Nickel-Catalyzed Alkylation and Transfer Hydrogenation of α,β-Unsaturated Enones with Methanol

Complexes of the type [{(dippe)­Ni}n(η2-Cα,Cβ-1,4-dien-3-one)]
(dippe = 1,2-bis­(diisopropylphosphino)­ethane); n=
1, 2; enone = aromatic 1,4-pentadien-3-ones) were synthesized. The
“[(dippe)­Ni]” moiety derived from [(dippe)­Ni­(μ-H)]2 η2-coordinated to the C,C double bonds of
the corresponding α,β-unsaturated enone and was fully
characterized using a variety of spectroscopic techniques, for instance,
single-crystal X-ray diffraction, nuclear magnetic resonance (NMR),
and mass spectrometry. The complexes were assessed in a catalytic
transfer hydrogenation process using methanol (CH3OH) as
a hydrogen donor. This alcohol turned out to be a very efficient reducing
and alkylating agent of 1,4-pentadien-3-ones, under neat conditions.
The current methodology allowed the selective reduction of CC
bonds in α,β-unsaturated enones to yield enones and saturated
ketones by a homogeneous catalytic pathway, whereas by a heterogeneous
pathway, the process leads to the formation of mono- and dimethylated
ketones. In the latter case, the occurrence of nickel nanoparticles
in the reaction media was found to participate in the catalytic alkylation
of such dienones.