A practical procedure for iron-catalyzed cross-coupling reactions of sterically hindered aryl-grignard reagents with primary alkyl halides

Article abstract of DOI:10.1002/adsc.201301089

Although iron-catalyzed cross-coupling reactions of arylmagnesium halides with alkyl halides are well established and proceed effectively under a variety of experimental conditions, they often find limitations when working with sterically hindered aryl-Grignard reagents. Outlined in this paper is a practical solution that allows this gap in coverage to be filled. Specifically, it is shown that bis(diethylphosphino)ethane (depe) crafts an effective coordination environment about Fe(+2). This commercially available ligand is slim enough not to interfere with the loading of the iron center even by ortho,ortho-disubstituted arylmagnesium halides, yet capable of preventing premature reductive coupling of the resulting organoiron species, which seem to be hardly basic either. The reaction is compatible with various polar functional groups as well as with substrates containing β-heteroatom substituents. Moreover, the procedure even allows encumbered neopentylic electrophiles to be arylated with donors as bulky as mesitylmagnesium bromide, whereas secondary alkyl halides tend to eliminate.

Full text of DOI:10.1002/adsc.201301089

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DOI: 10.1002/adsc.201301089
A Practical Procedure for Iron-Catalyzed Cross-Coupling
Reactions of Sterically Hindered Aryl-Grignard Reagents with
Primary Alkyl Halides
Chang-Liang Sun,a Helga Krause,a and Alois Fꢀrstnera,*
a
Max-Planck-Institut fꢀr Kohlenforschung, D-45470 Mꢀlheim/Ruhr, Germany
Fax : (+49)-208-306-2994; e-mail: fuerstner@kofo.mpg.de
Received: December 5, 2013; Revised: February 3, 2014; Published online: March 27, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201301089.
Abstract: Although iron-catalyzed cross-coupling re- preventing premature reductive coupling of the re-
actions of arylmagnesium halides with alkyl halides sulting organoiron species, which seem to be hardly
are well established and proceed effectively under basic either. The reaction is compatible with various
a variety of experimental conditions, they often find polar functional groups as well as with substrates
limitations when working with sterically hindered containing b-heteroatom substituents. Moreover, the
aryl-Grignard reagents. Outlined in this paper is procedure even allows encumbered neopentylic elec-
a practical solution that allows this gap in coverage trophiles to be arylated with donors as bulky as mesi-
to be filled. Specifically, it is shown that bis(diethyl- tylmagnesium bromide, whereas secondary alkyl hal-
phosphino)ethane (depe) crafts an effective coordi- ides tend to eliminate.
nation environment about Fe(+2). This commercially
available ligand is slim enough not to interfere with
the loading of the iron center even by ortho,ortho- Keywords: alkyl halides; cross-coupling; diphos-
disubstituted arylmagnesium halides, yet capable of phines; Grignard reagents; iron; radicals
Introduction
NMe2, N
ACHTUNGTERN(NUNG SiMe3)2] with functionalized primary and
secondary alkyl halides with remarkable rates at tem-
Although the dominance of palladium catalysts over peratures as low as À208C, failed when MesMgBr
the field of cross-coupling is likely to persist,[1] other was used.[18] Likewise, the combination of FeCl3
metals do provide valuable alternative solutions for (5 mol%) and bulky NHC ligands did not allow mesi-
specific applications. In this context, the interest in tyl groups to be transferred either, although the cata-
iron was rejuvenated during the last decade because lysts formed in situ from these components accept
of the obvious benefits that this cheap, benign and even unreactive alkyl chlorides otherwise.[24,25]
readily available metal can provide.[2–12] A particularly
Amongst the few cases of successful cross-couplings
valuable facet of its chemistry is the ease with which of hindered arylmagnesium halides, two reports by
different iron catalysts promote cross-coupling reac- Nakamura and co-workers stand out (Scheme 1).[26–29]
tions of alkyl halides,[13–16] which do not belong to the These authors showed that FeCl3 (5 mol%) in the
privileged substrates in the realm of palladium cataly- presence of excess tmeda (1.2 equiv.) effects the cou-
sis.[17] Such reactions are distinguished by an apprecia- pling of 1-bromooctane 2 (n=1) with MesMgBr,
ble functional group tolerance, including many sub- albeit in modest yield (32%).[26] A stoichiometric con-
stituents which are a priori susceptible to uncatalyzed trol experiment using the relatively stable iron com-
attack by an organomagnesium species as the most plex 1 furnished product 3 in 76% yield after 9.5 h at
commonly used nucleophilic partners.[13–16,18–23]
This considerable scope notwithstanding, successful nucleo
cross-coupling reactions of sterically hindered aryl- Shortly thereafter, the same group demonstrated that
magnesium halides with alkyl halides are conspicu- complex 4 endowed with a very bulky 1,2-bis{bis[3,5-
ously rare. Even the highly reactive ferrate complex (di-tert-butyl)phenyl]phosphino}benzene [3,5-(t-Bu)2-
[Li(tmeda)]2[Fe(C2H4)4] (5 mol%), which effects the SciOPP] ligand fares much better in such challenging
reaction of p-XC6H4MgBr [X=H, Me, Ph, OMe, Cl, transformations.[27,28]
308C, suggesting that 1 might be the competent
phile generated in situ in the catalytic manifold.
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
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Scheme 1. Important literature precedent for iron-catalyzed cross-coupling reactions of alkyl halides with MesMgBr. Mes=
mesityl (2,4,6-Me3C6H2-); tmeda=N,N,N’,N’-tetramethylethylenediamine.
Outlined below we present a practical alternative remains largely unexplored.[35] To fill this gap, we pre-
solution for iron-catalyzed cross-coupling of ortho- pared a representative set of complexes of this type
substituted aryl-Grignard reagents with primary alkyl by following the literature route (Scheme 2) and en-
halides, based on the use of bis(diethylphosphino)- gaged them into stoichiometric reactions with differ-
ethane (depe) as ligand for FeCl2. The fact that this ent electrophilic partners that are known to be ame-
low molecular weight catalyst system is uniquely ef- nable to iron-catalyzed cross-coupling. To calibrate
fective for the transfer of sterically hindered nucleo- our results, this investigation included complexes
philes such as MesMgBr but hardly useful for slimmer 1 and 14[36] carrying the ligands that had already been
donors has mechanistic significance. Reductive elimi- successfully used by the Nakamura group.[26,27] More-
nation of two encumbered aryl groups from a single
iron center is impeded on steric grounds, whereas less
bulky nucleophiles likely generate low-valent iron
species. This conclusion corroborates our previous
view that iron-catalyzed cross-coupling reactions are
mechanistically by no means uniform; rather, they
can follow different pathways depending on the
chosen nucleophiles.[2,18,30]
Results and Discussion
The Stoichiometric Regime
Mechanistic investigations into iron-catalyzed reac-
tions are challenging, not least because of the excep-
tional sensitivity of many organoiron compounds.
One important exception, however, are iron mesityl
complexes of the general type [L2FeACTHNUTRGEN(UGN Mes)2] (Mes=
2,4,6-trimethylphenyl), many of which are thermally
surprisingly stable. They are readily accessible by ad-
dition of the appropriate ligand(s) L to the dinuclear
complex [Fe
many complexes of the general type [L2Fe
were fully characterized in structural terms in the
past,[34] their reactivity vis-ꢀ-vis organic electrophiles tronic character.
2ACHTUNGTRENNUNG
(Mes)4] (6) (Scheme 2).[31–33] Although
AHCTUNGERTG(NNUN Mes)2]
Scheme 2. Assortment of iron bis
ancillary ligands of greatly different steric demand and elec-
ACHTUNGERTN(NUNG mesityl) complexes with
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A Practical Procedure for Iron-Catalyzed Cross-Coupling Reactions
Table 1. Results of the reaction of 1-iodoundecane (15) with
stoichiometric amounts of different iron mesityl complexes
in THF at 708C.[a]
the yields varied surprisingly little with the electronic
character of the ancillary chelate ligand. Thus, the
tmeda complex 1 (entry 1) and the 1,10-phenanthro-
line complex 7 (entry 3) gave an almost identical
product distribution, although tmeda is a strong s-
donor whereas phenanthroline has substantial p-ac-
ceptor properties; as one might expect, however, the
reaction rate was higher for complex 1. By far the
best results in terms of yield, reaction rate and purity
of the crude material were obtained with 1,2-bis(di-
Entry Complex t [h] Mes–Mes [%, GC] 16 [%, GC]
1
2
3
1
6
7
2.5
2.5
4
3
12
4
82
66
83
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
4
8
2
3
54
the crude mixture, thus rendering product isolation
very facile. Entry 8 shows that this complex is able to
transfer both mesityl groups to the alkyl iodide.
Entry 9 illustrates that bromoalkanes are also amen-
5
9
24
4[c]
0.5
1[d]
2
6.5
2.5
2.5
2.5
1
17
3.7[c]
<1
1[d]
2
10
7
7
65
6[c]
7
10
10
10
10
11
12
13
14
19
21
68[c]
91
8[d]
9[e]
10
11
12
13
14
15
91[d]
83[e]
60
AHCTUNGTREGaNNUN ble to cross-coupling with complex 10 with similar
efficiency. Comparison of entries 7 and 13 reveals that
the depe ligand (C10H24P2, 206.3) clearly outperforms
its more engineered cousin 3,5-(t-Bu)2-SciOPP
(C62H88P2, 895.6) contained in complex 14[36] and has
the distinct advantage of a much lower molecular
weight.
These encouraging results prompted us to study
whether complex 10 can also be generated under con-
ditions that emulate the elementary step of a catalytic
process more closely than the procedure shown in
Scheme 2. In this context, one has to keep in mind
that mixing of depe and FeCl2 in THF does not lead
to [(depe)FeCl2]; rather, the known 2:1 adduct trans-
[(depe)2FeCl2] (17) is formed, which can be obtained
in analytically pure form when 2 equivalents of depe
are utilized (Scheme 3).[41,42] Significantly though,
treatment of 17 with MesMgBr (2.2 equiv.) in THF at
708C releases the excess phosphine and furnishes
<50[b]
63
<1
15
24
77
66
1
43[f]
[a]
Unless stated otherwise, the mass balance is made up by
mesitylene formed by partial decomposition of the com-
plexes (ref.[39]) and, in some cases, residual starting mate-
rial; unless stated otherwise, the crude mixture contained
<5% of 1-undecene.
[b]
The exact determination is difficult because of large
amounts of Bu3P in the GC trace.
[c]
[d]
[e]
[f]
The reaction was performed at ambient temperature.
1
Using only = equivalent of the complex.
2
Using 1-bromoundecane as the substrate.
33% of 1-undecene were detected in the crude mixture.
[(depe)FeACTHUNRGTNEUNG(Mes)2] (10) as the only iron-containing
over, three representative complexes of proven con- complex that could be isolated from the reaction mix-
stitution bearing monodentate ligands were also in- ture in 81% yield by crystallization. The constitution
cluded in this screening exercise.[34,37]
of this sample and its identity with the material pre-
Whereas attempted cross-coupling reactions of pared from 6 by the literature route (Scheme 2) was
these iron nucleophiles with representative alkenyl unambiguously
proven
by
X-ray
diffraction
and aryl chlorides were not rewarding,[38] complex (Figure 1).[33,43] The ease of formation of 10 from
1 reacted cleanly with 1-iodoundecane (15) to give adduct 17 is interpreted in the light of a previous
product 16, in line with the result of Nakamuraꢂs study which suggested that [(depe)2FeCl2] might un-
model study alluded to above.[26] At ambient tempera- dergo reversible dissociation of one of the bidentate
ture, the coupling proceeded rather slowly (68% GC ligands from the iron center in solution.[42]
after 6 h), but it became effective when performed at
708C (82% GC yield after 2.5 h) (Table 1, entry 1).
Diphosphines that are sterically more encumbered
than depe led to a different outcome (Scheme 3,
However, the ability of 1 to react with 1-iodounde- bottom). Although they afford 1:1 adducts 18 with
cane is not unique and other complexes of the type FeCl2, treatment with excess MesMgBr in THF does
[L2FeACHTUNGTRENNUNG(Mes)2] worked similarly well as long as L2 is not lead to the putative complexes 20, likely for steric
a chelating ligand set; in contrast, the ligandless com- reasons.[44] Black mixtures were produced instead,
plex 6 (entry 2) as well as complexes 12 and 13 con- from which small amounts of the ate-complex 21
taining monodentate phosphines or phosphites (en- could be isolated by crystallization in one case. Its
tries 11 and 12), respectively, were less productive.[40] constitution was confirmed by X-ray diffraction
As can be seen from the results compiled in Table 1, (Figure 2);[33] related mesityl ate-complexes, differing
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Scheme 3. Reaction of different [(diphosphine)FeCl2] complexes with MesMgBr in THF.
Figure 1. Structure of complex 10[34e] in the solid state, which
was formed by reaction of complex 17 with MesMgBr
(Scheme 3).
Figure 2. Structure of the trisACTHNUTRGENUGN(mesityl)-ate complex 21 in the
solid state; co-crystallized toluene in the unit cell has been
removed for clarity (the full structure is shown in the Sup-
porting Information).
only in the escorting cation, are known in the litera-
ture.[31a,34f] Next, we checked whether this result has
any implications for catalysis:[45] in a catalytic process, tions of primary alkyl halides outlined below are not
the Grignard reagent is necessarily present in large plagued by competing elimination either, we conclude
excess relative to the chosen iron precatalyst, at least that tris
ACHTUNGTRENUN(NG mesityl)-ate complexes of type 21 cannot be
in the early stages of the reaction. One might there- the reigning nucleophiles.[56]
fore speculate that ligand-free 21 could also be gener-
Upon limiting the amount of the MesMgBr to
ated in situ from complexes 17 or 10. Although 21 re- 1 equivalent, the precursor complex 18 (R=t-Bu) was
acted with 1-iodoundecane (15) to give the product converted into the rather unusual halide-bridged
16, competing elimination of HI gained prominence, dimer 19, in which the chelate ring has been opened
leading to the formation of substantial amounts of 1- and one of the phosphine units resides unbound on
undecene (Table 1, entry 15). This result shows that a dangling side arm (Figure 3).[33] This structural fea-
21 is significantly more basic than 10, which produced ture notwithstanding, 19 reacts with 1-iodoundecane
hardly any alkene under otherwise identical condi- (15) to give the desired cross-coupling product 16 in
tions (entry 7). Since the catalytic cross-coupling reac- fair yield under the standard conditions of our assay
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A Practical Procedure for Iron-Catalyzed Cross-Coupling Reactions
in the catalytic set-up (Table 2). As expected,
[(depe)Fe(Mes)2] (10) outperformed the other tested
AHCTUNGTRENNUNG
complexes in terms of yield and purity of the crude
mixture. Although the reaction proceeded at ambient
temperature (Table 2, entry 5), the best result was
again obtained at 708C (entry 6). Neither the forma-
tion of Mes–Mes nor elimination of HI with forma-
tion of 1-undecene was a serious complication with
this catalyst. 1-Bromoundecane reacted with similar
efficiency as its iodinated congener 15 (entry 7),
which echoes the results of the stoichiometric regi-
men. As one might gather from the results outlined
above (Scheme 3), there is no need to use the pre-
formed complex 10; the robust adduct 17 (with and
without added FeCl2 meant to sequester the second
diphosphine) basically led to the same preparative
outcome (entries 8 and 9), which therefore constitutes
a convenient and practical choice.
Next, we used this complex as an effective and
user-friendly iron source to study the scope of the re-
action. To this end, MesMgBr and other sterically hin-
dered aryl-Grignard reagents were cross-coupled with
an assortment of primary alkyl iodides and bromides.
Figure 3. Structure of complex 19 in the solid state.
(entry 14). Yet, the higher efficacy of 10, the commer- Good to excellent results were obtained and the com-
cial availability of depe and its significantly lower mo- patibility of this method with various polar functional
lecular weight suggested that our further studies be groups was proven (Figure 4). Specifically, esters,
focused on this particular complex as the most prom- ethers, silyl ethers, acetals, nitriles, aryl bromides and
ising candidate en route to a practical catalytic mani- perfluoroalkyl chains are tolerated. Two carbohydrate
fold.
derivatives were also found to react without incident
to give products 38 and 39 in high yield. These latter
examples, together with the tetrahydropyranyl deriva-
tive 37 and chromanes 40 and 41, demonstrate that
the reaction is applicable to alkyl halides bearing b-
heteroatom substituents. However, a substrate con-
Catalytic Cross-Coupling of ortho-Substituted Aryl-
Grignard Reagents
The favorable outcome of the stoichiometric reactions taining an aromatic nitro group was decomposed,
of various well defined organoiron species is mirrored whereas an alkyl halide comprising an internal alkyne
Table 2. Catalytic cross-coupling of MesMgBr with 1-iodoundecane (15) as induced by different iron complexes.[a]
Entry
Complex
t [h]
Mes–Mes [%, GC]
1-undecene [%, GC]
16 [%, GC]
16 [isolated,%]
1
2
3
1
7
11
14
10
10
10
17
2.5
2.5
2.5
2.5
3[c]
2.5
20
2
1
4
1
2
5
<1
5
<1
<1
4[e]
1
5
13
3
<1
4
<1
2
<1
<1
1
75[b]
35
80
nd
nd
67
4
84
79
5[c]
6
59[c]
91
58[c]
88
7[d]
8
nd
nd
85[d]
83
9
10
11
17+FeCl2
18 (R=t-Bu)
19
2
nd
86
2.5[e]
2
88[e]
83
84[e]
nd
2
[a]
Unless stated otherwise, all reactions were performed with 5 mol% of the iron complex in THF at 708C.
[b]
When a mixture of FeCl
was reached.
2ACTHNUGTRENN(GNU THF)1.5 (5 mol%) and tmeda (1 equiv.) was used instead of preformed 1, a GC yield of 63%
[c]
[d]
[e]
At ambient temperature.
Using 1-bromoundecane as the substrate.
Using 10 mol% of the iron complex; nd=not determined.
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Figure 4. Assortment of products formed by iron-catalyzed cross-coupling of primary halides RX with Grignard reagents
bearing at least one ortho-substituent; unless stated otherwise, all reactions were performed using complex 17 as the precata-
lyst (5 mol%) in THF at 708C.
underwent competitive carbometalation of the p- bromides and iodides had been found similarly effec-
system; this reaction is currently under investiga- tive otherwise. Primary tosylates per se do not
tion.[46]
react,[47] but they can be used if the reaction mixture
It is rewarding that even sterically hindered electro- is supplemented with catalytic amounts of NaI to
philic partners can be coupled, as evident from the ensure in situ formation of the corresponding iodide
successful arylation of neopentyl iodide to afford as the actual coupling partner (see products 16 and
products 32 and 33 in respectable yields. The equally 30).[48] Unfortunately though, secondary halides tend
“neopentylic” environments in 31 and in the crowded to eliminate under the standard conditions, probably
chromane derivatives 40 and 41 further illustrate this due to the elevated temperature of 708C that is
aspect. In these cases, however, the use of the corre- needed to ensure meaningful reaction rates.
sponding iodide was mandatory, although primary
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A Practical Procedure for Iron-Catalyzed Cross-Coupling Reactions
Scheme 4. An iron-catalyzed cross-coupling delivers the starting material for the known meta-photocycloaddition that en-
ACHTUNGTRENNUNG
abled the total syntheses of retigeranic acid and the silphiperfolenes.[50,51]
With regard to the nucleophile, an assortment of plex [(depe)FeACTHNURGTNEUNG(Mes)2] (10) and relatives are hence
sterically hindered aryl-Grignard reagents was suc- unlikely to operate via a regime that involves low-
cessfully used. Specifically, 2,6-disubstituted Grignard valent iron species.[18] On the other hand, it is equally
reagents other than mesityl-MgBr also reacted cleanly unlikely that 10 reacts via a simple SN2-type alkyla-
to give the expected coupling products 41–44. Like- tion pathway, because primary tosylates per se are
wise, organomagnesium reagents with only one ortho- inert whereas primary halides are well suited sub-
substituent performed well, as evident from the pre- strates.[47] The smooth conversion of neopentylic hal-
ACHTUNGTRENNUNGparation of compounds 33–35. The scope is further il- ides, which tend to perform poorly in SN2-type reac-
lustrated by the reaction of 2,5-dimethylphenylmagne- tions, also argues for a more involved pathway. It is
sium bromide with the unsaturated iodide 45 which assumed that the reactions are triggered by single
gave product 46 in good yield (Scheme 4). Upon irra- electron transfer, as evident from the conversion of 6-
diation with light of appropriate wave length, this par- bromo-1-hexene (51) into a mixture of 52 and the cy-
ticular compound is known to undergo an intramolec- clized isomer 53 (Scheme 5). The faster-ticking radical
ular arene–alkene meta-photocycloaddition that en- clock of the cyclopropylmethyl halides 54 resulted in
À
genders a significant increase in structural complexi- quantitative ring opening prior to C C bond forma-
ty.[49] As previously reported by Wender and Singh,
products 47 and 48 are initially formed in a 1.9:1
ratio; further irradiation (pyrex filter) in the presence
of formACHTUNGTRENNUNGamide shifts the composition toward isomer 48
which undergoes homolytic cleavage of the cyclopro-
pane ring with concomitant attachment of the amide
group.[50] Product 49 thus formed served as the core
building block for an elegant total synthesis of retiger-
anic acid. Replacement of formamide by acetalde-
hyde delivers product 50, which opened a concise
entry into the angular sesquiterpenes of the silphiper-
folene family.[51]
If one takes the amount of homocoupling product
Mes–Mes as a proxy for the amount of low-valent
iron in solution, one must infer from the data shown
in Table 2 that MesMgBr does not reduce the iron
salt precursors tested in this study under the chosen
reaction conditions to any significant extent.[52] Com- Scheme 5. Radical clock experiments.
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(20.3 mg, 5 mol%, 0.041 mmol) in THF (4 mL) at 708C.
Once the addition was complete, stirring was continued at
this temperature for 30 min before the mixture was allowed
to reach ambient temperature. The reaction was quenched
with water (20 mL) and the organic phase extracted by
MeO-t-Bu (3ꢃ20 mL). The combined organic layers were
dried over Na2SO4 and evaporated, and the residue was pu-
rified by flash chromatography (silica, hexane) to give the
title compound as a colorless liquid; yield: 200.2 mg (90%).
Method B: 1-Iodoundecane (44.6 mg, 0.158 mmol) was
tion to give 55 as the only product. Collectively, these
observations suggest that the coupling of sterically
hindered arylmagnesium halides with alkyl halides de-
scribed herein proceeds via a catalytic cycle based
upon an Fe(II)/FeACTHNUTRGNE(NUG III) redox couple and the interven-
tion of alkyl radical intermediates. This conclusion ba-
sically reiterates the proposal made by Nakamura and
co-workers for [(tmeda)FeACHTNUGRTNEUNG
(Mes)2] (1).[53]
Slimmer Grignard reagents almost certainly react
by other pathways.[54] This notion is evident from the
fact that 10 is highly effective in catalyzing the cross-
coupling of 1-bromoundecane with o-MeC6H4MgBr
to give product 34 in 77% yield (Figure 4), whereas
the positional isomer p-MeC6H4MgBr as well as the
parent reagent PhMgBr both performed poorly under
otherwise identical conditions (22–32% GC yield).
Substantial amounts (25–40%, GC) of Ar–Ar were
detected in these cases, which is likely formed by re-
ductive coupling of a transient [(depe)Fe(Ar)2] (Ar=
Ph, para-tolyl) intermediate with concomitant forma-
tion of an Fe(0) species.[54]
Hence we conclude that the commercial and rea-
sonably low molecular weight diphosphine ligand
depe crafts a coordination environment about Fe(II)
that is particularly effective for the cross-coupling of
sterically hindered arylmagnesium halides with alkyl
halides. On the one hand, little barrier seems to exist
for the crucial diarylation of the central metal even
by bulky ortho,ortho’-disubstituted Grignard reagents.
The resulting loaded complexes such as 10 are averse
against reductive ligand coupling, which renders them
thermally surprisingly robust. Moreover, they are
added to
a stirred solution of [FeACTHUNTGRNUEGN(depe)Mes2] (10)
(102.9 mg, 0.206 mmol) in THF (4 mL). The mixture was
stirred at 708C until complete conversion of the starting ma-
terial was reached (the reaction was monitored by GC-MS;
the samples were quenched with MeOD). For work-up, the
mixture was allowed to reach ambient temperature, the re-
action was quenched with water (20 mL) and the organic
phase extracted by MeOtBu (3ꢃ20 mL). The combined or-
ganic layers were dried over Na2SO4 and evaporated, and
the residue was purified by flash chromatography (silica,
hexane) to give the title compound as a colorless liquid;
1
yield: 39.4 mg (91%). H NMR (400 MHz, CDCl3): d=1.02
(t, J=3.6 Hz, 3H), 1.40–1.54 (m, 18H), 2.36 (s, 3H), 2.40 (s,
6H), 2.68 (t, J=8.0 Hz, 2H), 6.93 (s, 2H); 13C NMR
(100 MHz, CDCl3): d=14.1, 19.7, 20.8, 22.7, 29.4, 29.5, 29.6,
29.7, 30.3, 32.0, 128.9, 134.7, 135.8, 136.7; IR (film): n=2921,
2853, 1614, 1578, 1484, 1466, 1376, 1205, 1117, 1029, 1012,
849, 721 cmÀ1; MS (EI): m/z (%)=274 (31), 133 (100), 120
(3), 105 (3), 91 (2), 55 (2), 41 (4); HR-MS (EI): m/z=
274.26605, calcd. for C20H34 [M]: 274.26618.
Acknowledgements
hardly basic but effective in transferring a single elec- Generous financial support by the MPG, the Alexander-von-
Humboldt Foundation (fellowship to C.-L. S.) and the Fond
der Chemischen Industrie is gratefully acknowledged. We
thank Dr. R. Goddard and Mr. J. Rust for solving the X-ray
structures reported in this paper, and Dr. Th. Netscher, DSM
Nutritional Products, for a gift of vitamin E-related chroman-
yl halide substrates.
tron to alkyl halides substrate as the likely key step
en route to the desired cross-coupling products. Since
the functional group compatibility is good and even
congested starting materials are amenable to produc-
À
tive C C bond formation, we believe that this meth-
odology provides a benign and practical solution for
a challenging problem and should therefore be of in-
terest for advanced organic synthesis.[55,56]
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[32] Complex 6 was prepared according to the literature
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solid state is known, our sample differed in that it con-
tained co-crystallized Et2O in the unit cell; for details
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[33] CCDC 973605 (21), CCDC 973606 (6), CCDC 973607
(10), CCDC 973608 (17), and CCDC 973609 (19) con-
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[44] Girolami et al. previously reported that the attempted
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with stoichiometric 10 under the standard conditions.
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[35] a) One piece of information concerns the reaction of
[(dppe)FeACHTUNGTRENNUNG(Mes)2] with CH2Cl2, which affords
[(dppe)Fe(Mes)Cl] within minutes; unfortunately, the
N
ACHTUNGTRENaNGNU tes in the presence of magnesium halides, see: S. Ito,
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able for X-ray diffraction could be grown so far, the as-
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[38] Treatment of (E)-1-(2-chlorovinyl)-4-methoxybenzene
with a stoichiometric amount of [(tmeda)Fe
furnished only small amounts (<41% GC) of the de-
sired stilbene derivative together with bis(mesityl)
ACHTUNGRTENN(UNG Mes)2] (1)
N
[53] Nakamura suggested
[(tmeda)Fe(Mes)2] (1) and [(tmeda)FeCl
ref.[26]; our data for complex [(depe)Fe
(Mes)2] (10)
a
cycle shuttling between
(31%) and mesitylene (ꢀ10%); other substrates con-
A
ACHTUNGTRENNUNG
2
À
taining C
G
ACHTUNGTRENNUNG
[39] Most of the detected mesitylene is supposed to derive
from decomposition of the respective complex, since
quenching of the reaction mixture with MeOD after
6 h reaction time led to only marginal deuterium incor-
poration.
seem to imply that the overall process might be more
complex: since 10 is able to transfer both mesityl
groups, either a second cycle shuttling between a mono-
aryliron halide and an iron dihalide species or a kineti-
cally competent ligand redistribution process must be
involved.
[40] It is currently unclear whether this is a consequence of
the fact that these complexes are trans-configured in
the ground state.
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which may in part be explained by the different combi-
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and solvents that were investigated. For studies sug-
[43] The constitution of 10 was confirmed by single crystal
X-ray diffraction; its structure was basically identical to
that reported in the literature (ref.[42]), except that two
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A Practical Procedure for Iron-Catalyzed Cross-Coupling Reactions
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[56] While this paper was in press, a report was published
that claims trisACTHNUTRGNEUNG(mesityl)-ate complexes related to 21 to
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