The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-Coupling with Simple Ferric Salts and MeMgBr

Article abstract of DOI:10.1002/anie.201802087

The use of N-methylpyrrolidone (NMP) as a co-solvent in ferric salt catalyzed cross-coupling reactions is crucial for achieving the highly selective, preparative scale formation of cross-coupled product in reactions utilizing alkyl Grignard reagents. Desp

Full text of DOI:10.1002/anie.201802087

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Accepted Article
Title: The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-
Coupling with Simple Ferric Salts and MeMgBr
Authors: Salvador B Munoz III, Stephanie L Daifuku, Jeffrey D
Sears, Tessa M Baker, Stephanie H Carpenter, William W
Brennessel, and Michael Neidig
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The N-Methylpyrrolidone (NMP) Effect in Iron-Catalyzed Cross-
Coupling with Simple Ferric Salts and MeMgBr
Salvador B. Muñoz III,† Stephanie L. Daifuku,† Jeffrey D. Sears, Tessa M. Baker, Stephanie H. Carpenter,
William W. Brennessel, Michael L. Neidig*
Abstract: The use of N-methylpyrrolidone (NMP) as a co-solvent in
ferric salt catalyzed cross-coupling reactions is crucial for achieving
the highly selective, preparative scale formation of cross-coupled
product in reactions utilizing alkyl Grignard reagents. Despite the
critical importance of NMP, the molecular level effect of NMP on in-
situ formed and reactive iron species that enables effective catalysis
remains undefined. Herein, we report the isolation and
characterization of
a
novel trimethyliron(II) ferrate species,
[Mg(NMP)6][FeMe3]2 (1), which forms as the major iron species in-
situ in reactions of Fe(acac)3 and MeMgBr under catalytically relevant
conditions where NMP is employed as a co-solvent. Importantly,
combined GC analysis and 57Fe Mössbauer spectroscopic studies
identified 1 as a highly reactive iron species for the selective formation
generating cross-coupled product. These studies demonstrate that
NMP does not directly interact with iron as a ligand in catalysis but,
alternatively, interacts with the magnesium cations to preferentially
stabilize the formation of 1 over [Fe8Me12]- cluster generation which
occurs in the absence of NMP.
Iron-catalyzed cross-coupling reactions are of significant
synthetic interest as they can provide more sustainable
alternatives to precious metal cross-coupling catalysis while
achieving complementary reactivity.[1] Simple ferric salt catalyzed
cross-couplings involving alkyl nucleophiles are especially
attractive due to their simplicity and tolerance of nucleophiles
containing b-hydrogens.[2] Originally developed by Kochi in the
1970s, recent studies have identified [Fe8Me12]- as the key
reactive iron species formed in these reactions with MeMgBr
(Scheme 1).[3] In 1998, Cahiez overcame many of the limitations
of the Kochi ferric salt system through the use of N-
methylpyrrolidone (NMP) as a co-solvent.[4] With NMP present,
cross-couplings of various vinyl halides and alkyl Grignard
reagents could be achieved with high chemoselectivity without the
requirement of excess alkenyl halide (required in the absence of
NMP), enabling preparative scale iron-catalyzed cross-coupling
reactions (Scheme 1). Furthermore, NMP has been demonstrated
to be critically important for extending the use of ferric salt-
catalyzed cross-coupling to substrates such as aryl halides,
alkenyl triflates and acyl chlorides.[5] While NMP has been
Scheme 1. Iron-catalyzed alkyl-alkenyl cross-coupling reactions with ferric salts
reported by Kochi and Cahiez and the molecular structure of [Fe8Me12]-.
performance remains poorly defined. For example, while a NMP-
ligated iron complex has previously been isolated under non-
catalytically relevant conditions, it remains ambiguous as to
whether NMP is a ligand to iron during cross-coupling or serves
another effect.[7] The ability to define the role of NMP in simple
ferric salt catalyzed cross-coupling, a mystery for twenty years,
would be a significant step forward in our molecular-level
understanding of the nature of reactive iron species required for
effective cross-coupling catalysis.
Herein, we report the use of 57Fe Mössbauer spectroscopy,
electron paramagnetic resonance (EPR) and magnetic circular
dichroism (MCD) to evaluate the effects of NMP on in-situ
generated iron species from reactions of simple ferric salts with a
methyl Grignard reagent (MeMgBr). This study represents the first
direct evaluation of the effect of NMP on iron speciation and
reactivity with electrophile in cross-coupling. Critically, these
studies demonstrate that the presence of NMP as an additive
results in the stabilization of an unprecedented and reactive,
three-coordinate homoleptic iron(II)-methyl species under
catalytically relevant conditions, where NMP acts as a ligand to
the Mg(II) counterion.
While previous studies demonstrated that generation of
[Fe8Me12]- could be accomplished by reacting Fe(acac)3 with 20
equiv of MeMgBr,[3] it was important to directly evaluate if the
Cahiez protocol involving NMP has any effect on cluster formation.
The addition of 20 equiv of MeMgBr to a 3 mM solution of
57Fe(acac)3 in 1:1 THF:2-MeTHF with 180 equiv of NMP (9 equiv
employed
extensively
in
iron-catalyzed
cross-coupling
methodology,[5b, 6] its role in enabling improved catalytic
[*]
Dr. S. B. Muñoz III, Dr. S. L. Daifuku, J. D. Sears, T. M. Baker, S. H.
Carpenter, Dr. W. W. Brennessel and Prof. M. L. Neidig
Department of Chemistry, University of Rochester
B31 Hutchison Hall, 100 Trustee Road
Rochester, NY 14627-0216 (USA)
[†]
Dr. S. B. Muñoz III and Dr. S. L. Daifuku contributed equally to this
work.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802087
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of NMP relative to Grignard reagent; Cahiez protocol ratio[4]) at
RT led to the rapid formation of a pale yellow solution. Note that
the drop-wise addition of Grignard reagent using a syringe pump
(12 equiv of Grignard reagent/min relative to iron) was utilized to
mimic the use of an addition funnel as described in the original
Cahiez report.[4] The 80 K 57Fe Mössbauer spectrum of the in-situ
formed iron species freeze-trapped 30 s after completion of the
Grignard reagent addition indicates the formation of a single major
iron species (97 % of total iron, green component) with 57Fe
Mössbauer parameters of δ = 0.25 mm/s and ΔEQ = 1.36 mm/s
(Figure 1A), whereas in the absence of NMP exclusive formation
of a broad doublet signal associated with [Fe8Me12]- was
previously observed (δ = 0.30 mm/s and ΔEQ = 0.85 mm/s). A
minor additional iron species is also observed by 57Fe Mössbauer
spectroscopy with parameters δ = -0.28 mm/s and ΔEQ = 0.73
mm/s (3 % of total iron, pink component) that corresponds to a S
= 3/2 species (axial, g ~ 4.3, 2.0, ~ 3% of total iron by spin
quantitation), which is also observed by 10 K EPR spectroscopy
(Figure 1A, inset). There is also a minor amount of an additional
broad S = 1/2 (isotropic, g ~ 2) species observed in the EPR
spectrum which is consistent with the previously identified iron
cluster, [Fe8Me12]- though present in too minor an amount to be
observed by 57Fe Mössbauer spectroscopy. The 5 K, 7 T near-
infrared (NIR) MCD spectrum of the in-situ formed iron species
shows two intense ligand field (LF) transitions at ~ 10200 cm-1
with a shoulder at ~ 9200 cm-1 (Figure 1C), assigned to the major
iron species formed in solution. While most studies were
performed in 1:1 THF:2-MeTHF for glassing purposes, the same
iron speciation was also observed in pure THF by 57FeMössbauer
(see SI, Figure S1).
It was important for subsequent single-turnover reactivity
studies to determine the minimum quantity of MeMgBr required to
form the major iron species. In fact, it was determined that 5 equiv
of MeMgBr is required for the formation of this species. The
reaction of 57Fe(acac)3 with 5 equiv of MeMgBr and 45 equiv of
NMP at RT generates the same major iron species as determined
Figure 1. 80 K 57Fe Mössbauer spectra of a frozen solution of 57Fe(acac)3 (9
equiv of NMP relative to MeMgBr) with (A) 20 equiv and (B) 5 equiv of MeMgBr
(black dots), total fit (black line) and individual fit components are shown. (C) 5
K, 7 T NIR MCD spectrum of 20 equiv of MeMgBr reacted with Fe(acac)3 and
180 equiv of NMP at RT in 1:1 THF:2-MeTHF.
by 80 K 57Fe Mössbauer (Figure 1B, δ = 0.25 mm/s and ΔEQ
=
1.36 mm/s, 82% of total iron, green component). The same
additional minor iron species is observed (δ = -0.28 mm/s and EQ
= 0.73 mm/s, 3 % of total iron, pink component) in addition to a
Figure 2. (A) Structure of [Mg(NMP)6][FeMe3]2 (1), drawn at 50% probability level. Hydrogen atoms omitted for clarity. (B) 80 K 57Fe Mössbauer spectrum of a frozen
solution of 1 (black dots), total fit (green line). (C) 5 K, 7 T NIR MCD of 1 dissolved in 1:1 THF:2-MeTHF. (D) Saturation magnetization data (dots) and best fit (lines)
collected at 10486 cm-1.
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new iron species with parameters δ = 0.64 mm/s and ΔEQ = 1.27
mm/s (15% of total iron, blue component) which, from reactivity
studies (vide infra), is assigned to an under-transmetallated iron
complex that forms prior to the major iron species (Figure 4). The
10 K EPR spectrum indicates minor contributions from a S = 3/2
species (~ 3 %) and a S = 1/2 cluster (Figure 1B, inset).
Spectroscopic investigations of the in-situ generated iron
species indicated a limited window of thermal stability at 23 °C (t
~ 3 min) before iron species associated with decomposition were
formed (Figure S2). Thus, efforts were directed toward isolating
the major iron species. The treatment of a red solution containing
Fe(acac)3 and 9 equiv of NMP in THF with 5 equiv of MeMgBr at
-5 °C provided a yellow colored solution. Rapid cooling of the
solution to -80 °C (to disfavor decomposition) followed by layering
with diethyl ether rendered large pale-yellow crystals. A single
crystal X-ray diffraction study (XRD) revealed three independent
instances of [Mg(NMP)6][FeMe3]2, 1, in the asymmetric unit in
general positions, with the cations and anions well separated
(Figure 2A). Although the crystals diffracted very weakly,
precluding a detailed discussion of the structural parameters, the
overall connectivity, geometry, and chemical formulation of this
species are unambiguous. Notably, the magnesium ion is
coordinated through the carbonyl oxygens of the six NMP
molecules (Note: NMP O-coordination mode for a magnesium ion
and other metals has been previously reported[6f, 7-8]) and no NMP
coordination to iron is present. A previous report showed similar
effects of co-solvent on iron speciation in which ligation of
tetramethylethylenediamine (TMEDA) to magnesium cations in
homoleptic triaryl-ferrate complexes was observed.[9]
Figure 3. Calculated molecular orbital energy diagram for 1.
To date, very few examples of homoleptic alkyl- and aryl-
iron complexes devoid of stabilizing ligands exist in the
literature.[8] The first example incorporating methyl ligands was
reported by Fürstner in which a distorted tetrahedral homoleptic
tetramethyliron(II) ferrate complex, [(Me4Fe)(MeLi)][Li(OEt2)]2,
was identified.[10] This inspired later studies from Neidig and
coworkers in which a distorted square-planar tetramethyliron(III)
ferrate, [MgCl(THF)5][FeMe4], was identified.[11] The unique
coordination geometry of the iron species was determined to be
strongly influenced by the choice of solvent and cation. This
assertion is further supported by the generation of 1 instead of
[Fe8Me12]- when utilizing NMP as a co-solvent demonstrating the
powerful effect of NMP coordination to cations versus other
solvents. Both Bedford and Fürstner have independently reported
the isolation of triaryl-ferrate complexes which are reactive with
electrophile, but unlike 1 lack high selectivity in producing cross-
coupled product.[9, 12]
electronic structure and bonding of this novel low-coordinate iron
(II) species. The PBE0 functional and def2-TZVP basis set were
employed to yield calculated structures for the S = 2 system which
are in good agreement with crystallographic data in both the gas
phase and THF solvent models (see SI). Additionally,
B3LYP/TZVP was used to evaluate the molecular orbitals (see SI).
The ground-state electronic structure can be described by
examination of the frontier molecular orbitals of the β-manifold
(Figure 3), which shows the highest occupied (β25) and the four
lowest unoccupied MOs (β26, β27, β28 and β29) with significant
d-orbital character. Strong Fe s-orbital character is also present
in β26 and modest antibonding interactions are observed
between the methyl carbon atoms in β26, β28, and β29.
In order to understand the role of 1 in cross-coupling,
reactions of 1 with sub-stoichiometric amounts of electrophile, b-
bromostyrene, were tracked by 80 K 57Fe Mössbauer, with
concurrent reaction aliquots quenched to determine product
yields by gas chromatography (GC). These experiments
demonstrate that 1 is consumed within 10 s in the presence of
While
1
represents an interesting, three-coordinate
homoleptic iron-methyl structure, spectroscopic characterization
of 1 in solution was necessary to verify its identity as the major
iron species formed under catalytically relevant conditions. The 5
K, 7 T NIR MCD spectrum of 1 in solution (Figure 2C), prepared
from dissolution of crystals of 1 in 1:1 THF:2-MeTHF at -80 °C to
prevent thermal decomposition, is consistent with the analogous
spectrum (Figure 1C) of the iron species generated from reaction
of Fe(acac)3 with 20 equiv of MeMgBr in 180 equiv of NMP.
Saturation magnetization data collected at 10846 cm-1 is well-fit
to S = 2 positive zero-field split (+ZFS) non-Kramers doublet
model with ground-state spin Hamiltonian parameters of D = 13 ±
3 cm-1, |E/D| = 0.07 ± 0.03 with giso = 2.20 ± 0.10 (Figure 2D).
Additionally, the solid 80 K 57Fe Mössbauer spectrum of crystals
of 1 (Figure 2B) yields identical parameters (δ = 0.25 mm/s and
ΔEQ = 1.36 mm/s, green component) to those obtained from in-
situ reaction of Fe(acac)3 with MeMgBr in the presence of NMP
(Figure 1A). Together, this data demonstrates that 1 is the major
iron species formed in-situ under the catalytic conditions
previously reported by Cahiez when using NMP as an additive.
Spin-unrestricted DFT calculations were performed on 1
with a focus on the ferrous trimethyl anion subunit, excluding the
[Mg(NMP)6]2+ dication in order to gain further insight into the
electrophile to generate exclusively cross-coupled product (kobs
>
6 min-1, Table 1 and Figure 4). Furthermore, the minor iron side-
products (S = 3/2 species and iron cluster (S = 1/2)), monitored
by EPR, remain unchanged when reacted with sub-stoichiometric
amounts of electrophile suggesting they are unreactive or less
reactive than 1 (Figure S3). While the iron products formed upon
reaction with electrophile could not be isolated, it was
demonstrated that the addition of 1 equiv of MeMgBr leads to
rapid regeneration of 1 (Figure 4D) suggesting that these are
under-transmetallated iron species (blue and brown
components). These results indicate that 1 is a very reactive
species for the productive formation of cross-coupled product.
Importantly, unlike [Fe8Me12]- which has been implicated as
the major reactive species in Kochi’s catalysis, 1 reacts with
electrophile without the requirement of an additional equivalent of
MeMgBr to produce cross-coupled product.[3] From reactivity
studies it appears that
1 produces less reactive under-
transmetallated iron species (Figure 4), whereas [Fe8Me12]-
generates an intermediate iron species intimately associated with
electrophile which was apparent from the disappearance of its S
= 1/2 EPR signal and the lack of formation of cross-coupled
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product. The requirement of additional MeMgBr for turnover from
this intermediate species may be the reason for the excess
electrophile required in Kochi’s protocol, as this intermediate
species is expected to be unstable and, hence, susceptible to
unproductive reaction pathways. Therefore, this observed
difference in reactivity is likely a key contributing factor to the
improved catalysis observed by Cahiez when using NMP.[4]
proved difficult, further demonstrating the strong influence of NMP
on the stability of 1 leading to greater ease of preparation and
handling. Future studies will be aimed at exploring the stability of
NMP analogs of the tetramethyliron(III) ferrate and its effect on
the overall reduction pathway of ferric salts with MeMgBr.
Table 1. GC analysis of the products formed upon reaction of in-situ generated
1 with b-bromostyrene at 23 °C
5 equiv MeMgBr
45 equiv NMP
β-bromostyrene
Me
(x equiv)
Fe(acac)3
23 °C, 10 s
1:1 THF:2-MeTHF
23 °C, 30 s
β-bromostyrene (equiv)
Me-styrene yield (%)a,b
0.25
0.50
0.75
25
50
75
[a] Yields are with respect to iron. No other side products were observed. [b]
Reactions are stereoselective and retain the same conformation in the cross-
coupled product as in the starting reagent (E- and Z-isomers present). Yields
account for both observed isomers.
Acknowledgements
This work was supported by a grant from the National Institutes of
Health (R01GM111480 to M.L.N.), an NSF Graduate Research
Fellowship (T.M.B.), an Alfred P. Sloan Fellowship (M.L.N.) and a
Ruth L. Kirchstein National Research Service award
(F32GM120823 to S.B.M. III).
Keywords: iron • cross-coupling • NMP • 57Fe Mössbauer
spectroscopy • MCD spectroscopy
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Figure 4. 80 K 57Fe Mössbauer spectra of a frozen solution of 57Fe(acac)3 (9
equiv of NMP relative to MeMgBr) with (A) 5 equiv of MeMgBr and subsequent
addition of (B) 0.25 equiv, (C) 0.75 equiv of β-bromostyrene and (D) 0.75 equiv
of β-bromostyrene followed by 1 equiv of MeMgBr (black dots), total fit (black
line) and individual fit components are shown.
[2]
[3]
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S. B. Muñoz III, S. L. Daifuku, W. W. Brennessel, M. L. Neidig, J.
Am. Chem. Soc. 2016.
While a previously identified tetramethyliron(III) ferrate,
[MgCl(THF)5][FeMe4], was implicated as an intermediate along
the reduction pathway of FeCl3 and MeMgBr to generate
[Fe8Me12]-, it is interesting to consider whether a trimethyliron(II)
ferrate such as 1, is accessible along this pathway in the absence
of NMP as co-solvent since 57Fe Mössbauer parameters
consistent with [Fe8Me12]- are generated upon prolonged mixing
of 1 (δ = 0.30 mm/s and ΔEQ = 0.87 mm/s, Figure S2).[11] To
explore this possibility, we attempted to synthesize a [FeMe3]- at
low temperature without NMP (see SI). Crystals of
[Mg3Cl3(OMe)2(THF)6][FeMe3]·2THF were identified by XRD.
Note that the methoxy-substituents of the magnesium cation are
attributed to a methanol impurity in the Grignard reagent.
Synthesis of this analogous structure suggests the possibility of 2
being an intermediate species between the tetramethyliron(III)
ferrate and iron-cluster. However, the consistent preparation of 2
[4]
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a) A. Fürstner, A. Leitner, M. Méndez, H. Krause, J. Am. Chem. Soc.
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Org. Chem. 2004, 69, 3950-3952.
a) L. K. Ottesen, F. Ek, R. Olsson, Org. Lett. 2006, 8, 1771-1773; b)
G. Cahiez, O. Gager, J. Buendia, C. Patinote, Chem. Eur. J. 2012,
18, 5860-5863; c) M.Gotta, B. W. Lehnemann, A. Jaboi von
Wangelin, S. Guelak, US 9,024,045 B2, 2015; d) S. Gülak, T. N.
Gieshoff, A. Jacobi von Wangelin, Adv. Synth. Catal. 2013, 355,
2197-2202; e) S. Malhotra, P. S. Seng, S. G. Koenig, A. J. Deese,
K. A. Ford, Org. Lett. 2013, 15, 3698-3701; f) D. Gärtner, A. L. Stein,
S. Grupe, J. Arp, A. Jacobi von Wangelin, Angew. Chem., Int. Ed.
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K. Ding, F. Zannat, J. C. Morris, W. W. Brennessel, P. L. Holland, J.
Organomet. Chem. 2009, 694, 4204-4208.
a) M. M. Williamson, C. M. Prosser-McCartha, S. Mukundan, C. L.
Hill, Inorg. Chem. 1988, 27, 1061-1068; b) Q. Zhou, T. W. Hambley,
B. J. Kennedy, P. A. Lay, P. Turner, B. Warwick, J. R. Biffin, H. L.
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[6]
[7]
[8]
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COMMUNICATION
Salvador B. Muñoz III, Stephanie L.
Daifuku, Jeffrey D. Sears, Tessa M.
Baker, Stephanie H. Carpenter, William
W. Brennessel and Michael L. Neidig*
Page No. – Page No.
The N-methylpyrrolidone (NMP) Effect
in Iron-Catalyzed Cross-Coupling with
Simple Ferric Salts and MeMgBr
The N-methylpyrrolidone (NMP) effect promoting highly selective formation of cross-
coupled products in reactions utilizing simple ferric salts and alkyl Grignard reagents
is explored.
A
new three-coordinate homoleptic iron-alkyl complex,
[Mg(NMP)6][FeMe3]2 (1), is identified as the key in-situ formed iron species in
reactions with MeMgBr and shown to be highly reactive for the selective generation
of cross-coupled product.
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