Direct Observation of Transmetalation from a Neutral Boronate Ester to a Pyridine(diimine) Iron Alkoxide

Article abstract of DOI:10.1021/acs.organomet.9b00733

Transmetalation of the neutral boronate esters, (2-benzofuranyl)BPin and (2-benzofuranyl)BNeo (Pin = pinacolato, Neo = neopentylglycolato), to a representative pyridine(diimine) iron alkoxide complex, (^iPrPDI)FeOEt (^iPrPDI = 2,6-(2,6-ⁱPr₂-C₆H₃Na? CMe)₂C₅H₃N; R = Me, Et, SiMe₃), to yield the corresponding iron benzofuranyl derivative was studied. Synthesis of the requisite iron alkoxide complexes was accomplished either by salt metathesis between (^iPrPDI)FeCl and NaOR (R = Me, Et, SiMe₃) or by protonation of the iron alkyl, (^iPrPDI)FeCH₂SiMe₃, by the free alcohol R′OH (R′ = Me, Et). A combination of magnetic measurements, X-ray diffraction, NMR, and M?ssbauer spectroscopies and DFT calculations identified each (^iPrPDI)FeOR compound as an essentially planar, high-spin, S = 3/2 compound where the iron is engaged in antiferromagnetic coupling with a radical anion on the chelate (S_Total = 3/2; S_Fe = 2, S_PDI =-1/2). The resulting iron benzofuranyl product, (^iPrPDI)Fe(2-benzofuranyl), was characterized by X-ray diffraction and in combination with magnetic measurements, spectroscopic and computational data, was identified as an overall S = 1/2 compound, demonstrating that a net spin-state change accompanies transmetalation (S_Fe = 1, S_PDI =-1/2). These findings may be relevant to further development of iron-catalyzed Suzuki-Miyaura cross-coupling with neutral boronate esters and alkoxide bases.

Full text of DOI:10.1021/acs.organomet.9b00733

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Direct Observation of Transmetalation from a Neutral Boronate
Ester to a Pyridine(diimine) Iron Alkoxide
Paul O. Peterson, Stephan M. Rummelt,‡ Bradley M. Wile,† S. Chantal E. Stieber,#
Hongyu Zhong, and Paul J. Chirik*
Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States
S
* Supporting Information
ABSTRACT: Transmetalation of the neutral boronate esters,
(2-benzofuranyl)BPin and (2-benzofuranyl)BNeo (Pin =
pinacolato, Neo = neopentylglycolato), to a representative
pyridine(diimine) iron alkoxide complex, (iPrPDI)FeOEt
(iPrPDI = 2,6-(2,6-iPr2−C6H3NCMe)2C5H3N; R = Me,
Et, SiMe3), to yield the corresponding iron benzofuranyl
derivative was studied. Synthesis of the requisite iron alkoxide
complexes was accomplished either by salt metathesis
between (iPrPDI)FeCl and NaOR (R = Me, Et, SiMe3) or
by protonation of the iron alkyl, (iPrPDI)FeCH2SiMe3, by the free alcohol R′OH (R′ = Me, Et). A combination of magnetic
iPr
̈
measurements, X-ray diffraction, NMR, and Mossbauer spectroscopies and DFT calculations identified each ( PDI)FeOR
compound as an essentially planar, high-spin, S = 3/2 compound where the iron is engaged in antiferromagnetic coupling with a
radical anion on the chelate (STotal = 3/2; SFe = 2, SPDI = −1/2). The resulting iron benzofuranyl product, (iPrPDI)Fe(2-
benzofuranyl), was characterized by X-ray diffraction and in combination with magnetic measurements, spectroscopic and
computational data, was identified as an overall S = 1/2 compound, demonstrating that a net spin-state change accompanies
transmetalation (SFe = 1, SPDI = −1/2). These findings may be relevant to further development of iron-catalyzed Suzuki−
Miyaura cross-coupling with neutral boronate esters and alkoxide bases.
required for transmetalation. Byers and co-workers have
recently reported that amide bases enabled C(sp2)−C(sp3)
cross coupling promoted by a cyano bis(oxazoline)-supported
iron(II) precatalyst (Scheme 1).5 A key finding in this study
was that no coupling was observed when the amide−borate
he success of Suzuki−Miyaura cross-coupling (SMC)
Twith palladium catalysts has motivated the search for
similar reactivity with more earth-abundant first row transition
metals.1 Iron is particularly attractive given its high terrestrial
abundance and low toxicity. While iron-catalyzed Kumada-type
coupling has been known since the 1940s2 and has been
performed on an industrial scale,3 iron-catalyzed Suzuki−
Miyaura cross coupling is by comparison less developed. Alkyl
lithium-activated arylborate nucleophiles (activated with n-
BuLi or t-BuLi) are often necessary for transmetalation to
iron,4 but these anionic nucleophiles likely function more like
organomagnesium reagents applied in Kumada coupling,
limiting functional group tolerance and the ease of handling
and operation associated with palladium-catalyzed SMC.
Achieving transmetalation to iron and using comparatively
mild bases (e.g., hydroxide, carbonate, alkoxide) applied
routinely in palladium catalysis remains an unmet need and
attractive target toward the further development of iron-
catalyzed Suzuki−Miyaura cross couplings.
Scheme 1. Transmetallation of Boron Nucleophiles to Iron
Nakamura and co-workers first reported an iron-catalyzed
cross-coupling with a boron nucleophile, where lithium
arylborates were coupled with primary and secondary alkyl
halides to achieve C(sp2)−C(sp3) bond formation.4a Lithium
aryl borate species have also been shown by Bedford and co-
workers to be nucleophiles for directed C(sp2)−C(sp2)
coupling.4c In both cases, formation of the borate nucleophile
was essential for a successful catalytic reaction and is likely
Received: October 26, 2019
© XXXX American Chemical Society
A
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Scheme 2. Synthesis of (iPrPDI)FeOR Complexes by (a) Salt
Metathesis and (b) Solvolysis of Iron Alkyls
complex was preformed prior to the reaction, suggesting
transmetalation occurred from a neutral, rather than anionic,
boron nucleophile. Alkoxide bases were ineffective due to
aggregation of the resulting iron complexes. Such a pathway is
analogous to that proposed for palladium SMC, whereby
transmetalation from a neutral boron nucleophile to a hydroxy
palladium intermediate is kinetically preferred over the
alternative involving a hydroxide-coordinated borate.6
Examples of boron-based stoichiometric transmetalation to
generate an iron aryl complex are primarily limited to lithium
aryl borate nucleophiles.7 Neidig and co-workers demonstrated
that the reaction of (SciOPP)FeCl2 (SciOPP = spin-control-
intended o-phenylene bisphosphine) with lithium phenyl-
borate resulted in two successive transmetalation events to
form (SciOPP)Fe(Ph)2, which undergoes reductive elimina-
tion to form (SciOPP)Fe(η6-biphenyl).8 Other examples
include the reaction of triarylboranes with iron alkyl
complexes, forming the corresponding iron aryl complex.9 In
these examples, it is proposed that initial alkyl abstraction by
the borane forms an activated borate, which becomes the
nucleophile for transmetalation.
To explore the possibility of transmetalation from a neutral
boron reagent to iron, pyridine(diimine) iron complexes were
selected for study using an approach previously applied by our
group for cobalt.10 This class of iron compounds exhibits a rich
and diverse catalytic chemistry as well as the potential for
electronic metal−ligand cooperativity, giving rise to a high
density of states.11−13 More germane to iron-catalyzed cross
coupling, both iron aryl, (iPrPDI)Fe(Ar) (Ar = phenyl; iPrPDI =
2,6-(2,6-iPr2−C6H3NCMe)2C5H3N)),14 and alkoxide
(iPrPDI)Fe(OR) (R = Et, allyl, etc.) derivatives have been
synthesized with the latter class of compounds isolated
following C−O bond cleavage in esters and ethers.15 Notably,
the pyridine(diimine) iron aryl complexes possess different
magnetic ground states from the iron alkoxides,16 suggesting
that a spin-state change may occur during transmetalation,
similar to observations previously reported by our group in
cobalt-catalyzed SMC.
preparation of iron alkoxide complexes,18,19 pentane solutions
of (iPrPDI)Fe(CH2SiMe3) were independently treated with
methanol and ethanol, and, following filtration and removal of
volatiles, (iPrPDI)FeOCH3 and (iPrPDI)FeOEt were obtained
in 71% and 66% yields, respectively, as green solids (Scheme
2b).
1
These iron alkoxide compounds exhibit the number of H
NMR resonances consistent with overall C2v symmetry. As has
been previously reported for (iPrPDI)FeX (X = Cl, Br)
compounds,22 1H NMR resonances for (iPrPDI)FeOEt were
observed over a wide chemical shift range (−300 to 300 ppm)
at 23 °C. Specifically, the pyridine 4−CH resonance (371.51
ppm for (iPrPDI)FeOEt) and imine methyl CH3 resonances
(−209.74 ppm for (iPrPDI)FeOEt) are diagnostic. For
(iPrPDI)FeOSiMe3, a benzene-d6 magnetic moment of 4.0 μB
was measured (Evans method),20 consistent with the spin-only
value of 3.87 μB for three unpaired electrons and an overall S =
3/2 iron complex.
The solid-state structures of (iPrPDI)FeOSiMe3, (iPrPDI)-
FeOEt, and (iPrPDI)FeOMe were determined by single crystal
X-ray diffraction and representations of the molecular
structures are presented in Figure 1. For all three compounds,
a distorted square planar geometry was observed around the
iron center with the sum of the bond angles approximating
360° in each case (R = SiMe3: 360.43(17)°, R = Et:
360.20(13)°, R = Me: 359.96(4)°). Near linear N(2)−
Fe(1)−O(1) bond angles of 174.39(11)° ((iPrPDI)-
FeOSiMe3), 177.30(6)° ((iPrPDI)FeOEt), and 175.46(2)°
((iPrPDI)FeOCH3) were observed in all cases, suggesting
some degree of π-donation from the alkoxide oxygen to iron.
Two representative pyridine(diimine) iron alkoxide com-
Suzuki-type transmetalation of a neutral aryl boronic acid or
ester nucleophile to an iron amide or alkoxide complex has yet
to be demonstrated in a stoichiometric reaction and could
provide key mechanistic insights on optimal bases, iron
precursors, and spin states for promoting this essential
fundamental organometallic transformation. Here we describe
the observation of stoichiometric transmetalation of furanyl-
substituted, neutral boron reagents with iron alkoxide
derivatives (Scheme 1). The electronic structure implications
associated with conversion of an iron alkoxide to the
corresponding iron heteroaryl complexes are elucidated, and
the potential for iron to undergo palladium-like SMC
transmetalation reactivity is demonstrated.
Because conversion of a metal-halide to the corresponding
alkoxide derivative is a key step in palladium-catalyzed SMC,
the salt metathesis of (iPrPDI)FeBr17 with various alkoxide
sources was initially studied. Addition of NaOR (R = SiMe3,
Me, Et) to a diethyl ether solution of the iron bromide
complex generated the corresponding (iPrPDI)FeOR deriva-
tives (Scheme 2a). Pure (iPrPDI)FeOSiMe3 was isolated in
57% yield following recrystallization from pentane. However,
using NaOCH3, the salt metathesis protocol yielded para-
magnetic iron-containing side products in addition to the
targeted (iPrPDI)FeOCH3 complex. Because protonolysis of
iron alkyls by free alcohols is a known method for the
pounds, (iPrPDI)FeOSiMe3 and (iPrPDI)FeOEt, were also
57
̈
characterized by solid-state Fe Mossbauer spectroscopy.
(Figure 1). Isomer shifts of 0.75 and 0.74 mm/s were obtained
for (iPrPDI)FeOSiMe3 and (iPrPDI)FeOEt, respectively, with
quadrupole splittings, |ΔEQ|, of 1.20 and 1.21 mm/s. Both
isomer shifts are comparable with the previously reported value
of δ = 0.74 mm/s for (iPrPDI)FeCl, although there is a slight
increase in quadrupole splitting when the X-type ligand is
changed from Cl to OR. (|ΔEQ|= 0.73 mm/s for (iPrPDI)-
FeCl).22 Both the magnetic moment and Mo
ssbauer isomer
̈
shift support high spin iron(II) complexes, in analogy to
(iPrPDI)FeCl and related complexes assigned previously.21,22
B
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Figure 1. Solid-state structures of (a) (iPrPDI)FeOEt, (b) (iPrPDI)FeOSiMe3 and (c) (iPrPDI)FeOCH3 at 30% probability ellipsoids, with hydrogen
iPr
iPr
atoms omitted for clarity. Zero-field 57Fe Mossbauer spectra of (a) ( PDI)FeOEt and (b) ( PDI)FeOSiMe3 at 80 K in the solid state. Gray circles
̈
represent the experimental spectrum, while the blue line indicates the best fit of the data.
To provide additional support for the electronic structure
assignment of the (iPrPDI)FeOR compounds (R = Et, SiMe3),
full-molecule density functional theory studies were conducted
at the B3LYP level of theory. The optimized electronic
structures are best described as high spin Fe(II), S = 3/2
(broken symmetry (4,1)), with an iron-centered radical
antiferromagnetically coupled to a radical anion delocalized
on the pyridine diimine ligand (see SI for discussion). A
qualitative d-orbital splitting diagram is shown in Figure S7.
This electronic structure is comparable to the electronic
structure of (iPrPDI)FeX (X = Cl, Br) compounds previously
described, which were also computed to exhibit an S = 3/2
broken symmetry (4,1) ground state.22 The computed
̈
Mossbauer parameters were δ = 0.65 mm/s and |ΔEQ|= 1.23
mm/s for (iPrPDI)FeOEt and δ = 0.64 mm/s and |ΔEQ|= 1.28
mm/s for (iPrPDI)Fe(OSiMe3), in good agreement with
experimentally determined data.
Figure 2. Top: A spin state change accompanies transmetalation.
Bottom: Solid-state structure (left) of (iPrPDI)Fe(benzofuranyl) at
30% probability ellipsoids, with hydrogen atoms omitted for clarity.
Mulliken spin density plot (right) of (iPrPDI)Fe(2-benzofuranyl).
With pyridine(diimine)iron alkoxide complexes in hand,
their transmetalation reactivity with neutral aryl boronic esters
was studied. Addition of PhBPin to a benzene-d6 solution of
(iPrPDI)FeOEt produced no reaction at ambient temperature
or upon heating to 80 °C for 24 h. Taking inspiration from
previous work on transmetalation to cobalt,10 the trans-
metalation of furan-type boron nucleophiles was investigated.
Use of either (2-benzofuranyl)BPin or (2-benzofuranyl)BNeo
(Pin = pinacolato, Neo = neopentylglycolato) resulted in rapid
(<15 min) transmetalation at 23 °C in C6D6 as judged by an
immediate color change from light to dark green and
the benzofuranyl group is stabilized by π-stacking with the aryl
groups or simply sterically preferred.
Experimental and full-molecule DFT studies (B3LYP) on
(iPrPDI)Fe(2-benzofuranyl) support an intermediate spin
iron(II) center engaged in antiferromagnetic coupling with a
chelate radical anion, resulting in an overall S = 1/2 complex
(Figure 2; see SI for discussion). With the electronic structure
of (iPrPDI)Fe(2-benzofuranyl) established, the flow of
electrons during transmetalation reaction can be discussed.
The starting iron alkoxides, (iPrPDI1−)FeIIOR, are high spin
ferrous compounds with a monoreduced pyridine(diimine)
chelate. Exchange of the weak field alkoxide ligand for a
stronger field heteroaryl induced a spin state change at iron
from S = 2 to S = 1 and the radical anion nature of the chelate
is maintained throughout the reaction (Scheme 3). This
behavior is comparable to the analogous transmetalation
reaction between (iPrPNP)CoOR (PNP = bis(phosphino)-
pyridine, R = iPr or CH(Ph)Me), where a spin-state change
from S = 1 (tetrahedral) to S = 0 (planar) accompanies the
1
observation of (iPrPDI)Fe(benzofuranyl) by H NMR spec-
troscopy. In each case, a second, paramagnetic iron compound,
tentatively assigned as the iron-boryl, (iPrPDI)Fe(BPin) or
(iPrPDI)Fe(BNeo), accompanied formation of the benzofur-
anyl transmetalation product. Recrystallization of the product
of the reaction with (2-benzofuranyl)BNeo from diethyl ether
resulted in isolation of (iPrPDI)Fe(2-benzofuranyl) as dark
green crystals in 90% yield. Attempted transmetalation with (2-
benzofuranyl)B(OH)2 and (iPrPDI)FeOEt generated an
intractable mixture of products with just trace quantities of
(iPrPDI)Fe(2-benzofuranyl) (see SI).
Single crystals of (iPrPDI)Fe(2-benzofuranyl) were obtained
from a concentrated solution of (iPrPDI)Fe(2-benzofuranyl) in
diethyl ether at −35°, and the solid-state structure confirmed
the generation of the iron heteroaryl complex (Figure 2). The
bond distances observed in (iPrPDI)Fe(2-benzofuranyl) are
consistent with one-electron reduction of the chelate,23 where
Nimine−Cimine distances of 1.319 (6) Å and 1.315(6) Å, and
Cimine−Cipso bond distances of 1.445(7) Å and 1.437(7) Å are
within the range expected for (RPDI)1−. The benzofuranyl
moiety is rotated by 51.5(4)° relative to the plane of the
pyridine(diimine) chelate. It is possible that this orientation for
Scheme 3. Reaction of (iPrPDI)Fe(2-Benzofuranyl) with
Electrophiles
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transmetalation step.10 Likewise, the propensity of 2-
benzofuranyl boronate esters to undergo transmetalation as
compared to the phenyl variant suggests that initial π-
coordination of the furan ring facilitates formation of the
oxygen boron bond, similar to the model proposed in cobalt-
catalyzed SMC.10
#S.C.E.S.: Department of Chemistry and Biochemistry,
California State Polytechnic University, Pomona, California
91768, United States
Notes
The authors declare no competing financial interest.
The potential of (iPrPDI)Fe(2-benzofuranyl) to participate
in carbon−carbon bond-forming chemistry was evaluated with
both aryl and alkyl halide electrophiles. Addition of one
equivalent of cycloheptyl bromide, iodobenzene, bromoben-
zene, or phenyl triflate produced blue solids identified as the
pyridine(diimine) iron dihalide, (iPrPDI)FeX2 (X = Br, I, OTf)
(Scheme 3). Low yields (<15%) of the cross-coupled product
were obtained, and the remainder of the organic material was
identified as hydrodehalogenated products (R−H) and
benzofuran (Scheme 3; see Table S1 for organic product
distribution).
In summary, the transmetalation of a neutral boron
nucleophile to an iron alkoxide complex was demonstrated.
In contrast to previous examples of transmetalation to iron,
boronic acid ester nucleophiles commonly employed for
palladium SMC are capable of promoting transmetalation. A
net spin state change from S = 3/2 to S = 1/2 occurs during
transmetalation, which was previously observed for cobalt and
may assist in transmetalation. These results suggest that
Suzuki-type transmetalation from boron to iron may be a
viable pathway for iron-catalyzed cross coupling.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health (R01 GM121441)
for financial support.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00733.
■
S
NMR spectra (PDF)
Cartesian coordinates (XYZ)
Accession Codes
CCDC 1953963, 1953965, 1954249, and 1956370 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: pchirik@princeton.edu.
ORCID
Paul O. Peterson: 0000-0002-0602-2695
Stephan M. Rummelt: 0000-0003-3996-0271
Bradley M. Wile: 0000-0001-6856-2796
S. Chantal E. Stieber: 0000-0003-3778-207X
Hongyu Zhong: 0000-0002-6892-482X
Paul J. Chirik: 0000-0001-8473-2898
■
Present Addresses
†B.M.W.: Donald J. Bettinger Department of Chemistry and
Biochemistry, Ohio Northern University, 525 South Main
Street, Ada, OH 45810, United States
‡S.M.R.: Department of Process Research and Development,
Merck Research Laboratories, Rahway, NJ 07065, United
States
D
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DOI: 10.1021/acs.organomet.9b00733
Organometallics XXXX, XXX, XXX−XXX