Synthesis and Reactivity of Organometallic Intermediates Relevant to Cobalt-Catalyzed Hydroformylation

Article abstract of DOI:10.1002/anie.202001677

Intermediates relevant to cobalt-catalyzed alkene hydroformylation have been isolated and evaluated in fundamental organometallic transformations relevant to aldehyde formation. The 18-electron (R,R)-(^iPrDuPhos)Co(CO)₂H has been stru

Full text of DOI:10.1002/anie.202001677

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Accepted Article
Title: Synthesis and Reactivity of Organometallic Intermediates
Relevant to Cobalt-Catalyzed Hydroformylation
Authors: Paul James Chirik, Connor MacNeil, Hongyu Zhong, Tyler
Pabst, and Lauren Mendelsohn
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10.1002/anie.202001677
Angewandte Chemie International Edition
COMMUNICATION
Synthesis and Reactivity of Organometallic Intermediates
Relevant to Cobalt-Catalyzed Hydroformylation
Connor S. MacNeil, Lauren N. Mendelsohn, Hongyu Zhong, Tyler P. Pabst, and Paul J. Chirik*[a]
Abstract: Intermediates relevant to cobalt-catalyzed alkene
hydroformylation have been isolated and evaluated in fundamental
organometallic transformations relevant to aldehyde formation.
Among these, the 18-electron cobalt carbonyl hydride with a chiral
bis(phosphine), (R,R)-(iPrDuPhos)Co(CO)2H has been structurally
characterized. Despite being an analog of rhodium precatalysts
synthetically useful enantioselectivities as well as detailed
understanding of catalyst speciation and mechanism of action.9
In these and other enantioselective asymmetric hydroformylation
reactions,10 bis(phosphine) rhodium carbonyl hydrides, (P-
P)Rh(CO)2H have been invoked and identified as catalyst
resting states although solid-state structures are rare.11 Given
the remarkable activity, enantioselectivity and functional group
tolerance of bis(phosphine) cobalt(0)12 and (I)13 complexes in
asymmetric hydrogenation, the synthesis of the first row metal
analogs of the well-studied rhodium complexes was targeted
(Scheme 1b). The dehydrogenation of methanol by
[(iPrDuPhos)Co(0)] complexes to yield catalytically active
carbonyl derivatives provided additional motivation.12c Here we
describe the synthesis of (R,R)-(iPrDuPhos)Co(CO)2H, explore its
catalytic activity and demonstrate fundamental transformations
relevant to cobalt-catalyzed alkene hydrogenation and
hydroformylation.
widely
used
in
asymmetric
hydroformylation,
(R,R)-
(
iPrDuPhos)Co(CO)2H promoted exclusive hydrogenation of styrene
in the presence of 750 psi of H2/CO gas (1:1) at 100 ºC. Deuterium
labeling studies established reversible 2,1 insertion of styrene into
the Co–D bond of (R,R)-(iPrDuPhos)Co(CO)2D. Whereas rapid β-H
elimination from cobalt alkyls occurred under an atmosphere of N2,
alkylation of the cobalt chloride (R,R)-(iPrDuPhos)Co(CO)2Cl in the
presence of CO enabled the interception of a cobalt acyl complex,
(R,R)-(iPrDuPhos)Co(CO)2C(O)CH2CH2Ph. Hydrogenolysis of the
cobalt acyl under 4 atm H2 produced the corresponding aldehyde
and cobalt hydride, demonstrating the feasibility of elementary steps
in hydroformylation. Both the hydride and chloride derivatives (R,R)-
(
iPrDuPhos)Co(CO)2X (X = H–, Cl–) underwent exchange with free
13CO. Under reduced pressure, the chloride derivative underwent
CO dissociation to form the tetrahedral, S = 1 complex, (R,R)-
(
iPrDuPhos)Co(CO)Cl.
Hydroformylation or the “oxo process” involves the addition of
H2 and CO to alkenes and is a widely applied industrial method
for the conversion of commodity hydrocarbons to more value
added detergents, plasticizers, surfactants and other consumer
goods.1 One of the earliest examples of a metal-catalyzed
reaction practiced in chemical industry, alkene hydroformylation
has evolved into the largest scale application of homogeneous
catalysis. Roelen’s seminal discoveries relied on precatalysts
such as Co2(CO)8 and HCo(CO)4 (Scheme 1a),3 and while
cobalt catalysis remains responsible for >1 million tons of C10-
C16 aldehydes annually,4 phosphine-supported rhodium catalysts
are often preferred due to higher linear selectivity and milder
operating
conditions.5
Recently,
bis(phosphine)cobalt(II)
precatalysts that exhibit remarkable activity and selectivity have
been reported but organometallic intermediates have thus far
remained elusive.6
More recent emphasis on rhodium catalysis has focused on
enantioselective hydroformylation reactions that benefit from a
shift to branch selectivity with terminal olefins, particularly
derivatives.7
Landis’
advances
with
Scheme 1. (a) Precatalyst activation in cobalt-catalyzed hydroformylation. (b)
Rhodium precatalyst in enantioselective hydroformylation by Landis and co-
workers. (c) Synthesis of bis(phosphine) cobalt(I) analogs (this work).
styrene
bis(diazaphosholane) ligands8 have provided rhodium catalysts
with enantiopure ligands (Scheme 1b) that exhibit high activity,
Despite
long-standing
interest
in
cobalt-catalyzed
hydroformylation, there is a paucity of isolated (P-P)Co(CO)2H
derivatives14
=
and
1,1ʹ-
[a]
C. S. MacNeil, L. N. Mendelsohn, H. Zhong, T. P. Pabst, Prof. P. J.
Chirik
Department of Chemistry, Princeton University
Frick Laboratory 292, Princeton, 08544 (USA)
E-mail: pchirik@princeton.edu
compounds
with
(dppf)Co(CO)2H
(dppf
(BINAP)Co(CO)2H15
bis(diphenylphosphino)ferrocene;
BINAP
being
=
2,2ʹ-
notable
bis(diphenylphosphino)-1,1ʹ-binaphthyl)
examples.16 The bis(phospholane), (R,R)-(iPrDuPhos) was
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202001677
Angewandte Chemie International Edition
COMMUNICATION
selected for these studies given its performance in asymmetric
hydrogenation and ability to support unusual cobalt(I)
intermediates.12a,c Several synthetic routes to (R,R)-
material was observed under catalytic conditions. The lack of
aldehyde raised question as to the feasibility of migratory
insertion processes in these complexes. Studies by Clarke and
(
iPrDuPhos)Co(CO)2H, a cobalt analog of rhodium asymmetric
co-workers
on
(bisphosphine)rhodium
hydroformylation
hydroformylation precatalysts, were developed. Exposure of a
THF solution containing (R,R)-(iPrDuPhos) and Co2(CO)8 to 4
atm of H2 gas resulted in isolation of (R,R)-(iPrDuPhos)Co(CO)2H
as a pale yellow crystalline solid in quantitative yield (Scheme
1c). The corresponding cobalt deuteride was obtained in an
identical manner using D2 gas. Alternative routes to (R,R)-
established that the nature of the alkyl ligand (branched versus
linear) affects the thermodynamics of CO migratory insertion by
as much as 1.8 kcal mol–1.11 The influence of a bidentate
phosphine, however, is unknown.18
Because the catalytic hydrogenation reaction provided no
insight for the preference for 1,2- or 2,1 insertion of styrene,
stoichiometric studies were undertaken. Addition of five
(
iPrDuPhos)Co(CO)2H included addition of excess HBPin to
(R,R)-(iPrDuPhos)Co(O2CtBu)2 or addition of NaBEt3H to the
chloride derivative, (R,R)-(iPrDuPhos)Co(CO)2Cl (vide infra).
The solid-state structure of (R,R)-(iPrDuPhos)Co(CO)2H
(Scheme 2, top) establishes an idealized trigonal bipyramidal
cobalt center with one phospholane and the two carbonyl
ligands defining the equatorial plane. The benzene-d6 1H NMR
spectrum exhibits a broad peak centered at –10.9 ppm assigned
as the cobalt hydride that has shoulders but does not resolve
upon cooling to -82 ºC in THF-d8. At 23 ºC in THF-d8, a single
31P NMR resonance was observed at 96.0 ppm that upon
cooling to -82 ºC resolved into two distinct resonances (91.5 and
100.9 ppm), consistent with a Berry pseudorotation faster than
the NMR time scale at ambient temperature.
equivalents of styrene to a benzene-d6 solution of (R,R)-
1
(
iPrDuPhos)Co(CO)2H produced no change as judged by H and
31P NMR spectroscopies. Repeating the experiment with (R,R)-
iPrDuPhos)Co(CO)2D in benzene and analysis by 2H NMR
(
spectroscopy established deuterium incorporation in the β-sites
of the alkene after three hours at 60 ºC (Scheme 3). Notably, no
deuterium incorporation was observed at the benzylic
(α) position supporting reversible 2,1-insertion to generate the
branched cobalt alkyl followed by rapid β-hydrogen elimination.
Trace amounts of ethylbenzene-dn (n = 1-3) were also observed.
Repeating the procedure in the presence of 1 atm of CO
resulted in no incorporation of the isotopic label into styrene after
5 hours at 60 ºC. Taken together, these results support a
pathway whereby reversible alkene coordination and insertion is
suppressed by added CO. Preference for 2,1 insertion is
consistent with the kinetic model established by Landis for
bis(diazaphospholane) (BDP) rhodium and styrene.19 In contrast
to rhodium where linear and branched acyls are observed,18 the
rate of β-H elimination for cobalt is faster than CO insertion
prohibiting observation of acyl intermediates in the absence of
added CO.
Scheme 3. Demonstration of reversible 2,1-insertion of styrene with (R,R)-
(
Scheme 2. Top (left): Molecular structure of (R,R)-(iPrDuPhos)Co(CO)2H at
30% probability ellipsoids with H atoms (except Co–H) omitted for clarity.
Co1–P1 = 2.151(1) Å; Co1–P2 = 2.175(1) Å; Co1–H1 = 1.44(6) Å. Top (right):
Variable-temperature 31P NMR spectra of (R,R)-(iPrDuPhos)Co(CO)2H (THF-
iPrDuPhos)Co(CO)2D and associated NMR spectra of styrene isotopologues.
To gain insight into the behavior of the carbonyl ligands and
understand migratory aptitude, the substitutional lability of the
CO ligands in (R,R)-(iPrDuPhos)Co(CO)2H was established.
Addition of excess 13CO gas (1.2-6 equiv) to a benzene-d6
solution of the compound resulted in exchange of the CO
ligands with the free gas as judged by 13C NMR spectroscopy
(Figure S7). These findings are consistent with the reported
observations that CO exchange between HCo(CO)4 and 13CO is
too fast to monitor at -30 ºC.18 Analogous rhodium complexes,
(P-P)Rh(CO)2H also undergo rapid carbonyl-CO exchange19b
with a half-life of 4 seconds determined for the Xantphos
d8).
(
Bottom:
Catalytic
hydrogenation
of
styrene
using
(R,R)-
iPrDuPhos)Co(CO)2H.
The catalytic performance of (R,R)-(iPrDuPhos)Co(CO)2H
was evaluated using styrene as a representative substrate. With
5 mol% of the cobalt precatalyst and 50 bar of a 1:1 H2/CO
mixture at 100 ºC for 16 hours, complete conversion to
ethylbenzene was observed without any evidence for
hydroformylation products (Scheme 2, bottom). With 1-octene,
1-octadecene or α-methylstyrene, no conversion of starting
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10.1002/anie.202001677
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COMMUNICATION
= 2.262(2) Å; Co1–P2 = 2.270(2) Å; Co1–Cl = 2.197(2) Å; Co1–C24 = 1.81(1)
Å.
example at 40 ºC (Xantphos = 4,5-bis(diphenylphosphino)-9,9-
dimethylxanthene).21
Despite the lability of the CO ligands in (R,R)-
The presence of π-donors increases the substitutional lability
of cis ligands in octahedral complexes such as (CO)5MnBr.22,23
Although such a “cis effect” has not been demonstrated in five-
coordinate compounds, (R,R)-(iPrDuPhos)Co(CO)2Cl was
(
iPrDuPhos)Co(CO)2Cl, the stability of the complex under an N2
atmosphere allowed exploration of cobalt intermediates relevant
to hydrogenation and hydroformylation. Alkylation of (R,R)-
(
iPrDuPhos)Co(CO)2Cl with PhCH2CH2MgCl under an N2
targeted. Addition of 1 atm of CO gas to a benzene-d6 solution
11a
of [(R,R)-(iPrDuPhos)Co(µ-Cl)]2
yielded a metastable, bright
identified as (R,R)-
iPrDuPhos)Co(CO)2Cl as determined by multinuclear NMR and
atmosphere resulted in loss of styrene and formation of (R,R)-
iPrDuPhos)Co(CO)2H after 10 minutes at 23 °C, consistent with
(
red,
diamagnetic
product
rapid β-H elimination (Scheme 5). This result, in combination
with the deuterium labelling study with styrene, demonstrates
that both linear and branched cobalt phenethyl complexes are
unstable with respect to β-H elimination. Repeating the alkylation
procedure in the presence of 1 atm of CO produced a product
that exhibited an intense band at 1636 cm-1, signalling formation
of the corresponding cobalt acyl complex (Scheme 5).24 The
identity of the product was confirmed by X-ray diffraction and the
31P NMR spectrum in benzene-d6 at 23 ºC exhibited two peaks
of equal intensity at 73.6 and 77.6 ppm, consistent with a slow
pseudorotation process. Isolation of the cobalt acyl in the
presence of excess CO demonstrates that migratory insertion is
operative from the putative alkyl but a trapping ligand is required
to suppress competing β-H elimination.
(
IR spectroscopies. The CO stretches in the IR spectrum are
shifted to higher energies relative to the hydride complex (Figure
S14-15), reflecting the donor strength of the X-type ligand in the
axial site. A low-quality solid-state structure was determined that
established the connectivity of the complex as isostructural with
(R,R)-(iPrDuPhos)Co(CO)2H (Figure S29). Exposure of
a
benzene-d6 solution of the compound to 13CO at 23 ºC resulted
in rapid CO exchange as judged by the enhancement of the Co–
CO 13C NMR resonance at δ 204 ppm.
While the relative lability of the carbonyl ligands in (R,R)-
(
iPrDuPhos)Co(CO)2X (X = H–, Cl–) did not vary significantly
under the present conditions, exposure of a bright red benzene
solution of (R,R)-(iPrDuPhos)Co(CO)2Cl to vacuum for 10
minutes produced (R,R)-(iPrDuPhos)Co(CO)Cl (Scheme 4) as a
deep-green powder. Treatment of [(R,R)-(iPrDuPhos)Co(µ-Cl)]2
with excess CO followed by comproportionation with 0.5 equiv of
[(R,R)-(iPrDuPhos)Co(µ-Cl)]2 was also an effective route to this
compound (see SI for details). The S = 1 (µeff = 2.7(1) µB, 23 ºC,
Evans method) product exhibited a sharp carbonyl band at 1956
cm-1 that shifted to 1913 cm–1 when the same product was
prepared using 13CO. The solid-state structure of (R,R)-
The competency of the cobalt acyl for formation of aldehyde
and key fundamental step in hydroformylation was demonstrated
by addition of 4 atm of H2 gas to a benzene-d6 solution of (R,R)-
(
iPrDuPhos)Co(CO)2C(O)CH2CH2Ph
(Scheme
5).
Both
hydrocinnamaldehyde and (R,R)-(iPrDuPhos)Co(CO)2H were
observed after 48 hours at 50 ºC.
(
iPrDuPhos)Co(CO)Cl was determined by X-ray diffraction
(Scheme 4) and established tetrahedral cobalt center
a
consistent with the S = 1 ground state. Attempts to prepare the
analogous four-coordinate hydride by treatment of (R,R)-
(
iPrDuPhos)Co(CO)Cl with NaBEt3H produced
a
dark-green
crystalline material that was identified as the previously reported
dicobalt bridging carbonyl complex [(R,R)-
iPrDuPhos)Co(µ−CO)]2.12c These results demonstrate a “cis-
(
effect” whereby the chloride ligand increases the lability of the
CO ligands compared to hydride such that dissociation resulted
in isolation of a four-coordinate product.
Scheme
5.
Synthesis
and
hydrogenolysis
Molecular structure
of
of
(R,R)-
(R,R)-
(
(
iPrDuPhos)Co(CO)2C(O)CH2CH2Ph.
iPrDuPhos)Co(CO)2C(O)CH2CH2Ph at 30% probability ellipsoids with H atoms
omitted for clarity. Co1–C1 = 1.994(2) Å; C1–O1 = 1.215(3) Å; Co1–P1 =
1.994(2) Å; Co1–P2 = 2.175(1) Å.
The demonstration of the fundamental organometallic
transformations relevant to hydroformylation highlights several
unusual features of the (R,R)-(iPrDuPhos)Co(CO)2X (X = H, Cl,
alkyl, acyl) family of complexes and provides insight into the lack
of catalytic hydroformylation. While observed alkyl derivatives
remain elusive, alkene insertion is fast and reversible and
liberation of alkane in the presence of H2 must therefore be
faster than formation of the acyl. Remarkably both of these
bimolecular processes are fast from coordinatively saturated,
18-electron (R,R)-(iPrDuPhos)Co(CO)2R (R
=
H, alkyl)
Scheme 4. Synthesis and molecular structure of (R,R)-(iPrDuPhos)Co(CO)Cl.
ORTEP at 30% probability ellipsoids with H atoms omitted for clarity. Co1–P1
suggesting that outer sphere, proton coupled electron transfer
reactions may be favoured over more traditional organometallic
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10.1002/anie.202001677
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pathways.25,26 Exploration of these possibilities and leveraging
them for productive catalysis, including oxidation to cobalt(II), is
currently ongoing.
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CCDC numbers 1972035, 1972036 and 1973713 contain the
supplementary
crystallographic
data
of
compounds
and
(R,R)-
(R,R)-
(
(
iPrDuPhos)Co(CO)2H,
(R,R)-(iPrDuPhos)Co(CO)2Cl,
iPrDuPhos)Co(CO)2C(O)CH2CH2Ph,
respectively. These data are
provided free of charge by The Cambridge Crystallographic Data Centre.
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Entry for the Table of Contents
COMMUNICATION
Connor S. MacNeil, Lauren N.
Mendelsohn, Hongyu Zhong, Tyler P.
Pabst, Paul J. Chirik*
Page No. – Page No.
Synthesis and Reactivity of
Organometallic Intermediates
Relevant to Cobalt-Catalyzed
Hydroformylation
The 18-electron cobalt hydride, (R,R)-(iPrDuPhos)Co(CO)2H has been synthesized. Despite being coordinatively saturated this
compound promotes reversible 2,1-insertion of styrene and catalytic hydrogenation raising questions as to the mechanism of action
of modified cobalt catalysts.
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