Transition Metal Ru (â…¡) - N Compound Ester Catalyzed Hydrogenation Of Theoretical Research

In this thesis, we mainly theoretically studied the reaction mechanisms forhydrogenation of esters to produce alcohols where the transition metal complexesRu(â…¡)PNN (1) and Ru(â…¡)PNP (a) were employed as catalysts by means of densityfunctional theory (DFT) calculations at the B3LYP level. The different catalyticbehaviors of the two catalysts have been comparatively discussed.1) David Milstein, et al. proposed a mechanism for hydrogenation of acidamides catalyzed by another Ru(â…¡)PNN complex. Following this mechanism, weproposed a similar mechanism for the reaction studied in this work. In thismechanism, there is a coordination isomeric process in which the coordination ofthe formed aldehyde replaces the coordination of nitrogen, for the purpose ofcreating an empty site for subsequent hydrogenation. Our study found that theactivation barrier for this process is too high (44.0kcal/mol). Thus, this mechanismis believed to be inavailable kinetically.2) David Milstein, et al. proposed amechanism for the reaction studied in this work using Ru(â…¡)PNN (1) as a catalyst.The overall activation energy for the reaction mechanism is calculated to be33.5kcal/mol.3) We proposed a new mechanism for the reaction, in which thehydrogenation of the formed aldehyde by H2(7â†'TS7-13â†'13) was proposed to bedistinguished with those proposed in the two mechanisms mentioned above. Ourcalculated results show that the overall activation energy for the reactionmechanism is calculated to be30.1kcal/mol. It is found that the mechanismproposed by us is slightly more kinetically favorable than the second one proposedby David Milstein, et al. It should point out, although the mechanism proposed byus is the best based on our calculation results, the second mechanism proposed byDavid Milstein, et al. should not be excluded since the activation energy for themechanism is only slightly higher than the one proposed by us.In additon, the catalytic behavior of the catalyst Ru(â…¡)PNP (a) for the reactionstudied in this work was also investigated theoretically. Only the process involvinginsertion of C=O of the ester into Ru-H is studied. The overall activation energyfor this process is calculated to be34.2kcal/mol, higher the overall activationenergy (30.1kcal/mol) catalyzed by Ru(â…¡)PNN (1). Therefore, it is predicted thatthe catalyst Ru(â…¡)PNP (a) is less effective than Ru(â…¡)PNN (1) for the reaction,which is in agreement with the experimental observation (99%yield with1and 10%with a). The reason is due to the fact that the Ru-P bond is stronger than theRu-N bond.In summary, we proposed a new mechanism for the reaction of esters withdihydrogen, which is helpful for designing such kinds of new catalytic reactionsystems. We also explained why the Ru(â…¡)PNN-type catalysts are more effectivethan the Ru(â…¡)PNP-type in activating the hydrogenation of esters.