Dihydrogen activation at non-metallic centers

The work presented in this thesis was dedicated to the development of novel systems based on pyridinium salts for the heterolytic dihydrogen activation and evaluation of their suitability as catalysts for catalytic hydrogenation. These systems contain a pyridinium salt as hydride acceptor and a basic nitrogen or oxygen function as proton acceptor. Initially, an intramolecular system was envisioned, in which the pyridinium salt is linked with a basic amine and the hydrogen cleavage should occur through a concerted transition state. Due to the low stability of the starting materials, mainly decomposition products were observed after test reactions at high temperatures and hydrogen pressures ranging from 50-100 bar.
As a next step, a bimolecular system based on pyridinium salts and sterically hindered alcoxides was evaluated. For this purpose, electrophilic N-acyl ammonium salts and phenolates derived from commercially available BHT derivatives were prepared. However, in the presence of hydrogen gas no reaction was observed and starting materials were recovered.
ITAMI and co-workers demonstrated the formation of pyridylidenes from pyridinium salts in the presence of a strong base. These highly reactive intermediates could then undergo formal addition of H2. Known pyridinium salt/dihydropyridine systems are Hantzsch esters and nicotinamide derivatives. Both classes of pyridinium salts were tested for H2 activation reactions. Although the formation of the corresponding dihydropyridines was observed, deuteration experiments proved that the reaction did not proceed via dihydrogen splitting. Furthermore, the ability of these N protected pyridinium salts to reduce selected substrates was rather poor.
Finally, the activation of dihydrogen was achieved by reaction of a pyridinium salt as described by ITAMI in the presence of base. First, a 1,3,5 triarylpyridinium salt was synthesized, which could be transformed into the corresponding pyridylidene in the presence of LiHMDS. This intermediate was trapped by quenching with S8. Reaction with H2 led to its activation and the corresponding 1,2 dihydropyridine was formed.
Dihydrogen activation was confirmed by reaction with deuterium gas, as the 2H NMR spectra showed the signal of the corresponding CD2 group. Studies towards catalytic applications of that system showed that imine 255 was reduced to the corresponding amine in the presence of stoichiometric amounts of base and 20 mol% of catalyst loading.