Seminar by Prof. Dr. Christophe Michon, University of Strasbourg and CNRS, France on "Towards chemoselective and sustainable catalysts for the reduction of organic compounds"

31 Oct 2023
Seminar Room # 350, second floor annex

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

Talk Title : “Towards chemoselective and sustainable catalysts for the reduction of organic compounds”
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.