Unusual structure and reactivity of a homoleptic "super-ate" complex of iron: Implications for Grignard additions, cross-coupling reactions, and the Kharasch deconjugation

Article abstract of DOI:10.1002/anie.200502859

The additional molecule of methyllithium present in [(Me ₄Fe)(MeLi)][Li(OEt₂)]₂ (1), the first structurally characterized ate complex of Fe^II bearing only alkyl substituents without any further stabilizing ligands (see picture: Fe: magenta; Li: cyan; O: red; C and H: gray), is a particularly remarkable structural feature and contributes to the fascinating reactivity profile of 1 in the title reactions. (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200502859

Communications
Synthetic Methods
DOI: 10.1002/anie.200502859
Unusual Structure and Reactivity of a Homoleptic
“Super-Ate” Complex of Iron: Implications for
Grignard Additions,Cross-Coupling Reactions,
and the Kharasch Deconjugation**
Alois Fürstner,* Helga Krause, and
Christian W. Lehmann
Our recent investigations on iron-catalyzed cross-coupling
reactions of organomagnesium reagents with various electro-
philes were guided by the perception that bare, low-valent
iron clusters formed in situ according to Scheme 1 might
Scheme 1. Proposed formation of low-valent, intermetallic iron clusters
serving as the actual catalysts in cross-coupling reactions of aryl
chlorides and alkyl Grignard reagents with two or more carbon atoms.
NMP=N-methylpyrrolidone.
account for the observed turnover.[1–5] Since the analysis of
the released gas suggests that this reduction process involves a
b-hydride elimination/reductive coupling event,[6] it should
proceed only with Grignard reagents containing at least two
[*] Prof. A. Fürstner, H. Krause, Dr. C. W. Lehmann
Max-Planck-Institut für Kohlenforschung
45470 Mülheim/Ruhr (Germany)
Fax: (+49)208-306-2994
E-mail: fuerstner@mpi-muelheim.mpg.de
[**] Generous financial support by the MPG and the Fonds der
Chemischen Industrie is gratefully acknowledged. We thank Dr. B.
Scheiper for performing the reaction of enol triflate 8.
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carbon atoms. In line with this notion, MeMgBr essentially
fails to react with methyl 4-chlorobenzoate in the presence of
[Fe(acac)3](acac = acetylacetone) or FeCln (n = 2, 3) as
precatalysts, whereas all higher alkyl Grignard reagents are
cross-coupled with exceptional ease under these conditions
(Scheme 1).[1] This strikingly different behavior is general; in
fact, methyl groups are transferred only to highly activated
substrates such as enol triflates, acid chlorides, and very
electron-deficient heteroarenes.[7,8]
To confirm this dichotomy in the proposed mechanism
further, we set out to elucidate the structure of the reagent
derived from FeXn and MeMgBr. It was previously proposed
that a host of homoleptic non-stabilized alkyl-iron species
and/or iron-ate complexes of different composition is formed
depending on the chosen stoichiometry, although none of
these putative intermediates has ever been confirmed by
spectroscopic or structural data.[9,10] Only the rapid reduction
of FeCl3 to FeCl2 on treatment with the first equivalent of
MeLi or MeMgBr is formally secured.[11]
Reaction of an anhydrous ethereal solution of FeCl3 with
MeLi at low temperature (< À308C), removal of the precipi-
tated LiCl, and evaporation of the solvent at À788C afforded
a red solid 1, which dissolves in THF to give bright yellow
colored solutions (Scheme 2). This material was previously
Figure 1. Structure of the iron “super-ate” complex 1 in the solid state.
Li1 is the unlabeled ellipsoid in the back of the tetrahedral metallic
À
À
frame. Selected bond lengths [Š]: Fe1 C1 2.095(4), Fe1 C2 2.188(3),
À
À
À
À
Fe1 C3 2.182(4), Li1 C2 2.279(4), Li1 C4 2.181(7), Li2 C3 2.298(5),
À
Li2 C4 2.190(6). The hydrogen atoms of methyl group C1 are
disordered. Anisotropic displacement parameter ellipsoids are drawn
at the 50% probability level.
mentioned methyl groups, the fourth face is capped sym-
À
metrically by a fourth methyl group (av Li C distance
2.186(6) Š) devoid of any direct contact to the iron center.
The coordination sphere of two of the lithium atoms is
completed by O-ether contacts (Li2 and Li2*), whereas Li1
forms a close contact of 2.301(6) Š to the carbon atom of the
terminal Fe-methyl group of a neighboring molecule.
Scheme 2. Formation of the iron “super-ate” complex 1 from FeCl3
and MeLi.
Overall, the structure of 1 shows a striking similarity to the
solid-state structure of the methyllithium tetramer.[17] The
thought to consist of [Me4Fe][Li(OEt2)]2.[12] Although the
reported protonolysis data roughly matched this composi-
tion,[12] all previous attempts to further characterize this
compound remained unsuccessful.[13] Gratifyingly, however,
we managed to obtain crystalline samples suitable for crystal
structure analysis upon slow cooling of saturated solutions of
1 in Et2O from about À408C to À788C. Greatest possible care
has to be taken due to the exceptional sensitivity of this
product, which ignites in air and rapidly decomposes both in
solution and in solid form at ꢀ 08C.
structure of (MeLi)4 has crystallographic Td or 43m symmetry,
¯
À
resulting in only one independent Li Li distance (2.68(5) Š),
which is within one standard uncertainty of the average length
À
of the tetrahedral edge in 1. The comparison between the Li
C distances in (MeLi)4 and 1 underlines this similarity
further.[18]
Based only on interatomic distances, it is difficult to
deduce a “simple” model for the chemical bonding in 1.
Graphical representations emphasizing either the ferrate
substructure with its “intertwined” lithium cap or the
tetrahedral metallic frame are depicted in Scheme 3. Overall,
the product derived from FeCl3 and excess MeLi analyzes as
[(Me4Fe)(MeLi)][Li(OEt2)]2 (1). As such, 1 represents the
first alkyl-ate complex of iron devoid of any stabilizing ligands
that has been structurally characterized and adds one more
example to the very small family of homoleptic iron
complexes with h1-bound carbon entities other than CO.[19,20]
Next, the reactivity of 1 was checked in a set of prototype
transformations. As expected, this compound essentially fails
to alkylate p-XC6H4COOMe (2; X = Cl, I, OTf; Scheme 4),
thus corroborating the notion that ate-complexes play no role
in the iron-catalyzed cross-coupling reactions of higher alkyl
Grignard reagents mentioned above (cf. Scheme 1).[1] Like-
wise, alkenyl halides such as 4 were found to be rather poor
substrates; this result is in striking contrast to previous reports
that an iron-ate complex of the putative composition
The remarkable structure of the resulting complex 1[14]
comprises a homoleptic ferrate moiety in which four methyl
groups surround the FeII atom in an almost ideal tetrahedral
arrangement (Figure 1).[15] The molecule possesses crystallo-
graphic mirror symmetry with the iron atom, one lithium, and
two methyl groups situated in special positions. The average
of the four independent C-Fe-C angles is 109(3)8. For three
(only two are crystallographically independent) of the four
À
methyl groups the average Fe C distance is 2.185(4) Š,
whereas the distance to the fourth methyl group is shortened
to 2.095(4) Š. The three former methyl groups each exhibit
two further short distances to three lithium atoms (2.279(4)–
2.321(5) Š). These lithium atoms form an equilateral trian-
gle[16] and, together with the iron atom, a tetrahedral metal
framework that shows Td symmetry. Three of the four sides of
this cluster are capped in a m3-fashion by the already
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Communications
chloride 10 affords the corresponding acetophenone deriva-
tive 11, again without any competing side reaction of the aryl
chloride moiety interfering (Scheme 6). When 1 is used in
excess, the reaction proceeds further to give the carbonyl
Scheme 3. Two different representations of the structure of the FeII-ate
complex 1: Top: Graphical illustration of the metal cluster forming the
inner core; the methyl units are represented as balls, four of which
reside over the faces of this tetrahedron. Bottom: The ferrate unit is
color-coded in red; its negative charges are counterbalanced by the
intertwined lithium “cap” carrying an extra methyl group.
Scheme 6.
addition product 12 and the corresponding pinacol 13,
suggesting that the ate-complex also acts as a single-electron
transfer (SET) agent towards the ketone initially formed. This
result is highly reminiscent of early work of Ashby et al. who
found that the amount of pinacol produced in attempted
additions of MeMgBr to aromatic ketones is directly propor-
tional to the iron impurities of the magnesium metal used to
form the Grignard reagent.[22] Our results, therefore, suggest
that the reagent responsible for this striking correlation is an
ate-complex of the type described herein.[23] More generally
speaking, this finding sheds light into the well established but
hardly understood “alchemistic” effects exerted by certain
transition-metal residues on the course of Grignard reac-
tions.[24]
Scheme 4.
The SET potential of complex 1 is also responsible for the
conversion of dibromide 14 to carbazole 15 (Scheme 7). This
“Me4FeLi2” formed in situ methylates bromide 4 in high yield
and selectivity.[9,21] Moreover, the claimed stability of the in
situ species up to 408C[9] is inconsistent with our data.
Complex 1, however, transfers its methyl groups to more
activated electrophiles such as enol triflates, as previously
communicated (Scheme 5).[7] Along the same lines, acid
Scheme 7.
Ullmann-type coupling was recently effected with the puta-
tive complex “Me4FeLi2” formed in situ.[25] Since the defined
metalate 1 provides the same result, it seems reasonable to
assume that it constitutes the actual intermediate operative
under the in situ conditions.
Finally, we propose that ate-complexes similar to 1 also
play a vital role in the deconjugation of a,b-unsaturated
enones pioneered by Kharasch et al. using catalytic amounts
of FeCl3 and excess MeMgBr.[26] A modified version of this
process allows the formation of thermodynamic silyl enol
ethers that are difficult to obtain otherwise.[27] Although this
method has undergone significant empirical optimiza-
tion,[27,28] the identity of the active species remained obscure,
Scheme 5.
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except for the vague statement that it might consist of a
“mixture of activated zero-valent iron and Grignard
reagent”.[27] To check whether ate-complexes play any
significant role in this transformation, we treated dihydro-
carvone 16[29] according to the original recipe (FeCl3 cat.,
MeMgX)[30] as well as with complex 1 as the catalyst
(Scheme 8). The crude product spectrum obtained after
defined homoleptic complexes of iron, in contrast to those
of the neighboring elements in the periodic table, are
extremely scarce,[20] we hope that our data will help to
unravel the as of yet largely unknown chemistry and catalytic
potential of such species. Concerning its reactivity profile,
compound 1 obviously combines many different facets
including nucleophilic behavior, a prominent electron-trans-
fer capacity, as well as a significant basicity. With this in mind,
we like to express a “caveat” concerning selectivity claims for
reactions invoking putative iron-ate complexes of different
“formal” compositions found in the early literature,[9] notably
in those cases where the reactions have been performed at
higher temperatures.
Received: August 11, 2005
Published online: December 2, 2005
Keywords: ate complexes · cross-coupling · Grignard reaction ·
.
iron · single-electron transfer
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the corresponding 1,2- and 1,4-addition products 18 and 19, and the
dimer 20. TMSCl=trimethylsilyl chloride, DMPU=N,N’-dimethyl-
N,N’-propylene urea.
quenching of the reaction with TMSCl/Et3N is virtually
identical for both cases. The silyl enol ether 17 is the major
compound accompanied by the corresponding 1,2- and 1,4-
addition products 18 and 19 as well as dimer 20 (diastereo-
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without any further stabilizing ligands; the fact that 1
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Communications
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[29]Dihydrocarvone was prepared as described in ref. [30.] The
reported 13C NMR data of this compound, however, are
incorrect; compound 16 analyzes as follows: 13C NMR
(75 MHz, CDCl3): d = 200.5, 145.2, 142.0, 141.9, 135.2, 31.9,
29.7, 19.5, 19.4, 15.5 ppm.
100 K, Z = 4, 1calcd = 1.017 gcmÀ3
, l = 0.71073 Š, m(MoKa) =
0.762 mmÀ1, Nonius Kappa CCD diffractometer, 3.30 < q <
27.088, 17506 measured reflections, 2231 independent reflec-
tions, 1601 reflections with I > 2s(I), structure solved by direct
methods and refined by full matrix least-squares on F2o to R1 =
0.050 [I > 2s(I)], wR2 = 0.114, 116 parameters, H atoms mixed,
S = 1.038, residual electron density + 0.5/À0.3 eŠÀ3. CCDC-
279956 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
[15]The four independent C-Fe-C angles are 113.67(15), 109.24(9),
109.28(9), and 105.83(15)8.
[16]The standard uncertainty of the average edge is within three
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À
À
[18]The C Li distance in Me4Li4 is 2.31(5) Š, the average C Li
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