New Pyrazole-Based Ligands and Their Complexes for Application in Transfer Hydrogenation and Hydrosilylation
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A series of bidentate and tridentate ligands bearing pyrazolyl moiety in combination with phosphine, oxazoline, amine, and sulfide were synthesized. These ligands were applied for the synthesis of ruthenium complexes, that would be efficient in catalyzing transfer hydrogenation reaction in alcohol. From a number of obtained complexes, a mixture of two isomeric ruthenium complexes was found to be the most efficient in reduction of acetophenone and N-benzylideneaniline, as model substrates, with 2-propanol. These ruthenium complexes were successfully applied in transfer hydrogenation of nitriles, heterocyclic compounds, olefins, and alkynes. Activated esters were reduced under similar catalytic conditions when ethanol was used as a hydrogen source. These isomeric ruthenium complexes were also applied in the synthesis of secondary amines via hydrogen borrowing methodology. A number of primary amines and anilines were combined with primary alcohols under the conditions, optimized for transfer hydrogenation of nitriles, resulting in corresponding secondary amines. Furthermore, ammonium formate was used as a nitrogen source for alcohol amination. Thus, secondary and tertiary amines were obtained from primary alcohols. Another project was focused on transfer hydrogenation of carbonyl compounds with lithium isopropoxide. Addition of various ligands and small molecules was found to improve the reaction efficiency for aromatic substrates. Further studies revealed that lithium cation forms stable adduct with aromatic alcohols, while different additives help to break this interaction, thus resulting in significant improvement of the conversion to alcohols. Another strategy that was applied to improve the reaction yields was the addition of a cheap source of lithium cations, such as LiCl. Finally, a new zinc complex was synthesized and applied in the catalytic hydrosilylation of carbonyl compounds. The optimization of reaction conditions reviled that the presence of substoichiometric amounts of methanol in the system significantly accelerates the process. The reaction can proceed at very low catalyst load (down to 0.1mol%) under relatively mild reaction conditions. The substrate scope analysis showed the tolerance to carbon-carbon double bond. Thus, this procedure is efficient for the synthesis of allylic alcohols from α,β-unsaturated aldehydes and ketones.