Theoretical Studies On Hydrogenation Of Ketones Catalyzed By Transition Metal Complex

Asymmetric hydrogenation of ketones is one of the most important and widely used synthetic methods in chemical industry and chemical synthesis. Currently,two synthetic methods were developed to achieve homogeneous enantioselective hydrogenation of ketones.These methods are different in the use of hydrogen sources:one of them,transfer hydrogenation(TH),uses organic hydrogen sources(alcohols or formic acid),while the other, H₂-hydrogenation(HH),utilize molecular dihydrogen.Among the catalysts, the Ru(â…¡)-diphosphine-diamine catalyst developed by Noyori and co-workers performed HH with high reactivity and enantioselectivity. Another type catalystsη⁶-arene Ru(â…¡) is efficient in TH and shown also intelligent catalytic reactivity and enantioselectivity.In this work,we studied the mechanism of hydrogenation of ketones catalyze by these two catalysts using DFT method.And the results are shown below:1,Density functional theory(DFT) method was employed to study the asynchronous hydrogen transfer in hydrogenation of ketones catalyzed by Ru(â…¡)-diphosphine-diamine complex.Hydrogenation processes of eight different substrates were compared.Results indicate there are two mechanisms:concerted but asynchronous hydrogen transfer,and stepwise hydrogen transfer.In generally,the hydrogen transfer process is a stepwise mechanism in nature.The differences of substrates or ligands in catalysts may lead to the disappearance of one barrier,resulting in concerted but asynchronous hydrogen transfer process.2,In asymmetric hydrogenation of ketones,â…§metals complex always showed intelligent activity.The catalysts using Fe,Ru and Os as centers with diphosphine and dianmine ligands were developed for H2-hydrogenaion of ketones.We compared the hydongenation of ketones catalyzed by eightâ…§metals centerd diphosphine-diamine complexes.The results indicate there are three hydrogen transfer modes:(1) hydride transfers first then promotes proton transfer,(2) hydride and proton transfer synchronously and(3) proton transfers preceding hydride.And the barrier of hydrogen activation is relevant to the strength of M=N bond in 16e complex.The stronger the M=N bond is,the barrier is higher.Fe,Ru and Os systems showed good catalytic activity in hydrogenation of ketones,which is in agreement with experimental results.3,In neutral and basic conditions,η⁶-arene-Ru(â…¡) are good catalysts for transfer hydrogenation.But in acidic conditions,they could carry out H₂-hydrogenation rather than transfer hydrogenation.We focused on the mechanism of dihydrogen activation in acidic condition using DFT method. Six possible hydrogen activation paths were examined.In the three possible paths in acidic conditions,the barriers of dihydrogen activation were reduced to 3.7,7.1 and 7.8 kcal/mol because of the participation of acid group in reaction.In neutral and basic conditions,dihydrogen can not coordinate with Ru center in 16e complex RuN and INT2 to form stableη²-H₂ complexes.This leads to great decrease of entropy and increase of free energy barrier.In contrary,in acidic condition,dihdyrogen can coordinate with Ru complexes,resulting in the conversion of hydrogenation types and dihydrogen coordination is essential to H₂-hydrogenation.