Entwicklung neuartiger homogener Rheniumkatalysatoren für die Deoxydehydratisierung biogener Alkohole und selektive Umsetzung von Alkoholen und Aminen zu Estern und Amiden

The activation and transformation of alcohol functions in biomass based molecules is a central topic in chemical science. Therefore, the development of novel homogeneous catalysts to enable these transformations and apply them on platform chemicals to receive products of our daily life is a substantial issue at present time. Platform chemicals represent easy accessible and quantitative available biomass based molecules, and hence the only sustainable source for carbon to compensate the shortage of crude oil. Apart from isoprene the molecules have distinctive oxygen content resulting in seven of these thirteen molecules have an alcohol function. The transformation of alcohols and amines to ester and amides under liberation of hydrogen is an atom efficient and sustainable oxidative reaction. In this work Re(Triphos)H5 was identified as an active catalyst in the dehydrogenative coupling of aliphatic and aromatic alcohols to ester. Yields of > 99 % and selectivities of > 99 % could be reached due to structured optimization and detailed analysis of deactivation path ways. The furthermore amides could be obtained from mixtures of alcohols and primary amines resulting in yields of up to 79 % and corresponding selectivity of > 97 %. Those studies motivated to a modification of the catalyst resulting in Re(Triphos)(CO)H3 which is able to work completely without additives in the ester and amid synthesis. DFT calculations were done using M06/def2-TZVP and included solvent and temperature effects to establish reaction paths. The obtained results are consistent with the observed performance of the catalytic system.The CH3ReO3 catalyzed reduction of diols to alkenes with hydrogen or sec.-alcohols emerged to an attractive reaction in biomass conversion. In this work the stability of CH3ReO3 under reaction conditions was analyzed, its decomposition proven and HReO4 was identified as the actual catalytic species. Subsequent work with 1,4-sorbitan as an substrate enables the selective reduction of this tetraol to the alkene-diol (2R,3R,4S)-2-vinyltetrahydrofuran-3,4-diol. This novel, chiral, heterocyclic and multifunctional molecule is a potential building block for biomass based products. The furthermore the condensation behavior of catalyst and substrate under reactions conditions as well as a new reaction path were described by DFT calculations.In summary a new rhenium based reaction network was developed allowing the release and consumption of Hydrogen in the transformation of alkohols and biogenic polyols to valuable ester, amides and alkenes. Detailed investigations, optimizations and characterizations of these systems enable a profound understanding of underlying reaction mechanisms and constitute a fundament for a sustainable future work with platform molecules

Polarization Transfer Efficiency in PHIP Experiments

Parahydrogen induced polarization (PHIP) is a hyperpolarization method for NMR signal enhancement with applications in spectroscopy and imaging. Although parahydrogen can be easily enriched up to nearly 95%, the polarization detected on the hydrogenated substrate is substantially lower, where numerous loss mechanisms between the start of the hydrogenation reaction and detection affect polarization levels. The quality of PHIP systems is commonly determined by stating either the polarization degree or the enhancement factor of the product at the time of detection. In this study, we present a method that allows the distinction of polarization loss due to both the catalytic cycle and T1 relaxation of the formed product prior to detection. We determine the influence of homogeneous catalysts and define a rigorous measure of the polarization transfer efficiency (PTE). Our results show that the PTE strongly depends on the concentration of all components and the chemical structure of the catalyst as well as on the magnetic field of detection