Generation of Aryl and Heteroaryl Magnesium Reagents in Toluene by Br/Mg or Cl/Mg Exchange

Article abstract of DOI:10.1002/anie.201802123

The alkylmagnesium alkoxide sBuMgOR?LiOR (R=2-ethylhexyl), which was prepared as a 1.5 m solution in toluene, undergoes very fast Br/Mg exchange with aryl and heteroaryl bromides, producing aryl and heteroaryl magnesium alkoxides (ArMgOR?LiOR) in toluene. These Grignard reagents react with a broad range of electrophiles, including aldehydes, ketones, allyl bromides, acyl chlorides, epoxides, and aziridines, in good yields. Remarkably, the related reagent sBu₂Mg?2 LiOR (R=2-ethylhexyl) undergoes Cl/Mg exchange with various electron-rich aryl chlorides in toluene, producing diorganomagnesium species of type Ar₂Mg?2 LiOR, which react well with aldehydes and allyl bromides.

Full text of DOI:10.1002/anie.201802123

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Accepted Article
Title: Aryl and Heteroaryl Magnesium Reagents in Toluene via a Br/
Mg- or a Cl/Mg-Exchange
Authors: Dorothée Sophia Ziegler, Konstantin Karaghiosoff, and Paul
Knochel
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802123
Angew. Chem. 10.1002/ange.201802123
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10.1002/anie.201802123
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Aryl and Heteroaryl Magnesium Reagents in Toluene via a Br/Mg-
or a Cl/Mg-Exchange
Dorothée S. Ziegler, Konstantin Karaghiosoff, and Paul Knochel*[a]
Dedication ((optional))
Abstract: The alkylmagnesium alkoxide sBuMgOR·LiOR (R = 2-
ethylhexyl) prepared as a 1.5 M solution in toluene undergoes very
fast Br/Mg-exchanges with aryl and heteroaryl bromides, producing
aryl and heteroaryl magnesium alkoxides (ArMgOR·LiOR) in toluene.
These Grignard reagents react with a broad range of electrophiles,
including aldehydes, ketones, allyl bromides, acyl chlorides, epoxides
and aziridines in good yields. Remarkably, the related reagent
sBu2Mg·2LiOR (R = 2-ethylhexyl) allows a Cl/Mg-exchange with
various electron-rich aryl chlorides in toluene producing
diorganomagnesium species of type Ar2Mg·2LiOR, which react well
with aldehydes and allyl bromides.
in cyclohexane) to the colorless gel (2) at 25 °C for 2 h provided
slightly yellow solutions of 1a and 1b. Removal of the solvent in
vacuo produced a foam which readily dissolves in toluene,
affording a 1.0-1.5 M solution of 1a and a 0.6-0.85 M slightly yellow
solution of 1b (Scheme 1).
Organomagnesium halides are key intermediates in organic
synthesis.[1] They are usually prepared by a direct insertion of
magnesium turnings,[2] Rieke magnesium,[3] or magnesium
powder, and lithium chloride[4] to organic halides. The
heterogenous nature of this reaction complicates scale-up and
Scheme 1. Preparation of the new exchange reagents 1a and 1b.
Both 1a-b are very powerful exchange reagents, which for the first
time, allow the synthesis of arylmagnesium reagents in toluene.[13]
In preliminary experiments, the Br/Mg-exchange on
4-bromoanisole (4a) was examined (Table 1).
industrial use.[2]
A
deprotomagnesiation of arenes or
heteroarenes can also be accomplished using soluble hindered
magnesium amides.[5] Alternatively, various organomagnesium
halides can be prepared via a halogen-magnesium exchange by
treating aryl or heteroaryl iodides or bromides with an
alkylmagnesium halide[6] or better with iPrMgCl·LiCl (turbo-
Grignard).[7] Furthermore, synthetically useful reagents for
halogen-metal exchange are lithium organomagnesiates
Table 1. Br/Mg-exchange for 4-bromoanisole (4a) using various magnesium-
exchange reagents at 25 °C.
(R3MgLi).However, in this case, lithium triorgano magnesiates are
[8]
produced.
All these preparation methods provide Grignard
entry
Mg-exchange reagent
equiv
solvent
yield(%)[a]
reagents in ethereal solvents such as diethyl ether or THF and
only few methods have been reported describing the
a
1
2
3
4
5
iPrMgCl·LiCl
1.2
0.6
1.2
1.2
1.2
THF
1 [0][b]
13 [0][b]
85
preparation of magnesium organometallics in non-polar
solvents.[9] The synthesis of Grignard reagents in hydrocarbons
or toluene is of great interest, since these weakly-coordinated
Grignard reagents may display an original and unusual reactivity.
Also, Grignard reagents in hydrocarbons or toluene are
industrially friendly reagents, as such solvents improve aqueous
extraction during work-ups.[10]
sBu2Mg·2LiCl
THF
sBuMgOR·LiOR (1a)
sBuMgOR·LiOR (1a)
sBuMgOR·LiOR (1a)
toluene
toluene
THF
99[c]
14[c]
Herein, we report new halogen-magnesium exchange reagents
sBuMgOR·LiOR (1a) and sBu2Mg·2LiOR (1b) (R = 2-ethylhexyl),
that allow very fast Br/Mg-exchanges and, for the first time, a
Cl/Mg-exchange on various electron-rich aryl chlorides. First, a
convenient synthesis of 1a and 1b was developed. Thus, the
magnesium alkoxide 2 was prepared by treating nBu2Mg (0.7 M
in hexane) with 2-ethylhexanol (3, 2.0 equiv) at 25 °C for
24 h.[11,12] The addition of one or two equivalents of sBuLi (1.2 M
[a] Yield of 5a determined by GC-analysis of water quenched reaction aliquots.
[b] Yield obtained in toluene. [c] Yield obtained in the presence of TMEDA
(1.2 equiv).
In THF, iPrMgCl·LiCl[7] required 27 h at 25 °C to complete the
exchange reaction and produced the desired arylmagnesium
halide 5a (X = Cl). However, less than 1% of 5a was formed with
this exchange reagent after 15 min reaction time (entry 1).[14] The
currently most powerful available exchange reagent
sBu2Mg·2LiCl[7b] led to 13% of 5a (X = 4-anisyl) after 15 min (entry
2). A full conversion with this reagent required 8 h reaction time.
Switching to toluene showed, that both of these reagents were
unreactive. However, the addition of the new reagent
sBuMgOR·LiOR (1a) to 4a in toluene provided 5a (X = OR·LiOR)
in 85% yield within 15 min reaction time (entry 3). The addition of
[a]
D. S. Ziegler, Prof. Dr. K. Karaghiosoff, Prof. Dr. P. Knochel
Department Chemie, Ludwig-Maximilians-Universität München
Butenandtstrasse 5–13, Haus F, 81377 München (Germany)
E-mail: paul.knochel@cup.uni-muenchen.de
Supporting information for this article is given via a link at the end of
the document.
TMEDA
(N,N,N´,N´-tetramethylethylenediamine;
1.2 equiv)
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further improved the reaction conversion to 99% (entry 4).[15] The
addition of TMEDA to iPrMgCl·LiCl[7] or sBu2Mg·2LiCl[7b] had no
effect. Interestingly, the rate of the Br/Mg-exchange using 1a in
THF instead of toluene was considerably decreased and only a
conversion of 14% was observed (entry 5). This showed, that
there is a real advantage for carrying out the Br/Mg-exchange in
non-polar solvents such as toluene.
smoothly with various ketones. The structure of the magnesium
reagents 5 were tentatively proposed to be A (Scheme 2).
Furthermore, the Grignard reagents 5 (X = OR·LiOR·TMEDA)
underwent a range of copper-catalyzed reactions such as
acylations with various acyl chlorides in toluene leading to
ketones 6o-6p in 60-95% yield (10% CuI, 0 °C, 1 h; Scheme 4).
Also, allylations of arylmagnesium alkoxides were realized at
25 °C with allylic bromides, providing products 6q-6s in 89-92%
yield. The opening of epoxides and aziridines with arylmagnesium
reagents is also a challenging reaction, that often required Lewis
acids or harsh reaction conditions.[20] We found, that epoxides or
aziridines were opened under mild conditions (25 °C, 4-10 h),
leading to the expected products 6t-6w in 73-95% yield.
Quenching of 5a (X = OR·LiOR·TMEDA) with iodine furnished the
aryl iodide 6a in 70% yield (25 °C, 30 min; Scheme 2). Usually,
arylmagnesium reagents do not add well to ketones.[16,17]
However, trapping 5a in toluene with ketones furnished the
tertiary alcohols 6b-c in 80-86% yield (25 °C, 2 h). Related aryl
bromides such as 3-bromoanisole (4b) or 1-bromo-3,5-
dimethoxybenzene (4c) underwent a complete Br/Mg-exchange
by treatment with 1a (25 °C, 15 min). These Grignard reagents
were added to PhCHO or were acylated with a Weinreb amide[18]
,
furnishing the desired products 6d and 6e in 89-95% yield (25 °C,
1 h).
Scheme 3. Br/Mg-exchange for aryl bromides of type 4 leading, via intermediate
organomagnesiums of type 5, to functionalized arenes of type 6 in the presence
of 10% CuI.
The arylmagnesium alkoxides of type 5 (X = OR·LiOR·TMEDA)
also participate in Pd-catalyzed cross-couplings (Scheme 4).
Scheme 2. Br/Mg-exchange for aryl bromides of type 4 leading, via intermediate
organomagnesiums of type 5, to functionalized arenes of type 6.
Also, bromoanilines 4d-f which are very reluctant to undergo a
Br/Mg-exchange with previously known exchange reagents,[7b-d]
were converted to the corresponding arylmagnesium alkoxides
within 1-4 h at 25 °C in toluene. Their trapping with MeSO2SMe,
morpholino(phenyl) methanone[19] or dicyclopropyl ketone
produced the polyfunctional aniline derivatives 6f-h in 65-84%
yield (25 °C, 1-2 h). 2-Bromonaphthalene (4g) was converted
within 15 min to the Grignard reagent and its addition to a
thienylaldehyde, led to the alcohol 6i in 75% yield (25 °C, 1 h).
Similar electron-rich aryl bromides such as 4h and 4i were
converted to the Grignard reagents and were quenched with
cyclohexanone or N-Benzylideneamine, leading to 6j and 6k in
75-91% yield (25 °C, 2 h). Finally, electron-poor aryl bromide 4j
underwent the Br/Mg-exchange at 25 °C within 15 min and gave,
after quenching, 6l in 72% yield (25 °C, 2 h). These results
demonstrated, that arylmagnesium alkoxides of type 5 complexed
with LiOR and TMEDA are superior nucleophiles, since they react
Scheme 4. Cross-couplings of organomagnesium intermediate 5a.
Thus, p-anisylmagnesium derivative (5a; X = OR·LiOR·TMEDA)
underwent a Kumada cross-coupling[21] in the presence of 2%
Pd(OAc)2 and 3% SPhos[22] (25 °C, 2 h), producing the biphenyl
8a in 70% yield. Transmetalation with Zn(OPiv)2[23] in toluene led
to a milky suspension which reacted with 7b (25 °C, 15 h) via a
Negishi cross-coupling using 4% of PEPPSI-iPr,[24,25] furnishing
the desired product 8b in 72% yield (Scheme 4). We also
extended our method to the preparation of several heterocyclic
magnesium alkoxides. Quenching with various electrophiles led
to the desired products 11a-11g in 60-75% yield. Interestingly, the
treatment of 2,3,4,5-tetrachlorothiophene[26] (9e) with 1a
(2.4 equiv) led to a double Cl/Mg-exchange, providing a bis-
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magnesium species, which after a copper-catalyzed allylation
with allyl bromide, gave 2,5-bis-allylthiophene 11g in 60% yield
(Scheme 5).
and the addition of PMDTA (0.6 equiv) instead of TMEDA further
increased the conversion to 75% yield (entries 5 and 6). The
addition of benzaldehyde (1.5 equiv) led to the alcohol 14a in 61%
yield (25 °C, 1 h). This Cl/Mg-exchange was extended to a range
of aryl chlorides bearing a methoxy group[27,28] in ortho-position.
For all these substrates the Cl/Mg-exchange was completed
within 1 h at 25 °C and produced a diarylmagnesium species 13
of tentative structure B. Quenching with various electrophiles (1.5
equiv) furnished the expected products 14b-14f in 50-75% yield
(Scheme 7).
Scheme 5. Br/Mg-exchange for heteroaryl bromides and chlorides of type 9
leading, via intermediate organomagnesiums of type 10, to functionalized
heteroarenes of type 11.
This excellent propensity of magnesium exchange reagent 1a for
undergoing Cl/Mg-exchanges led us to explore the scope of such
an
almost
unknown
Cl/Mg-exchange.[26]
2-Chloro-1,4-
dimethoxybenzene (12a) was used as substrate and was
submitted to various Grignard exchange reagents. We found that
iPrMgCl·LiCl[7] or sBu2Mg·2LiCl[7b] did not react with 12a, even in
the presence of TMEDA or PMDTA (N,N,N´,N´´,N´´-penta-
methyldiethylenetriamine).
Scheme 6. Cl/Mg-exchange for aryl chlorides of type 12 leading via
intermediate organomagnesiums of type 13 to functionalized arenes of type 14.
In conclusion, we have reported a method for preparing well
soluble aryl and heteroaryl magnesium alkoxides complexed with
one or two equivalents of lithium alkoxide, respectively, (LiOR;
R = 2-ethylhexyl) in toluene using the new exchange reagents
sBuMgOR·LiOR (1a) and sBu2Mg·2LiOR (1b).[29] These
exchanges are ca. 30 times faster than the previous exchange
reagent sBu2Mg·2LiCl[7b] and ca. 110 times faster than
iPrMgCl·LiCl[7] (turbo-Grignard). Furthermore, the resulting
Grignard reagent of type ArMgOR·LiOR (5) or HetArMgOR·LiOR
(10) display an excellent reactivity in toluene adding well to
ketones and opening epoxides and aziridines under mild
conditions. Furthermore, sBu2Mg·2LiOR (1b) is able to trigger a
Cl/Mg-exchange leading to diarylmagnesium species of the type
13 (ArMg2·2LiOR). Further applications of these new aryl
magnesium alkoxide reagents in toluene are currently studied in
our laboratories.
Table 2. Cl/Mg-exchange on aryl chloride 12a using various magnesium-
exchange reagents.
entry
exchange reagent
iPrMgCl·LiCl
equiv.
1.2
additive
PMDTA
PMDTA
PMDTA
-
yield (%)[a]
0 [0][b]
0 [0][b]
0 [0][b]
48
1
2
3
4
5
6
sBu2Mg·2LiCl
0.6
sBuMgOR·LiOR (1a)
sBu2Mg·2LiOR (1b)
sBu2Mg·2LiOR (1b)
sBu2Mg·2LiOR(1b)
1.2
0.6
0.6
TMEDA
PMDTA
59
Experimental Section
0.6
73 [20][c]
See Supporting Information.
[a] Yield of 13a determined by GC-analysis of water quenched reaction aliquots.
[b] Yield obtained in the presence of TMEDA. [c] Yield obtained in THF.
Also, sBuMgOR·LiOR (1a, R = 2-ethylhexyl) gave no exchange
under various conditions (Table 2, entries 1-3). After extensive
experimentations, we found, that sBu2Mg·2LiOR (1b, 0.6 equiv)
led to 48% of a Cl/Mg-exchange affording a bis-magnesium
reagent of type 13 after 1 h reaction time at 25 °C (entry 4). The
addition of TMEDA (0.6 equiv) improved the conversion to 59%
Acknowledgements
We thank the LMU Munich and the SFB 749 for financial support.
We also thank Albemarle for the generous gift of chemicals.
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[26] Previously, we found that 9e is the only substrate, which undergoes a
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exchange reagent (1b) and therefore accelerates the Cl/Mg-exchange.
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[29] A patent application has been filed.
[12] Alternatively, a magnesium alkoxide solution (0.94 M in heptane) is
commercially available from Albemarle, Frankfurt: U. Wietelmann, U.
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[13] The reactions were also performed in other apolar solvents such as
hexane and cyclohexane or in cyclopentyl methyl ether (CPME).
[14] Yield of 5a determined by GC-analysis of water quenched reaction
aliquots.
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
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Dorothée S. Ziegler, Konstantin
Karaghiosoff, and Paul Knochel*
Page No. – Page No.
Aryl and Heteroaryl Magnesium
Reagents in Toluene via a Br/Mg- or a
Cl/Mg-Exchange
Grignard reagents in toluene: sBuMgOR·LiOR (R = 2-ethylhexyl) undergoes very
fast Br/Mg exchanges producing aryl and heteroaryl magnesium alkoxides
(ArMgOR·LiOR) in toluene (30-110 times faster than previous exchange reagents).
Remarkably, these Grignard reagents react with a broad range of electrophiles. The
related reagent sBu2Mg·2LiOR (R = 2-ethylhexyl) allows for the first time Cl/Mg-
exchanges with aryl chlorides in toluene producing Ar2Mg·2LiOR.
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