Ethylaluminum as an ethylene source for the Mizoroki-Heck-type reaction. Rhodium-catalyzed preparation of stilbene derivatives

Article abstract of DOI:10.1039/c4cc09306b

Treatment of an organoaluminum reagent bearing aryl and ethyl groups furnishes 1,2-diarylethene derivatives in good to excellent yields by the catalysis of a rhodium complex, in which the ethyl group of the aluminum reagent serves as an ethylene source in the product formation.

Full text of DOI:10.1039/c4cc09306b

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Ethylaluminum as an ethylene source for the
Mizoroki–Heck-type reaction. Rhodium-catalyzed
preparation of stilbene derivatives†
Cite this: Chem. Commun., 2015,
51, 1949
Received 21st November 2014,
Accepted 11th December 2014
Shota Tanaka, Kazuki Itami, Kazuhiro Sunahara, Go Tatsuta and Atsunori Mori*
DOI: 10.1039/c4cc09306b
www.rsc.org/chemcomm
Treatment of an organoaluminum reagent bearing aryl and ethyl recently reported that the related rhodium-catalyzed reaction of
groups furnishes 1,2-diarylethene derivatives in good to excellent vinylarenes also occurs with arylaluminum leading to stilbene
yields by the catalysis of a rhodium complex, in which the ethyl derivatives.11–13 During the course of our studies on such reactions,
group of the aluminum reagent serves as an ethylene source in the we observed several unexpected findings: the reaction of diethyl-
product formation.
(phenyl)aluminum (1a) with styrene afforded exceeding amounts of
trans-stilbene (2a)tothestoichiometryofthe employed styrene. When
Incorporation of small molecules such as carbon monoxide, the reaction of styrene with diethyl(4-methylphenyl)aluminum (1b)
carbon dioxide, hydrogen, nitrogen, oxygen, etc. into organic was performed, unexpected (E)-1,2-bis(4-methylphenyl)ethane
compounds is an attractive approach for the functionalization (2b) was obtained along with desired (E)-1-(4-methylphenyl)-2-
of the organic framework in organic synthesis. Transition phenylethene (2ab). No such byproduct was found in the reaction
metals as catalysts play a key role in the reaction with small of an aluminum reagent bearing a methyl substituent, instead
molecules, where gaseous reagents have been utilized in a (Scheme 1).
certain reaction.1 Several surrogate organic compounds as
These findings suggest that the two carbon units in styrene
synthetic equivalents have been employed for reagents to avoid are derived from the ethyl group of 1a to induce the double
the use of occasionally toxic or explosive gasses, e.g. aldehyde as Mizoroki–Heck-type reaction to afford diarylethene 2. Herein,
a CO source, etc.2 Several metallic alkyls have also been employed we describe that two mol amounts of aluminum aryls and
as synthetic equivalents of the metal hydrides for the reduction ethylaluminum species form stilbene derivatives using a rhodium
of carbonyl compounds through b-hydride elimination accom- catalyst, in which the ethyl group of aluminum serves as an
panied by the formation of the corresponding olefins.3 Compared ethylene source in stilbene synthesis.
with the use of metal alkyls as reducing agents as hydrides, little
When diethyl(phenyl)aluminum (1a) was treated with 2.5 mol%
attention has been paid so far to the produced olefins as building [RhCl(cod)]2 (5 mol% of Rh) in the presence of diisopropyl ketone
blocks of organic compounds.4
(2 eq.), 76% of trans-stilbene (2a) was obtained after stirring in
On the other hand, reactions using rhodium catalysts have
been widely used as essential tools in forming a variety of C–C
bonds in organic synthesis.5 In particular, various reactions
using organometallic species (boron, silane, zinc, tin, aluminum,
etc.) as nucleophiles, such as cross coupling with organic halides,6
conjugated addition to enones,7 1,2-addition to carbonyl com-
pounds,8 and the oxidative Mizoroki–Heck-type reaction toward
olefins,9 have been developed so far. We have shown related
Mizoroki–Heck-type reactions of organosilicon species with olefins
in the presence of a rhodium or an iridium catalyst,10 and we have
Department of Chemical Science and Engineering, Kobe University, 1-1 Rokkodai,
Nada, Kobe 657-8501, Japan. E-mail: amori@kobe-u.ac.jp; Fax: +81-78-803-6181;
Tel: +81-78-803-6181
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cc09306b
Scheme 1 Unexpected reaction of aryl(diethyl)aluminum.
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Scheme 4 The reaction of triethylaluminum with triphenylaluminum.
a rhodium catalyst also afforded stilbenes in 43% yield. In
addition, the reaction with phenylmagnesium bromide (PhMgBr)
instead of Ph3Al afforded 2a in 57% yield, whereas the use of
diethylzinc (Et2Zn) and Ph3Al did not allow the reaction to proceed
at all. These reactions suggest that it is essential to use ethyl-
aluminum species to serve as an ethylene source in stilbene
synthesis, while arylmetallic species other than aluminum allow
the rhodium-catalyzed reaction to occur (Scheme 4).
Table 1 summarizes the rhodium-catalyzed reaction of various
arylated diethylaluminum. The reaction of 4-methylphenyl sub-
stituted diethylaluminum with the rhodium catalyst and ketone
took place to give dimethylstilbene 2b in 63% yield. The reaction
of 2-methylphenyl or 3,5-dimethylphenyl substituted diethyl-
aluminum also proceeded to give the corresponding stilbene
derivatives 2c and 2d in excellent yields. Biphenyl or 1-naphthyl
diethylaluminum also reacted in the presence of the rhodium
Scheme 2 Rhodium catalyzed reaction of diethyl(phenyl)aluminum.
THF/hexane at 60 1C, whereas no reaction took place in the
absence of diisopropyl ketone.11 The reaction of ethyl(diphenyl)-
aluminum 3a, which was obtained by the reaction of ethylalu-
minum dichloride with 2.0 equivalents of PhMgX, also afforded
2a in 43% yield under similar conditions. The yield was found to
improve to 57% when the amount of ketone employed was
increased to 5.0 equivalents (Scheme 2).
Concerning the transition metal-catalyzed reactions with
main group organometallics such as zinc, aluminum, silicon,
and tin, metal alkyl species have been recognized as dummy
substituents, which are hardly introduced into the component
of the target molecule, as shown in Scheme 3 (eqn (1)).14
Although the alkyl groups of main group organometallics are
shown to induce b-hydride elimination to give the corre-
sponding metal hydrides in the presence or absence of transi-
tion metals, to serve as reducing agents for unsaturated bonds
or organic halides, the thus produced olefinic species have only
served as waste molecules15 (eqn (2)). Worthy of note in the
present rhodium-catalyzed reaction with ethylaluminum is that
the reaction shows unprecedented utilization of the ethyl group
as an ethylene source, as shown in eqn (3).
Table
1
Rhodium-catalyzed coupling reaction of various arylated
diethylaluminuma
The presence of both aryl and ethyl groups in the same
molecule is not the requirement in the rhodium-catalyzed stilbene
synthesis. The use of triethylaluminum (Et3Al) as an ethyl source
with triphenylaluminum (Ph3Al) in the presence of the ketone and
a
Unless otherwise noted, the reaction was performed with
aryl(diethyl)aluminum (0.5 mmol), ketone (2.5 mmol) and a rhodium
b
catalyst (0.0125 mmol) at 60 1C for 24 h. The reaction was carried out
with 1.0 mmol of aryl(diethyl)aluminum and 2.0 mmol of ketone.
Scheme 3 Ethyl-metal species in organometallic reactions.
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Scheme 7 Reaction of 2-phenylethyl metal reagent as a styrene source.
Scheme 5 The reaction of aluminum chloride and Grignard reagents
leading to stilbene.
catalyst and ketone to give 2e or 2h. The use of diethylaluminum
with the methoxy group as an electron-donating group afforded 2f
in 53% yield. The reaction of diethyl(4-fluorophenyl)aluminum
proceeded to furnish 2g in 55% yield.
The combined use of ethyl and aryl Grignard reagents in the
presence of aluminum chloride was also examined, as shown in
Scheme 5. The reaction was carried out with several different
ratios of EtMgBr, PhMgBr, and AlCl3. When double the amount of
PhMgBr was employed toward EtMgBr and AlCl3, the reaction
occurred to furnish 2a in 50% yield in the presence of a rhodium
catalyst (5.0 mol%) and diisopropyl ketone (5.0 eq.). The reaction
of AlCl3, EtMgBr and PhMgBr in a 1:2:1 ratio improved the yield
to 87% to afford 2a. However, reducing the amount of AlCl3
(30 mol%) to catalytic amounts toward Grignard reagents afforded
29% of 2a and the reaction with the Grignard reagent in the
absence of AlCl3 did not produce stilbenes at all.12 Although the
stilbene synthesis only proceeds based on the employed amount
of aluminum species, it is remarkable that the reaction with
readily available Grignard reagents and AlCl3 induces the for-
mation of both aryl and ethyl aluminum species in situ.
This combined use of AlCl3 and Grignard reagents was then
employed for several 1,2-diarylethenes, as shown in Scheme 6.
The reaction of (4-fluorophenyl)magnesium bromide occurred to
afford the product in 57% yield. Other aryl Grignard reagents,
4-methylphenyl and 4-methoxyphenyl, also reacted to deliver
stilbene derivatives in 56% and 60% yields, respectively.
Although we also used several metal alkyl species such as
n-propyl, isobutyl, etc. instead of an ethyl group, the reaction
was mostly found to be unsuccessful in giving the desired
addition-elimination product. However, the reaction bearing
the 2-phenethyl substituent took place smoothly. Treatment of
(2-phenylethyl)magnesium bromide bearing b-hydrogen with
aryl Grignard reagents in the presence of aluminum chloride
afforded stilbene derivatives in 57–58% yields (Scheme 7).
A plausible reaction mechanism is shown in Scheme 8. Trans-
metalation reactions of both ethyl and aryl groups of aluminum
Scheme 8 Plausible mechanism of 1,2-diarylethene synthesis.
species to rhodium afford arylrhodium A and the ethylated
rhodium species. The ethylated rhodium immediately induces
b-hydride elimination to release ethylene along with the for-
mation of hydridorhodium. Insertion of ethylene to the thus
formed carbon–rhodium bond of arylrhodium species A leads to
the addition product B, which undergoes b-hydride elimination to
give styrene. Since formation of styrene has been hardly observed
throughout the present rhodium-catalyzed reactions, the gener-
ated styrene reacts with arylrhodium A much faster than ethylene
to form C. Beta-hydride elimination of C affords stilbene accom-
panied by the formation of rhodium hydride D. The ketone was
reduced by D to furnish rhodium alkoxide E, which is capable
of undergoing transmetalation of arylaluminum to regenerate
arylrhodium complex A.16
In conclusion, we have shown that the ethyl group of aluminum
species serves as an ethylene source in rhodium-catalyzed stilbene
synthesis. The reaction proceeded in the presence of diisopropyl
ketone and a rhodium catalyst to give trans-stilbene in good to
excellent yields. The reaction can be recognized as a novel type of
incorporation of a small molecule surrogate employed as a metal-
alkyl species into the organic framework. The reaction proceeded
using an organoaluminum reagent bearing ethyl and aryl groups as
well as by the combined use of ArMgX, EtMgX, and aluminum
chloride, which merits procedural simplicity.
This work was supported financially by Kakenhi B by MEXT,
Japan. ST thanks JSPS for the Research Fellowship for Young
Scientists.
Scheme 6 Rhodium-catalyzed reaction of various ArMgX with EtMgBr
and AlCl3.
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1952 | Chem. Commun., 2015, 51, 1949--1952
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