DOI: 10.1002/cctc.201500729
Communications
Catalytic Semireduction of Internal Alkynes with All-Metal
Aromatic Complexes
Pierre-Alexandre Deyris,[a] Tatiana CaÇeque,[a] Yanlan Wang,[a] Pascal Retailleau,[a]
Franca Bigi,[c] Raimondo Maggi,[c] Giovanni Maestri,*[a, c] and Max Malacria*[a, b]
Dedicated to Professor Iwao Ojima on the occasion of his 70th birthday
A simple catalytic method involving all-metal aromatic frame-
works as precatalysts ensures an efficient route to (Z)-alkenes.
Aromatic triangular palladium clusters were used to reduce in-
ternal alkynes without any trace of the formation of alkane
side products. These trinuclear complexes provide a catalytic
system that parallels the activity and selectivity of their best
Scheme 1. Strategies to exploit triangular metal–aromatic clusters.
mononuclear peers, and the catalyst likely operates through
complementary mechanisms.
Trinuclear metal–aromatic complexes are interesting plat-
forms for catalytic applications (Scheme 1). Featuring noncoor-
All-metal aromatics are cyclic molecules that present delocal-
ized molecular orbitals similar to those of regular aromatics
but involve metal atoms in their cores.[1] They can therefore
display a greater variety of bonding modes and represent an
ideal bridge that connects discrete homogeneous complexes
with heterogeneous metallic nanoparticles thanks to their de-
localized metal–metal bonding.[2] We wanted to gain some in-
sight into the consequences of metal aromaticity by studying
the catalytic behavior of stable and easily accessible triangular
d-orbital-aromatic palladium clusters.[3]
dinating counteranions, the positive charge of these clusters
should provide them with Lewis acid character. At the same
time, their delocalized HOMO might grant Lewis basic proper-
ties to the triangular core. Their resonance-enhanced stabiliza-
tion should increase their robustness. From a practical point of
view, these complexes are definitely stable to oxygen and
moisture, are formed in high yields from commercial reagents
in one-pot, and feature easily tunable organic fragments.[3]
We report the application of a metal–aromatic framework in
a catalytic reaction. Triangular Pd3 clusters were used to effi-
ciently reduce internal alkynes to (Z)-alkenes under transfer-hy-
drogenation conditions, completely excluding over-reduction
to alkanes and preserving other reducible groups, even in the
presence of a large excess amount of the donor.
We reasoned that semihydrogenation of alkynes would be
both a challenging and meaningful playground for aromatic
palladium clusters.[4] Whereas these reactions are well studied
in homogenous catalysis,[5] no catalytic method for the semire-
duction of alkynes has been reported by using discrete Pd
clusters, and examples with other clusters are rare.[6] (Z)-Al-
kenes can be formed in high yields by using electron-rich Pd0
complexes bearing suitable N-heterocyclic carbenes or chelat-
ing diphosphines as ligands.[7] Their efficiency represents
a probing test and therefore a tool to check the potential of
metal aromatics.[8]
We used 1-phenylpropyne (1a) as a model substrate. Its in-
ternal triple bond is conjugated with the aromatic ring, which
makes it the candidate of choice to evaluate catalyst efficiency
(Table 1). In a typical experiment, 1a (0.3 mmol) was stirred
with a Pd3 catalyst (1 mol%) and triethylammonium formate
(5 equiv.) in THF. The mixture was heated to reflux, and sam-
ples were periodically taken for GC–MS analyses. The complex
with p-tolyl groups on bridging thiolates and fluorinated tri-
arylphosphines provided proof of principle of the feasibility of
the reaction (Table 1, entry 1). The activity of this complex was
fairly low and conversion ceased at 86% after 4 days. It
showed good Z selectivity (92%) and no trace of over-reduc-
tion; (E)-phenylpropene (3a) and allylbenzene (4a) were the
sole co-products (4% selectivity each). Scrambling electron-do-
nating and electron-withdrawing groups on fragments proved
beneficial for activity but detrimental for selectivity (Table 1,
entry 2). The alkyne was consumed in 64 h but 3a and 4a
were formed in yields of 11 and 7%, respectively. The Z selec-
tivity was similar with the all-phenyl-substituted cluster (83%;
Table 1, entry 3), which became inactive after 2 days. We then
tried to tune the steric hindrance around the metal aromatic
core. Surprisingly, complexes with aliphatic phosphines were
[a] P.-A. Deyris, Dr. T. CaÇeque, Dr. Y. Wang, P. Retailleau, Dr. G. Maestri,
Prof. M. Malacria
ICSN CNRS (UPR 2301)
1 Av. de la Terrasse, Bat. 27, 91198 Gif-sur-Yvette (France)
[b] Prof. M. Malacria
IPCM (UMR CNRS 7201)
UPMC Sorbonne UniversitØs
4 place Jussieu, C. 229, 75005 Paris (France)
[c] Prof. F. Bigi, Prof. R. Maggi, Dr. G. Maestri
Dip. di Chimica
Università degli Studi di Parma
17 A Parco Area delle Scienze, 43124 Parma (Italy)
Supporting Information for this article is available on the WWW under
ChemCatChem 2015, 7, 3266 – 3269
3266
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