B.Basu et al./ Tetrahedron Letters 46 (2005) 8591–8593
8593
ployed, with the expectation that other functional
groups would not react, to force the reaction to comple-
tion. The reduction of the C–C double bond proceeded
smoothly at 50–80 ꢂC requiring only gentle agitation;
work-up was then achieved by simple filtration, extrac-
tion with diethyl ether and evaporation. The reduced
product was purified by column chromatography over
silica gel and isolated in 75–86% yield. Other reducible
groups such as the ketone, nitrile or halogen and ester
groups remained unaffected under the reaction
conditions.
ladium) of PSF-Pd along with newer applications for
other transformations are currently underway.
Acknowledgements
We thank the Council of Scientific & Industrial Re-
search, New Delhi for financial support (Grant No.
01(1863)/03/EMR-II). P.D. and S.D. thank the CSIR
for research fellowships.
References and notes
To extend the scope and generality of the PSF-Pd in
CTH, we explored reduction of the C–N double bonds
of imines. Since imines are generally derived from the
corresponding aldehydes or ketones, the overall reaction
in one-pot constitutes a method for Ôdirect reductive
aminationÕ and is an attractive synthetic route to sec-
ondary and tertiary amines. Using the PSF-Pd, the imi-
nes could be reduced efficiently at 50–70 ꢂC (entries 6–8
in Table 2). A nitro group and heteroaromatic moiety
remained unaffected under the reaction conditions.
1. (a) Brieger, G.;Nestrick, T. J. Chem.Rev. 1974, 74, 567–
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3. Noyori, R.;Yamakawa, M.;Hashiguchi, S. J.Org.Chem.
2001, 66, 7931–7944, and references cited therein.
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Chem.Rev. 2000, 100, 2159–2232;(b) Noyori, R.;Takaya,
H. Acc.Chem.Res.
1990, 23, 345–350;(c) Ram, S.;
Ehrenkaufer, R. E. Synthesis 1988, 91–95.
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S.;Leach, A. G.;Longbottom, D. A.;Nesi, M.;Scott, J.
S.;Storer, I.;Taylor, S. J. J.Chem.Soc,. Perkin.Trans.1
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B.;Bhuiyan, M. M. H.;Das, P.;Hossain, I. Tetrahedron
Lett. 2003, 44, 8931–8934.
7. (a) Ley, S. V.;Mitchell, C.;Pears, D.;Ramarao, C.;Yu, J.
Q.;Zhou, W. Org.Lett. 2003, 5, 4665–4668;(b) Yu, J. Q.;
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1998, 63, 4172–4173.
The reduction of nitroarenes to anilines is a synthetically
important transformation both in the laboratory and in
industry. To broaden the scope of PSF-Pd, reduction of
the nitro group was investigated with nitroarenes as the
substrates. While the nitro group was not reducible at a
lower temperature (70 ꢂC) it could be reduced at 100–
110 ꢂC to yield the corresponding anilines (entries 9–12
in Table 2). Several other reducible groups such as a hal-
ogen, ester or ketone were inert to these conditions illus-
trating a clear advantage in terms of chemoselectivity.
Further applications of this new heterogeneous reduc-
tive system were tested with 1,2-dicarbonyl compounds.
When benzil or substituted benzils were used as the sub-
strate, the reduction of one of the carbonyl groups with
PSF-Pd in DMF at 100 ꢂC reached completion after 10–
12 h to give the corresponding a-hydroxyketone (benz-
oin) in a 77–88% isolated yield (entries 13–15 in Table
2).
8. Ley, S. V.;Mitchell, C.;Pears, D.;Ramarao, C.;Yu, J. Q.;
Zhou, W. Org.Lett. 2003, 5, 4665–4668.
9. (a) Akiyama, R.;Kobayashi, S. Angew.Chem,. Int.Ed.
2001, 40, 3469;(b) Ramarao, C.;Ley, S. V.;Smith, S. C.;
Shirley, I. M.;DeAlmeida, N. Chem.Commun. 2002,
1132–1133;(c) Yu, J. Q.;Wu, H. C.;Ramarao, C.;
Spencer, J. B.;Ley, S. V. Chem.Commun. 2003, 678–679;
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33.
10. A representative procedure for the CTH of 1,2-diketones
using PSF-Pd: To a solution of 4,40-dimethoxybenzil
(2 mmol) in freshly distilled DMF (2 cm3) was added
PSF-Pd (1 g) and the reaction mixture was stirred at
110 ꢂC for 10 h. The reaction mixture was diluted with
ether (15 cm3) and filtered through a bed of cotton. The
filtrate was extracted with ether (2 · 15 cm3) and the
combined organic layers were washed brine (2 · 10 cm3)
and dried (Na2SO4). Evaporation under reduced pressure
and purification by column chromatography afforded 4,40-
dimethoxybenzoin as a pale yellow solid (88% yield);mp
111–112 ꢂC;FT IR (Nujol): mmax 3466, 1666, 1597, 1512,
The novel combination PSF-Pd was easily used for four
successive recycling runs without any significant drop of
reactivity. With methyl 4-nitrobenzoate as the substrate,
the reduction proceeded to completion giving excellent
yields for up to four runs.
In summary, formic acid as its formate anion and a pal-
ladium catalyst from palladium acetate have been co-
immobilized effectively on an inexpensive Amberlyst
ion-exchange resin. This resin (PSF-Pd) proved to be a
versatile and heterogeneous reductive combination for
transfer hydrogenation of functionalized alkenes, imi-
nes, nitroarenes and 1,2-diketones. This new technique
also demonstrates high chemoselectivity in the reduction
of alkenes, imines and nitro groups, thus establishing an
efficient, environmentally benign, economically friendly
and sustainable process. Further studies on structural
aspects and the reaction behaviour (leaching of the pal-
1466 cmꢀ1 1H NMR (300 MHz, CDCl3): d = 7.91 (d,
;
J = 9 Hz, 2H), 7.26 (d, J = 8.7 Hz, 2H), 6.87 (d, J = 9 Hz,
2H), 6.85 (d, J = 8.7 Hz, 2H), 5.87 (s, 1H), 4.85–4.3 (br s,
1H), 3.82 (s, 3H), 3.76 (s, 3H); 13C NMR (75 MHz,
CDCl3): d = 197.3, 163.9, 159.6, 131.8, 131.5, 129.0, 126.2,
114.5, 113.9, 75.2, 55.4, 55.2.
11. Zheng, T.;Stacchiola, D.;Saldin, D. K.;James, J.;Scholl,
D. S.;Tysoe, W. T. Surf.Sci. 2005, 574, 166–174.