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the application of the optimized conditions to 49-bromochalcone
resulted in a mixture of debrominated products. When benzyloxy
(1c), carboxyl (1d), hydroxy (1e), or amino (1f) groups were attached
to the 49-position of the chalcone, the reaction proceeded
smoothly and selectively at the olefin moiety (Table 3, entries 2–
5). The olefin moiety of an a,b-unsaturated carboxylic acid (1g) was
also successfully reduced when the solvent was changed to THF to
inhibit the formation of an ester in the presence of EtOH (Table 3,
entry 6). A benzyl ester (1h), which is a common protecting group
for carboxylic acids, was not deprotected through the selective
reduction of olefin moiety (Table 3, entry 7). Furthermore, primary
(1i), secondary (1j), and tertiary (1k) amides all were well tolerated
under the optimized conditions (Table 3, entries 8, 9 and 10,
respectively). Compounds bearing alkyl substituents at both ends
of the a,b-unsaturated carbonyl moiety did not inhibit the reaction
(Table 3, entries 11 and 12). Unfortunately, the selective mono-
reduction of diene system proved difficult and both the olefins
were eventually completely reduced without any loss of the
carbonyl group (Table 3, entries 13 and 14). The selective reduction
of an a,b-unsaturated aldehyde proved to be a more challenging
reaction.14 When the optimized conditions were applied to the
a,b-unsaturated aldehyde (1p), both the olefin and carbonyl
groups were reduced to give a mixture of the aldehyde (2p) and the
corresponding alcohol (Table 3, entry 15). It has been reported that
the solvent can control the chemoselectivity,15 therefore, various
that GO is hydrophilic, it was envisaged that substrates bearing
polar or ionic functional groups would show a high level of affinity
for the catalyst (Table 4). D-Glucal (3a), possessing three hydroxy
groups, was successfully reduced under the optimized conditions
without any loss of its hydroxy groups nor deactivation of the
catalyst (Table 4, entry 1). An imidazolium salt (3b), which
represents a framework widely employed in ionic liquids, was also
reduced without any deactivation of the catalyst (Table 4, entry 2).
In addition, a diastereoselective reduction was achieved when (2)-
terpinen-4-ol (3c) was employed as a substrate (Table 4, entry 3),
likely because of a hydrogen bonding interaction with an oxygen-
containing functional group on the GO. The application of
commercial Pd/C catalyst under the same conditions produced a
6 : 4 mixture of diastereomers.
Conclusions
We have achieved the preparation of a reactive, selective, and
recyclable Pd/GO precatalyst for the reduction of a,b-unsaturated
carbonyl compounds. The active catalyst species, Pd(0) nanopar-
ticles on rGO, is formed during the catalytic reaction, and can be
recycled without any leaching or aggregation of the Pd species.
The existence of minimal essential amounts of oxygen function-
alities on graphene would effectively coordinate to Pd species,
enhancing the chemoselectivity and stability of Pd nanoparticles.
solvents were examined (See ESI ). Pleasingly, the chemoselectivity
3
Acknowledgements
of
the
process
could
be
enhanced
by
adding
N,N-dimethylacetamide (DMA) to give 2p in high yield (Table 3,
entry 16).
We then investigated the potential for further expanding the
substrate scope of the selective olefin reduction process. Given
Financial support for this study was provided by the
Development of Human Resources in Science and
Technology, Ministry of Education, Culture, Sports, Science,
and Technology of Japan, and RSK culture foundation, and
The Kyoto Technoscience Center. This work was partly
supported by Nanotechnology Platform Program of the
Ministry of Education, Japan.
Table 4 Expanded substrate scope of the Pd/GO-catalyzed reductiona
Notes and references
1 T. Ikawa, H. Sajiki and K. Hirota, Tetrahedron, 2005, 61,
2217–2231.
2 (a) H. Sajiki, Tetrahedron Lett., 1995, 36, 3465–3468; (b) B.
P. Czech and R. A. Bartsch, J. Org. Chem., 1984, 49, 4076–4078.
3 (a) H. Sajiki, K. Hattori and K. Hirota, J. Org. Chem., 1998, 63,
7990–7992; (b) K. R. Campos, D. W. Cai, M. Journet, J. J. Kowal,
R. D. Larsen and P. J. Reider, J. Org. Chem., 2001, 66, 3634–3635.
4 (a) K. W. Rosenmund, Ber. Dtsch. Chem. Ges., 1918, 51, 585–593;
(b) A. Mori, T. Mizusaki, Y. Miyakawa, E. Ohashi, T. Haga,
T. Maegawa, Y. Monguchi and H. Sajiki, Tetrahedron, 2006, 62,
11925–11932.
Entry
1
Substrate
Product
Yield (%)b
96
2
98
5 (a) V. Pandarus, G. Gingras, F. Beland, R. Ciriminna and
M. Pagliaro, Org. Process Res. Dev., 2012, 16, 1230–1234; (b)
G. Neri, G. Rizzo, L. De Luca, A. Donato, M. G. Musolino and
R. Pietropaolo, Appl. Catal., A, 2009, 356, 113–120; (c) M.
L. Kantam, R. Kishore, J. Yadav, M. Sudhakar and
A. Venugopal, Adv. Synth. Catal., 2012, 354, 663–669; (d) M.
L. Kantam, T. Parsharamulu and S. V. Manorama, J. Mol. Catal.
A: Chem., 2012, 365, 115–119; (e) M. Lim, K. A. De Castro, S. Oh,
K. Lee, Y. W. Chang, H. Kim and H. Rhee, Appl. Organomet.
Chem., 2011, 25, 1–8.
3c
100 (85 : 15)d
a
General conditions: Pd (0.1 mol%), 3 (0.3 mmol), H2O (1.5 mL),
and EtOH (1.5 mL) were mixed and stirred under a H2 atmosphere
(1 atm). Isolated yield. 0.5 mol% of Pd was added. The ratio of
diastereomers was determined by GC.
b
c
d
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RSC Adv., 2013, 3, 15608–15612 | 15611