Browsing Ph.D. Chemistry by Subject "Transfer hydrogenation"
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Half-sandwich Complexes of Ruthenium; Synthesis and Application to CatalysisThis thesis describes syntheses and catalytic reactivity of several half-sandwich complexes of ruthenium. The neutral ruthenium trihydride complex, Cp(PPri3)RuH3(1), can efficiently catalyse the H/D exchange reaction between various organic substrates and deuterium sources, such as benzene-d6. Moreover, the H/D exchange reactions of polar substrates were also observed in D2O, which is the most attractive deuterium source due to its low cost and low toxicity. Importantly, the H/D exchange under catalytic conditions was achieved not only in aromatic compounds but also in substituted liphatic compounds. Interestingly, in the case of alkanes and alkyl chains, highly selective deuterium incorporation in the terminal methyl positions was observed. It was discovered that the methylene units are engaged in exchange only if the molecule contains a donating functional group, such as O-and N-donors, C=C double bonds, arenes and CH3. The cationic half-sandwich ruthenium complex [Cp(PPri3)Ru(CH3CN)2]+(2) catalyses the chemoselective mono-addition of HSiMe2Ph to pyridine derivatives to selectively give the 1,4-regiospecific, N-silylated products. An ionic hydrosilylation mechanismis suggested based on the experiments. To support this mechanistic proposal, kinetic studies under catalytic conditions were performed. Also, the 1,4-regioselective mono-hydrosilylation of nitrogen containing compounds such as phenanthroline, quinoline and acridine can be achieved with the related Cp*complex [Cp*(phen)Ru(CH3CN)]+(3) (phen = 1,10-phenanthroline) and HSiMe2Ph under mild conditions. The cationic ruthenium complex 2 can also be used as an efficient catalyst for transfer hydrogenation of various organic substrates including carbonyls, imines, nitriles and esters. Secondary alcohols, amines, N-isopropylidene amines and ether compounds can be obtained in moderate to high yields. In addition, other ruthenium complexes, 1,3 and [Cp*(PPri3)Ru(CH3CN)2]+(4), can catalyse transfer hydrogenation of carbonyls although the reactions were sluggish compared to the ones of 2. The possible intermediate, Cp(PPri3)Ru(CH3CN)(H), was characterized by NMR at low temperature and the kinetic studies for the transfer hydrogenation of acetophenone were performed. Recently, chemoselective reduction of acid chlorides to aldehydes catalysed by the complex 2 was reported. To extend the catalytic reactivity of 2, reduction of iminoyl chlorides, which can be readily obtained from secondary amides, to the corresponding imines and aldehydes was investigated. Various substituted iminoyl chlorides were converted into the imines and aldehydes under mild conditions and several products were isolated with moderate yields.
New Pyrazole-Based Ligands and Their Complexes for Application in Transfer Hydrogenation and HydrosilylationA 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.