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Tetrahedron 63 (2007) 3899–3906
Scope, limitations and mechanistic aspects in the selective
homogeneous palladium-catalyzed reduction of alkenes
under transfer hydrogen conditions
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Jean Michel Brunel
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Laboratoire Syntheꢀse et Etude de Substances Naturelles aꢀ Activites Biologiques (SESNAB), Faculte des Sciences et Techniques de
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St Jerome, Case 342, Universite Paul Cezanne, Aix-Marseille III, Avenue Escadrille Normandie Niemen,
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13397 Marseille Cedex 20, France
Received 2 January 2007; revised 22 January 2007; accepted 25 January 2007
Available online 30 January 2007
Abstract—A new and efficient mild Pd/P(t-Bu)3 catalyst for selective reduction of various alkenes under transfer hydrogen conditions has
been developed leading to the corresponding saturated derivatives in chemical yields varying from 65 to 98%. Mechanistic rationale of this
reaction has been also demonstrated.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
marked changes in the behavior of the free ligands and of
their transition metal complexes. Thus, it has appeared in
this area that P(t-Bu)3 (tri-tert-butyl phosphine) may have a
singular behavior since numerous reactions can be only per-
formed using this ligand.9 In this context, we recently found
its successful use as ligand in a homogeneous palladium-
catalyzed reduction of numerous alkenes under transfer hy-
drogen conditions with high selectivities and isolated yields.10
In continuation of our work, we report herein the scope,
limitations, and mechanistic aspects of such a reaction.
The growing importance of green chemistry and the concept
of atom economy in organic synthesis have increased the
search for transformations resulting in less waste.1,2 An im-
portant advance in this field is the development of multifunc-
tional reagents capable of promoting one or more distinct
transformations sequentially in the same pot. Such reactions
not only make better use of precious reagents but, as an
added benefit, eliminates inefficient separation and purifica-
tion after each step.3,4 In this context, palladium reagents,
well known to provide a myriad of different reactions, ap-
peared particularly well suited.5,4b Hydrogenation of unsat-
urated compounds is one of the most important reactions in
organic chemistry and hence several reagents have been used
for this purpose.6 Addition of hydrogen to an alkene to form
the corresponding alkane is highly thermodynamically
favored. However, the reaction rate is negligible under ordi-
nary conditions in the absence of a catalyst. Selective, mild,
and effective reducing agents in transition metal-catalyzed
transfer hydrogenation have been of considerable interest.
Among the various available processes, catalytic transfer hy-
drogenation (CTH) is emerging as a viable alternative to the
commonly used reduction processes involving hazardous
molecular hydrogen or a metal hydride donor.7 Nevertheless,
to date only few homogeneous catalytic palladium systems
have been reported for selective reduction of alkenes under
transfer hydrogen conditions.8 It has long been recognized
that changing substituents on phosphorus ligands can cause
A systematic analysis of solvent and ligand nature as well as
temperature and palladium source involved has been real-
ized since this reaction seems to be highly experimental
conditions dependent as we have already noticed. Thus, 4-al-
lylanisole 1 was chosen as test substrate and the reaction
was carried out using HCO2H as the hydrogen source under
various experimental conditions (Table 1).
Although isolated chemical yield of up to 95% has been ob-
tained performing the reaction in THF (Table 1, entry 6), it
clearly appears that the nature of the solvent used has an
important influence on the outcome of the reaction. Thus, po-
lar solvents such as dioxane and diethylether led to the forma-
tion of the expected hydrogenated product in low chemical
yields (Table 1, entries 5 and 6, 15 and 25% yield, respec-
tively), whereas no conversion occurred using dichloro-
methane or acetonitrile as solvents (Table 1, entries 8 and 9).
In a second way, the influence of the phosphorus ligand in-
volved on the outcome of the reaction has been studied
(Table 2). Thus, the choice of the nature of the ligand appears
crucial since only P(t-Bu)3 afforded compound 2, whereas
Keywords: Palladium; Reduction of alkenes; Tri-tert-butyl phosphine;
Transfer hydrogen; Catalysis.
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.01.053