Indium-catalyzed reduction of allyl bromide with gallium or aluminum. Formation of allylgallium and allylaluminum sesquibromides

Article abstract of DOI:10.1021/ol025784v

Matrix presented Fast transmetalation of an allyl group from indium to gallium enables the assembly of a catalytic cycle of indium, which accelerates the formation of allylgallium sesquibromide. A solution of allylaluminum sesquibromide in THF is also pre

Full text of DOI:10.1021/ol025784v

ORGANIC
LETTERS
2002
Vol. 4, No. 10
1727-1729
Indium-Catalyzed Reduction of Allyl
Bromide with Gallium or Aluminum.
Formation of Allylgallium and
Allylaluminum Sesquibromides
Kazuhiko Takai* and Yoshito Ikawa
Department of Applied Chemistry, Faculty of Engineering, Okayama UniVersity,
Tsushima, Okayama 700-8530, Japan
ktakai@cc.okayama-u.ac.jp
Received February 27, 2002
ABSTRACT
Fast transmetalation of an allyl group from indium to gallium enables the assembly of a catalytic cycle of indium, which accelerates the
formation of allylgallium sesquibromide. A solution of allylaluminum sesquibromide in THF is also prepared from allyl bromide and aluminum
metal under indium catalysis.
Although there are many methods for preparing organome-
tallic compounds, direct reduction of organic halides with
low-valent metals has several advantages.1 In particular, this
approach allows many types of organometallic compounds
to be made. A key issue when using a direct reduction is
how to accelerate the electron transfer from the metal to the
organic compound. For this purpose, several methods such
as the Rieke’s activation method1a and the metal-graphite
method1d have been developed. Another method for the
acceleration of electron transfer is to add a catalytic amount
of a second metal element to the target metal.2 This
sometimes has the advantage of activating the target metal
under mild conditions. We report here a novel activation
method for metallic gallium or aluminum using a catalytic
amount of indium. The method is useful for the preparation
of both allylgallium3 and allylaluminum sesquibromides4 in
THF under mild conditions.
Despite its reducing power (E°: GaIII/Ga0, -0.549 V),5
gallium metal has not been used widely in organic synthesis.3b,6
In contrast to allylations involving indium metal, where the
allylic indium species can be prepared allowing Grignard-
(3) (a) Araki, S.; Ito, H.; Butsugan, Y. Appl. Organomet. Chem. 1988,
2, 475. (b) Han, Y.; Huang, Y.-Z. Tetrahedron Lett. 1994, 35, 9433. (c)
Yamaguchi, M.; Sotokawa, T.; Hirama, M. Chem. Commun. 1997, 743.
(d) Araki, S.; Horie, T.; Kato, M.; Hirashita, T.; Yamamura, H.; Kawai,
M. Tetrahedron Lett. 1999, 40, 2331. (e) Usugi, S.-i.; Yorimitsu, H.; Oshima,
K. Tetrahedron Lett. 2001, 42, 4535. (f) Takai, K.; Ikawa, Y.; Ishii, K.;
Kumanda, M. Chem. Lett. 2002, 172.
(1) For reviews of activated metals in organic synthesis, see: (a) Rieke,
R. D. Top. Curr. Chem. 1975, 59, 1. Rieke, R. D. Aldrichimica Acta 2000,
33, 52. (b) Fu¨rstner, A. Angew. Chem., Int. Ed. Engl. 1993, 32, 164. (c)
Cintas, P. ActiVated Metals in Organic Synthesis; CRC Press: Boca Raton,
1993. (d) Fu¨rstner, A. ActiVe Metals: Preparation, Characterization,
Application; VCH: Weinheim, 1996.
(2) (a) Li-Na: Kamienski, C. W.; Esmay, D. L. J. Org. Chem. 1960,
25, 1807. Smith, W. N., Jr. J. Organomet. Chem. 1974, 82, 1. (b) Zn-Cu:
Shank, R. S.; Shechter, H. J. Org. Chem. 1959, 24, 1825. LeGoff, E. J.
Org. Chem. 1964, 29, 2048. (c) Mn-Pb: Takai, K.; Ueda, T.; Hayashi, T.;
Moriwake, T. Tetrahedron Lett. 1996, 37, 7049.
(4) Some selected examples of the Barbier-type reactions. (a) Al-
PbBr2: Tanaka, H.; Yamashita, S.; Hamatani, T.; Ikemoto, Y.; Torii, S.
Synth. Commun. 1987, 17, 789. (b) Tanaka, H.; Nakahata, S.; Watanabe,
H.; Zhao, J.; Kuroboshi, M.; Torii, S. Inorg. Chim. Acta 1999, 296, 204.
(c) Al-InCl3: Araki, S.; Jin, S.-J.; Idou, Y.; Butsugan, Y. Bull. Chem.
Soc. Jpn. 1992, 65, 1736.
(5) Standard reduction potential (E°) of InIII/In0 is -0.3382 V; see: CRC
Handbook of Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton,
2001; p 8-21.
(6) Zhang, X.-L.; Han, Y.; Tao, W.-T.; Huang, Y.-Z. J. Chem. Soc.,
Perkin Trans. 1 1995, 189.
10.1021/ol025784v CCC: $22.00 © 2002 American Chemical Society
Published on Web 04/24/2002

type allylations to be achieved,7 the reported gallium-
mediated reactions are only conducted in Barbier-type
procedures. Moreover, the gallium-mediated Barbier-type
allylation in THF requires heating at reflux.3b,8 Therefore,
we explored the formation of allylgallium species by addition
of a catalytic amount of a second metal element.9
A Barbier-type addition of allylic bromide to cyclodo-
decanone with gallium(0) was chosen as a probe for the
activating effects. Because of the low melting point (29.8
°C) of the gallium metal, the allylation was examined at 10
°C in order to avoid the formation of spherical gallium liquid,
which has the smallest surface area.10 Among the reactions
examined, addition of an indium metal to gallium(0) was
effective to promote the Barbier-type allylation. In addition,
the amount of indium metal could be reduced to 1 mol % of
gallium(0). For example, treatment of a mixture of cy-
clododecanone (1.0 mmol) and allyl bromide (1.5 mmol) in
THF with gallium(0) (0.90 mmol) and indium(0) (0.009
mmol) at 10 °C for 5 h gave 1-allylcyclododecanol (1) in
95% yield (Scheme 1). This reaction only proceeded when
with the THF solution of allylgallium at 10 °C for 5 h, 1
was obtained in 96% yield.15
The 1H and 13C NMR spectra of the prepared allylgallium
solution in THF-d8 exhibited two sets of allyl peaks (see
Supporting Information).16 The allylic methylene of the
allylgallium appeared at δ 1.63 and 1.93 ppm in a 2:1 ratio,
supporting the corresponding sesquibromide structure
(allyl)3Ga2Br3.17
A possible catalytic cycle for the formation of allylic
gallium compounds by indium is shown in Scheme 2. The
Scheme 2. Possible Catalytic Cycle for the Formation of 3
surface of commercially available indium metal is covered
with oxidized indium(III). Because the reducing power of
gallium is stronger than that of indium, the indium(III) is
reduced with gallium(0) to generate indium(0) with no
oxidized layer (step A).5 Indeed, a fine powder having a
metal luster appeared when a catalytic amount of indium
metal was added to a suspension of gallium(0) in THF at 10
°C. Allylic bromide is reduced with a catalytic amount of
this indium(0) to produce the allylindium species 2 (step B).7
Reduction of allylic bromide with the indium(0) proceeds
smoothly because it is freshly generated in situ. Then,
transmetalation of the allylindium species 2 with gallium-
(III) proceeds to give the allylgallium species 3 and indium-
(III) (step C). As expected from the mechanism, the
allylgallium species 3 could also be prepared by addition of
a catalytic amount of InCl3 instead of the indium metal.
Scheme 1. Addition of Allylgallium Species to
Cyclododecanone
the catalytic amount of indium was added. When indium(0)
was not used, cyclododecanone was recovered in 95% yield.
On the other hand, when gallium(0) was not used in the
Barbier-type allylation, the reaction proceeded with only a
stoichiometric amount of indium(0) (0.90 mmol), but the
yield of 1 was 64% and the ketone was recovered in 29%
yield.
The allylation of the ketone could also be conducted using
the Grignard procedure. For example, when a catalytic
amount of indium(0) (powder, 0.045 mmol)11 was added to
a mixture of allyl bromide (1.5 mmol) and gallium metal
(powder, 0.90 mmol)11 in THF (5 mL), all of the gallium
powder dissolved after 2 h stirring at 10 °C.12-14 On treatment
of a solution of cyclododecanone (1.0 mmol) in THF (5 mL)
(13) A standard solution of allylgallium in THF (ca. 1.0 M solution,
allyl bromide excess) was prepared as follows. A catalytic amount of indium
powder (0.90 mmol) was added to a mixture of allyl bromide (30 mmol)
and gallium metal (shot, 18 mmol) in THF (28 mL), and the mixture was
stirred at 10 °C for 24 h. When gallium powder was used, all of the gallium
dissolved at 10 °C within 2 h. The solution of allylgallium in THF could
be stored at 5 °C (in a refrigerator) for 1 month without decreasing the
reactivity, but the reactivity decreased gradually at 25 °C. An allylgallium
solution without contamination of allyl bromide could be prepared by using
an excess amount of gallium metal; however, the reactivity of this solution
decreased faster at 25 °C than that of the solution containing an excess
amount of allyl bromide, and the solution became viscose.
(14) A solution of allyl gallium in DME or DMF could also be prepared
in the same fashion.
(7) (a) Araki, S.; Shimizu, T.; Johar, P. S.; Jin, S.-J.; Butsugan, Y. J.
Org. Chem. 1991, 56, 2538. (b) Araki, S.; Ito, H.; Butsugan, Y. J. Org.
Chem. 1988, 53, 1831.
(8) Tertiary-amine accelerated allylgallation of terminal alkynes does not
proceed in DMF.3f Thus, a preparative method for a solution of allylgallium
sesquibromide in THF is required.
(15) Addition of 3 to 3-phenylpropanal in THF occurred at 10 °C in 30
min; however, the yield of 1-phenyl-5-hexen-1-ol (4) was 74% as a result
of the formation of the corresponding ketone derived by Oppenauer-type
oxidation in 7% yield and the diallylation product in 5% yield. See ref 4b.
Formation of these byproducts was suppressed by addition of DMF (1.0
equiv of the gallium metal) in the reaction mixture, and 4 was obtained in
93% yield.
(16) To consume all of the allyl bromide, an excess amount of gallium
metal was used for the preparation of the allylgallium species in THF-d8
solution. The supernatant solution was used for the NMR measurement.
(17) Araki reported the NMR data of the allylindium species. (Allyl)3-
In2I3: ref 7b. AllylInI2: Araki, S.; Ito, H.; Katsumura, N.; Butsugan, Y. J.
Organomet. Chem. 1989, 369, 291. Allylindium(I): Chan, T. H.; Yang, Y.
J. Am. Chem. Soc. 1999, 121, 3228.
(9) Activating effects by addition of PbCl2 or KI-LiCl on gallium were
not observed at 25 °C in our experiments.
(10) The melting point of gallium lowered to 15.3 °C when 21.4 wt %
of indium was added.
(11) Gallium powder (99.9% purity, particle size ca. 0.85 mm diameter),
shot (99.99%, ca. 1 g), and indium powder (99% purity, particle size ca.
75 µm diameter) were purchased from Kojundo Chemical Lab Co., Ltd.
(12) When a shot of gallium(0) was used, the metal dissolved in 24 h.
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Org. Lett., Vol. 4, No. 10, 2002

This is not a simple process of recycling indium with
gallium(0) as a reductant but involves generating the allyl-
gallium species. Therefore, the transmetalation (step C)
should proceed quickly in order to construct the catalytic
cycle with a reasonable turnover frequency.18 In step C,
indium(III) is consumed by reduction with gallium(0),
whereas the amount of the allylindium species 2 is increased
by reaction of the regenerated indium(0) and allyl bromide.
In addition, indium has the largest difference of the bonding
energies between Mtl-Br and Mtl-C in the metal elements
of the 12-15 groups,19 and this provides an important driving
force for the transmetalation. During the transmetalation step,
the In-C and Ga-Br bonds rearrange to Ga-C and In-Br
bonds. Because the sum of the bond energies on the right
side (Ga-C + In-Br) of the arrows (step C) is larger than
the left side (In-C + Ga-Br), formation of carbon-gallium
bonds is more favorable and the transmetalation proceeds
smoothly.
To examine the transfer of the allyl group from indium to
gallium in step C, the following NMR experiments were
conducted. A solution of gallium(III) bromide (1 M solution)
in THF-d8 was added portionwise at 25 °C to a 1 M solution
of allylindium sesquibromide derived from indium metal (2
mmol) and allyl bromide (3 mmol) in THF-d8. When 0.5
mmol of gallium(III) bromide was added to the solution, the
allylic proton peak of the diallylindium bromide at δ 1.74
ppm disappeared with the appearance of that of diallylgallium
bromide at δ 1.63 ppm and increase of the allylindium
dibromide peak at δ 2.03 ppm. Allylic protons of allylgallium
dibromide at δ 1.90 ppm were not observed. On further
addition of gallium(III) bromide, proton peaks of allylgallium
dibromide appeared and both peaks of allylindium dibromide
and diallylgallium bromide decreased. When 3 mmol of
gallium(III) bromide was added to the solution, the peak of
allylgallium dibromide was only observed in the allylic
proton region.20 These results suggest the fast transmetalation.
Further, we examined this concept by designing a new
indium-catalyzed cycle for activation of a different metal.
To clarify the effectiveness, a candidate metal should have
a stronger reducing power than indium(0); however it should
not reduce allyl bromide directly (or at least the reducing
rate should be quite slow). Also, for the fast transmetalation
step, the metal should satisfy the following inequality for
the bond energy: (In-Br - In-C) > (Mtl-Br - Mtl-C).
Following these guidelines, we selected aluminum metal.
Aluminum metal has been used as a source of electrons,
e.g., as a reducing agent of allylic bromides to allylic
aluminum species, with a catalytic amount of PbCl2 or InCl3.4
However, the reported additions of allylic aluminum species
to carbonyl compounds were not conducted under the
Grignard but the Barbier conditions. By using a catalytic
amount of indium, allylic aluminum species could be
prepared and used in Grignard-type additions. For example,
allyl bromide (1.5 mmol) was added to a mixture of
aluminum (cut foil, 2 mm × 2 mm, 1.0 mmol) and a catalytic
amount of indium(0) (powder, 0.050 mmol) in THF (5 mL),
and the mixture was stirred at 25 °C. All of the aluminum
foils were dissolved in 30 min. Grignard-type addition of
the allyl aluminum compound to cyclododecanone (1.0
mmol) proceeded at 10 °C within 30 min to give 1 in 98%
yield. InCl3 could be used instead of the indium(0) in this
reaction.4c
The 1H and 13C NMR spectra of the prepared allylalumi-
num solution in THF-d8 revealed two sets of allyl peaks,
suggesting the allylaluminum sesquibromide (allyl)3Al2Br3.
All of the proton signals of the allylaluminum shifted upfield
compared to those of the allylgallium sesquibromide; the
allylic methylene of the allylaluminum appeared at δ 1.19
and 1.32 ppm in a 2:1 ratio (Supporting Information).21
In conclusion, allylgallium and allylaluminum sesquibro-
mides were prepared by direct reduction of allylic bromides
using a catalytic amount of indium as a mediator.
Acknowledgment. Financial support from the Ministry
of Education, Science, Culture, and Technology of Japan is
gratefully acknowledged. We would also like to thank
Professor Shuki Araki of Nagoya Institute of Technology
for valuable information about the NMR of allyl indium
species.
Supporting Information Available: Proton and carbon
spectra for allylgallium and allylaluminum sesquibromides.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(18) Gallium(III) tribromide is soluble in the ethereal solvents to some
extent.
(19) The bond energies between metals and carbon were estimated from
their trimethyl compounds. In-C, 160 kJ mol-1; Ga-C, 247 kJ mol-1
;
Al-C, 274 kJ mol-1. See: O’Neill, M. E.; Wade, K. In ComprehensiVe
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon: Oxford, 1982; Vol. 1, p 5. The bond energies between
metals and bromine are as follows: In-Br, 414 ( 21 kJ mol-1; Ga-Br,
444 ( 17 kJ mol-1; Al-Br, 429 ( 6 kJ mol-1. See: CRC Handbook of
Chemistry and Physics, 82nd ed.; CRC Press: Boca Raton, 2001; p 9-51.
(20) A solution of allylgallium dibromide in THF-d8 was also generated
by addition of gallium(III) bromide to a solution of allylgallium sesquibro-
mide in THF-d8. Allylic protons appeared at δ 1.90 ppm.
OL025784V
(21) Barbier-type allylation of carbonyl compounds with allyl bromide
and indium(I) regenerated on an aluminum sacrifical anode in THF was
reported quite recently. Although the authors think indium(I) plays an
important role in the reaction, they do not eliminate the possibility of the
aluminum species for the allylation. Hilt, G.; Smolko, K. I. Angew. Chem.,
Int. Ed. 2001, 40, 3399.
Org. Lett., Vol. 4, No. 10, 2002
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