| Acceptorless alcohol dehydrogenation(AAD)and imine hydrogenation are the most important reactions among chemical research.On the one hand,AAD reactions provide a template for the synthesis of carbonyl compounds such as ketones,aldehydes,acids from the corresponding alcohols.On the other hand,hydrogen is the product of the AAD reactions,which is an ideal replacement of the fossil-based sources.The asymmetric imine hydrogenation(AIH)provides an important way to construct chiral carbon-nitrogen bond.In this thesis,a systematic theoretical study on transition metal catalyzed AAD and AIH reactions was carried out,the main contents are shown as follows:1.The mechanism for the production of acetic acid from ethanol and water mixture catalyzed by iridium complex bearing a functional bipyridonate ligand was theoretically investigated.The reaction proceeds via two sequential catalytic cycles:the dehydrogenation of ethanol to acetaldehyde,the dehydrogenative of coupled acetaldehyde and OH-to produce acetate.The cooperation between iridium and bpyO ligand are highlighted and the reason for reactivity.The energy barrier for the hydrogen transfer step in favorable outer-sphere pathway is 16.3 kcal/mol(Al→TSA2-3).In the hydrogen release step,the ethanol assisted hydrogen release is more favorable than the water assisted pathway and the direct hydrogen-release pathway.The electronic effect was investigated,electron-donating substituents in the catalyst will facilitate the cleavage of the O-H bond and the release of hydrogen,which agree well with experimental observations.2.Transition metal free catalysis can avoid the use of transition metal catalysts and is a very popular research in modern chemistry.We investigated the mechanism of transition metal-free diastereoselective N-alkylation of amines with racemic alcohols.The hydrogen transfer process from racemic alcohols to imine is proceeded via the similar Meerwein-Ponndorf-Verley(MPV)mechanism,and the sodium cation involved in the reaction as the alkoxide,which coordinates to the oxygen and nitrogen atoms of imine to form an[6+4]ring transition state.The chirality of product is determined by the chirality of imine regardless of the configuration of the attacking alkoxide.We actually explored twelve possible reaction pathways for the attack of the alkoxide at the R-17,three reaction modes were identified,which could lead to productive transition state.Mode 1,displaying a[6+4]ring structure,and the Na+ coordinates to both the oxygen and nitrogen atoms.Mode 2,displaying an eight membered ring structure,and features no chelating interaction with the Na+.Mode 3,with a six-membered ring structure features no chelating interaction with the Na+.We also found that only when both the Na6-N4 and Na6-O5 distances fall within a bonding distance can the energy barrier of the hydride transfer be lowered considerably.The importance of the sodium cation is further seen when it is replaced with H atom,the free energy barrier of the hydride transfer rises to 34.0 kcal/mol,corroborating the experimental observation that without using a base,the amination is infeasible.Similarly,replaying the Na with a Li atom also rises the energy barrier,which also consistent with the observation that a sodium-containing base gives faster amination.3.The mechanism of asymmetric imine hydrogenation and the origin of enantioselectivity catalyzed by ruthenium(Ⅱ)PNP-type pincer complex was studied in detail using density functional theory(DFT).The mechanism involves dehydrogenation of racemic alcohols,hydrogen transfer from catalyst to imine fundamental reactions.The calculation results indicate that both alcohols dehydrogenation and imine hydrogenation are stepwise.In alcohol dehydrogenation step,the proton transfer prior to the hydride transfer while in the imine hydrogen step the hydride transfer prior to the proton transfer.The hydride transfer from 5 to imine is supposed to be the chirality-determining step in the whole catalytic cycle.In the pro-(R)pathway for the formation of R configuration products,the energy barrier of hydride transfer is 8.2 kcal/mol,and the energy barrier along the pro-(S)pathway is 11.5 kcal/mol,the ΔΔG value between the two pathways is 11.0 kcal/mol,consistent with the experimental observation that the R-amine is the main product.It was found that the diastereoselectivity was mainly based on the hydrogen bond interaction between the O of the sulfinyl moiety and the H atom of the-NH group of the ligand. |
