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reaction of 1-butanol to 2-ethyl-1-hexanol under homogeneous
and heterogeneous Pd-based catalysts such as PdCl2(dppe) and
Pd/C, respectively, in the presence of sodium butoxide but using
a higher reaction temperature (200 °C).6
Guerbet Reaction of Primary Alcohols Leading to
â-Alkylated Dimer Alcohols Catalyzed by Iridium
Complexes
Previously, we reported the R-alkylation of ketones with
alcohols leading to R-alkylated ketones by Ir complexes in the
presence of a small amount of KOH (10 mol %) without any
solvents.7 This method provides a very convenient synthetic tool
which makes it possible to introduce a carbonyl function to the
desired position of ketones by reacting methyl ketones with
various primary alcohols. In the course of this study, our
attention has been focused on the Guerbet reaction, alkylation
of primary alcohols giving â-alkylated higher alcohols.8,9 In this
paper, we wish to report a highly efficient Ir-catalyzed Guerbet
reaction of various primary alcohols (eq 1).
Toyomi Matsu-ura, Satoshi Sakaguchi, Yasushi Obora, and
Yasutaka Ishii*
Department of Applied Chemistry, Faculty of Engineering &
High Technology Research Center, Kansai UniVersity, Suita,
Osaka 564-8680, Japan
ReceiVed July 6, 2006
[IrCl(cod)]2 and [Cp*IrCl2]2 complexes catalyzed efficiently
the Guerbet reaction of primary alcohols to â-alkylated dimer
alcohols in good yields. For instance, the reaction of
1-butanol in the presence of [Cp*IrCl2]2 (1 mol %), t-BuOK
(40 mol %), and 1,7-octadiene (10 mol %) produced 2-ethyl-
1-hexanol in 93% yield. Various primary alcohols undergo
the Guerbet reaction under the influence of Ir complexes to
give the corresponding dimer alcohols in good yields. This
method provides an alternative direct route to â-alkylated
primary alcohols which are prepared by aldol condensation
of aldehydes followed by hydrogenation.
1-Butanol (1a) was chosen as a model substrate for the
Guerbet reaction and allowed to react under various iridium
complexes (Table 1).
A p-xylene solution (0.5 mL) of [IrCl(cod)]2 (1 mol %), 1,7-
octadiene (10 mol %), and KOH (20 mol %) was stirred for 2
h at room temperature, and then 1a (2 mmol) was added and
reacted at 120 °C for 4 h to afford 2-ethyl-1-hexanol (2a) in
62% yield at 84% conversion of 1a (entry 1). The reaction using
[Ir(OH)(cod)]2 under these conditions gave 2a in a higher yield
(70%) (entry 2), whereas IrCl(PPh3)3 resulted in 2a in poor
selectivity (entry 3). IrCl3 did not catalyze the present reaction
(entry 4). Among the iridium complexes used, [Cp*IrCl2]2 was
found to be the best catalyst and led to 2a in 77% yield (entry
5). The use of t-BuOK in place of KOH as a base resulted in a
slight improvement of the yield of 2a (entry 6).
It is interesting to note that the present reaction could be
carried out using KOH as a base and without continuous removal
of the resulting water, in contrast to the Rh-catalyzed Guerbet
reaction of 1a and 1-pentanol (1b) where sodium butoxide for
1a and sodium pentoxide for 1b are used as bases and the
resulting water is continuously removed through a column
packed with 4 Å molecular sieves. Sodium alkoxides are
hydrolyzed by the water generated to form NaOH, which
induces the formation of carboxylate salts and esters by the
Tischenko reaction and/or the Canizzaro reaction of aldehydes.3
In the present Ir-catalyzed Guerbet reaction, however, a small
amount of a hydrogen acceptor such as 1,7-octadiene was
needed. The reaction without 1,7-octadiene resulted in the
The Guerbet reaction is recognized as a useful synthetic tool
to obtain â-alkylated dimer alcohols by the self-condensation
of primary alcohols as exemplified by the conversion of
1-butanol to 2-ethyl-1-hexanol.1 The reaction is usually carried
out in the presence of alkali metal hydroxides or alkoxides and
hydrogenation/dehydrogenation catalysts such as Raney Ni at
higher temperature (220 °C) and pressure.2 In 1972, the Guerbet
reaction was first examined using transition-metal complexes
such as Rh, Ru, Pt, and Ir which makes it possible to carry out
the Guerbet reaction under much milder conditions of 110-
140 °C at atmospheric pressure.3 Among the metal complexes
examined, the catalytic activity of RhCl3‚3H2O/4P(C4H9)3 is
shown to be very high compared with that of Ru and Ir
complexes, but Pd and Au complexes are inactive because of
the difficulty of the generation of stable hydride metal com-
plexes.3 Burk et al. reported the Rh-promoted Guerbet reaction
of 1-butanol and 1-pentanol and clarified the multiple roles of
Rh and alkoxide catalysts.4,5 Carlini et al. disclosed the Guerbet
(6) Carlini, C.; Macinai, A.; Raspolli Galletti, A. M.; Sbrana, G. J. Mol.
Catal. A: Chem. 2004, 212, 65.
(7) Taguchi, K.; Nakagawa, H.; Hirabayashi, T.; Sakaguchi, S.; Ishii,
Y. J. Am. Chem. Soc. 2004, 126, 72.
(1) Guerbet, M. C. R. Acad. Sci. 1909, 49, 129.
(2) Falbe, J.; Bahrmann, H.; Lipps, W.; Mayer, D. Alcohols, Aliphatic
In Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; release 2006
on the web.
(3) Gregorio, G.; Pregaglia, G. F.; Ugo, R. J. Organomet. Chem. 1972,
37, 385.
(8) Quite recently, â-alkylation of secondary alcohols with primary
alcohols is reported via the in situ generation of methyl ketones. Fujita, K.;
Kitatsuji, S.; Yamaguchi, T.; Hanasaka, F.; Yamaguchi, R. Org. Lett. 2005,
7, 4017.
(9) (a) Cho, C. S.; Kim, B. T.; Kim, T.-J.; Shim, S. C. Tetrahedron Lett.
2002, 43, 7987. (b) Cho, C. S.; Kim, B. T.; Kim, T.-J.; Shim, S. C. J. Org.
Chem. 2001, 66, 9020. (c) Cho, C. S.; Kim, B. T.; Kim, H.-S.; Kim, T.-J.;
Shim, S. C. Organometallics 2003, 22, 3608.
(4) Burk, P. L.; Pruett, R. L.; Campo, K. S. J. Mol. Catal. 1985, 33, 1.
(5) Burk, P. L.; Pruett, R. L.; Campo, K. S. J. Mol. Catal. 1985, 33, 15.
10.1021/jo061400t CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/21/2006
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J. Org. Chem. 2006, 71, 8306-8308