Gerald Bauer
The aim of the project was to investigate iron bis(oxazolinylphenyl)amino (bopa) pincer complexes as pre-catalysts in the enantioselective Kumada cross-coupling of non-activated alkyl halides with aryl Grignard reagents. From this preliminary cross-coupling studies we were able to isolate potential intermediate iron species that were then investigated for their catalytic activity. Further reactivity studies, kinetic studies, and DFT computations revealed the feasible catalytic cycles. Some intermediate species also proved to be relevant in the enantioselective hydrosilylation reaction of ketones. In the first chapter, a short overview is given of state-of-the-art iron pincer complexes and their catalytic application. The focus was put on symmetric pincer systems which are rigidified with aromatic rings in the ligand backbone. They proved themselves as highly active catalysts for the hydrogenation and hydrosilylation reaction of olefins, acetylenes, and carbonyls. There are also some examples given for trapping and releasing of hydrogen as potential applications for the storage of hydrogen on a molecular level. Examples for C-C bond formations are given to a lesser extent. In chapter two, the enantioselective Kumada cross-coupling of alkyl halides with aryl Grignard reagents was investigated. Iron(III) bopa pincer complexes are efficient pre-catalysts for the cross-coupling of non-activated primary and secondary alkyl halides with aryl Grignard reagents. The reactions proceed at room temperature in moderate to excellent yields. A variety of functional groups can be tolerated. The enantioselectivity of the coupling of secondary alkyl halides is low. The cross-coupling with secondary benzylic bromides solely yielded the homo-coupling products, 2,3-diphenylbutane and biphenyl. Modifications on the oxazoline, and diphenylamino moiety showed similar reactivity but did not improve the enantiomeric excess (ee). In chapter three, we isolated and characterised well-defined iron complexes and probed and supported their catalytic roles. Reactivity studies identified an Fe(II) "ate" complex, [Fe(Bopa-Ph)(Ph)2]-, as the active species for the oxidative addition of alkyl halide. Experiments using radical-probe substrates and DFT computations reveal a bimetallic and radical mechanism for the oxidative addition. The kinetics of the coupling of an alkyl iodide with PhMgCl indicates that formation of the "ate" complex, rather than oxidative addition, is the turnover determining step. In chapter four, the preliminary results on a possible mechanism in the enantioselective hydrosilylation of 4-acetylbiphenyl is presented. We could show that [(Fe(bopa)OAc)2], a 5-coordinated dimeric iron complex, is formed as the catalytic active species from the reaction of bopa ligands with Fe(OAc)2. The gathered results are consistent with an inner sphere reaction pathway, including the formulation of an iron-hydride species. For the reactions with zinc, no mechanism could yet be proposed.
EPFL2015
