Reductive elimination from arylpalladium cyanide complexes

Article abstract of DOI:10.1021/ja300827t

We report the isolation and characterization of arylpalladium cyanide complexes that undergo reductive elimination to form arylnitriles. The rates of reductive elimination from a series of arylpalladium cyanide complexes reveal that the electronic effects on the reductive elimination from arylpalladium cyanide complexes are distinct from those on reductive reductive eliminations from arylpalladium alkoxo, amido, thiolate, and enolate complexes. Arylpalladium cyanide complexes containing aryl ligands with electron-donating substituents undergo reductive elimination of aromatic nitriles faster than complexes containing aryl ligands with electron-withdrawing substituents. In addition, the transition state for the reductive elimination of the aromatic nitrile is much different from that for reductive eliminations that occur from most other arylpalladium complexes. Computational studies indicate that the reductive elimination of an arylnitrile from Pd(II) occurs through a transition state more closely related in structure and electronic distribution to that for the insertion of CO into a palladium-aryl bond.

Full text of DOI:10.1021/ja300827t

Communication
pubs.acs.org/JACS
Reductive Elimination from Arylpalladium Cyanide Complexes
Jessica L. Klinkenberg and John F. Hartwig*,†
Department of Chemistry, University of Illinois, 600 South Mathews Avenue, Urbana, Illinois 61801, United States, and Department
of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
However, the absence of an observed arylpalladium(II) cyanide
ABSTRACT: We report the isolation and characterization
of arylpalladium cyanide complexes that undergo reductive
elimination to form arylnitriles. The rates of reductive
elimination from a series of arylpalladium cyanide
complexes reveal that the electronic effects on the
reductive elimination from arylpalladium cyanide com-
plexes are distinct from those on reductive reductive
eliminations from arylpalladium alkoxo, amido, thiolate,
and enolate complexes. Arylpalladium cyanide complexes
containing aryl ligands with electron-donating substituents
undergo reductive elimination of aromatic nitriles faster
than complexes containing aryl ligands with electron-
withdrawing substituents. In addition, the transition state
for the reductive elimination of the aromatic nitrile is
much different from that for reductive eliminations that
occur from most other arylpalladium complexes. Compu-
tational studies indicate that the reductive elimination of
an arylnitrile from Pd(II) occurs through a transition state
more closely related in structure and electronic distribu-
tion to that for the insertion of CO into a palladium−aryl
bond.
complex suggests that such complexes could be unstable; they
could degrade in the presence of excess cyanide to form anionic
species, or the sp2−sp carbon−carbon bond-forming reductive
elimination to form arylnitriles could be fast. If arylpalladium
cyanide complexes could be isolated and induced to undergo
reductive elimination (Scheme 1), then one could gain
Scheme 1. Reductive Elimination of an Arylnitrile from a
Proposed Arylpalladium Cyanide Complex
information on the factors controlling the rates of this process.
Moloy and Nolan proposed that reductive elimination of
alkylnitriles occurs by a mechanism related to that for carbon
monoxide insertion on the basis of relative rates that coincided
with those for CO insertion into metal−alkyl bonds. An
accelerating effect of Lewis acids on the reductive elimination
further supported a mechanism related to insertion. However,
coupling of a cyano ligand with an aryl group could occur by a
process much different than that for coupling with an alkyl
group, and elimination of an arylnitrile allows the electronic
effects on this class of reductive elimination to be probed in the
absence of steric effects.7,8
Here we report an isolated arylpalladium cyanide complex
and reductive eliminations from this complex and its congeners
to form aromatic nitriles. The experimental and computational
data we report assess the relationships between this reductive
elimination and related reductive eliminations from aryl Pd(II)
complexes or migratory insertions of CO. The effects of the
electronic properties of the aryl ligand on the rates and
thermodynamics for reductive elimination to form arylnitriles
contrast with those of reductive eliminations from Pd(II) to
form other types of C−C bonds9 or C−N,10 C−O,11 or C−S12
bonds.13 Instead, our data and computational studies imply that
the transition state maps closely onto that for insertion of CO
into a metal−aryl bond.
he palladium-catalyzed cyanation of aryl halides has
T
become a common method for the synthesis of
arylnitriles.1 Since the first report by Takagi in 1973,2 a large
number of transition metal-catalyzed cyanations of aryl halides
have been reported, and recently developed protocols occur
under conditions suitable for commercial production.1,3
Despite these advances, little information on the productive
steps of the catalytic cycle has been reported. Grushin and co-
workers observed anionic Pd(0) and Pd(II) cyanide complexes
from displacement of the phosphine ligand with cyanide and
showed that these complexes do not undergo the oxidative
addition and reductive elimination steps proposed to occur
during the catalytic cycle.4 Subsequently, Beller and co-workers
demonstrated the advantageous effect of diamines on the
oxidative addition of aryl halides to Pd(PPh3)4 in the presence
of excess cyanide ion and on the rate of transmetalation
proposed to form an arylpalladium cyanide that undergoes
reductive elimination. However, the arylpalladium cyanide
complex was not observed.5
Our studies began with efforts to isolate a stable
arylpalladium cyanide complex. A variety of bisphosphine-
ligated p-tolylpalladium bromide complexes were treated with
sources of cyanide. The reactions of p-tolylpalladium(II)
bromide complexes containing 1,1′-bis(diphenylphosphino)-
Thus, no reports have described the isolation of an
arylpalladium cyanide complex or evaluated its propensity to
undergo reductive elimination. Metal−cyanide linkages are well
known to be thermodynamically stable,6 and the facile
formation of anionic Pd(II) cyanide complexes4 underscores
the fact that Pd forms strong bonds with cyanide ligands.
Received: January 25, 2012
Published: February 21, 2012
© 2012 American Chemical Society
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ferrocene (DPPF) and 1,1′-bis(diisopropylphosphino)ferrocene
(DiPPF) with NBu4CN at 20 °C led to complete conversion of
1
the complexes, as determined by 31P NMR spectroscopy. H
NMR analyses of the reaction mixtures indicated the formation
of p-tolylnitrile in 89 and 98% yield, respectively, but the
presumed arylpalladium cyanide complex was not observed
directly. However, the Josiphos (CyPF-t-Bu)-ligated p-
tolylpalladium bromide complex reacted with NBu413CN at
−78 °C to form a new complex that was sufficiently stable to be
and CD2Cl2 to form the corresponding arylnitrile products in
80−95% yield (Scheme 3 and Table 1). In DMF, the Pd(0)
1
characterized at −40 °C by H and 31P NMR spectroscopy.
The two doublets of doublets in the 31P NMR spectrum
confirmed the binding of one labeled cyanide ligand to Pd (see
Supporting Information for spectra). After the solution warmed
to room temperature, p-tolylnitrile was formed in 88% yield.
The arylpalladium cyanide complex (aryl = p-Cl-C6H4)
containing the rigid bisphosphine ligand 1,3-bis-
(diphenylphosphino)-2,2-dimethylpropane (dppdmp) gener-
ated in situ from the reaction of (dppdmp)Pd(C6H4-p-Cl)(F)
and TMSCN was even more stable. This complex persisted for
hours at room temperature, as determined by 31P NMR
spectroscopy, whereas the complex containing the more
electron-rich p-anisyl group was unstable at room temperature.
Ultimately, we found that 1, containing two trifluoromethyl
groups on the Pd-bound aryl ligand, was stable enough to be
isolated in 91% yield (Scheme 2). Complex 1 was characterized
Scheme 3. Reductive Elimination from Arylpalladium
Cyanide Complexes 3a−e
Table 1. Yields and Rates of Reductive Elimination from 3a−
e
a
a
b
complex
R
kDCM (10−4 s−1
)
kDMF (10−4 s−1
)
yield (%)
g
3a
3a
3a
3b
3b
3c
3d
3e
p-CF3
p-CF3
p-CF3
p-Cl
0.23
1.8
−
0.73
81
94
81
ef
f
,
3.2
3.6
d
h
0.62
−
1.3
11
21
22
77
Scheme 2. Synthesis and Reductive Elimination of
Arylpalladium Cyanide Complex 1
e
p-Cl
6.7
98
80
94
93
d
H
2.5
c
p-OMe
o-Me
7.4
5.9
a
First-order rate constants determined by monitoring the decay of the
cyanide complex by 1H NMR spectroscopy ( kDCM, in CD2Cl2) or 31
P
b
NMR spectroscopy (kDMF, in DMF). Yield of the arylnitrile for the
reductive elimination step as determined by 1H NMR spectroscopy (in
c
d
CD2Cl2). Average of three separate runs. Average of two separate
runs. At 40 °C. With 2 equiv of B(C6F5)3. Yield after 81%
conversion of 3a. Yield after 87% conversion of 3b.
e
f
g
1
by H and 31P NMR and IR spectroscopy. This complex
h
undergoes reductive elimination at elevated temperatures over
an extended time; heating a solution of 1 at 80 °C for 16 h led
to the conversion of >95% of 1 to form 3,5-trifluoromethyl-
product was Pd(dppdmp)2, which was isolated in 72% yield by
heating 2a, TMSCN, and dppdmp at 80 °C in DMF for 30 min.
In CD2Cl2, the palladium product was (dppdmp)PdCl2, which
crystallized from the solution of 2e and TMSCN (Scheme 3,
3e) apparently, the Pd(0) product reacts with the CD2Cl2
solvent.
1
benzonitrile in 45% yield, according to H NMR spectroscopy
(Scheme 2).14
These initial studies indicated that the arylpalladium cyanide
complex containing an electron-poor aryl group is more stable
toward reductive elimination than the analogous complex
containing a more electron-rich aryl group. This relative
reactivity contrasts with that for reductive eliminations to
form C−C bonds from arylpalladium(II) and arylplatinum(II)
complexes, which occur faster from complexes containing
electron-withdrawing substituents on the aryl group than from
analogous complexes containing electron-donating substitu-
ents.9,13
The rate constants for reductive elimination were measured
1
by monitoring the reaction by H and 31P NMR spectroscopy
over a period of 2−56 h and are shown in Table 1. Indeed,
complexes containing electron-withdrawing substituents on the
aryl ligand (3a,b, Scheme 3) underwent reductive elimination
more slowly than those containing an unsubstituted aryl ligand
(3c) or an electron-donating substituent on the aryl ligand (3d)
(Scheme 3), and a ρ value of −2.0 in CD2Cl2 and DMF was
obtained from Hammett plots (see the Supporting Informa-
tion). This trend in the electronic effects suggests that the aryl
ligand acts as a nucleophile in the coupling with an electrophilic
cyanide ligand. As a side note, 3e containing an ortho-
substituted tolyl ligand reacted more rapidly than the less
congested arylpalladium cyanide complexes.
To investigate the effect of the electronic properties of the
reacting aryl ligand quantitatively, a series of dppdmp-ligated
arylpalladium cyanide complexes were prepared from fluoride
complexes 2a−e. Reaction of these fluoride complexes with
TMSCN formed arylpalladium cyanide complexes 3a−e (eq 2),
1
which were characterized at low temperature by H and 31P
NMR spectroscopy and at room temperature by IR spectros-
copy in some cases. Only the 3,5-trifluoromethyl-substituted 1
described above was stable enough to be isolated.
Complexes 3a−e underwent reductive elimination at 20 or
40 °C at convenient rates in N,N-dimethylformamide (DMF)
Prior reductive eliminations to form alkylnitriles from
alkylpalladium cyanide complexes were accelerated by coordi-
nation of Lewis acids to the nitrogen of the cyanide ligand.7
Reactions of 3a with added B(C6F5)3 at 40 °C were only 2−5
times as fast as those without added B(C6F5)3, whereas the
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prior reactions of an alkylpalladium cyanide complex with
added Lewis acids were 1−2 orders of magnitude faster than
the reactions without an added Lewis acid. Although small, this
effect of Lewis acid is consistent with a pathway in which the
aryl group acts as a nucleophile and the cyanide ligand as an
electrophile.
A series of computational studies was conducted to gain
insight into the origin of the electronic effects on the reductive
elimination of arylnitriles from arylpalladium(II) complexes.
The ground-state and transition-state structures of three model
PMe3-ligated arylpalladium cyanide complexes were calculated,
and the results of these calculations are shown in Figure 1. The
oxidative addition is computed to be lower for complexes with
electron-withdrawing substituents on the aryl ligand.
In contrast to this trend, the magnitude of the effect of the
aryl substituent on the transition-state energy for reductive
eliminations of arylamines, which occur faster from complexes
containing more electron-poor Pd-bound aryl groups, was
computed previously to be similar to the magnitude of the
effect on the ground state energies (8.2 vs 8.9 kcal/mol,
respectively).15 These effects are similar because arylpalladium
complexes and arylamines are both stabilized by electron-
withdrawing substituents. Thus, we propose that the strong and
opposite electronic effects of the substituents on the stabilities
of the starting Pd complexes and the nitrile products causes the
unusual electronic effect on the rate of reductive elimination of
arylnitriles.16
A second contribution to the observed electronic effect could
stem from the relationship between the reductive elimination of
arylnitriles and insertion of CO into a metal−aryl bond. As
noted in the introduction, the reductive elimination of
alkylnitriles from a neopentylpalladium cyanide complex was
suggested to be related to the insertion of CO.8 Consistent with
this proposed relationship, insertion of CO into a palladium−
aryl bond is known to be faster for complexes containing
electron-donating substituents on the aryl ligand,17 and CO
insertion into a M−C bond in which the hydrocarbyl ligand is
more electron-donating is more thermodynamically favored
than CO insertion into a M−C bond in which the hydrocarbyl
ligand is more electron-withdrawing.18
Computational data provide a more precise view of the
relationships between these two reactions. The computed
ground-state structures of neutral PMe3-ligated
phenylpalladium(II)-cyanide and cationic PMe3-ligated
phenylpalladium(II)−CO complexes, as well as the transition
states for reductive elimination of benzonitrile and insertion of
CO into a palladium−phenyl bond, are overlaid in Figure 2.
Figure 1. Ground-state, transition-state, and overall free energies in
kcal/mol for PMe3-ligated arylpalladium cyanide complexes under-
going reductive elimination of arylnitriles.
reductive elimination of benzonitrile from the phenylpalladium
cyanide complex containing two cis-disposed PMe3 ligands was
computed to have a barrier of 16.7 kcal/mol. The barrier to
elimination from the complex containing the more electron-
donating p-aminophenyl group (16.1 kcal/mol) was computed
to be lower than that for elimination from the complex
containing the more electron-poor p-trifluoromethylphenyl
group (17.2 kcal/mol). These results agree with the
experimental observation that reductive elimination from the
more electron-rich arylpalladium cyanide complexes is faster
than that from the more electron-poor analogues.
The effect of the electronic properties of the aryl group on
the thermodynamic driving force for the reductive elimination
was parallel to, but much larger, than that on the relative rate.
Reductive elimination of the arylnitrile from p-aminobenzoni-
trile was computed to be more downhill (ΔG⧧ = −22.7 kcal/
mol) than reductive elimination to form benzonitrile (ΔG⧧ =
−19.3 kcal/mol), which in turn was predicted to be more
downhill than reductive elimination of p-trifluoromethylbenzo-
nitrile (ΔG⧧ = −15.9 kcal/mol) (Figure 1). The large
differences in the driving force arise from the contrasting
electronic effects of the substituents on the arylpalladium
cyanide and arylnitrile products. Electron-withdrawing sub-
stituents stabilize the arylpalladium species but destabilize the
arylnitrile product.
Figure 2. Computed structures of a PMe3-ligated phenylpalladium
cyanide complex (blue labels) and a PMe3-ligated arylpalladium CO
complex (red labels) in the ground state (GS) and transition state
(TS) prior to reductive elimination of benzonitrile or insertion of CO.
Atom colors: Pd, turquoise; P, pink; C, gray; N, blue; O, red; H, white.
The reaction coordinate for reductive elimination of aromatic
nitriles resembles that for insertion of CO into a palladium−
aryl bond. The C−N bond length in the ground-state cyanide
complex and the C−O bond length in the ground-state
carbonyl complex differ by only 0.03 Å. Likewise, the C−N and
C−O bond lengths in the transition states differ by only 0.01 Å.
The bond angles involving the CX (X = N, O) ligands and the
phenyl ligand in the two complexes also change by a similar
amount during the two reactions. The C−Pd−C angle changes
from 85.7° in the ground state to 59.5° in the transition state
(Δ = 26.2°) during reductive elimination of the arylnitrile,
This range of computed driving forces (6.8 kcal/mol) is
much larger than the range of computed transition-state
energies (1.5 kcal/mol). As a result, the barrier for reductive
elimination is lower for complexes containing electron-donating
substituents on the aryl ligand, but the barrier for the reverse
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Notes
while the C−Pd−C angle changes from 81.8° to 54.8° (Δ =
27.0°) during insertion of CO into the palladium−aryl bond.
The distortion from planarity reported for a related transition
state involving oxidative addition of aromatic nitriles to Ni(0)
was not observed for the second-row Pd system.19
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Giang Vo for the synthesis of dppdmp and the
characterization of complex 2b. We thank the NIH (GM-
58108) for support of this work and Johnson-Matthey for a gift
of PdCl2.
The difference between a more synchronous coupling of two
ligands and a more asynchronous migratory bond formation
has been proposed to indicate distinct forms of concerted
reductive elimination. Calhorda20 reported extended Huckel
̈
REFERENCES
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and shortening of the Pd−C bond of the unsaturated ligand to
which migration occurs. Data in more recent work by
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“migratory” reductive elimination.
■
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ASSOCIATED CONTENT
■
S
* Supporting Information
(19) Atesi̧ n, T. A.; Li, T.; Lachaize, S.; García, J. J.; Jones, W. D.
Organometallics 2008, 27, 3811.
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(22) This result also parallels the accelerating effect of electron-
donating substituents on the aryl groups during reductive elimination
from arylpalladium phosphonate complexes, although the origins of
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All experimental procedures and spectroscopic data for new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
jhartwig@berkeley.edu
Present Address
†Department of Chemistry, University of California Berkeley,
718 Latimer Hall, Berkeley, CA 94720.
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