Transition-metal-free homocoupling of organomagnesium compounds

Article abstract of DOI:10.1002/anie.200600772

(Chemical Equation Presented) Without palladium or copper: A wide range of functionalized Grignard compounds were coupled by using diphenoquinone 1 as an electron acceptor. The oxidative dimerization of alkenylmagnesium reagents proceeds with complete retention of the stereochemistry (see scheme; TBDMS = tert-butyldimethylsilyl).

Full text of DOI:10.1002/anie.200600772

Communications
ꢀ
C C Coupling
DOI: 10.1002/anie.200600772
Transition-Metal-Free Homocoupling of
Organomagnesium Compounds**
Arkady Krasovskiy, Alexander Tishkov,
Vicente del Amo, Herbert Mayr,* and Paul Knochel*
Dedicated to Professor Siegfried Hünig
on the occasion of his 85th birthday
ꢀ
C C coupling reactions are among the most powerful tools in
modern organic chemistry.[1,2] For most types of cross-
couplings, transition metals are required as mediators or
catalysts.[3] Usually CuI salts[4] (for Ullmann-type coupling
reactions[5]), TiCl4,[6] or the addition of catalytic amounts of
other transition metals is needed.[2,7] The importance of
finding new catalytic systems[8] and using atmospheric
oxygen[9] or its derivatives[10] for the performance of oxidation
reactions is well-recognized. However, such oxidations are
often unselective since they are governed by the chemistry of
high-energy zwitterions, (di)radicals, or by electron-transfer
reactions without stereochemical control.
Herein, we report a new concept which allows the
performance of coupling reactions by using only main-
group-metal derivatives. We have envisioned that the coor-
dination of a main-group-metal center with a readily reduci-
ble ligand would function as an electron shuttle and would
allow a reductive coupling to take place. Thus, the organic
oxidant (Ox) converts the intermediate A to the key
intermediate B, which can undergo an intramolecular redox
ꢀ
process leading to C C bond formation (oxidative coupling)
and reduction of the ligand Ox, which is thereby converted
into the reduced ligand (Red) by accepting two electrons
(Scheme 1). The main-group metal keeps the same oxidation
state during the entire process.
Thus, mono- and diorganomagnesium reagents that are
complexed with lithium chloride[11] can be efficiently coupled
by treatment with readily available 3,3’,5,5’-tetra-tert-butyldi-
phenoquinone (1),[12] which acts as a two-electron acceptor
(Scheme 2 and Table 1).
[*] Dr. A. Krasovskiy, Dr. A. Tishkov, Dr. V. delAmo, Prof. Dr. H. Mayr,
Prof. Dr. P. Knochel
Ludwig-Maximilians-Universität München
Department Chemie und Biochemie
Butenandtstrasse 5–13, Haus F, 81377 München (Germany)
Fax: (+49)89-21-80-776-80
[**] We thank the Fonds der Chemischen Industrie and Merck Research
Laboratories (MSD) for financialsupport. We asl o thank V.
Malakhov for the performance of some preliminary experiments as
well as Chemetall GmbH (Frankfurt) and BASF AG (Ludwigshafen)
for the generous gift of chemicals. V. del Amo thanks the Alexander
von Humboldt Foundation for financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Table 1: Formation of biaryls.
Entry Grignard reagent
Biaryl( 4)
Yield [%][a]
1
2
3
4
5
3a: FG=H, X=Br
3b: FG=MeO, X=Br
3c: FG=CF3, X=Cl
3d: FG=CN, X=Cl
4a: FG=H
96
94
92
96
93
4b: FG=MeO
4c: FG=CF3
4d: FG=CN
3e: FG=CO2Et, X=Cl 4e: FG=CO2Et
Scheme 1. Coupling reactions of MgII reagents.
6
7
8
3 f: FG=CN
3g: FG=CO2Et
3h: FG=tBu
4 f: FG=CN
4g: FG=CO2Et
4h: FG=tBu
85
88
83
9
3i
4i
88
10
11
3j: FG=H
3k: FG=OMe
4j: FG=H
4k: FG=OMe
99
90
Scheme 2. Coupling of organomagnesium reagents with 1.
12
13
3l
4l
80
96
The resulting biphenyldiolate 2 can be easily separated
(> 90% yield) from the reaction mixture by the addition of
pentane and subsequent filtration. By oxidation of 2 with air,
1 can be recovered in nearly quantitative yield,[13] which
makes this methodology especially attractive from ecological
and economical standpoints.
3m
4m
[a] Yield of isolated, analytically pure product.
The reaction of phenylmagnesium bromide with
0.5 equivalents of 1 at ꢀ208C led to the formation of biphenyl
(4a; Table 1, entry 1) in quantitative yield. The reaction
proceeded well with electron-rich (3b) and electron-poor
(3c) arylmagnesium halides and afforded the corresponding
biaryls 4b and 4c, respectively, in high yields (Table 1,
entries 2 and 3). At low reaction temperature functionalized
organomagnesium compounds that bear a nitrile (3d) or an
ester group (3e) could be coupled in excellent yields (4d and
4e; Table 1, entries 4 and 5). Functional groups in the ortho
position do not disturb the reaction, and the corresponding
ortho,ortho’-disubstituted biaryls 4 f and 4g were formed in 85
and 88% yield, respectively (Table 1, entries 6 and 7). Even
the sterically hindered ortho-tert-butyl- (3h) and mesitylmag-
nesium (3i) derivatives gave biaryls 4h and 4i in 83 and 88%
yield, respectively (Table 1, entries 8 and 9). 1-Naphthylmag-
nesium reagents 3j and 3k are also suitable substrates and
afforded the corresponding binaphthyls 4j and 4k, respec-
tively, in good yields (Table 1, entries 10 and 11). Heterocyclic
organomagnesium reagents could also be coupled by 1. Thus,
5-bromopyridin-3-ylmagnesium chloride led to the corre-
sponding dipyridine 4l in 80% yield (Table 1, entry 12). The
organomagnesium reagent 3m, which was generated from
1,1’-oxybis(2-iodobenzene), underwent selective intramolec-
ular coupling with quantitative formation of dibenzofuran
(4m; Table 1, entry 13).
Although coupling of the ortho-iodophenyl Grignard
reagent 5a led only to a moderate yield of biaryl 5b,
compound 5b was obtained in 80% yield when the diorga-
nomagnesium reagent 5c was used (Scheme 3). Coupling of
the allyloxy-substituted organomagnesium reagent 6b, which
was prepared by selective Br/Mg exchange from the corre-
sponding dibromide 6a and iPrMgCl·LiCl,[14] gave rise to
biaryl 6c. We did not observe any ring-closure products
arising from radical cyclization. The diester 7a could be
selectively deprotonated with the mixed Mg/Li base 8[15] and
coupled to form the highly substituted biaryl 7b. This example
shows that the presence of an NH group (2,2,6,6-tetramethyl-
piperidine) is tolerated.
We have also examined the coupling of alkynylmagne-
sium compounds, which are easily available by deprotonation
of the corresponding acetylenes with iPrMgCl·LiCl. Although
the Glaser coupling,[16] the Eglinton procedure,[17] and modi-
fications thereof[18] are well-known, each of them necessitates
the addition of a transition metal (usually CuI) that requires
subsequent recycling or disposal. Reactions of alkynylmag-
nesium reagents with 1 proceed cleanly with the formation of
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chloranil (14, 1.05 equiv, ꢀ608C!ꢀ108C, 12 h)
afforded the expected dimer 13c in 90% yield
(Scheme 6). The use of the zinc reagent in association
with chloranil is complementary to the homocoupling
of Grignard compounds, since attempts to perform
the coupling with the Grignard reagent corresponding
to 13b and 1 did not lead to 13c.
The mechanism of this reaction is still under
investigation. By using a stopped-flow instrument
with a UV/Vis detector, we were able to show that the
interaction of 1 with Grignard reagents proceeds via
the intermediate radical anion 1a (Scheme 7).
When 1 was mixed with a large excess of
mesitylmagnesium bromide 3i, the UV/Vis spectrum,
which was taken 7 ms after mixing of the reagents,
showed the complete consumption of 1 (lmax
=
423 nm, Figure 1b). A new species with an absorption
maximum at lmax = 459 nm had appeared, which is
assigned to 1a (Scheme 7). Treatment of 1 with
Scheme 3. Formation of biaryls.
only the desired diacetylenes and easily
recyclable 2. Thus, phenyl- (9a), n-hexyl-
(9b), trimethylsilyl- (9c), and cyclohexenyle-
thynylmagnesium chloride (9d) react with 1
within 12 hours at 258C to give the corre-
sponding diynes 10a–d in 80–90% yield
(Scheme 4).
Scheme 4. Formation of diynes.
Alkenylmagnesium reagents also could
be coupled in this way. Bis(a-styryl)magne-
sium (11a) reacted with 1 to afford 2,3-
diphenyl-1,3-butadiene (12a) in 87% yield.
Stereoselective couplings of terminal alkenes
are of great interest since the resulting
Scheme 5. Stereoselective coupling of alkenylmagnesium reagents; TBDMS=tert-butyldi-
methylsilyl.
isomerically pure 1,3-dienes cannot be prepared by conven-
tional Wittig reactions.[19] This methodology allows the
coupling of E- (11b, 11d) or Z-alkenylmagnesium reagents
(11c, 11e) with complete retention of the double-bond
stereochemistry to afford the isomerically pure E,E (12b,
12d) and Z,Z dienes (12c, 12e), respectively (Scheme 5).
Interestingly, the coupling reaction could also be per-
formed by using organozinc reagents. Thus, the reaction of
2,5-dibromothiophene (13a) with iPrMgCl·LiCl (258C, 1 h)
and subsequent transmetalation with ZnCl2 produced the zinc
reagent 13b. The reaction of this thiophene–zinc species with
Scheme 6. Coupling of an organozinc compound.
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pseudo-first-order rate constant for SET1 must be greater
than 300 sꢀ1, thus corresponding to a second-order rate
constant of greater than 3700mꢀ1 sꢀ1. The consumption of 1a
is much slower and can be followed photometrically (Fig-
ure 1a). It was not possible, however, to find a simple rate law
which describes SET2 (Scheme 7).
The reaction of 1 (c = 1.25 ” 10ꢀ5 m) with 1-naphthylmag-
nesium bromide (c = 0.042m) also proceeded with immeasur-
ably fast formation of 1a, which disappeared within 3 s, which
is much faster than in the corresponding experiment with
mesitylmagnesium chloride (Figure 1). Since 1a, generated
from 1 and Na in THF, did not react with 1-naphthylmagne-
sium bromide, a mechanism in which the arylmagnesium
reagents are oxidized by 1a can be ruled out.[21]
Scheme 7. Stepwise reduction of 1 in the course of homocoupling of
Grignard reagents; SET=single-electron transfer.
The oxidation of the organomagnesium reagents does not
yield a significant amount of free radicals as only traces of by-
products, which emerge from the abstraction of HC from THF,
are detected by GC–MS analysis of the crude reaction
mixtures. Complete retention of the configuration of the
ꢀ
C C double bonds in the coupling of alkenylmagnesium
reagents (Scheme 5) also indicates that free radicals are not
involved in this homocoupling reaction. These findings are in
line with the mechanism in Scheme 8.
Scheme 8. Mechanism of homocoupling of organomagnesium
reagents; Ox=oxidizing agent, 1.
The species that are formed by fast transfer of an electron
from RMgX (or R2Mg) to 1 ([Eq. (1)], Scheme 8) can be
formally considered as radicals RC that are bound to the
cationic magnesium center. The formation of analogous
ꢀ
intermediates, in which the C Mg bond is retained, was
proposed in reactions of organomagnesium reagents with
benzophenones and benzils.[22] These highly reactive species
were reported to effect transfer of the RC group to a radical
center of the reduced carbonyl group or form stable dimeric
dications that contain two ketyl molecules as counterions.[23]
Furthermore, it was reported that exchange of ligands in these
intermediates (analogous to [Eq. (2)]) is fast and precedes the
product-determining step.[24]
Figure 1. a) UV/Vis monitoring of the interaction of mesitylmagnesium
bromide (c=0.079m) with 1 (c=1.25”10ꢀ5 m). b) UV/Vis spectrum of
1 (c=2.01”10ꢀ6 m). c) UV/Vis spectrum of 1a, obtained from the
reduction of 1 (c=5.12”10ꢀ6 m) with Na in THF.
sodium metal in THF gave a green solution with a UV/Vis
spectrum (Figure 1c) that was identical to that from the
reaction of 1 with organomagnesium reagents (compare
Figures 1a and 1c). Since both 1 and 1a have previously
It is likely that the transfer of the RC group to the radical
center of 1a is hindered by the a-tert-butyl groups. This
hindrance may favor the consumption of the radical species
through oxidative dimerization (SET2, [Eq. (3)]). Similar
dimerization pathways that give rise to the formation of
biaryls or biaryl anion radicals are known.[25]
In conclusion, we have shown that the use of 3,3’,5,5’-
tetra-tert-butyldiphenoquinone (1) as an electron acceptor
allows a simple, high-yield preparation of a broad range of
been reported to have very similar absorption coefficients at
[20]
lmax
,
one can conclude that immediately after mixing, the
concentration of 1a is similar to the initial concentration of 1.
In all cases studied, the formation of 1a proceeded faster
than the mixing of the reagents in the stopped-flow instru-
ment. Assuming that the mixing time of the stopped-flow
system (ca. 7 ms) corresponds to more than three half-lives of
the substrate 1 in the presence of 0.08m Grignard reagent, the
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Communications
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functionalized biaryls, diynes, and dienes through coupling
reactions of readily available organomagnesium reagents. The
coupling of alkenylmagnesium reagents proceeds with high
stereoselectivity. All of the reactions take place within a
convenient range of temperatures (ꢀ208C to room temper-
ature) and can be easily extended to large-scale preparations.
We have performed for the first time an effective transition-
metal-free coupling of a broad range of organomagnesium
reagents by using a conceptually new process (Scheme 1).
Extension of this work to other organometallic compounds,
such as zinc reagents, has already been demonstrated
(Scheme 6), and further such investigations are currently
underway.
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Experimental Section
Representative procedure: Synthesis of 4e: A dry and argon-flushed
flask (10 mL), equipped with a magnetic stirrer and a septum, was
charged with ethyl 4-iodobenzoate (552 mg, 2.0 mmol) in THF
(2 mL). The reaction mixture was cooled to ꢀ208C, and iPrMgCl·LiCl
(2 mL, 1.05m in THF, 2.1 mmol) was added dropwise. After 20 min at
ꢀ208C, the I/Mg-exchange was complete (checked by GC analysis of
reaction aliquots), and a solution of 1 (449 mg, 1.1 mmol) in THF
(5 mL) was added dropwise. The reaction mixture was stirred for 2 h
at 08C. Conventional work up of the crude residue by flash
chromatography (pentane/CH2Cl2 1:1) yielded 4e (184 mg, 93%) as
white crystals.
Received: February 28, 2006
Published online: July 3, 2006
ꢀ
Keywords: biaryls · C C coupling · dienes · diynes ·
Grignard reagents
.
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