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Developments and Mechanistic Investigations of Ester, Imide, and Ketone Hydrogenations
- Author / Creator
- Takebayashi, Satoshi
The Noyori-type catalyst trans-Ru((R)-BINAP)(H)2((R,R)-dpen) and its analogues are among the most active and enantioselective ketone hydrogenation systems reported to date. Its applications towards other types of carbonyl compounds are, however, understudied. This dissertation describes the first applications of this catalyst system towards hydrogenations of esters and imides under mild reaction conditions. Further, a detailed mechanistic study of this system is presented using ketones, esters, and imides as substrates.
The dihydride 6 was highly active towards the hydrogenation of esters. Stoichiometric reactions between 6 and lactones proceeded at –80 °C to form the net hydride insertion products, Ru-hemiacetaloxides. The hemiacetaloxides were further hydrogenated at –40 °C under ~2 atm of H2 to form the corresponding Ru-alkoxides. Catalytic hydrogenations could be carried out, even at –20 °C under 4 atm of H2, however, these hydrogenations slowed over time due to deactivation of the catalyst by primary alcohol products.
The first homogeneous monohydrogenation of imides was developed using 6 and related compounds as catalysts. Further, upon optimization of reaction conditions and imide structure, meso-cyclic imides were desymmetrized in high ee via the monohydrogenation to form kinetically unfavoured trans-hydroxy lactams with up to 5 stereogenic centres. Furthermore, the number of stereogenic centres was increased
from 5 to 7 using N-acyliminium ion chemistry. A model for the origin of enantioselection was proposed using substrate-catalyst steric interactions. Low temperature NMR studies revealed that base catalyzes the rapid cis-trans isomerization to form the thermodynamically more stable trans-isomer. It was proposed that this rapid isomerization prevents racemization of a product.
Transition states for the formation of Ru-alkoxides from addition between 6 and acetophenone were studied using an intramolecular trapping experiment. Addition between 6 and 4-hydroxymethylacetophenone at –80 °C exclusively formed the net hydride insertion product, Ru-secondary-alkoxide. Combined with controlled experiments, this result is strong evidence for the formation of a Ru–O interaction in the transition state, which supports concerted formation of the Ru-alkoxides from 6 and acetophenone.
- Graduation date
- Spring 2011
- Type of Item
- Doctor of Philosophy
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