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J. M. Brunel
LETTER
(2) (a) Tsuji, J. Palladium Reagents and Catalysts: Innovations
in Organic Synthesis; John Wiley & Sons: New York, 1995.
(b) Poli, G.; Giambastiani, G.; Heumann, A. Tetrahedron
2000, 56, 5959. (c) Tsuji, J. Transition Metal Reagents and
Catalysts: Innovations in Organic Synthesis; John Wiley &
Sons: New York, 2000.
(3) For reviews on hydrogen transfer reactions, see:
(a) Zassinovich, G.; Mestroni, G. Chem. Rev. 1992, 92,
1051. (b) Brieger, G.; Nestrick, T. Chem. Rev. 1974, 74,
567. (c) de Graauw, C. F.; Peters, J. A.; van Bekkum, H.;
Huskens, J. Synthesis 1994, 1007.
32
33
34
36
COOH
Ph
NHCOMe
35
HO
(4) (a) Zoran, A.; Sasson, Y. J. Mol. Catal. 1984, 26, 321.
(b) Cortese, N. A.; Heck, R. F. J. Org. Chem. 1978, 43,
3985. (c) Selvam, P.; Sonavane, S. U.; Mohapatra, S. K.;
Jayaram, R. V. Tetrahedron Lett. 2004, 45, 3071.
(5) (a) Baán, Z.; Finta, Z.; Keglevich, G.; Hermecz, I.
Tetrahedron Lett. 2005, 46, 6203. (b) Arcadi, A.;
Bernocchi, E.; Cacchi, S.; Marinelli, F. Synlett 1991, 27.
(c) Prasad, K.; Jiang, X.; Slade, J. S.; Clemens, J.; Repič, O.;
Blacklock, T. J. Adv. Synth. Catal. 2005, 347, 1769. (d) We
must also mention the work of Arterburn involving the
formation of Pd-colloids species leading to an homogeneous
transfer hydrogenation of various substrates: Arterburn, J.
B.; Pannala, M.; Gonzaleza, A. M.; Chamberlin, R. M.
Tetrahedron Lett. 2000, 41, 7847. (e) Nevertheless, in the
present study even if no evidence has been obtained for the
exclusion, or no formation, of such Pd colloids, it seems that
our catalytic system was totally homogeneous. Studies are
now underway to determine the exact nature of the catalyst
and will be reported in due course.
< 5% yield in all cases
Figure 1 Palladium-catalyzed reduction of disubstituted and/or
functionalized alkenes under transfer-hydrogenation conditions
R
P(t-Bu)3
O
H
OH
Pd0[P(t-Bu)3]2
L = P(t-Bu)3
R
H
Ln
38
Pd
37
Pd
H
Ln
O
H
(6) Brunel, J. M. Mini Rev. Org. Chem. 2004, 1, 249.
(7) For a review on green chemistry, see: Anastas, P. T.;
Kirchhoff, M. M. Acc. Chem. Res. 2002, 35, 686.
(8) For a discussion of atom economy, see: (a) Trost, B. M.
Acc. Chem. Res. 2002, 35, 695. (b) Trost, B. M. Science
1991, 254, 1471.
O
R
Ln
CO2
R
Pd
H
O
H
O
(9) Selective Reduction of 4-Allylanisole(1) under Transfer
Hydrogen Conditions; General Procedure (Table 1,
Entry 4): CAUTION: All reagents (solvent, phosphine
ligand, formic acid) must be purified before use in order to
obtain good conversions and reproducible results. In a two-
necked round-bottomed flask placed under argon was added
Pd(OAc)2 (5.4 mg, 2.4 × 10–5 mol) at 20 °C in freshly
distilled anhyd THF (5 mL). To this solution, P(t-Bu)3 (20
mg, 10–4 mol; purchased from Acros) was successively
added and the solution was heated to reflux for 10 min. To
this mixture 4-allylanisole (1; 148 mg, 10–3 mol) and formic
acid (230 mg, 5 × 10–3 mol) were added. The resulting
solution was stirred for 15 min to reflux and then cooled to
20 °C for 12 h. After filtration over Celite and evaporation of
the solvents, the crude residue was purified by chromatog-
raphy on a silica gel column using PE–EtOAc (100:0 →
50:50) as eluent affording the expected product 2 in 95%
yield.
Scheme 1 Mechanistic rationale of selective palladium-catalyzed
reduction of alkenes under transfer-hydrogenation conditions
In conclusion, a new and efficient mild Pd/P(t-Bu)3
catalyst for selective reduction of various alkenes under
transfer-hydrogenation conditions has been developed.
Additional studies dealing with the scope and limitation of
such a method are currently underway and will be report-
ed in due course.
References and Notes
(1) (a) Johnstone, R. A. W.; Willby, A. H.; Entwistle, I. D.
Chem. Rev. 1985, 85, 129. (b) Kovacs, J.; Todd, T. D.;
Reibenspies, J. H.; Joo, F.; Darensbourg, D. J.
(10) Kinetic experiments performed on the selective reduction of
4-allylanisole (1) under transfer-hydrogenation conditions
indicate that such a reaction takes place in less than 4 h.
Nevertheless, this reaction time seems to be highly substrate-
dependent and we have chosen to perform all experiments
for 12 h.
Organometallics 2000, 19, 3963. (c) Lee, S. H.; Park, Y. J.;
Yoon, C. M. Tetrahedron Lett. 2000, 41, 887.
Synlett 2007, No. 2, 330–332 © Thieme Stuttgart · New York