Cross-coupling of alkyl halides with aryl Grignard reagents catalyzed by a low-valent iron complex

Article abstract of DOI:10.1002/anie.200460504

A striking reversal of the usual reactivity pattern of aryl Grignard reagents is observed for reactions in the presence of catalytic amounts of the "bare" ferrate complex [Li(tmeda)]₂[Fe(C₂H ₄)₄] (1). Highly reduced iron-magnesium clusters may play a decisive role in the exceptionally facile and chemoselective cross-coupling reaction with alkyl halides (see scheme).

Full text of DOI:10.1002/anie.200460504

Angewandte
Chemie
Reversal of Reactivity
weakly ligated by four ethylene molecules, and strong ion-pair
interactions between the ferrate unit and the peripheral
lithium cations are observed in the solid state. Since these
structural features are somewhat reminiscent of the assumed
bonding situation in the intermetallic cluster [Fe(MgX)2]n,
complex 1 could qualify as a catalyst for similar purposes.
As can be seen from Scheme 1, this is indeed the case.
Thus, reaction of chloride 2 with hexylmagnesium bromide in
THF/NMP (NMP = N-methylpyrrolidone) at 08C proceeded
within minutes, delivering the desired product 3 in 85% yield.
This outcome compares well to the result obtained under “in
situ” conditions.[10]
Cross-Coupling of Alkyl Halides with Aryl
Grignard Reagents Catalyzed by a Low-Valent
Iron Complex**
RubØn Martin and Alois Fürstner*
While palladium- and nickel-catalyzed cross-coupling reac-
tions of aryl and vinyl halides have evolved over decades into
mature tools for advanced organic synthesis,[1] it was only
recently that extensions of this chemistry to alkyl halides as
the substrates have been possible.[2–7] The use of special
ligands and additives as well as careful optimization of the
reaction conditions were necessary to overcome the reluc-
tance of alkyl halides to undergo oxidative addition and to
suppress the proclivity of the resulting alkyl metal reagents
for destructive b-hydride elimination.[2–7] Therefore it may
come as a surprise that bare, low-valent iron species under
“ligand-free” conditions offer a simple and powerful alter-
native. Prompted by recent reports in the literature,[8] we
disclose our results on the remarkable efficiency and excellent
selectivity profile of alkyl–aryl cross-coupling reactions
catalyzed by a well-defined FeÀII complex.
Scheme 1.
Even more gratifying is the fact that complex 1 also
catalyzes the cross-coupling of alkyl halides and various aryl
Grignard reagents or phenyllithium with exceptional ease
(Scheme 2). Primary alkyl iodides and secondary alkyl
Encouraged by early reports of Kochi et al.,[9] our group
has launched a program to explore in more detail the
potential of iron catalysts as substitutes for palladium and
nickel. Various types of substrates were found to undergo
effective cross-coupling reactions with Grignard reagents in
the presence of FeXn (n = 2 , 3; X= Cl, acac (acac = acetyl-
acetonate)) as precatalysts.[10–17] These applications are dis-
tinguished by the low cost, ready availability, and benign
character of the required iron salts as well as by exceptionally
high reaction rates and notably mild conditions. Although the
mechanisms are far from clear, it was speculated that highly
reduced iron–magnesium clusters of the formal composition
Scheme 2.
bromides, as well as propargyl and allyl halides react
smoothly, affording the desired arylated products in virtually
quantitative yields in most cases; only tertiary halides and
alkyl chlorides were found to be inert. Since these reactions
usually proceed within minutes even at À208C, they compare
favorably to the palladium- and nickel-based procedures for
alkyl–aryl cross-coupling known to date[2–7] and are clearly
more effective than related arylations employing stoichio-
metric amounts of organocopper reagents.[20]
The unprecedented rate of productive cross-coupling of
alkyl halides in the presence of complex 1 translates into an
excellent chemoselectivity profile. Although one might
assume that the use of organomagnesium reagents as the
nucleophiles inherently restricts the functional-group toler-
ance of the method, the results compiled in Table 1 show that
this is not the case. The iron-catalyzed activation of the alkyl
halide turned out to be significantly faster than the uncata-
lyzed attack of the Grignard reagent to various other polar
groups in the substrates, thus leaving ketones, esters, enoates,
chlorides, nitriles, isocyanates, ethers, acetals, and trimethyl-
silyl groups intact. Moreover, tertiary amines do not interfere
[18]
[Fe(MgX)2]n generated in situ may play a decisive role in
the catalytic cycle.[10,12]
To probe this hypothesis, we investigated whether the
reactivity of these presumed clusters can be emulated by
structurally well-defined complexes containing an FeÀII
center. Of these, the tetrakis(ethylene)ferrate complex
[Li(tmeda)]2[Fe(C2H4)4] (1) (tmeda = N,N,N’,N’-tetramethyl-
ethylenediamine) described by Jonas et al.[19] seemed to be
most promising. Its highly reduced metal center is only
[*] Dr. R. Martin, Prof. A. Fürstner
Max-Planck-Institut für Kohlenforschung
45470 Mülheim an der Ruhr (Germany)
Fax: (+49)208-306-2994
E-mail: fuerstner@mpi-muelheim.mpg.de
[**] Generous financial support from the Deutsche Forschungsge-
meinschaft (Leibniz Award Program) and the Fonds der Chem-
ischen Industrie is gratefully acknowledged. R.M. thanks the
Alexander-von-Humboldt Foundation for a postdoctoral fellowship.
We express our gratitude to Prof. K. Jonas, Mülheim, for valuable
discussions and for providing a generous sample of the iron
complex 1, as well as to Dr. A. Leitner for performing preliminary
experiments.
À
with productive C C bond formation. This remarkable
tolerance allowed us, for example, to convert even ethyl a-
Angew. Chem. Int. Ed. 2004, 43, 3955 –3957
DOI: 10.1002/anie.200460504
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Communications
Table 1: Cross-coupling reactions of alkyl halides catalyzed by [Li(tmeda)]2[Fe(C2H4)4] (1) (5 mol%). All
reactions were performed in THF at À208C unless stated otherwise.
bromobutyrate into ethyl 2-phenylbutyrate
in
excellent
yield
within
minutes
(entry 23).[21] Substrates bearing more than
one halide function are subject to exhaustive
arylation (entries 26, 29; for an exception
see entry 19).
Entry
Substrate
RMgX/RLi
Product
Yield [%]
1
2
3
4
5
6
PhMgBr
PhLi
p-MeOC6H4MgBr
p-ClC6H4MgBr
p-PhC6H4MgBr
m-(Me3Si)2NC6H4MgBr
94/89[a] (X=H)
92 (X=H)
95 (X=OMe)
67 (X=Cl)[b]
93 (X=Ph)[b]
88 (X=NH2)[b,c]
Despite previous reports in the literature
on iron-catalyzed transformations of allylic
phosphates,[22] effective cross-coupling of
the more abundant allylic halides have not
yet been described. Entries 14 and 24–29,
however, show that complex 1 is highly
adequate for this purpose. In all cases
investigated, the aryl group is introduced
regioselectively at the least hindered site of
the allylic system. Propargyl bromides
behave equally well; only minor amounts,
if any, of allenic by-products are formed
under the chosen conditions (entries 30–32).
Although the development of this
method was driven by the assumption that
the ferrate complex 1 might mimic the
behavior of the alleged iron clusters that
supposedly operate under “in situ” condi-
tions,[8,10] a concise mechanistic interpreta-
tion of the results outlined above is not yet
possible. While the complete loss of optical
purity in the reaction of (R)-2-bromooctane
(98% ee) with PhMgBr (entry 12) could be
explained by either an organometallic or by
a radical pathway, the fact that the 2-
iodoacetal derivative shown in entry 33
undergoes ring-closure prior to cross-cou-
pling seems to indicate radical intermediates
in this particular case.[23] Care, however,
must be taken in generalizing this, because
several other compounds set up for analo-
gous 5-exo-trig pathways do not cyclize
under otherwise identical conditions (cf.
entries 13, 14, 25, 32). Moreover, tertiary
halides remain unchanged, although they
would afford the most stabilized alkyl rad-
icals. In further studies we aim to unravel the
mechanistic basis of this versatile iron-
catalyzed alkyl–aryl coupling process and
fully explore its preparative scope.
7
p-MeC6H4MgBr
95
8
9
PhMgBr
PhMgBr
96 (X=I)
61 (X=Br)
10
2,4-(CH3)2C6H3MgBr
94
11
12
PhMgBr
PhMgBr
74[b]
93
13
14
PhMgBr
PhMgBr
89
84
15
16
PhMgBr
PhMgBr
91
88
17
PhMgBr
83 (X=I)
18
19
PhMgBr
PhMgBr
90
86
20
21
PhMgBr
p-PhC6H4MgBr
87 (X=H)
85 (X=Ph)[b]
22
23
PhMgBr
PhMgBr
95
87
24
25
PhMgBr
PhMgBr
94
93
26
27
PhMgBr[d]
PhMgBr
96
87
Experimental Section
28
29
PhMgBr
PhMgBr[d]
97
98
Representative example: Table 1, entry 26: A
solution of 2-benzoyl-6-bromo-2-(4-bromo-but-2-
enyl)-hex-4-enoic acid ethyl ester (228 mg,
0.5 mmol) in THF (1 mL) was added to a solution
of [Li(tmeda)]2[Fe(C2H4)4] (1) (9 mg, 5 mol%) in
THF (3 mL) at À208C under argon, causing an
immediate color change from green to red. At that
point, PhMgBr (1m in THF, 1.2mL, 1.2mmol)
was added dropwise, and the resulting mixture
was stirred at À208C for 5 min. Quenching of the
reaction with saturated aq NH4Cl followed by a
30
31
PhMgBr
PhMgBr
93[e]
96[f]
32
PhMgBr
87[g]
3956
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Chemie
Table 1: (Continued)
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Substrate
RMgX/RLi
PhMgBr
Product
Yield [%]
85[b]
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33
alkyl halides using structurally
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34
PhMgBr
77[h]
Matsuo, S. Ito, E. Nakamura, J.
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ethyl acetate 4:1) of the crude product afforded 2-benzoyl-6-phenyl-
2-(4-phenyl-but-2-enyl)-hex-4-enoic acid ethyl ester as a colorless
solid (217 mg, 96%). 1H NMR (CDCl3, 400 MHz): d = 0.92(t, J =
6.8 Hz, 3H), 1.98 (m, 2H), 2.37 (m, 2H), 3.44 (m, 2H), 3.89 (m, 4H),
4.61 (d, J = 6.2Hz, 2H), 5.23 (m, 2H), 7.31 ppm (m, 15H); 13C NMR
(CDCl3, 100 MHz): d = 13.6, 30.8, 30.9, 52.8, 52.9, 60.3, 60.4, 77.8, 77.9,
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Received: April 29, 2004 [Z460504]
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