Nichtklassische Rutheniumhydridkomplexe in Wasserstofftransfer-Reaktionen (2012)

This work investigates the activity of pincer stabilised Ruthenium complexes. The main emphasis was put on the complexes [Ru(dtbpmp)(H2)H2] 2 and [Ru(dtbpoet)(H2)H2] 3, whose activity was compared with the well-known Chaudret complex [Ru(PCy3)2(H2)2H2] 1. Complexes 1 to 3 were active in the transfer hydrogenation of ketones, olefins and alpha,beta-unsaturated ketones with 2-propanol as hydrogen donor – without any base additives necessary. Thereby the activity for the reduction of ketones, with TOFs up to 9780 h-1, was significantly higher than for olefins, for which TOFs were not higher than 490 h-1. Olefins tended to isomerisation of the C=C double bond. In the transfer hydrogenation of ketones the activity decreased in the order 1 > 3 > 2. This trend reversed in the case of olefins.C=C bonds in alpha,beta-unsaturated ketones were easily and, particularly with catalyst 2, very selectively hydrogenated. This is due to the activation by the adjacent C=O bond. Compared to transfer hydrogenation, the reactivity profile changed in case of the reduction under hydrogen atmosphere. C=C bonds were efficiently hydrogenated without any isomerisation products formed. In contrast to transfer hydrogenation, the selectivity towards C=C bonds was higher than for C=O bonds.Despite good activities for the transfer hydrogenation of ketones, aldehydes were only slowly or not at all reduced. This is due to the high decarbonylation activity of the Ruthenium catalysts which leads to inactive carbonyl complexes. Furthermore glycerine, ethanol and formic acid were tested as alternative hydrogen sources for transfer hydrogenation. With glycerine the activities were relativley low and only small conversions under formation of the couple product dihydroxyacetone were possible. Complex 2 catalyses the transfer hydrogenation of cyclohexanone with ethanol as hydrogen source, which is converted to ethyl acetate, though only with low activity. Formic acid decomposes in the presence of the catalysts 1 to 3. Deuterium labelling experiments showed that the transfer hydrogenation with catalysts 1 to 3 and 2-propanol as hydrogen donor proceeds via a monohydride mechanism. In this type of mechanism no change of the metal´s oxidation state occurs. The monohydride route applies only in the presence of a hydrogen donor, because complex 2 also catalyses the reduction of cyclohexanone without the presence of 2-propanol via a dihydride mechanism under the formation of an unstable Ruthenium(0) species. An activation barrier of 18.4±2 kcal mol-1 was experimentally found for the transfer hydrogenation of cyclohexanone with complex 2. Investigations on the kinetic istope effect showed that a proton transfer is part of the rate determining step. By means of density functional calculations different catalytic cycles for the transfer hydrogenation of cyclohexanone in 2-propanol with complex 2, that are compatible with the experimental findings, were investigated. An outer-sphere mechanism was found. This mechanism contains a Ruthenium alcoholate complex as active species. A simultaneous transfer of a Ruthenium hydride ligand and an alcohol proton to the substrate proceeds from the outer coordination sphere of the active species. Investigations on the dehydrogenative lactonisation of diols with [Ru(dtbpmp)(H2)H2] 2 gave good activities for the synthesis of five to seven membered lactones. TOFs up to 760 h-1 were found and the catalyst to substrate ratio could be reduced to 1:1000. Lactonisation was still efficient at reaction temperatures of 80°C and in the presence of a hydrogen donor the reaction was possible at room temperature. Yields of macrocycles with rings larger than eight members were less than 1%. Instead long chained diols with more than six CH2-units were selectively transformed into linear oligo esters, which gave chain lengths of more than six monomeric units. Reaction of 2 with ethanol gave a hydrido carbonyl complex 18, which is the putative active species in esterification and lactonisation reactions, respectively. In addition to diols, aminoalcohols could also be converted into the respective lactams with catalyst 2. Furthermore cyclic imines were formed in similar amounts due to dehydration of the intermediately formed hemiaminals. To conclude, the combination of non-classical Ruthenium hydrides and pincer ligands constitutes very interesting catalyst systems for hydrogen transfer reactions. Different functional groups can be hydrogenated and transfer hydrogenated with distinctive activities and selectivities without the need of catalytic additives. Besides transfer hydrogenation, acceptorless dehydrogenation reactions are also possible. These reactions work under mild conditions and also without catalytic additives.