Speaker: Prof. Dr. Christophe Michon
University of Strasbourg and CNRS, LIMA UMR7042,
ECPM 25 rue Becquerel FR-67087 Strasbourg, France.
Email: cmichon@unistra.fr
Title: “Towards chemoselective and sustainable catalysts
for the reduction of organic compounds”
Day and Date: Tuesday, October 31, 2023
Time: 4.00 pm.
Venue: Room no. 350, Chemistry Department
Second floor, Annex
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Hosted by Prof. Maheswaran Shanmugam
Abstract From laboratory to industrial scale, the reduction of organic compounds is one of the
major pathways for the synthesis of bulk and fine chemicals of interest and can be performed
through hydrogenation,[1] hydrogen transfer[2] or hydrosilylation.[3] Hydrogenation is the
most important and relevant of these three catalytic methods and is widely applied in the
industry.[4] For example, the catalytic semi-hydrogenation of internal or terminal alkynes is
useful in numerous organic syntheses by providing alkene building blocks, both at laboratory
and industrial scale.[5] In particular, semi-hydrogenation is critical in the reduction of
polyfunctionalized bioactive molecules and in polymerization reactions in order to eliminate
alkynes and dienes from alkene raw materials. In this context, various homogeneous and
heterogeneous transition metal-based catalysts have been developed but, overall, several
challenges remain to be met in order to overcome catalyst deactivation and develop highly
active, selective, stable and reusable hydrogenation catalysts.[6] By comparison,
hydrosilylation enables the mild and selective reduction of organic compounds using mainly
transition-metal catalysts.[3] Indeed, the use of hydrosilanes as reductants, which proceeds
without any high-pressure equipment or high temperatures, is an interesting alternative to
hydrogenation. Because the reactivity of such reagents and related reaction intermediates is
modular and depends on the substituents on the silicon atoms, the hydrosilylation reaction can
become a highly chemo- and regioselective reduction method that tolerates various other
reducible functional groups.[3] Due to economic, environmental and societal reasons,
attention is now turned on catalysts based on abundant and non-precious metals, i.e. first row
transition metals, as well as organocatalysts.[7]
At first, we have shown iridium(III) metallacycles[8a] can catalyze the hydrosilylation
of alkynes,[8b] imines,[8c] and various carbonyl and carboxylic acid derivatives.[8d-g] We
have specifically highlighted in details the selective hydrosilylation of esters to aldehydes[8d],
the efficient hydrosilylation of amides into amines[8f] and the controlled hydrosilylation of
ene-amides into amides or amines through a tandem process.[8g] These reactions are fast,
selective and performed with low catalyst loadings using mild conditions. The reaction
mechanisms were detailed through studies based on NMR, HRMS, DFT calculations and on
the isolation of key intermediates.
Second, we have shown cobalt(II) salts combined with NaBHEt3 and eventually a base
catalyse efficiently and selectively the reduction of esters to aldehydes or alcohols through
hydrosilylation by using phenylsilane. Catalyst characterisations by XRD, XPS, TEM and
STEM analyses indicate the materials were partially crystalline with the presence of cobalt
nanoparticles. Control experiments suggested low valent Co(0) was the active catalytic
Lecture at IIT Mumbai - October 31th 2023
species involved. Interestingly, one catalyst was successfully reused up to 4 cycles without
significant loss of selectivity.[9]
Third, we have reported an organocatalytic and transition metal-free reductive
deoxygenation of esters to ethers through the use of a hydrosilane and a fluorinated borate
BArF salt as organocatalyst. Experimental in-situ IR and theoretical studies (including NCI
and NBO analyses) supported the role of noncovalent interactions between the fluorinated
organocatalyst, the hydrosilane and the ester substrate in the activation of silane and therefore
in the reaction mechanism.[10]
Finally, we have shown a half-sandwich nickel NHC-picolyl complex effectively
catalyzed the hydrosilylation of aldehydes and ketones. Studies by DLS, STEM, XPS, ICPAES and elemental analyses showed evidence for the involvement of NHC-stabilized Ni(0)
nanoparticles when potassium t-butoxide is used as an activator.[11a] Interestingly, we have
subsequently found that the reduction of a [NiCpBr(NHCcinnamyl)] complex with an excess
of MeMgBr leads to coordinated Ni(0) nanoparticles. The combination of a strong -donor
NHC ligand with a -coordinating appended cinnamyl moiety likely prevents Ni(0) particle
aggregation into larger inactive species and allows rapid exchanges with reaction substrates.
The resulting NHC-olefin-Ni(0) particles effectively catalyse the (Z)-selective semihydrogenation of various alkynes and ynamides (Figure 1), the latter being a rare example of
direct semi-hydrogenation of ynamides without the use of any salt or additive. The developed
catalyst proved to be reusable over six runs.[11b] It is worth to note such NHC-olefin-Ni(0)
particles can be applied further as catalyst in full hydrogenations and reductive aminations in
organic solvents or in water using micellar conditions.