Identification of the Active Catalyst for Nickel-Catalyzed Stereospecific Kumada Coupling Reactions of Ethers

Article abstract of DOI:10.1002/chem.202000215

A series of nickel complexes in varying oxidation states were evaluated as precatalysts for the stereospecific cross-coupling of benzylic ethers. These results demonstrate rapid redox reactions of precatalysts, such that the oxidative state of the precatalyst does not dictate the oxidation state of the active catalyst in solution. These data provide the first experimental evidence for a Ni⁰–Ni^II catalytic cycle for a stereospecific alkyl–alkyl cross-coupling reaction, including spectroscopic analysis of the catalyst resting state.

Full text of DOI:10.1002/chem.202000215

A Journal of
Accepted Article
Title: Identification of the Active Catalyst for Nickel-Catalyzed
Stereospecific Kumada Coupling Reactions of Ethers
Authors: David D Dawson, Victoria F Oswald, A S Borovik, and
Elizabeth R Jarvo
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To be cited as: Chem. Eur. J. 10.1002/chem.202000215
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Identification of the Active Catalyst for Nickel-Catalyzed
Stereospecific Kumada Coupling Reactions of Ethers
David D. Dawson, Victoria F. Oswald, A. S. Borovik,* Elizabeth R. Jarvo*[a]
Ni catalyst
Abstract: A series of nickel complexes in varying oxidation states
OR'
R"
R"MgX
were evaluated as precatalysts for stereospecific cross-coupling of
benzylic ethers. These results demonstrate rapid redox reactions of
precatalysts, such that the oxidative state of the precatalyst does not
dictate the oxidation state of the active catalyst in solution. These
data provide the first experimental evidence for a Ni(0)-Ni(II) catalytic
Ar
R
Ar
R
active catalyst:
Ni(0) or Ni(I)?
1
2
Scheme 1. Nickel-catalyzed stereospecific Kumada cross-coupling.
We aimed to synthesize precatalysts at the Ni(0), Ni(I), and
Ni(II) oxidation states that could be employed under our cross-
coupling reaction conditions. The ligand in these complexes
must both stabilize the nickel complexes sufficiently to provide
isolable complexes, yet also allow for a reactive catalyst in our
cross-coupling method. The bidentate phosphine ligand BINAP
is an effective ligand in nearly all Kumada-type cross-coupling
protocols disclosed by our group;17 for this reason, it was chosen
as the ligand for these experiments. The single enantiomer (R)-
BINAP was preferentially chosen over the racemic ligand, as
isolation and crystallization of enantioenriched complexes was
more straightforward than the racemic analogs. Previous
experiments in our laboratory have demonstrated that there are
no reactivity differences or matching effects when using (R)-,
(S)-, and rac-BINAP in Kumada cross-coupling reactions with
chiral substrates.17
cycle for
a stereospecific alkyl-alkyl cross-coupling reaction,
including spectroscopic analysis of the catalyst resting state.
Nickel complexes can participate in a range of elementary
reactions, including polar and single-electron events, and
distinguishing mechanistic possibilities for a catalytic reaction
can be challenging. 1 For many cross-coupling reactions,
fundamental features of the active nickel catalyst including
oxidation state have not yet been experimentally established.2 In
most nickel-catalyzed cross-coupling protocols, a Ni(0) or Ni(II)
precatalyst is employed, and an active catalyst is formed in situ.
The oxidation state of this active catalyst depends on several
factors, and does not necessarily correspond to the oxidation
state of the precatalyst.3 Across the field, there is support for
both Ni(I) and Ni(0) complexes as active catalysts, depending on
the reaction details including supporting ligand and substrate.
Evidence of Ni(I) complexes as active catalysts in cross-coupling
reactions began with stoichiometric reactions of organonickel
complexes. Kochi and co-workers demonstrated reductive
Three (R)-BINAP-ligated nickel precatalysts at varied
oxidation states were synthesized (Scheme 2). To synthesize
the Ni(0) complex 3, a procedure by Buchwald and co-workers
was employed.18 Ni(cod)2 and (R)-BINAP were stirred in toluene,
which resulted in the clean formation of ((R)-BINAP)Ni(cod) as a
red crystalline material (Scheme 2a). The product was
4
elimination of dialkylnickel(III) complexes; more recently,
experimental data supporting key Ni(I) intermediates have been
reported.
catalytically active Ni(0) and Ni(II) complexes in related cross-
coupling reactions,
recrystallized and characterized by 1H & 31P NMR and X-ray
5 , 6 , 7 , 8 , 9 , 10
However, there is also evidence for
19
crystallographic analysis.
as well as UV-vis absorption
11
spectroscopy, which showed a strong absorbance at λ = 535 nm.
including characterization of Ni(II)
complexes that can enter catalytic cycles and off-cycle Ni(I)
complexes.12,13,14
In the context of cross-coupling reactions of alkyl
electrophiles, most efforts have focused on the mechanism of
stereoablative reactions of alkyl halides. These reactions can
proceed via halogen atom abstraction,15 and Ni(I)/Ni(II)/Ni(III)
catalytic cycles are typically invoked.5,6,9 Alkyl electrophiles that
undergo stereospecific cross-coupling, such as ethers, likely
react with distinct and different mechanistic features.16 Thus far,
interrogation of these mechanisms has relied heavily on DFT
calculations. In this manuscript, we present spectroscopic
studies of a stereospecific Kumada coupling reaction that are
consistent with a Ni(0)/Ni(II) couple (Scheme 1). We also
demonstrate that both Ni(I) and Ni(II) precatalysts provide rapid
entry to the catalytic cycle, underscoring the rapid redox activity
of typical catalyst precursors under these reaction conditions.
(R)-BINAP
((R)-BINAP)Ni(cod)
3
24% yield (crystals)
a)
b)
Ni(cod)2
PhMe, rt, 24 h
(R)-BINAP
then
chlorobenzene
(((R)-BINAP)NiCl)2
4
31% yield (crystals)
Ni(cod)2
C6H6, rt, 24 h
(R)-BINAP
((R)-BINAP)NiCl2
5
34% yield (crystals)
c)
d)
NiCl2•6H2O
MeCN, reflux, 24 h
((R)-BINAP)Ni(cod)
3
(((R)-BINAP)NiCl)2
((R)-BINAP)NiCl2
4
5
Scheme 2. Synthesis and key characterization data for precatalysts.
[a]
Department of Chemistry
University of California, Irvine
Irvine, CA 92697 USA
A modified procedure by Schoenebeck was used in the
synthesis of Ni(I) complex 4.14a Ni(cod)2 and (R)-BINAP were
stirred in benzene to form a BINAP-ligated Ni(0) species in situ
(Scheme 2b). Subsequent addition of 0.5 equivalents of
chlorobenzene formed a Ni(II) species by an oxidative addition
E-mail: erjarvo@uci.edu
Supporting Information for this article is given via a link at the end of
the document.
pathway, which allowed for
a comproportionation reaction
between Ni(0) and Ni(II) species. The reaction afforded the
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paramagnetic ((R)-BINAP)NiCl, which was crystallized. Due to
the open coordination site on the Ni(I) center, ((R)-BINAP)NiCl
crystallizes as a dimer with pseudo-tetrahedral geometry.20 In
solution, however, the complex is monomeric; the EPR spectrum
shows an expected strong signal.
In contrast, Ni(0) precatalyst 3 provided substantially
suppressed rate of product formation when compared to Ni(I)
and Ni(II) catalysts 4 and 5 (Figure 1b, blue diamonds). We
reasoned that this was due to inhibition by cod by formation of a
stable off-cycle species. To determine whether or not cod
suppresses the rate of reaction, 5 mol % of cod was added to
the reactions employing ((R)-BINAP)NiCl and ((R)-BINAP)NiCl2
(Figure 1c). With cod present in all reactions, the rates of
formation of product 7 using Ni(I) and Ni(II) are slowed, and are
similar to the rate using the Ni(0) precatalyst. These
observations are consistent with formation of the same catalyst
resting state, e.g., (BINAP)Ni(cod), from all three precatalysts.
As shown above, coordination of cod to nickel species
clearly retards reaction progress, likely by stabilizing an off-cycle
nickel complex. Although this adversely affects the rate of
product formation, it could also slow or prevent catalyst
decomposition by reaction with Grignard reagent. To test this
hypothesis, nickel precatalysts were allowed to stir with Grignard
reagent for two hours prior to addition of substrate (Figure 1d).
Surprisingly, upon addition of ether 6, no product formation was
seen for either the Ni(I) or Ni(II) catalyst, even at six hours. This
lack of product formation is likely caused by reaction of the
nickel complexes with the Grignard reagent, which results in
reduction and formation of nickel black. In contrast, the Ni(0)
catalyst ((R)-BINAP)Ni(cod) is stabilized and protected by the
cod ligand during the two hour prestir with Grignard reagent;
upon addition of substrate, product formation is comparable to
that observed without the two hour prestir. This experiment
highlights the ability of cyclooctadiene to stabilize low valent
nickel complexes and protect from catalyst degradation. For
applications where a robust catalyst is required such as large
Finally, Ni(II) complex 5 was synthesized according to the
procedure reported by Jamison and co-workers.18,
21
, 22
NiCl2•6H2O was refluxed with (R)-BINAP in acetonitrile (Scheme
2c). The resultant black solid was recrystallized, which afforded
((R)-BINAP)NiCl2 as sparingly soluble black crystals.22
Crystallographic analysis confirmed pseudo-square planar
geometry around the nickel center.23 This pseudo-square planar
geometry combined with its low solubility in most common
solvents made additional characterization of the Ni(II) species
1
difficult. However, the H NMR spectrum correlated with related
complexes reported in the literature,24 and the magnetic moment
was measured to be 2.49 µB by Evan’s Method,25 consistent with
the assigned structure.
With nickel complexes 3–5 in hand, we examined them as
catalysts in a cross-coupling reaction of a benzylic ether. We
chose
cross-coupling
of
tetrahydropyran
6
with
methylmagnesium iodide, a prototypical stereospecific Kumada-
type coupling reaction that is robust, high-yielding, and scales
well (Figure 1a).26 The active catalyst is proposed to be either
Ni(0) or Ni(I) species ligated by the bidentate phosphine ligand;
under typical reaction conditions the precatalyst would undergo
reduction and oxidation pathways in situ to generate the active
catalyst. Figure 1 shows a comparison of the rate of product
formation with ((R)-BINAP)Ni(cod) (blue diamonds), ((R)-
BINAP)NiCl (pink squares), and ((R)-BINAP)NiCl2 (orange
triangles).
scale applications, use of
provide a protective effect.
a cod-ligated precatalyst would
a)
L[Nin]Xn (5 mol%)
Me Ph
O
To provide evidence as to the exact structure of the active
catalyst, including whether the oxidation state of the resting state
complex is Ni(0) or Ni(I), we undertook spectroscopic tests to
further characterize nickel complexes that are present in situ.
There are many spectroscopic techniques that are regularly
employed to analyze organometallic complexes, but due to the
electronic properties and coordination geometry of these nickel
compounds, absorption spectra proved most informative.
MeMgI (5 equiv)
PhMe, rt, 6 h
Nap
OH
Nap
Ph
6
7
b)
Ni(0)
Ni(I)
Ni(II)
((R)-BINAP)Ni(cod), 3
((R)-BINAP)NiCl, 4
((R)-BINAP)NiCl2, 5
We characterized each precatalyst using absorption
spectra (Figure 2a), and then compared to spectra of Kumada
reactions employing each catalyst. As shown in Figure 2a, 3
displayed a distinctive absorption at λmax = 535 nm, whereas the
spectra for Ni(I) and Ni(II) complexes 4 and 5 featured a series
of peaks of lower intensity.22 Addition of substrate to each of the
preformed catalysts did not alter the absorption spectra.27 Upon
the addition of Grignard reagent, there is no change in signal for
the solution of 3,27 which is consistent with the unchanged
catalytic activity of the Ni(0) catalyst upon stirring with Grignard
reagent (c.f. Figure 1d). In contrast, the Ni(I) and Ni(II) spectra
changed drastically upon addition of Grignard reagent, with
disappearance of most identifiable peaks (Figure 2b and 2c).
These results, in conjunction with lack of product formation from
these mixtures (Figure 1d), are consistent with reduction and
decomposition of 4 and 5 by the Grignard reagent to form
inactive complexes. We sought corroborate this premise using
electron paramagnetic resonance (EPR) spectroscopy. Upon
addition of Grignard reagent to complex 4, the strong Ni(I) signal
d)
c)
Prestir with
MeMgI
Ni(0)
Ni(I) + cod
Ni(II) + cod
Ni(0)
Ni(I)
Ni(II)
Figure 1. Relative rate experiments: a) Cross-coupling of 6. b) Formation
of product 7 over time using ((R)-BINAP)Ni(cod) (blue), ((R)-BINAP)NiCl
(pink), and ((R)-BINAP)NiCl2 (orange); c) Formation of product 7 over
time in the presence of cyclooctadiene: ((R)-BINAP)Ni(cod) (blue); ((R)-
BINAP)NiCl + 0.05 equiv cod (pink); and ((R)-BINAP)NiCl2 + 0.05 equiv
cod (orange); d) Formation of product 7 over time after a 2 hour pre-stir
of precatalyst with MeMgI, followed by addition of substrate 6: ((R)-
BINAP)Ni(cod) (blue); ((R)-BINAP)NiCl (pink); and ((R)-BINAP)NiCl2
(orange). See Supporting Information for full details.
(g
= 2.13) is immediately quenched, consistent with the
formation of an even electron species. Even-electron complexes
3 and 5 were EPR silent and showed no signal after addition of
the Grignard reagent to the EPR tube.27
Comparison of the rate profiles using Ni(I) precatalyst 4
(pink squares) and Ni(II) precatalyst 5 (orange triangles) show
that both form products with similar rates throughout the 6 hour
window (Figure 1b). This observation is consistent with rapid
reduction of both complexes with the Grignard reagent to
generate the same active catalyst in solution. The slightly lower
initial rate with Ni(II) precatalyst 5 can be attributed to slightly
slower reduction of 5 by the Grignard reagent (vide infra, Figure
2b and c).
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and EPR studies, which indicated an instantaneous reaction
between the precatalysts and MeMgI, and that resulted in a
substantial changes in the absorbance spectra, loss of the Ni(I)
EPR signal indicating formation of even-electron species. When
no substrate or cod is present (as in Figure 1d), the low valent
nickel complexes decompose, likely generating nickel black.
However, when cod is present, low-valent complexes can be
stabilized by forming off-cycle resting state 3; this resting state is
observed by absorption spectroscopy (Figure 2d–e). These
conclusions are consistent with the results of our prior
investigations of the mechanism of nickel-catalyzed coupling of
benzylic ethers, including DFT calculations of the reaction
coordinate diagram for the catalytic cycle.16a
a)
b)
4 + SM (6)
((R)-BINAP)Ni(cod) (3)
4 + SM (6) + MeMgI (10 s)
4 + SM (6) + MeMgI (45 m)
4 + SM (6) + MeMgI (2 h)
((R)-BINAP)NiCl (4)
((R)-BINAP)NiCl2 (5)
c)
d)
4 + cod + SM (6)
5 + SM (6)
4 + cod + SM (6) + MeMgI (1 m)
4 + cod + SM (6) + MeMgI (2 h)
5 + SM (6) + MeMgI (10 s)
5 + SM (6) + MeMgI (45 m)
5 + SM (6) + MeMgI (2 h)
(BINAP)NiICl
MeMgI
O
(BINAP)Ni0
(BINAP)Ni0(cod)
3
4
Ph
or
Reduction
observe by UV-vis
8
(BINAP)NiIICl2
5
active catalyst
Figure 3. Mechanistic hypothesis.
Conclusions
Our spectroscopic studies have provided clarity on the identity of
the active catalyst of our nickel-catalyzed Kumada-type cross-
coupling reaction. The use of preformed nickel catalysts allowed
us to probe the effect of varying nickel oxidation states as it
relates to catalytic activity. Two components of our reaction
manifold, the 1,5-cyclooctadiene ligand (cod) and Grignard
reagent (MeMgI), were found to have a dramatic influence.
Addition of MeMgI to solutions containing Ni(I) and Ni(II)
preformed catalysts resulted in a rapid reaction that altered the
oxidation state of the catalytic precursor. The presence of cod
provided a stabilizing effect on the catalyst, protecting it from
MeMgI induced decomposition, yet also mildly supressing the
rate of the cross-coupling reaction. When solutions of Ni(I) and
Ni(II) containing exogenous cod were subjected to MeMgI,
absorption spectra confirmed convergence on a single species,
which matched that of ((R)-BINAP)Ni(cod). These data,
combined with the absence of single-electron species in the
EPR spectra, provide experimental evidence that the
stereospecific Kumada-type cross-coupling reaction proceeds
via a Ni(0)/Ni(II) catalytic cycle. In contrast, stereoablative alkyl-
alkyl cross-coupling reactions are typically proposed to involve
Ni(I) intermediates.
e)
5 + cod + SM (6)
f)
3
5 + cod + SM (6) + MeMgI (1 m)
5 + cod + SM (6) + MeMgI (2 h)
4 + cod + SM (6) + MeMgI (2 h)
5 + cod + SM (6) + MeMgI (2 h)
Figure 2. Kumada reactions employing 5 mol% of precatalysts 3-5,
monitored by absorption spectroscopy. a) Absorbance spectra for
complexes ((R)-BINAP)Ni(cod) (blue), ((R)-BINAP)NiCl (pink), and ((R)-
BINAP)NiCl2 (orange); b) Addition of MeMgI to ((R)-BINAP)NiCl; c)
Addition of MeMgI to ((R)-BINAP)NiCl2; d) Addition of MeMgI to ((R)-
BINAP)NiCl + 0.05 equiv cod; e) Addition of MeMgI to ((R)-BINAP)NiCl2
+ 0.05 equiv cod; f) comparison to ((R)-BINAP)Ni(cod).
These absorption spectra, combined with reaction rate data, are
consistent with the Grignard reagent affecting a transformation
of the precatalysts in situ, generating an even electron species.
Considering our previous results that show the stabilizing effect
of the cod ligand in the reaction, we anticipated that adding an
equivalent of cod to the Ni(I) and Ni(II) solutions might produce a
more stable complex. Thus, solutions of Ni(I) complex 4 and
Ni(II) complex 5 were treated with Grignard reagent in the
presence of cod and analyzed by absorption spectroscopy. The
results show the influence of cod on the reaction (Figure 2d and
2e): after two hours, both Ni(I) and Ni(II) complexes in the
presence of cod converged on the same intense spectrum with
an intense signal that was previously observed with Ni(0)
complex 3, with a strong absobance at λmax = 535 nm. These
results are consistent with a pathway where upon addition of
Grignard reagent, Ni(I) and Ni(II) species are rapidly reduced to
Ni(0) (Figure 2f), at which point they are stabilized by the cod
ligand to generate the same complex, (BINAP)Ni(cod). The lack
of a substantial induction period in the cross-coupling rates
employing Ni(I) and Ni(II) complexes is likely because of the
rapid redox activity of these complexes that leads to rapid
reduction to Ni(0).
Acknowledgements
This work was supported by NSF-CHE-1464980 (ERJ) and and
NIH GM050781 (ASB). Dr. Joseph Ziller is acknowledged for X-
ray crystallographic data. Professor Heyduk and Kyle
Rosenkoetter at UC Irvine are acknowledged for assistance with
Evans’ Method Analysis of complex 5.
Keywords: nickel catalysis • precatalyst • absorption
spectroscopy • cross-coupling mechanism
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in the absence of cod, the active catalyst is arene-ligated
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Entry for the Table of Contents
COMMUNICATION
Evaluation of nickel precatalysts in
different oxidation states provided
evidence for active Ni(0) species in a
stereospecific alkyl-alkyl cross-
coupling reaction.
D. D. Dawson, V. F. Oswald, A. S.
Borovik,* E. R. Jarvo*
Page No. – Page No.
Identification of the Active Catalyst
for Nickel-Catalyzed Stereospecific
Kumada Coupling Reactions of
Ethers
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