- Novel P,N-Ligand Stabilized Transition Metal Complexes as Efficient Catalysts for Organic Syntheses (2009)
- In the context of this work a library of novel P,N-ligands was synthesized and reacted with several transition metals with the aim to prepare active complexes for the efficient application in the field of homogeneous catalysis. Besides the synthesis and full characterization of these P,N-ligand stabilized transition metal complexes, further important aspects, such as the elucidation of mechanistic pathways and the development of novel catalytic methodologies for organic syntheses were covered. The preparation of the P,N-ligands is carried out in a one-pot reaction, affording the desired ligands in excellent yields, even on a multi-gram scale. With rhodium precursor complexes these P,N-ligands afford highly active complexes for the activation of C-Cl bonds which, in the case of methylene chloride, can even react with the solvent, affording dimeric Rh(III) complexes with terminal chloromethyl groups and dinuclear Rh(III) complexes with a bridging µ-CH2 group. The obtained activation products were characterized by single crystal X-ray analysis, and further synthetic as well as NMR kinetic experiments were carried out in order to identify the active species for this reaction and to postulate a mechanism for this reaction. P,N-ligand stabilized iridium complexes were readily prepared in quantitative yields and evaluated as potential catalysts for the N-alkylation of amines with alcohols. After optimization of the reaction conditions by systematic variation of important parameters such as solvents, bases or catalyst loadings the possible substrate scope of this method was demonstrated. The obtained results showed that the P,N-iridium catalyst has an excellent activity in this reaction along with a very narrow selectivity profile, because in all reactions only the selective monoalkylation of the corresponding amine was observed without any side-product formation. In addition, only aromatic amines can be alkylated with this catalyst, whereas no conversion is observed with aliphatic amines. Further optimization of the reaction conditions for the N-alkylation of amines with alcohols led to a significant improvement, so that such a reaction could for the first time be performed at a temperature of only 70°C, along with a catalyst loading as low as 0.1 mol% Ir. Moreover, the excellent selectivity of the catalyst for monoalkylation was successfully exploited for the symmetric and non-symmetric N,N’-dialkylation of diamines under mild reaction conditions. Also, the specific selectivity of the P,N-iridium catalyst for the preferred alkylation of aromatic amines in comparison to aliphatic amines was exploited in order to develop the first simple method for the selective preparation of mono-N-aryl aliphatic diamines by using commercially available amino alcohols. The presented synthetic protocol allows the preparation of N-aryl alipatic diamines with a branched alkyl backbone that were hitherto highly difficult to obtain or not accessible at all. In the last chapter of the present work, the selective formation of C-C bonds by alkylation of methyl-N-heteroaromatic substrates with alcohols was examined. This reaction can be seen as a completely new extension of the so-called “borrowing hydrogen” mechanism and was therefore intensively studied and fully developed. A large variety of substituted benzylic as well as aliphatic alcohols can be employed and many methyl-substituted N-heteroaromatic substrates such as pyridimidines, pyrazines, pyridazines and even pyridines are perfectly tolerated.