High-activity cobalt catalysts for alkene hydroboration with electronically responsive terpyridine and α-diimine ligands

Article abstract of DOI:10.1021/cs501639r

Cobalt alkyl complexes bearing readily available and redox-active 2,2′:6′,2″-terpyridine and α-diimine ligands have been synthesized, and their electronic structures have been elucidated. In each case, the supporting chelate is reduced to the monoanionic, radical form that is engaged in antiferromagnetic coupling with the cobalt(II) center. Both classes of cobalt alkyls proved to be effective for the isomerization-hydroboration of sterically hindered alkenes. An α-diimine-substituted cobalt allyl complex proved exceptionally active for the reduction of hindered tri-, tetra-, and geminally substituted alkenes, representing one of the most active homogeneous catalysts known for hydroboration. With limonene, formation of an η³-allyl complex with a C-H agostic interaction was identified and accounts for the sluggish reactivity observed with diene substrates. For the terpyridine derivative, unique Markovnikov selectivity with styrene was also observed with HBPin. (Figure Presented).

Full text of DOI:10.1021/cs501639r

Letter
pubs.acs.org/acscatalysis
High-Activity Cobalt Catalysts for Alkene Hydroboration with
Electronically Responsive Terpyridine and α‑Diimine Ligands
W. Neil Palmer,† Tianning Diao,†,‡ Iraklis Pappas,† and Paul J. Chirik*,†
†Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Cobalt alkyl complexes bearing readily available
and redox-active 2,2′:6′,2″-terpyridine and α-diimine ligands
have been synthesized, and their electronic structures have
been elucidated. In each case, the supporting chelate is reduced
to the monoanionic, radical form that is engaged in
antiferromagnetic coupling with the cobalt(II) center. Both
classes of cobalt alkyls proved to be effective for the
isomerization−hydroboration of sterically hindered alkenes.
An α-diimine-substituted cobalt allyl complex proved excep-
tionally active for the reduction of hindered tri-, tetra-, and
geminally substituted alkenes, representing one of the most active homogeneous catalysts known for hydroboration. With
limonene, formation of an η3-allyl complex with a C−H agostic interaction was identified and accounts for the sluggish reactivity
observed with diene substrates. For the terpyridine derivative, unique Markovnikov selectivity with styrene was also observed
with HBPin.
KEYWORDS: cobalt, hydroboration, catalysis, redox-active ligands, boronates
Scheme 1. Examples of Bis(imino)pyridine-Cobalt-Catalyzed
Alkene Isomerization−hydroboration
rganoboronates are a valuable class of reagents owing to
Otheir stability, ease of handling, and versatility in various
carbon−carbon and carbon−heteroatom bond-forming reac-
tions.1 Metal-catalyzed alkene hydroboration has proven to be
an effective route to alkyl boronates with precious metal
catalysts using rhodium and iridium being the most common.2
Interest has recently shifted to catalysts that employ more
earth-abundant transition metals such as iron and cobalt.3,4 In
addition to their potential economic and environmental
benefits, catalysts based on first row transition metals, by virtue
of the smaller atomic radii and unique electronic structures,
offer the ability to promote new chemistry or expand substrate
scope not encountered with traditional precious metal
catalysts.5
Bis(imino)pyridine cobalt alkyl complexes have proven to be
highly active and selective catalysts for the hydroboration of
alkenes.6 Modification of C1 symmetric variants of these ligands
used in cobalt-catalyzed alkene hydrogenation7 has recently
been extended to the enantioselective hydroboration of certain
1,1-disubstituted aryl alkenes.8 Introduction of a pyrrolidinyl
substituent into the 4-position of the chelate, (4-pyrr-MesPDI)-
CoCH3 (1), resulted in improved performance and enabled
tandem isomerization−hydroboration of hindered alkenes, a
method pioneered by Crudden and co-workers in rhodium
catalysis.9 With trans-4-octene, methyl-cyclohexene and 2,3-
dimethyl-2-butene, cobalt-catalyzed hydroboration exclusively
furnished the terminal-substituted boronate ester arising from
remote hydrofunctionalization of the methyl positions (Scheme
1).6 Recently, the tandem isomerization−hydroboration was
replicated for 4-octene with N-phosphinoamidinate-supported
cobalt catalysts, but activity with more substituted internal
alkenes was not reported.10 Given the utility of this
transformation for the introduction of versatile boron
functionality at remote, saturated C−H bonds, more active
catalysts with broader substrate scope are of interest. For the
most hindered substrates, catalytic reactions with 1 required
neat conditions and elevated temperatures (Scheme 1). Here
we describe the synthesis and electronic structure determi-
nation of new cobalt alkyl complexes with readily available
Received: October 23, 2014
Revised: December 15, 2014
© XXXX American Chemical Society
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Letter
2,2′:6′,2″-terpyridine (terpy) and α-diimine ligands and their
exceptional activity for alkene isomerization−hydroboration.
The two different precatalyst classes also offer complementary
performance among different substrate types.
Scheme 3. Synthesis and Molecular Structure (30%
Probability Ellipsoids, Hydrogens Omitted) of 3
Given the success of redox-active bis(imino)pyridine ligands
in promoting base metal-catalyzed hydrofunctionalization
reactions,5 terpy was explored due to its commercial availability,
open steric environment, and established redox activity.11 Our
laboratory previously demonstrated the utility and electronic
participation of a terpy chelate in the iron-catalyzed hydro-
silylation of vinyl cyclohexane oxide.12 The synthesis of
(terpy)CoCH2SiMe3 (2) was accomplished by straightforward
addition of the free ligand to a diethyl ether solution of
(py)2Co(CH2SiMe3)2.13 Ejection of one of the alkyl ligands
accompanied displacement of the pyridine ligands and 2 was
isolated as a purple diamagnetic solid in 80% yield (Scheme 2).
Scheme 2. Synthesis, Solid-State Structure (30% Probability
Ellipsoids, Hydrogens Omitted) and DFT-Computed14
Mulliken Spin Density Plot of 2
yield (Scheme 3). The solid-state structure of 3 was determined
by X-ray diffraction and established a near tetrahedral
geometry.
The C(2)−C(3) distance of 1.417(2) Å in combination with
other α-diimine bond distortions are consistent with a ligand-
centered radical and hence an overall Co(II) oxidation state.15
The observed S = 1 magnetic ground state arises from
antiferromagnetic coupling of the α-diimine radical with the
high spin, S = 3/2 Co(II) center. Computational studies
corroborate this view of the electronic structure where a broken
symmetry (3,1) solution corresponding to a high-spin Co(II)
complex with an α-diimine radical anion was preferred. The
substitutional lability of the pyridine ligand was established by
addition of excess pyridine to a benzene-d6 solution of the
independently synthesized pyridine-d5 isotopologue (3-d5).
Over the course of 90 min at 23 °C, the paramagnetically
shifted resonances began to appear in the 1H NMR spectrum at
9.27, 20.72, and 79.61 ppm corresponding to coordinated
pyridine, signaling chemical exchange between the isotopo-
logues.
The X-ray structure established an idealized planar geometry,
expected for an S = 0 ground state. The N(1)−C(5), C(5)−
C(6) and N(2)−C(6) distances of 1.373(7), 1.447(8), and
1.366(7) Å, respectively, are consistent with one electron
reduction of the terpyridine chelate.11 DFT calculations14
converged to a broken symmetry (1,1) solution corresponding
to a low-spin cobalt(II) center engaged in antiferromagnetic
coupling to a terpyridine radical anion.
Cobalt alkyl complexes of aryl-substituted α-diimine (DI)
ligands were also targeted due to the ease of synthesis and
modularity of the chelate and its established redox-activity with
first row transition metal complexes.15 Iron dialkyl, diene, and
alkyne complexes of this ligand class have previously been
reported for alkene hydrogenation, although performance was
limited by deactivation due to formation of bis(chelate) or iron
arene compounds.16,17 Despite the ubiquity of this ligand class,
especially in the context of alkene polymerization,18 α-diimine
cobalt complexes have only been preliminarily investigated, and
reduced compounds include monohalide and η6-arene deriva-
tives.19,20
Pyridine displacement coupled with alkyl ejection was again
utilized as a synthetic entry point to monoalkyl cobalt
chemistry. Addition of 1 equivalent of iPrDI (iPrDI =
[2,6-iPr2−C6H3NC(CH3)]2) to a diethyl ether solution of
(py)2CoCl(CH2SiMe3) furnished (iPrDI)Co(py)Cl as a para-
magnetic green solid in 59% yield (Scheme 3). Subsequent
alkylation with 1 equivalent of LiCH2SiMe3 in diethyl ether
followed by recrystallization from pentane furnished red
crystals identified as (iPrDI)Co(py)CH2SiMe3 (3) in 70%
A pyridine-free α-diimine cobalt alkyl complex was also
targeted to avoid complications from pyridine inhibition during
catalysis (vide infra). The cobalt allyl complex, (iPrDI)Co(η3-
C3H5) (4) was isolated in 73% yield as diamagnetic green
18
crystals following allylation of [(iPrDI)CoCl]2 with CH2
CHCH2MgCl in diethyl ether (Scheme 4). The toluene-d8 1H
NMR spectrum of 4 recorded at 22 °C exhibits the number of
resonances consistent with a C2v symmetric α-diimine environ-
ment with the imine methyl groups located at −5.60 ppm,
diagnostic of participation of the chelate in the electronic
structure of the compound.21−23 The resonances for the allyl
ligand were located at −0.21 (anti), 10.28 (syn) and 11.19
(meso) ppm, consistent with η3 coordination and either rapid
rotation or facile η3, η1 interconversion. An EXSY experiment at
22 °C revealed no crosspeaks between the syn and anti proton
resonances, suggesting no chemical exchange is occurring, as
would be expected for η3, η1 interconversion. The distinguish-
ability and observation of coupling in these resonances further
indicates lack of exchange and support a rapid η3-rotation
mechanism.24 The solid-state structure was determined by
single crystal X-ray diffraction and established η3 coordination
of the allyl ligand in the solid state (Scheme 4). The methine
hydrogen is directed above the metal−chelate plane between
623
DOI: 10.1021/cs501639r
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ACS Catalysis
Letter
promoted by the previously reported bis(imino)pyridine cobalt
methyl complex, 1.6 Each of the cobalt alkyl complexes new to
this study proved to be active for the isomerization−
hydroboration of trans-4-octene (entry A). Both the terpyridine
derivative, 2, and the base-free allyl complex, 4, proved to be
more active than 1 reaching complete conversion in 8 and 1.5
h, respectively. Using 2 as a precatalyst, a mixture of
hydroborated products was obtained and composed predom-
inantly of the 1-substituted (55%) and the 2-substituted (38%)
octylboronate esters. With 4, the n-octyl boronate product was
obtained exclusively in high isolated yield following removal of
the cobalt catalyst by exposure to air and subsequent filtration
through silica.
Scheme 4. Synthesis, Molecular Structure (30% Probability
Ellipsoids, Hydrogens Omitted) and DFT-Computed
Mulliken Spin Density Plot of 4
The pyridine stabilized cobalt alkyl complex, 3, was the
slowest in the series for the catalytic isomerization−hydro-
boration of trans-4-octene, requiring 32 h to reach complete
conversion to the terminal alkylboronate ester product.
Suspecting pyridine dissociation was required to generate the
active catalyst, the effect of added pyridine on catalytic
performance was determined using 4. A more readily reduced
substrate, 1-octene was used for these studies. In the absence of
added pyridine, >98% conversion to n-octyl boronate ester was
observed in less than 5 min at 22 °C with 1 mol % of catalyst.
Addition of 5 mol % pyridine significantly slowed the reaction,
as 6 h was required for complete conversion. Saturation
behavior was achieved at 25 mol % pyridine as 18 h was
required for complete conversion. The same amount of time
was required with 50 mol % of added pyridine.
the large 2,6-diaryl imine substituents. The C(2)−C(2A) and
N(1)−C(2) distances of 1.427(4) and 1.329(3) Å are
consistent with one-electron reduction of the diimine chelate15
and establish a Co(II) oxidation state.
With new cobalt alkyl complexes bearing redox-active ligands
in hand, the performance of 2−4 in catalytic alkene
hydroboration was evaluated. Standard conditions employed
1% of the desired cobalt alkyl precatalyst with 1.0 M solution of
the substrate in methyl tert-butyl ether (MTBE) and 1.05 equiv
of HBPin (Pin = pinacolate) at 23 °C (Table 1). Included in
Table 1 are the selected examples of analogous reactions
Solvent effects were also explored with cyclohexyl methyl
ethylene (entry J) using 3 as the precatalyst. This substrate was
selected for these studies because it is relatively challenging and
could delineate relative rates. In toluene, only 47% conversion
Table 1. Room-Temperature Catalytic Alkene Hydroboration with Pinacolborane Using Precatalysts 1−4
a
1
Percent conversions based on GC-FID. Reaction times, isolated yields, and product ratios shown in parentheses. Ratios determined by H NMR,
b
quantitative 13C NMR, or GC-FID. Isolated as a mixtureof products: 55% 1-octylboronate ester, 38% 2-octylboronate ester, and 7% unidentified
hydroborated products. 14% of product mixture composed of a third unidentified hydroboration product. 7% ethylcyclohexane observed in
product mixture prior to isolation of hydroboration products. 10% ethylcyclohexane observed in product mixture prior to isolation of hydroboration
products. Isomerized to α-pinene. Only trace hydroborated product detected. 5% ethylbenzene observed in product mixture prior to isolation of
c
d
e
f
g
h
i
hydroboration products. Exclusively reduced to ethylbenzene. No hydroboration product detected. 6% of isolated mixture composed of two
j
additional unidentified products. Major diastereomer determined by oxidation to the corresponding alcohol and analysis by quantitative 13C NMR.
624
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Letter
was obtained in 1 h, reaching a maximum of 58% after 12 h. It
is likely that the η6-arene complex, (iPrDI)Co(η6-C7H8)20 is
formed under these conditions. Control experiments with
isolated compound demonstrated that it is inactive for
hydroboration. This deactivating effect was not seen in
nonaromatic solvents such as pentane, hexane, diethyl ether
and MTBE. As a consequence, all catalytic reactions reported in
this study were conducted in MTBE.
Scheme 5. Synthesis, Molecular Structure (30% Probability
Ellipsoids, Hydrogens Omitted Except for C−H Agostic
Interaction), and Diminished Catalytic Activity of 5
The cobalt-catalyzed isomerization−hydroboration of more
hindered tri- and tetrasubstituted alkenes was also explored as
few catalysts, either precious or base metal, are known to
reduce these substrates.6 With 2-methyl-2-pentene (entry B),
mixtures of the chain walking products, B-i and B-ii were
obtained. In all cases, except for 2, B-ii was favored. The
cyclohexyl-substituted trisubstituted alkene (entry C) also
underwent isomerization−hydroboration with 1−4. The α-
diimine-substituted precatalysts, 3 and 4, gave exclusive
selectivity, whereas with 1 and 2, small amounts of the
secondary alkyl boronate were observed. With a more
challenging tetrasubstituted alkene such as entry D, cobalt
precatalysts with tridentate ligands either required long reaction
times (1) or were inactive (2). By contrast, complete
conversion was observed with both 3 and 4 with the base-
free example exhibiting extremely high activity.
In the case of hindered, geminally substituted alkenes
(entries I−L), the α-diimine precatalysts again proved superior
and the base-free example, 4, was exceptionally active.
Complete conversion with the most hindered substrates was
achieved over the course of minutes at ambient temperature,
representing one of the most active alkene hydroboration
catalysts known. Hydroboration of 2,3,3-trimethyl-1-butene
(entry I) was conducted with 0.1 mol % of 4 over the course of
3 h at 23 °C; lowering the loading to 0.05% resulted in
irreproducible results likely due to the sensitivity of the catalyst.
The terpyridine precatalyst, 2 offered unique anti-Markovnikov
selectivity for the hydroboration of styrene with HBPin, a rare
feature among base metal catalysts.4d,e,i,6
In summary, new cobalt alkyl complexes bearing readily
available and redox-active 2,2′:6′,2″-terpyridine and aryl-
substituted α-diimine ligands have been synthesized, and their
catalytic performance has been proven to be complementary.
The lower electron count α-diimine-substituted cobalt catalyst
was exceptionally active for the reduction of hindered tri-,
tetra-, and geminally substituted alkenes. In substrates
containing dienes or arene groups, the terpyridine derivative
was more effective, likely a result of being more resistant to
competitive deactivation pathways or inhibition due to a more
open coordination environment. These results establish the
potential reactivity advantages available in base metal catalysts
and serve to highlight the importance of having a versatile
compound library to overcome the limitations with individual
members.
Precatalyst 4 proved sluggish with diene substrates such as
limonene (entry E) and β-pinene (entry F), whereas the terpy
derivative, 2, proved more effective reaching complete
conversion in 9 and 18 h, respectively. To elucidate the origin
of the catalytic inhibition, a diethyl ether solution of 4 was
treated with HBPin and a slight excess of limonene and yielded
orange crystals in 41% yield following recrystallization from
pentane at −35 °C (Scheme 5). A combination of NMR
spectroscopy and X-ray diffraction established the identity of
the product as a cobalt allyl complex, 5, where the gem−alkene
has been reduced to an isopropyl group. The allyl formed from
the six-membered ring likely arises from isomerization of the
gem−alkene to an endocyclic position followed by reduction by
a putative Co−H. Both the NMR and X-ray data support an
agostic interaction with the C−H bond in the 3-position of the
ring with Co(1)−H(33A) bond length of 1.8861(44) Å, and a
Co(1)-H(33A)-C(33) bond angle of 102.7(10)°. A doublet for
ASSOCIATED CONTENT
* Supporting Information
The following files are available free of charge on the ACS
■
S
Publications website at DOI: 10.1021/cs501639r.
Complete experimental procedures, characterization data
for all new compounds, and computational results (PDF)
Crystallographic data: CCDC 1025114−1025117,
1038682 (CIF)
For crystallographic data in CIF or other electronic
format, see DOI: 10.1039/b000000x/.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
■
1
Present Address
this proton was located at 5.00 ppm in the benzene-d6 H NMR
‡Department of Chemistry, New York University, New York,
New York 10003, United States
spectrum. Importantly, attempted hydroboration of 2,3,3-
trimethyl-1-butene in the presence of limonene with 1 mol %
of 4 resulted in <5% conversion of the substrate after 18 h.
Similar results were obtained using camphene with isolated 5 as
the precatalyst. The diminished catalytic activity when 5 was
used instead of 4 under identical conditions suggest slower
activation of the precatalyst and further establish the inhibitory
effect of the diene on cobalt catalyzed hydroboration.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Princeton University for financial support and the US
National Science Foundation (NSF) for a Grant Opportunities
■
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