Iron Catalyzed Double Bond Isomerization: Evidence for an FeI/FeIII Catalytic Cycle

Article abstract of DOI:10.1002/chem.202004980

Iron-catalyzed isomerization of alkenes is reported using an iron(II) β-diketiminate pre-catalyst. The reaction proceeds with a catalytic amount of a hydride source, such as pinacol borane (HBpin) or ammonia borane (H₃N?BH₃). Reactivity with both allyl arenes and aliphatic alkenes has been studied. The catalytic mechanism was investigated by a variety of means, including deuteration studies, Density Functional Theory (DFT) and Electron Paramagnetic Resonance (EPR) spectroscopy. The data obtained support a pre-catalyst activation step that gives access to an η²-coordinated alkene Fe^I complex, followed by oxidative addition of the alkene to give an Fe^III intermediate, which then undergoes reductive elimination to allow release of the isomerization product.

Full text of DOI:10.1002/chem.202004980

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doi.org/10.1002/chem.202004980
Chemistry—A European Journal
&
Catalysis
Iron Catalyzed Double Bond Isomerization: Evidence for an
FeI/FeIII Catalytic Cycle
Callum R. Woof,[a] Derek J. Durand,[b] Natalie Fey,*[b] Emma Richards,*[c] and
Ruth L. Webster*[a]
Abstract: Iron-catalyzed isomerization of alkenes is reported
using an iron(II) b-diketiminate pre-catalyst. The reaction
proceeds with a catalytic amount of a hydride source, such
as pinacol borane (HBpin) or ammonia borane (H3N·BH3). Re-
activity with both allyl arenes and aliphatic alkenes has been
studied. The catalytic mechanism was investigated by a vari-
ety of means, including deuteration studies, Density Func-
tional Theory (DFT) and Electron Paramagnetic Resonance
(EPR) spectroscopy. The data obtained support a pre-catalyst
activation step that gives access to an h2-coordinated alkene
FeI complex, followed by oxidative addition of the alkene to
give an FeIII intermediate, which then undergoes reductive
elimination to allow release of the isomerization product.
gen shift,[2] the simplicity of which may give increased likeli-
hood of OA/RE chemistry (Scheme 1b). Carbon–carbon double
bonds are important in synthetic chemistry: they are used to
introduce diverse functionality to organic molecules, including
pharmaceuticals, and they are also used extensively in the pet-
rochemical-based polymer industry. Double bond isomerization
can change the properties of a molecule without the need for
stoichiometric exogenous reagents.[3]
Introduction
Two-electron chemistry within the field of iron catalysis is in-
credibly rare.[1] We envisioned that if a controlled one electron
reduction of an Fe(II) species could be carried out with a well-
defined iron pre-catalyst, then it might be feasible to access an
oxidative addition (OA)/reductive elimination (RE) (i.e. FeI/FeIII)
catalytic cycle to afford activity similar to that typically ob-
served with precious metal congeners. Previous catalytic stud-
ies within our group using an iron(II) b-diketiminate complex
(hereafter referred to as complex 1) have focused on bond
forming transformations (for example hydrophosphination, hy-
drogenation and dehydrocoupling, Scheme 1a). However,
double bond isomerization gives the ideal opportunity to ex-
plore a unimolecular transformation that relies on a 1,3-hydro-
Although there are several elegant examples of earth abun-
dant metal-catalyzed isomerization/functionalization[4] (e.g. hy-
droboration,[5] hydrosilylation,[6] allylic alcohol to ketone[7]),
which ultimately provide an energetically favorable driving
force for the isomerization step, there are far fewer examples
of the use of abundant metal pre-catalysts to only invoke iso-
[a] C. R. Woof, Dr. R. L. Webster
School of Chemistry
University of Bath
Claverton Down, Bath, BA2 7AY (UK)
E-mail: r.l.webster@bath.ac.uk
[b] D. J. Durand, Dr. N. Fey
School of Chemistry
University of Bristol
Cantock’s Close, Bristol, BS8 1TS (UK)
E-mail: Natalie.Fey@bristol.ac.uk
[c] Dr. E. Richards
School of Chemistry
Cardiff University
Main Building, Park Place, Cardiff, CF10 3AT (UK)
E-mail: richardse10@cardiff.ac.uk
Supporting information and the ORCID identification numbers for the
authors of this article can be found under:
https://doi.org/10.1002/chem.202004980.
ꢀ 2021 The Authors. Chemistry - A European Journal published by Wiley-
VCH GmbH. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and repro-
duction in any medium, provided the original work is properly cited.
Scheme 1. a) Previous catalytic reactions undertaken using 1; b) this work.
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merization. Isomerization-only processes catalyzed by iron are
dominated by thermodynamically favored cis-to-trans and/or
terminal to internal double bond isomerization. Some of the
earliest examples of iron-catalyzed reactions date back to the
1960’s when Frankel, Emken and Davison reported the use of
Fe(CO)5 as a catalyst in double bond isomerization in fatty
acids and esters.[8] Triiron dodecacarbonyl was used by Casey
and Cyr to undertake isomerization of 3-ethyl-1-pentene and
detailed mechanistic studies suggested that the reaction likely
proceeded via an Fe(h3-allyl)(H) intermediate.[9] Beyond cataly-
sis performed using iron carbonyl complexes,[10] von Wangelin
has reported isomerization using Fe(acac)3 with a Grignard ad-
ditive at room temperature.[11] More recently, Koh and co-work-
ers have reported an elegant regiodivergent isomerization pro-
cess that uses B2pin2 or PhMe2SiBpin in conjunction with an
iron catalyst and LiOtBu. Mechanistic studies revealed that the
reaction is likely to proceed via an iron-hydride with subse-
quent olefin insertion and b-hydride elimination.[12] Pertinent
to this study, Smith has employed an FeI species, exploiting
the propensity for spin crossover, to achieve iron catalyzed
double bond isomerization via a two-electron catalytic cycle.[13]
From the synthetic studies we have already disclosed in hy-
drofunctionalization chemistry,[14] we postulated that access to
a two-electron isomerization catalytic cycle would be possible
by limiting the quantity of hydride source employed in order
to access FeI, meanwhile preventing competitive FeII hydrobo-
ration. We herein report evidence to support this hypothesis,
using pre-catalyst 1.
reactions, amongst others, proceed in a redox-neutral manner,
in particular operating via an FeII hydride. In light of this, we
sought to rule out an alkyl-based mechanism where HBpin or
an alternative reagent serves as a hydride source, forming an
iron(II) hydride which can consequently undergo addition
across a double bond and subsequent b-hydride elimination at
the more substituted position. Further investigation was able
to discount this mechanism taking place. Firstly, the iron(II) hy-
dride dimeric species 4 is poor in catalysis (16% conversion to
3a,16 h, 608C as opposed to 93% 3a with 1 and HBpin, see
Scheme 2d), contrasting with our dehydrocoupling work.[16]
Adding further hydride source to 4 does lead to catalysis, but
Results and Discussion
Following a short optimization procedure we found that a co-
catalytic quantity (10 mol%) of pinacol borane (HBpin), ammo-
nia borane (H3N·BH3) or dimethylamine borane (Me2HN·BH3)
are all similarly competent at transforming allyl benzene (2a)
into b-methylstyrene (3a) with 5 mol% 1 at 608C in 16 h
(Table 1, also see Supporting Information for further optimiza-
tion).
Our previous synthetic and theoretical studies with 1 have
indicated that hydroboration[14] and transfer hydrogenation[15]
Table 1. Optimization of iron catalyzed double bond isomerization using
allylbenzene as the substrate.
Catalyst (5 mol%),
additive (10 mol%)
Conversion to 3a
(%)
trans:cis
1
1, none
0
0
2
3
4
None, HBpin
1, HBpin
0
0
93
99
12
99
73
44
7.6:1
25:1
8:1
33:1
3.6:1
2.9:1
1, H3N·BH3
1, H3N·BH3
1, Me2HN·BH3
1, TMP·BH3
1, PhH2N·BH3
5[a]
6
7[b]
8
Standard reaction conditions: C6D6 (0.6 mL), allylbenzene (0.5 mmol), cata-
lyst (0.025 mmol), additive (0.05 mmol), 608C, 16 h. Conversion and selec-
1
tivity determined by in situ H NMR spectroscopy. [a] RT. [b] TMP=2,2,6,6-
tetramethylpiperidine. 808C, 48 h.
Scheme 2. a)–c) Deuterium labelling studies; d) Discounted redox-neutral
catalytic isomerization cycle.
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the lack of NMR evidence for 4 forming in situ suggests it is
not formed as part of a pathway to an active species. Secondly,
deuteration studies give outcomes contradicting what would
be expected for a redox neutral mechanism (Scheme 2).
2
Firstly, there is no H-incorporation in substrates when using
deuterated hydride sources (Scheme 2a), and full isoretention
is observed in the transformation of d-labelled substrate 2a-d2
(Scheme 2b) to 3a-d2. If 1 is activated to form an Fe-hydride
(or deuteride) then a small amount of H/D exchange would be
expected owing to the intermolecular nature of catalyst activa-
tion and H transfer.
Secondly, in a competitive reaction of 2a-d2 and 2b, isore-
tention is again observed (Scheme 2c), indicating hydride
transfer is intramolecular rather than intermolecular. At this
stage, the deuterium labelling studies closely match the results
one would expect from an h3-allyl mechanism.[3] Qualitatively,
the reactions are observed to change from the dark yellow of
1 to a vivid red; a color that is synonymous with the formation
of FeI. Furthermore, upon investigating our proposed catalytic
cycle with DFT calculations, we were able to demonstrate that
a redox-neutral FeII catalytic cycle of the form shown in
Scheme 2d is disfavored due to an exceptionally high energy
barrier for the b-hydride elimination step for the more stable,
high-spin quintet surface (DG° =37.4 kcalmolÀ1, see Support-
ing Information for details). Evans method NMR analysis[17]
gives mobs =5.7 for the pre-catalyst 1, which is consistent with
measurements from Hessen of similar FeII complexes.[18] How-
ever, addition of HBpin, H3N·BH3 or Me2HN·BH3 to 1 results in a
decrease in the magnetic moment to mobs =5.1. We tentatively
attribute this to a shift from pure FeII to a mix of oxidation/
spin states being present. Adding 2a does not appear to affect
mobs further. At this stage, we feel we have reasonable evidence
to rule out the entirely redox-neutral cycle of the form shown
in Scheme 2d. No reaction is observed with the radical clock 6-
bromo-1-hexene, therefore we are confident that there are no
organic radicals present or propagating in solution.
Figure 1. i) CW X-band EPR spectrum [T=140 K] of 1 + HBPin. ii) Experimen-
tal (a), and simulation (a’) of CW X-band EPR spectrum [T=140 K] of 1 +
HBPin, focusing on the center-field region. The broad signal originating from
high-spin FeIII (seen in i) and also in Figure S1) has been background sub-
tracted. See Supporting Information for corresponding simulations and dis-
cussion.
used to determine whether HBpin, H3N·BH3 or Me2HN·BH3 are
strong enough reductants to convert 1 into an FeI complex
(see Supporting Information for details). Cyclic Voltammetry
studies of 1 show two strong reduction peaks. We assign the
first to the reduction of our FeII pre-catalyst to an FeI species,
which we determine to be À1.48 vs. Fc/Fc +. The lack of a
large peak in the oxidative direction suggests the irreversibility
of the FeII/FeI reduction process, potentially ruling out a one-
electron oxidation/reduction catalytic cycle. The second peak
has a potential of À1.92 vs. Fc/Fc +, and we believe it is ligand-
based and is unlikely to have significant bearing on the catalyt-
ic pathway. The relatively strong reducing potentials of both
HBpin and H3N·BH3 indicate that such a reduction is thermody-
namically feasible.[20] Because of this, we propose that the
active species in catalysis is initially an FeI species. This com-
plex may be stabilized by either a molecule of solvent, as pre-
viously reported, or by a molecule of substrate. Substitution
coefficient studies from Holland[21] on 1A (Scheme 3a) indicate
that, once formed, the h2-allylbenzene complex 1B is more
stable, and we therefore believe that if 1A forms, displacement
to form 1B is rapid and facile. Wide sweep width NMR spectros-
copy of both stoichiometric and catalytic reactions (using
HBpin or H3N·BH3) give complex spectra that we attribute to
To support the NMR results, X-band CW EPR studies were re-
corded in benzene:toluene (200:10 mL) frozen solution. A solu-
tion of 1 in the absence of substrate indicates the presence of
a small amount of high-spin FeIII, which we assign to the side
formation of LFeCl2 during synthesis of the pre-catalyst (see
Supporting Information). Direct addition of 2a to 1 did not
lead to any changes in the EPR spectrum (see Supporting In-
formation), indicating no change in oxidation state induced by
the alkene. However, upon addition of HBpin or H3N·BH3 to 1,
the appearance of a well-resolved rhombic signal without any
observable hyperfine coupling is observed, consistent with a
low spin S=1/2 iron d7 center (Figure 1). Addition of 2a to this
mixture does not lead to further spectral changes. Computer
simulation of this species revealed the spin Hamiltonian pa-
rameters characterized by g=[1.984 2.018 2.200], which is
analogous to Holland’s characterization of an FeI species inter-
acting with benzene, and our previous report of FeI complexes
formed upon reaction of FeCl3 with aryl/alkyl Grignard re-
agents.[19] The FeI signal is superimposed on a broad signal as-
sociated with a high-spin FeIII center, vide infra (also see Sup-
porting Information for details). Cyclic Voltammetry was also
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ferred to the isomerized product and therefore it appears to
be acting as a pre-catalyst activator only.
Having ruled out a redox-neutral FeII pathway both experi-
mentally and computationally, we further investigated the iso-
merization of allylbenzene to b-methylstyrene using DFT calcu-
lations. We propose that the reaction proceeds from our active
FeI h2-alkene species 1B, in the doublet spin-state, via a selec-
tivity-determining oxidative addition of one of the phenyl adja-
cent CÀH bonds to form either of FeIII h3-allyl species, 1C
or
trans
1C cis. These intermediates then undergo a reductive elimina-
tion of the proton onto the terminal carbon to form the FeI
allyl species, 1D
or 1D cis. A ligand exchange with the sub-
trans
strate 2a then yields the major product, trans-b-methylstyrene
or the minor product, cis-b-methylstyrene, while also regener-
ating 1B (Scheme 3b).
We suggest that the doublet spin state of the complex is
largely preserved throughout the catalytic cycle, although we
have also investigated the other spin states, with a particular
focus on the trans-selective route. While the quartet 1B was
calculated to be more stable than the doublet by 21.0 kcal
molÀ1, we found, similarly to Smith,[13] that the activation barri-
er for the trans selective oxidative addition on the quartet sur-
face (DG° =45.0 kcalmolÀ1) was far in excess of what might be
considered accessible under the experimental conditions used
(see Supporting Information). In addition to this, our EPR re-
sults do not support the presence of a quartet FeI species, or a
quartet FeIII species.
Scheme 3. a) 1 reacts with borane to generate 1A, which can then undergo
further stabilization with alkene; b) proposed FeI/FeIII catalytic cycle for the
isomerization of allylbenzene.
Starting from our initial species 1B, rotation of the PhÀC
bond to give the appropriate orientation for each oxidative ad-
dition is a low-energy process (<1 kcalmolÀ1), with the subse-
quent activation barriers (DG° =25.7 kcalmolÀ1 for trans,
DG° =28.1 kcalmolÀ1 for cis) supporting the experimentally
observed trans selectivity of the reaction (Table 2). Due to
steric interactions arising from the proximity of the substrate
phenyl ring to the 2,6-diisopropylphenyl groups on the b-dike-
timinate ligand, the cis h3-allyl product of the oxidative addi-
tion, intermediate 1C cis, generates an overall positive DG for
that step (+19.7 kcalmolÀ1 relative to 1B), while the less hin-
off-cycle or catalytically inactive FeII complexes or FeIII formed
by disproportionation of FeII into FeI and FeIII (see Supporting
Information for extended discussion). Unfortunately, we have
not been able to isolate any key intermediates from reaction
mixtures[22] and characterize them crystallographically, and de-
tailed kinetics studies have been hampered by the reaction
temperature coupled to the paramagnetic iron center, which
have made in situ NMR monitoring challenging. However, in
situ UV-vis spectroscopy measurements give lmax =494 and
552 nm (for catalysis in the presence of H3N·BH3) or lmax =497
and 547 nm (for catalysis in the presence of HBpin); wave-
lengths that have been linked to both FeI and FeIII b-diketimi-
nate species.[21,23] Using the method reported by Scheer for the
synthesis of the h6-toluene analogue of 1A,[24] we obtain 87%
3a (7.5:1 trans:cis) after 16 h at 608C when 5 mol% of this FeI
species is used in catalysis (31% in a 6.3:1 ratio after 2 h);
these results are in-line with those obtained using the same
conditions with pre-catalyst 1 (5 mol% 1 and 10 mol% HBpin,
27%, 5.7:1 ratio after 2 h; and 93%, 7.6:1 ratio after 16 h) and
further support the proposal that the reaction proceeds via an
FeI species.
dered 1C
is 2.7 kcalmolÀ1 more stable than 1C and results
trans
cis
in a slightly reduced free energy difference, DG= +18.0 kcal
molÀ1, making the formation of 1C
slightly less reversible
trans
than 1C cis. From there, both isomers have relatively small barri-
ers for the reductive elimination (DG° =2.6 kcalmolÀ1 for trans
and 4.8 kcalmolÀ1 for cis), the rate determining barrier for both
is the oxidative addition, supported by a secondary kinetic iso-
tope effect (KD/KH =1.03Æ0.06) observed experimentally. Both
the trans (À4.6 kcalmolÀ1) and cis selective (À3.3 kcalmolÀ1
)
isomerization of allylbenzene are calculated to be exothermic
overall.
To summarize the experimental results thus far: 1 appears to
undergo a change in oxidation state in the presence of a hy-
dride source and this is supported by NMR and EPR studies.
Electrochemistry indicates that the hydride source can reduce
1 to an FeI species and work from Holland has demonstrated
that alkenes are able to form h2-FeI complexes (i.e. 1A and 1B).
An h6-arene FeI complex gives similar catalytic results to 1.
Deuterium labelling indicates that the hydride is not trans-
Table 2. Calculated activation barriers DG° (kcalmolÀ1) for the isomeriza-
tion of allylbenzene for the proposed FeI/FeIII catalytic cycle.
Selectivity
Oxidative
addition
Reductive
elimination
trans
cis
25.7
28.1
2.6
4.8
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The sextet FeIII intermediate 1C
lies below the doublet
trans
electronic configuration at our chosen level of theory. As
shown on the energy surfaces plotted in Figure 2, the high-
spin FeIII species detected by EPR is likely to be 1C
(3.1 kcal
trans
molÀ1 more stable than the doublet), although we concede
that computational method effects might affect this prediction.
The observed reaction outcome could thus be explained either
by considering only the low spin species, or by spin crossovers
onto the high spin surface near the oxidative addition step
and crossing back onto the low spin pathway near the reduc-
tive elimination. Unfortunately, we were unable to locate mini-
mum energy crossing points (MECPs) between these surfaces
with the method described by Harvey et al. (see Supporting In-
formation for further details).[25]
Figure 2. Calculated free energy surface for the trans-selective isomerization
of allylbenzene in doublet (black line) and sextet (red line) electronic config-
urations, relative to the initial on-cycle species 1B neutral. B3LYP-D3/6-31G(d),
PCM=benzene, 298 K.
To prove utility beyond allyl benzene we have explored a
range of substrates (Scheme 4). We have opted to use H3N·BH3
as our hydride source as it is an easy to handle, inexpensive re-
agent with good sustainability credentials. In most cases there
is good selectivity for the trans-styrene product (3a to 3d).
However, when the aromatic ring is functionalized with strong-
ly electron donating (3e and 3 f) or withdrawing groups (3g
and 3h), there is a drop-off in trans-selectivity and the cis
product becomes more favorable. We link this, and the drop-
off in trans selectivity observed with HBpin (see Table 1), to
rate of reaction: reactions of electron donating or withdrawing
substrates, or those mediated by HBpin, are slower, meaning
there is less opportunity for cis-to-trans isomerization to take
place prior to product isolation. Good turnover is observed for
the double bond isomerization of b- to a-pinene (3i) and a
modest amount of exo- to endo-double bond isomerization is
observed for valencene (2j). Unfortunately, no reactivity is ob-
served with secondary and tertiary amine-containing alkenes
(diallylamine, N-allylaniline and 4-methyl-diallylaniline), or with
1,1-disubstituted alkenes such as (2-methyl-2-propenyl)ben-
zene or multiple double bond containing terpenes such as b-
farnesene (which showed good reactivity during hydroboration
studies).[14] We have explored the selectivity obtained for the
isomerization of hexene (see Supporting Information) but
more forcing reaction conditions are required (808C, 48 h, in
comparison to 608C, 16 h).
Scheme 4. a) Double bond isomerization substrate scope. [a] 808C; b) Prepa-
ration of fragrance molecule piperonal using iron catalyzed double bond iso-
merization followed by ozonolysis.
to afford a key synthetic fragrance and flavor molecule
(Scheme 4b). In a reaction sequence involving iron-catalyzed
double bond isomerization followed by ozonolysis, we have
transformed safrole into piperonal, a floral/vanilla scented com-
pound with both fragrance and synthetic applications.
Conclusions
In summary, we have reported a rare example of iron catalysis
likely proceeding through a two-electron oxidation/reduction
mechanistic pathway. A catalytic amount of HBpin or H3N·BH3,
rather than act as a source of hydride, acts to reduce our FeII
pre-catalyst to an FeI species, a proposal supported by EPR
and electrochemical studies. An h3-allyl-type mechanism, pro-
ceeding via an FeI/FeIII redox active cycle, is supported by deu-
Finally, to demonstrate the applicability of this isomerization
catalysis to sustainable chemistry, we have utilized our catalysis
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Acknowledgements
The EPSRC Centre for Doctoral Training in Catalysis (EP/
L016443/1) is thanked for funding CRW and DJD. We thank the
EPSRC UK National Mass Spectrometry Facility at Swansea Uni-
versity for MS analysis and the Centre for Computational
Chemistry at the University of Bristol for providing access to
computing resources. The authors would also like to thank Dr.
Josh Tibbetts for assistance with ozonolysis experiments, Pro-
fessor Frank Marken and Mr. Jack Stewart for helpful discussion
on Cyclic Voltammetry studies, and Dr. Danila Gasperini for the
preparation of the FeI h6-toluene analogue of 1A.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: homogeneous catalysis · iron · isomerization ·
reaction mechanisms · redox chemistry
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Manuscript received: November 17, 2020
Revised manuscript received: January 19, 2021
Accepted manuscript online: January 25, 2021
Version of record online: March 15, 2021
Chem. Eur. J. 2021, 27, 5972 –5977
www.chemeurj.org
5977 ꢀ 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH