre-equilibrium between
2b and A in pathway a (in which case one would expect the
proportionality k2 ∝1/[py]).25 At this point, we gravitate
toward pathway b with a direct Cl+ transfer from hexacoordi-
nated complex 2b26 and conclude that alkene chlorinations by
LPdIV chlorides are responsive to the presence of positively
charged metal centers rather than to the presence of
pentacoordinated ligand environments.
Scheme 4. Mechanistic Considerations for Cl+ Transfer from
2b
Despite the irrelevance of pyridine dissociation in alkene
chlorinations, complex 2b underwent a facile py/py-d5 exchange
on a time scale of seconds when 10 equiv of py-d5 was added
(Scheme 6). Since small amounts of the reduced species
Scheme 6
LPdII(py)Cl+ were observed after py-d5 addition, the ligand
substitution is likely a PdII-catalyzed process analogous to the
Cl−/py exchange in LPdIVCl4 (vide infra).27An intriguing facet
of the equilibrium in Scheme 6 is the absence of potential fac
isomers of 2b-d5 even after a period of 1 h of continued ligand
exchange. As pentacoordinated (and presumably fluxional)28
species are likely intermediates in this process,29 we are inclined
toward a thermodynamic explanation for the exclusive formation
of the mer isomer. Such a bias may be driven by steric effects, due
to clashes of py with the bridge hydrogen Hendo or alternatively
with side-chain methylene groups CHα/Hβ and CHα′/Hβ′ if
pyridine were forced to adopt an axial substitution. On the other
hand, a possible stereoelectronic cause of the equatorial py
substitution may be rooted in Pearson’s antisymbiosis principle30
with a harder nitrogen ligand trans to a soft carbene ligand as
opposed to being aligned trans to a chloride ligand in a fac
isomer. Recently, Ritter has reported an octahedral PdIV complex
in which py ligands would also align themselves trans to a (soft)
carbon ligand rather than trans to nitrogen.15
demands on the olefin during this stepwise chlorination were
interrogated by a competition experiment involving a mixture
of 1-hexene and trans-3-hexene being subjected to a
substoichiometric amount of 2b (Scheme 5). We found that
Scheme 5. Chlorination Competition Experiment Probing
the Bias of 1 and 2b toward trans-3-Hexene/1-Hexene
Mixtures
Alcohol Oxidations with PdIV. Various alcohols were
mixed with LPdIVCl4 (1.05 equiv) in CDCl3, and reactions were
1
followed by H NMR spectroscopy at 25 °C (Table 3). After
24 h borneol had been converted to camphor (VIII) in 62.2%
yield (entry 10), primarily limited by incomplete borneol
conversion (67.4%). In comparison, benzyl alcohol produced
benzaldehyde (IX) in very low yield (entries 11a,b). Improve-
ments were found when oxidations were conducted in the
presence of pyridine (entry 12a) or 2,6-lutidine (entries 13a,b)
at 25 °C. This base effect increased in size at elevated
the preferred chlorination took place at the internal alkene over
the terminal alkene with a bias of 150:1. The preference for the
electron rich, yet sterically more encumbered trans-3-hexene
is indicative of a ligand-mediated, “SN2-type” transition state
(with the CC bond acting as a nucleophile) that does not
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temperatures; for example, chlorinator 1 produced only a 9.7%
yield of IX after 2 h 60 °C (entry 11c), while a mixture of 1 and
pyridine afforded 62.1% (entry 12b). This result was mirrored
by the behavior of 2-butanol, whose poor 4.9% oxidation yield
by 1 (25 °C, 24 h, entry 14) was improved to 53.6% yield after
24 h (66.0% after 48 h) merely through the addition of pyridine
(entries 15a,b). 1-Butanol was unreactive toward 1 even upon
heating (entries 16a,b). Although no PdIV−O intermediates
were spectroscopically detected during the experiments, we
propose the in situ formation of PdIV alkoxides C (Scheme 7).
of 1. Interestingly, mixtures of 1 and 2,6-lutidine did not form
the respective lutidine complex. A Cl−/py substitution also
occurred (within seconds) in LPdIICl2 (3) (equilibrium II) with
Keβq = [LPdIICl(py)+]eq[Cl−]eq/[3]eq[py]eq = 0.18. With values
for Keαq and Keβq in hand, the redox equilibrium constant Keγq
for the reaction 1 + LPdIICl(py)+ ⇔ 3 + LPdIVCl3(py)+ can be
estimated (Keγq = Keαq/Keβq = 2.3). A thermodynamic preference
for the right-hand side of equilibrium III implies that the neutral
couple 1/3 has a greater reduction potential than the cationic
counterpart LPdIVCl3(py)+/LPdIICl(py)+.
Following the Cl−/py exchange rates for 1 (CDCl3/
1
Bu4NBF4, 25 °C) via H NMR spectroscopy, we discovered
Scheme 7
that the reaction proceeded through an initial induction period,
after which velocities d[1]/dt reached a plateau (when plotted
versus time) followed by a decline in rates in approaching
equilibrium.34 Time-independent velocities are a consequence
of a rate law that is zero order in [1]; accordingly, respective
d[1]/dt vs [1] plots feature identical [1]-independent plateaus
(Figure 2). Interestingly, plateau heights were positively
The order of reactivity borneol > 2-butanol > 1-butanol suggests
that abstraction of Halc from complex C likely constitutes the
slow step of the reaction. This is reminiscent of the relative ease
of previously reported borneol oxidations by CrVI relative to
2-propanol that was attributed to the release of internal strain in
the scission of the Halc−CRR′OCrVI bond in the chromium
borneolate.31 The octahedral 18-VE complex C is unlikely to
undergo a β-hydride elimination in the absence of a vacant site,
and the prevalent reluctance of reported PdIV complexes1c,32

in particular PdIV alkoxides33to engage in such a process
attests to that. Our findings that the addition of bases such as
pyridine and 2,6-lutidine accelerated conversions and improved
yields is consistent with an E2 process in the slow step of
the reaction. Even though small amounts of LPdIVCl3(py)+Cl−
(2a) were formed in situ from 1 and pyridine during these
reactions, its presence appeared to have no particular benefit
on the progress of the reaction (as it did in the case of alkene
chlorinations), since the addition of 2,6-lutidine equally
improved oxidation yields without forming spectroscopically
detectable quantities of the analogous complex LPdIVCl3(2,6-
lutidine)+Cl−.
Figure 2. Velocity d[1]/dt versus [1] plots. Initial concentrations [1]0,
[py]0, and [3]0 for the three independent experiments are as follows:
○
(I) [1]0 = 27.4 mM, [py]0 = 274 mM, [3]0 = 0.11 mM (− −); (II)
■
[1]0 = 31.7 mM, [py]0 = 317 mM, [3]0 = 0.24 mM (− −); (III) [1]0 =
△
31.7 mM, [py]0 = 634 mM, [3]0 = 0.24 mM (− −). The
contiguous red line at the bottom represents the calculated addend
kdissoc[1] in eq 1.
In Situ Generation of 2a, 2a-d5, and 4a and Equilibria
in Cl−/py Exchanges for PdIV and PdII. After spectroscopi-
cally detecting the C1-symmetrical cation LPdIVCl3(py)+ in
entries 2, 13, and 15 (Table 1), we monitored the reactions of 1
and pyridine in the absence of organic substrates and found an
affected by concentrations [py] and [3]. A quantitative
analysis35 provided a two-term rate law with kassoc = 0.192
0.022 M−1 s−1 for an associative pyridine substitution on 3
(eqs 1 and 2).36
equilibrium between 1 and LPdIVCl3(py)+Cl− (2a) with Keαq
=
d[1]/dt = − k
[1] − k
[3][py]
assoc
(1)
(2)
dissoc
[LPdIVCl3(py)+][Cl−]/([1][py]) = 0.42 that was fully estab-
lished within hours (Scheme 8, equilibrium I) (analogously,
k
= ( − k
[1] − d[1]/dt)/[3][py]
assoc
dissoc
In chemically rationalizing the bimodal kinetics, a hypo-
thetical pyridine association to the octahedral 18-VE complex 1
was excluded from consideration.37 Our dual-track model
(Scheme 9) includes a slow dissociative route involving
PdIV−Cl heterolysis (pathway c); this is supplemented by a
competing two-step associative pathway d, in which Cl−/py
exchange occurs on PdII in the first step followed by an inner-
sphere Cl+ transfer38 between LPdIICl(py)+ and 1, similar to
Scheme 8
39
documented cases for PtII/PtIV and PdII/PdIV systems.40 Since
the associative term in eq 1 (−kassoc[3][py]) is independent of
[1], the rate-determining step in pathway d cannot be the Cl+
transfer between 1 and LPdIICl(py)+ and must therefore be the
Cl−/py substitution in 3. Few such exchanges on PdII have been
kinetically studied in the past, presumably due to the rapid
nature of the process. Basolo provided data that translate into
the deuterated version 2a-d5 was generated from 1 and py-d5).
The addition of Bu4NCl shifted the equilibrium back to the side
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however, instead of forming LPdIVCl3(ligand)+ complexes these
compounds were chlorinated by 1 to Cl2PPh3, BnNHCl·HCl,
and ClCH2S(O)CH3 (entries 1−3). Complex 3 was formed
quantitatively as the sole organometallic product in all cases.
The facile oxidation of heteroatom-containing substrates
matches our previous observation of Br− oxidation to BrCl by
1 (entry 4).8 The reactions in entries 1, 2, and 4 were virtually
instantaneous (on the basis of decolorations of yellow solutions
of 1), while the chlorination of DMSO (entry 3) proceeded with
t1/2 ≈ 480 s (presumably via initial S chlorination).42 The rapid
pace of heteroatom chlorinations is in contrast to relatively slow
Cl− dissociation from 1 (t1/2 = 275 min) (Scheme 1),8 which
precludes the intermediacy of A in these reactions. A reasonable
mechanism may instead entail a direct SN2-type attack at a Cl−
ligand of 1 with concomitant PdIV/PdII reduction (Scheme 10).
Scheme 9. Dissociative Pathway c and Redox Pathway d for
a
the Cl−/py Exchange in 1
Scheme 10
This proposal would be reminiscent of the inner-sphere Cl+
transfer between 1 and 4a (Scheme 9, pathway d). Alternatively,
an outer-sphere electron transfer to form LPdIIICl4−X•+ (X = N,
P, S, Br−) may precede the Cl transfer step. Our observation
that a suspension of 1 in CH2Cl2 instantaneously oxidizes Cp2Fe
to Cp2Fe+ supports the principal viability of PdIII intermediates.
Various mononuclear43 and binuclear44 PdIII systems have been
reported recently.45
a
Pathway d is the predominant route according to experimental data.
baseline values of k2 > 4.3 M−1 s−1 for cationic PdII complexes in
water.41 Our value of kassoc is at least 1 order of magnitude lower,
which is likely due to the low solvent polarity in our CDCl3
experiments as well as the absence of a positive charge in 3.
Attempts To Synthesize Other PdIV Complexes
Containing N,P,S Ligands. A few potential ligand candidates,
including PPh3, BnNH2, and DMSO (Table 4), were tested for
their compatibility in Cl−/ligand substitutions on LPdIVCl4 (1);
Relative Reactivity of PdII Complexes 3 and 4b toward
Cl2. In order to address the issue of facile PdII reoxidation in the
context of catalytic PdII/PdIV functionalizations (requirement I
in the Introduction), we needed to evaluate the respective
propensities of 3 and 4b toward chlorination. A direct
competition experiment between 3 and 4b and a chlorinator
was not feasible, since the initially established product ratio of 1
and 2b would be rapidly altered in the presence of unreacted
starting material 3 and 4b via channels outlined in Scheme 9 and
thermodynamically driven by equilibrium III. Therefore, we
resorted to an indirect comparison in which 3 and 4b were
individually pegged against a common substrate (cyclohexene)
in separate competition experiments.46 We found that Cl2
reacted with 3 more slowly than with cyclohexene by a factor
of 15 2 (Table 5, entry 1a), while the reactivities of 4b and
Table 4. Reaction of 1 with Heteroatom-Containing
Substrates
cyclohexene were virtually identical (krel = 1.1
0.2). Using
these two ratios, we were able to indirectly estimate that 4b is
13.6 times more reactive toward Cl2 than 3. Even though
mixtures of cyclohexene and 3 (or 4b) did not give rise to
discrete new signals or to changes in chemical shifts, a concern
was the potential relevance of endothermic π complexes that
may have formed in a rapid pre-equilibrium,47 in which case
respective product ratios would be a consequence of the
abundance of that π complex. To probe for this unsettling
scenario, we conducted separate competition experiments with
altered initial concentrations [3]0 (or [4b]0) and [cy]0 (entries
1b and 2b). Within error margins the relative rates were not
affected, which therefore precludes the relevance of potential π
complex intermediates.48
a
[substrate] = 10−20 mM; 150 mM Bu4NCl/CDCl3; 1.05 equiv of
b
1
(freshly prepared) 1 unless otherwise noted. Identified via H NMR
spectroscopy chemical shift analysis and comparison to either
c
authentic samples or published spectra. Determined via 1H NMR
d
integration using 1,1,2,2-tetrachloroethane as an internal standard. 1H
e
NMR was recorded in D2O. Precipitated from CHCl3 upon reaction
of benzylamine with 1; washed white solid with CHCl3; yield was
determined gravimetrically; loss of yield was primarily due to partial
product solubility in chloroform. 5 equiv of Bu4NBr was reacted with 1.
f
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PdII/PdIV/Pyridine Catalysis on the Chlorination of
Styrene by PdIV. In our studies on alkene chlorinations, we
elucidated that (I) LPdIICl(py)+Cl− (4a) is reversibly and swiftly
formed from LPdIICl2 (3) and pyridine, (II) complex 1 can
rapidly oxidize 4a to LPdIVCl3(py)+Cl− (2a), and (III)
LPdIVCl3(py)+ ions displayed a reactivity toward alkenes
significantly higher than that of LPdIVCl4. On the basis of
these parameters, it was plausible that otherwise slowly reacting
mixtures of 1 and an alkene such as styrene would experience
dramatic rate acceleration by simply adding pyridine to the
reaction mixture; key to our hypothesis was the fact that
noncatalyzed direct alkene chlorinations by 1 inadvertently
produce 3, which in turn reacts with pyridine to form 4a, which
ultimately allows for the catalytic cascade outlined in Scheme 12.
Table 5. Competition Experiment To Determine Relative
Rates (krel) for Alkenes and PdII Complexes toward
Chlorination with Cl2
a
i
entry
alkene → ACP
PdII → PdIV
krel
b
1a
cyclohexene → II + III
cyclohexene → II + III
cyclohexene → II + III
cyclohexene → II + III
1 → 3
15
14
2
2
c
1b
1 → 3
d
2a
4b → 2b
4b → 2b
4b → 2b
4b → 2b
1.1 0.2
1.3 0.3
e
2b
f
h
3
3-hexene → XI
7
1
g
4
1-hexene → I
0.34 0.04
a
Mixtures of PdII and the alkene were treated with a solution of Cl2 in
Scheme 12. Catalytic PdII/PdIV Styrene Chlorination Cycle
Based on the Couple 4a/2a
CDCl3, and a 1H NMR spectrum of the product mixture was acquired
immediately. From [cy]0 = 20.6 mM (0.667 equiv) and [3]0
30.9 mM (1.00 equiv). From [cy]0 = 5.7 mM (0.087 equiv) and
[3]0 = 65.3 mM (1.0 equiv). From [cy]0 = 67.6 mM (1.10 equiv) and
[4b]0 = 61.3 mM (1.00 equiv). From [cy]0 = 100.1 mM (4.3 equiv)
and [4b]0 = 23.4 mM (1.0 equiv). From [trans-3-hexene]0 = 14.9 mM
(1.10 equiv) and [4b]0 = 13.5 mM (1.00 equiv). From [1-hexene]0 =
a
b
=
c
d
e
f
g
h
10.4 mM (0.770 equiv) and [4b]0 = 13.5 mM (1.00 equiv). XI =
i
meso-3,4-dichlorohexane. krel = ([ACP]/[PdIV]) × ([PdII]0/[alkene]0).
a
While compound 1 functionalizes the alkene sluggishly (right), its
main function is to reoxidize 4a to 2a and to feed the catalytic cycle
(left) with additional supplies of 3.
To account for the surprisingly49 higher reactivity of the
electron-deficient cation 4b relative to 3, we assumed the initial
formation of pentacoordinated intermediates LPdIICl2(η1-Cl2)
and [LPdIICl(py)(η1-Cl2)]+ (D in Scheme 11), similar to van
Once the reaction is “jump-started” (dashed line), [3] would
build up continuously and so would the sum [4a] + [2b]. At this
point, 1 would adopt the dual role of a terminal oxidant for 4a
(left circle) and a source of additional 3; the system would
become autocatalytic in the PdII/PdIV couple 4a/2a.
Scheme 11
Experimentally, the impact of pyridine on the reaction of
various PdIV compounds (27.1 mM solutions in CDCl3) with
styrene (10-fold excess) is outlined in Figure 3. In the absence
of pyridine the reaction of styrene and 1 proceeded slowly
with t1/2 ≈ 275 min (−○−). However, the addition of pyridine
(10 equiv) caused a quantitative conversion of 1 to 3 and 4a
within 4 min (−△−) (with concomitant formation of styrene
chlorination products IV and V). On the basis of our 1H NMR
detection limit, we were confident of a minimum of 97%
conversion of PdIV (at least 5 half-lives; t1/2 ≤ 48 s) amounting
to a rate acceleration ≥344-fold relative to the pyridine-free
reaction. This finding supports the notion that separately
manifested individual steps can indeed operate in concert, as
postulated in Scheme 12. Interestingly, the overall pace of PdIV
disappearance from the mixture of 1/py (forming chloride salt
2a in situ) exceeded not merely the reaction rate of 1 alone but
also the rate of disappearance of triflate salt 2b in its reaction
with styrene (−×−). The elevated reactivity of 2a over 2b is
remarkable but not unexpected, considering our previous
finding that the chloride salt of cation A is about 1500 times
more reactive in Cl+ transfer reactions to cyclohexene than the
50
Koten’s end-on (η1-I2)PtII
and (η1-Cl2)PdII complexes.51
In the absence of counteranions, scission of the Cl−Cl bond
would then produce the pentacoordinated 16-VE cation
LPdIVCl3+ (A) (step a) followed by a collapse to 1. In contrast,
Cl−Cl bond breaking in LPdIICl(py)(η1-Cl2)+ may be assisted
by triflate ions52 via the formation of the octahedral complex E
and collapse to F.53 Precedence for this proposal can be found
2−
in the oxidative additions of Cl2 and Br2 to Pt(CN)4 that
were catalyzed by H2O acting as an axial ligand to an octahedral
(η1-X2)PtII intermediate.54,55 Interestingly, Lewis base pro-
moted accelerations of SN2-type oxidative additions have been
documented for square-planar d8 complexes of Rh56 and Ir.57
Regardless of the cause for the unexpected reactivity of 4b, its
rate of chlorination was comparable to that of cyclohexene and
even exceeded that of 1-hexene (entry 4). For the development
of novel PdII/PdIV catalyses the relatively easy oxidizability of
4b (relative to 3) in conjunction with the enhanced reactivity of
2b (relative to 1) underscores the dual benefit of py ligands in
both oxidation states.
respective BF4 salt of A.8
−
Equally, we found that the addition of 10 equiv of pyridine
enhances the performance of 2b as well (−□−);58 in this case
excess pyridine presumably prevented Cl− ions (formed as a
byproduct in the formation of β-chlorostyrene(V)) to
coordinatively displace py ligands in LPdIVCl3(py)+ and
LPdIICl(py)+. As a consequence of rising levels of chloride
ions, the ratio [2a]/[2b] continuously soared upon styrene
conversion, thus boosting the overall reactivity.59 Therefore,
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Scheme 13
Figure 3. Reaction of PdIV (27.1 mM) with a 10-fold excess of styrene
(271 mM) in CDCl3. Conversions to PdII complexes 3 and 4b were
monitored by 1H NMR spectroscopy with CHCl2CHCl2 as an internal
standard. (A) Use of LPdIVCl4 (1) as a chlorinating agent (5 equiv of
Bu4NBF4 was added to increase the solubility of 1): after 36 min the
lower reactivity of the terminal oxidant and its previous use in
NHC PdIV oxidation.61 A mixture of the arene, N-chlorosucci-
nimide (1.5 equiv), 3 (0.05 equiv), and pyridine (1 equiv)
(60 °C, CDCl3) was followed at low conversions (0−5%), and
the production of 1-chlorodurene (5% yield) (Scheme 13b) was
15 times faster in the presence of 3/py in comparison to the
background reaction (0.4% yield). Accounting for a catalyst
loading of 5 mol %, the rate acceleration on a per catalyst basis
was therefore 300-fold, even though only one turnover of catalyst
could be accomplished prior to significant decomposition after
10 h at elevated temperature. Despite the demonstration of a
principal extension of Scheme 12 to an economical terminal
oxidant in lieu of 1, a challenge that needs to be addressed more
forcefully in the future is the balance that a terminal oxidant has
to strike in being mild enough to not directly attack a substrate
yet being powerful enough to bring about PdII/PdIV oxidation.
○
conversion of 1 (− −) was 25.6% while the combined product yield of
1,2-dichloroethylbenzene (IV) and β-chlorostyrene (V) (ratio IV:V =
2.2:1) was 25.5% after 36 min; after 24 h (not shown) the conversion
of 1 was 87.5% % with a 87.4% combined yield of products. (B) Use of
LPdIVCl3(py)+TfO− (2b) as a chlorinating agent: after 32 min the
conversion of 2b (−×−) was 93.1% while the combined product yield
of IV and V was 90.5% (ratio IV:V = 5.6:1). (C) Use of 1 and 10 equiv
of pyridine as a chlorinating agent (forming a mixture of rapidly
equilibrating LPdIVCl3(py)+Cl− (2a) and 1 in situ): after 4 min the
conversion of all PdIV (− −) including 2a and 1 was 100% with a
△
combined product yield of IV and V of 93.2% (ratio IV:V = 1.3:1). (D)
Use of LPdIVCl3(py)+TfO− (2b) + py (10 equiv) as a chlorinating
□
agent: after 89 s the conversion of 2b (− −) was 100% with a
combined product yield of IV and V of 97.8% (ratio IV:V = 1.1:1).
the presence of pyridine amplified the reactivity of PdIV both
by imposing a positive charge on the metal in LPdIVCl3(py)+
and by enabling a chloride-rich counteranion environment.
In comparison, a recent literature report on PdII/PdIV-catalyzed
aromatic C−H acetoxylations in the presence of PhI(OAc)2/
Pd(OAc)2/py highlighted dramatic “pyridine effects” on
reaction rates and yields as well (presumably through the
action of (py)Pd(OAc)2).60
CONCLUSIONS
■
This body of work describes the synthesis and structural
elucidation of novel cationic bis-NHC PdIV complexes
LPdIVCl3(py)+X− (X = Cl−, TfO−) with equatorially bound
pyridine. The chloride salt could be generated from LPdIVCl4
and pyridine in situ. Octahedral LPdIVCl3(py)+ smoothly
dichlorinated alkenes via an ionic mechanism involving direct
ligand-mediated Cl+ transfer without the need for π-alkene
intermediates. Mixtures of LPdIVCl4 and pyridine constituted the
first reported PdIV system capable of alcohol oxidation, likely via
E2 elimination from in situ formed PdIV alkoxides. A total of five
features could be imparted by pyridine on the reactivity profile of
bis-NHC palladium systems by acting as a ligand or as a base. (I)
LPdIVCl3(py)+ is more reactive toward alkenes than LPdIVCl4,
due to its positive charge and its ability to react as an octahedral
complex. (II) LPdIICl(py)+ underwent faster PdII/PdIV oxidation
than LPdIICl2, possibly due to the assistance of triflate
coordination to reactive intermediates. (III) Cl−/py substitution
in LPdIVCl4 is effectively promoted by traces of LPdIICl(py)+
(formed in situ from LPdIICl2). (IV) In the presence of excess
pyridine in the reaction mixture coordinative Cl− sequestration
by LPdIVCl3(py)+ and LPdIICl(py)+ ions was effectively
diminished. As a consequence, the presence of Cl− counterions
further enhanced rates of alkene chlorination by LPdIVCl3(py)+.
(V) In alcohol oxidations pyridine acted as a base (rather than a
ligand) in slow E2-type β eliminations. Exploiting features I−IV,
we were able to devise a catalytic chlorination platform for
styrene with LPdIICl(py)+/LPdIVCl3(py)+ as a catalytic PdII/PdIV
couple and LPdIVCl4 as a sacrificial oxidant. Furthermore, an
An appealing extension from Scheme 12 would be the
replacement of 1 with a more economical terminal oxidant. To
this end, we selected benzyl alcohol and durene as substrate
targets, since their oxidations were comparatively slow relative
those of the other substrates given in Table 3 and the distinction
between catalyzed reaction and noncatalyzed background
reaction would be easier to make. Monitoring the oxidation of
benzyl alcohol (1.00 equiv) to benzaldehyde (CDCl3/Bu4NBF4,
25 °C) with PhICl2 (1.5 equiv) in the presence of a catalytic
amount of 3 (0.05 equiv, 13.2 mM) as well as pyridine
1
(10 equiv) by H NMR showed the formation of 4a (6% yield
from 3) as well as 2a (91% yield from 3) and 1 (3% yield from 3)
within 4 min, while the concentration of residual 3 was below
the detection limit. The high yield of PdIV confirmed that PhICl2
is capable of PdII/PdIV oxidation at a faster rate than alcohol
oxidation. Subsequently, benzaldehyde was indeed gradually
formed (50.2% yield after 31 min; 51.7% conversion of benzyl
alcohol); however, the control experiment in the absence of
palladium produced a nearly identical result (54.9% yield after
31 min, 57.5% conversion of benzyl alcohol) (Scheme 13a).
Clearly, the background reaction between the substrate and the
terminal oxidant was much faster than the attempted catalytic
PdIV oxidation. Next we explored if the chlorination of durene
(entry 9, Table 3) would be conducive to catalysis, given the
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room temperature. A solution of AgSO3CF3 (0.0535 g, 0.208 mmol,
1.01 equiv) in 0.1 mL of dry CH2Cl2 was then added, and the immediately
formed precipitate of AgCl was removed via centrifugation. A stream
of dry N2 was used to reduce the volume of the supernatant to 0.1 mL,
followed by the addition of hexanes (0.9 mL). The slightly gelatinous
solid was collected by centrifugation and carefully dried under vacuum
to afford 4b (yield: 0.110 g, 0.120 mmol, 58.2%). The supernatant was
subjected to further fractional precipitation via careful addition of
diethyl ether to obtain another 0.0340 g of product (combined yield
of 4b: 0.144 g, 0.157 mmol, 76.1%). Both fractions were >97% pure
LPdCl2/py/NCS mixture was able to monochlorinate durene,
albeit with only one turnover. Taken together, this work
demonstrates novel cationic PdIV pyridine complexes engaged in
direct intermolecular catalytic reactions with organic substrates
and breaks from the existing paradigm for common PdII/PdIV
catalyses that require (slow) PdII activation steps.
EXPERIMENTAL SECTION
■
General Considerations: Materials and Methods. All reactions
were carried out under atmospheric conditions unless otherwise noted.
Inert gas experiments were performed using standard Schlenk
techniques or in an inert-gas glovebox (MBRAUN) with extra-dry
solvents (<2 ppm of H2O) stored over heat-activated 4 Å molecular
sieves. NMR spectra were obtained on a Varian Unity/INOVA
1
by H NMR spectroscopy. Compound 4b does not show signs of
decomposition when stored under air at −20 °C for greater than
1
2 months. H NMR (400 MHz, dry CDCl3): δ 8.71 (dt, J = 5.08,
1.56 Hz, 2 H), 7.92 (tt, J = 7.6, 1.6 Hz, 1 H), 7.80 (d, J = 2.0 Hz, 1 H),
7.69 (d, J = 2.0 Hz, 1 H), 7.47 (ddd, J = 7.7, 5.0, 1.4 Hz, 2 H), 6.98 (d, J =
2.0 Hz, 1 H), 6.94 (d, J = 2.0 Hz, 1 H), 6.74 (d, J = 13.3 Hz, 1 H), 6.37
(d, J = 13.3 Hz, 1 H), 4.83 (ddd, J = 13.7, 8.6, 6.7 Hz, 1 H), 4.21 (ddd, J =
13.7, 8.6, 6.3 Hz, 1 H), 3.46 (ddd, J = 13.6, 9.3, 5.7 Hz, 1 H), 3.18 (ddd,
J = 13.4, 9.3, 6.3 Hz, 1 H), 1.79−2.01 (m, 2 H), 1.56−1.78 (m, 1 H),
1.07−1.48 (m, 43 H), 0.91−1.06 (m, 2 H), 0.87 (t, J = 6.7 Hz, 6 H).
13C NMR (100 MHz, CDCl3): δ 156.66, 152.76, 152.20, 139.33, 125.84,
122.97, 122.59, 121.79, 121.51, 120.68 (q, J = 320.3 Hz, CF3), 62.51,
50.99, 50.03, 31.85, 31.43, 30.79, 29.64, 29.60, 29.51, 29.35, 29.29, 29.20,
28.98, 26.53, 26.42, 22.61, 14.04. HRMS-ESI (m/z): [M]+ calcd for
C40H69N5ClPd, 760.4282, found 760.4262 (−2.1 ppm).
Synthesis of (1,1′-Ditetradecyl-3,3′-methylene-4-diimidazo-
line-2,2′-diylidene)bis(pyridine)palladium(II) Ditriflate, [LPd-
(py)2]2+(TfO−)2 (4c). In 1.0 mL of CH2Cl2 compound 3 (0.111 g,
0.145 mmol, 1.00 equiv) and pyridine (0.0368 g, 0.464 mmol,
3.00 equiv) were mixed in a centrifuge tube at room temperature. A
solution of AgSO3CF3 (0.0836 g, 0.0.325 mmol, 2.10 equiv) in 0.1 mL
of dry CH2Cl2 was then added, and the immediately formed
precipitate of AgCl was removed via centrifugation. The solvent was
removed with a stream of dry N2, and the oily residue was dissolved in
0.2 mL of diethyl ether and filtered over a cotton ball within a Pasteur
pipet. Addition of 1.5 mL of hexane caused precipitation of an oily
residue. After decanting of the supernatant solution the oil was
dissolved in a small amount of CH2Cl2 and this solution was filtered
over approximately 50 mg of charcoal over a cotton ball within a
Pasteur pipet. The filtrate was transferred into a centrifuge tube and
layered with 0.7 mL of hexane. Gradually a colorless gelatinous
precipitate formed. After 2 h, the supernatant solution was carefully
decanted and the residue was washed with hexane. After centrifugation,
the supernatant solution was discarded and the residue was washed two
more times in this fashion. After careful drying under high vacuum a
white powder of 4c was obtained (yield: 0.171 g, 0.0665 mmol, 42.9%).
1H NMR (400 MHz, dry CDCl3): δ 9.34 (broad doublet, J = 6.2 Hz,
1
400 spectrometer that operates at 400 MHz for H and 100 MHz for
13C acquisitions. VnmrJ software (Varian) was used in NMR data
acquisition, and ACD/Laboratories NMR processor software was used
for data analysis. High-resolution mass spectra were acquired on an
MDS SCIEX/Applied Biosystems QStar Elite hybrid quadrupole/
TOF mass spectrometer. Solvents and specialty chemicals were
purchased from Fisher Scientific or from Acros. All chemicals were
used without further purification.
Synthesis of (1,1′-Ditetradecyl-3,3′-methylene-4-diimidazo-
line-2,2′-diylidene)trichloro(pyridine)palladium(IV) Triflate
(2b). PdII complex 4b (0.014 g, 0.015 mmol, 1.0 equiv) was dissolved
in 0.75 mL of CH2Cl2 in a glass test tube under a blanket of N2, and
chlorine gas was bubbled through the solution until the solution
turned bright orange. Solvent was removed using a stream of dry N2,
and the orange solid of 2b was dried in vacuo for 2 h (yield: 0.015 g,
0.015 mmol, 100%). Compound 2b, which is moderately stable toward
air and moisture at room temperature, can be stored at −20 °C under
air without degradation for at least 2 months. Detailed structural
assignments of 2b were obtained from a 2D ROESY NMR spectra as
described in the Supporting Information. 1H NMR (400 MHz, CDCl3;
proton assignments are based on Figure 1): δ 9.22 (br s, 2 H, Ho) 8.15
(br s, 1 H, H5), 8.10 (br t, J = 7.4 Hz, 1 H, Hp), 8.04 (d, J = 1.95 Hz,
1 H, H5′), 7.68 (br t, J = 5.7 Hz, 2 H, Hm), 7.37 (br s, 1 H, H4), 7.29 (d,
J = 2.0 Hz, 1 H, H4′), 7.25 (d, J = 13.7 Hz, 1 H, Hexo), 7.12 (d, J = 13.3
Hz, 1 H, Hendo), 4.95 (ddd, J = 13.6, 11.4, 5.0 Hz, 1 H, Hα′ or Hβ′), 4.71
(ddd, J = 13.6, 11.5, 5.3 Hz, 1 H, Hα′ or Hβ′), 3.45 (ddd, J = 13.0, 11.6,
5.5 Hz, 1 H, Hα or Hβ), 3.17 (ddd, J = 13.0, 11.4, 4.7 Hz, 1 H, Hα or
Hβ), 1.96−2.12 (m, 1 H), 1.80−1.94 (m, 1 H), 1.58−1.73 (m, 1 H),
1.43−1.57 (m, 3 H), 1.34−1.43 (m, 3 H), 1.19−1.34 (m, 34 H), 1.00−
1.10 (m, 2 H), 0.91−1.00 (m, 2 H), 0.88 (t, J = 7.0 Hz, 6 H). 13C
NMR (100 MHz, CDCl3): δ 151.83, 140.68, 139.48, 129.87, 126.39,
125.27, 125.10, 124.16, 123.62, 120.60 (q, CF3), 62.42, 53.34, 50.63,
31.91, 31.25, 30.01, 29.58−29.75 (multiple overlapping signals), 29.55,
29.52, 29.45, 29.34, 29.27, 29.19, 28.72, 26.48, 26.20, 14.11. HRMS-
ESI (m/z): [M]+ calcd for C40H69N5Cl3Pd+ 832.3642, found 832.3681
(+4.7 ppm).
Synthesis of (1,1′-Ditetradecyl-3,3′-methylene-4-diimidazo-
line-2,2′-diylidene)trichloro(pyridine-d5)palladium(IV) Triflate
(2b-d5). The deuterated analogue 2b-d5 was synthesized according
to the procedure outlined for 2b. 1H NMR (400 MHz, CDCl3; proton
assignments are based in Figure 1): δ 8.08 (d, J = 2.0 Hz, 1 H), 7.99
(d, J = 2.0 Hz, 1 H), 7.41 (d, J = 2.0 Hz, 1 H), 7.30 (d, J = 2.0 Hz,
1 H), 7.18 (d, J = 13.3 Hz, 1 H), 7.11 (d, J = 13.3 Hz, 1 H), 4.94 (ddd,
J = 13.6, 11.4, 5.1 Hz, 1 H), 4.71 (ddd, J = 13.7, 11.5, 5.3 Hz, 1 H),
3.42 (ddd, J = 13.0, 11.6, 5.5 Hz, 1 H), 3.17 (ddd, J = 13.0, 11.4, 4.9
Hz, 1 H), 1.96−2.10 (m, 1 H), 1.79−1.94 (m, 1 H), 1.59−1.71 (m, J =
3.1 Hz, 1 H), 0.91−1.57 (m, 45 H), 0.88 (t, J = 6.8 Hz, 6 H). 13C
NMR (100 MHz, CDCl3): δ 139.39, 129.98, 125.13, 124.88, 124.20,
123.79, 120.60 (q, J = 319.5 Hz), 62.32, 53.32, 50.61, 31.89, 31.23,
29.99, 29.63 (multiple overlapping signals), 29.54, 29.51, 29.44, 29.33,
29.26, 29.17, 28.70, 26.46, 26.18, 22.66, 14.10.
4 H), 7.80 (broad t, J = 7.8, 1.6 Hz, 2 H), 7.69 (d, J = 2.0 Hz, 2 H),
7.48 (ddd, J = 7.7, 6.1 Hz, 4 H), 7.33 (d, J = 13.0 Hz, 1 H), 6.87 (d, J =
2.0 Hz, 2 H), 6.37 (d, J = 13.1 Hz, 1 H), 4.16 (ddd, J = 13.8, 10.5,
5.5 Hz, 2 H), 3.87 (ddd, J = 13.8, 10.2, 5.8 Hz, 1 H), 0.92−1.45 (m, 52 H),
0.86 (t, J = 6.7 Hz, 6 H). 13C NMR (100 MHz, CDCl3): δ 154.07,
152.01, 139.14, 126.35, 122.91, 121.09, 120.68 (q, J = 320.3 Hz, only
the two central prongs of the quartet were resolved within the
experimental signal-to-noise ratio), 62.90, 50.55, 31.89, 30.43, 29.67,
29.63, 29.51, 29.33, 29.30, 29.15, 26.11, 22.65, 14.09.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving 2-D NMR spectra and analysis
for 2b, detailed reaction protocols for kinetics experiments and
their analyses, equilibration experiments, competition experi-
ments, compound characterization by high-resolution Mass
spectrometry and NMR spectroscopy, and a detailed derivation
of eqs 1 and 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
Synthesis of (1,1′-Ditetradecyl-3,3′-methylene-4-diimidazo-
line-2,2′-diylidene)chloro(pyridine)palladium(II) Triflate (4b).
In 1.0 mL of CH2Cl2 compound 3 (0.148 g, 0.206 mmol, 1.00 equiv)
and pyridine (0.0244 g, 0.310 mmol, 1.50 equiv) were mixed at
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(4) For selected examples involving C−C bond formations from
PdII/PdIV couples: (a) Qiu, D.; Zheng, Z. T.; Mo, F. Y.; Xiao, Q.; Tian,
Y.; Zhang, Y.; Wang, J. B. Org. Lett. 2011, 13, 4988−4991. (b) Peng,
X. G.; Zhu, Y. G.; Ramirez, T. A.; Zhao, B. G.; Shi, Y. Org. Lett. 2011,
13, 5244−5247. (c) Xiao, B.; Fu, Y.; Xu, J.; Gong, T. J.; Dai, J. J.; Yi, J.;
Liu, L. J. Am. Chem. Soc. 2010, 132, 468−469. (d) Shabashov, D.;
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AUTHOR INFORMATION
■
Corresponding Author
*Tel: (785)341-8173. E-mail: skraft@ksu.edu.
Present Address
†Box 156, Light Hall, Vanderbilt University School of Medicine,
Nashville, TN 37205. E-mail: scott.mccall@vanderbilt.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the NIH/K-INBRE (P 20 RR16475)
(SM), the Terry C. Johnson Cancer Center, the Kansas
Lipidomics Research Center Analytical Laboratory (EPS
0236913, DBI 0521587), and Kansas State University is
gratefully acknowledged.
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assumption (d[A]/dt = 0), resulting in k2 = −kpykA/(kpy′[py]+
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(16) Pyridine-based palladium pincer complexes experienced down-
16a
field shifts of 0.16−0.34 ppm in going from PdII
to PdIV.16b
́ ́
(a) Vicente, J.; Arcas, A.; Julia-Hernandez, F.; Bautista, D.; Jones, P. G.
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alkene chlorinator. Reductive eliminations of dihalogens X2 from PdIV
complexes have been previously invoked by Webster (X = Cl, Br)26a
and spectroscopically studied by Vicente (X = I).26b However, the
trans-3-hexene/1-hexene chlorination bias for Cl2 as the chlorinating
agent was found to be 40.1:1,8 which evidently does not match with
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Dissertation, Universitat Tubingen, Tubingen, Germany, 2002.
̈
̈
̈
(18) In contrast, chelating (bis-NHC)PdII complexes with trans
donor substituents display downfield shifts on C2 and upfield shifts for
the average shifts of C4 and C5. For example, Douthwaite’s LPd(CH3)2
complex (L = t-Bu-(C3H2N2)-CH2-(C3H2N2)-t-Bu) has NHC
resonances at δ 195.5 (C2) and 117.3 (average C4, C5)18a which
approaches shifts for the free carbene (t-Bu-(C3H2N2)-CH2-
(C3H2N2)-t-Bu) with δ 214.5 (C2) and 117.3 (average C4, C5).18b
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coordinatively sequestered by the abundance of 2b and PdII product
4b, a process that is manifested in the 1H NMR detection of 1 and 3 in
the course of the reaction. The action of “chloride sponges” 4b and 2b
can be easily counteracted by the addition of excess pyridine to the
reaction mixture, thus allowing for a gradual buildup of free Cl− ions
and accordingly for the conversion of 2b into the more reactive 2a.
Small amounts of LPdIICl(py)+ and LPdIICl(py-d5)+ were detected
upon addition of py-d5 (10 equiv) to 2b, and in independent studies
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on a time scale of seconds when pyridine and pyridine-d5 were present
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(20) The only other structurally characterized NHC PdIV complex to
date is Arnold’s and Sanford’s LL′PdIVCl2 (L = i-Pr-NHC-CH2-
C(CH3)2O−; L′ = (N,C−)-benzo[h]quinoline), whose carbene
resonance (δ 150.6) is upfield shifted by −23.5 ppm relative to its
PdII precursor LL′PdII (δ 174.1).20a It should be noted that the parent
free imidazolium salt i-Pr-C3H3N2-CH2-C(CH3)2OH has its hetero-
cyclic 13C NMR C2 resonance at δ 134.1,20b which is shifted
significantly upfield from C2 in Arnold’s and Sanford’s PdIV complex.
(a) Arnold, P. L.; Sanford, M. S.; Pearson, S. M. J. Am. Chem. Soc.
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with 2b according to LPdIICl(py-d5)+ + LPdIVCl3(py)+
⇔
LPdIICl(py)+ + LPdIVCl3(py-d5)+ and thus regenerate LPdIICl(py)+
similar to the process in pathway d in Scheme 9.
(28) (a) Vicente, J.; Arcas, A.; Julia-Hernandez, F.; Bautista, D.
Angew. Chem., Int. Ed. 2011, 50, 6896−6899. (b) Vigalok, A.
Organometallics 2011, 30, 4802−4810.
+
(29) Potential intermediates include LPdIVCl3 (A) from py-d5
dissociation or LPdIVCl2(py-d5)2+ either from Cl− dissociation or as
a result of a Cl+ transfer from 2b to 4b. The latter process would be
analogous to the Cl+ transfer from 1 to 4b outlined later in the
discussion.
(21) Strassner and coworkers have recently found a 13C NMR upfield
shift on C2 by −11.7 ppm for the transformation LPtIICl2 → LPtIVCl4
(L = CH2-bridged bis-NHC ligand): Meyer, D.; Ahrens, S.; Strassner,
T. Organometallics 2010, 29, 3392−3396.
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(34) See Figure S9 in the Supporting Information.
(35) For details see the Supporting Information.
(36) kdissoc was previously quantified in ref 8.
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3537
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Organometallics
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1
appearance of extra NHC ligand signals in the H NMR spectrum,
which is evidence against the formation of hypothetical dipyridine
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(59) All of our styrene chlorinations produce HCl in the formation of
the byproduct β-chlorostyrene. In the case of 2b as the chlorinator (no
pyridine added), virtually all of these newly formed chloride ions are
coordinatively sequestered by the abundance of 2b and PdII product
4b, a process that is manifested in the 1H NMR detection of 1 and 3 in
the course of the reaction. The action of “chloride sponges” 4b and 2b
can be easily counteracted by the addition of excess pyridine to the
reaction mixture, thus allowing for a gradual buildup of free Cl− ions
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