Transition Metal Complex Catalyst C - H Activation Theory Research

The activation of C-H bond is an effective way for the concise construction ofC-C, C-N and C-O bonds. It plays an important role in the organic synthesis. In recentyears, transition metal catalyzed C-H bond activation and functionalization havereceived great attention and made great progress for their convenient and atomeconomical advantage. The research of the C-H activation mechanism from themolecular level contribute to the understanding of catalytic reaction process, theexplanation of experimental phenomena and the revelation of the relationship betweencatalyst structure and catalytic activity. It also provides some theoretical guidance forthe development of new and efficient catalyst. Therefore, the study has importanttheoretical and practical significance.The third to the fourth chapter of our dissertation, two typical experimentalreports about the transition metal catalyzed C-H activation were selected as theresearch object. By using quantum chemical method, we studied the reactionmechanism of catalytic activation of aryl compounds C-H bond from the molecularlevel, thus revealed the essence of the catalysis in the process of C-H activation. Thestudy unquestionable provide theoretical guidance for the development of noveltransition metal C-H activation catalyst.The valuable results in this dissertation can be summarized as follows:The reaction mechanism of Pd(II) and Pd(IV) complex catalyzed C-H bondactivation was investigated by density functional theory. The calculation resultssuggest that the main differences between Pd(II) and Pd(IV) complex catalyzedreaction mainly involve two following aspects:(1) In the reaction pathway, everyelementary reaction of C-H activation by Pd(IV) complex is exothermic, while it iscontrary by Pd(II) complex. Thus, the condition of Pd(IV) complex catalyzed reactionis milder.(2) The energy barrier difference (EAb-EAa) of Pd(II) complex catalyzedreaction is higher than that (EBb-EBa) of Pd(IV) complex catalyzed reaction in therate-determining step, which induces the higher site selectivity of Pd(II) complex catalyzed reaction. Moreover, the value of energy barrier difference should be relatedto the steric effect of Me and the space distribution of HOMO-C-p and LUMO-Pd-dorbitals.The detailed mechanism of the C-H activation catalyzed by Ir(III) complex,(bis(oxazolinyl)phenyl)Ir(OAc)2(H2O) has been investigated via the densityfunctional theory calculation. According to the calculation, we insure that the C-Hactivation catalyzed by Ir(III) complex proceed via a six-memerbered transitionstructure, which consist of the activating C-H bond, the metal Ir and the OAc group.The process follows a CMD mechanism. On this basis, the site selectivity of C-Hactivation of the monosubstituted arenes C6H5R were investigated. When the C-Hactivation follows the CMD mechanism, the higher atomic charge density around theC atom, the easier the formation of the transition states and the reaction is more facile.When the substituent group is the electron-rich group such as methyl or methoxy, theatomic charge density of the para-position is higher than any other position, so theC-H bond activation occur predominantly at the para-position. However, when thesubstituent group is electron-withdrawing group such as nitro or acetyl, the atomiccharge density of the meta-position is higher than any other position, so the C-H bondactivation occur predominantly at the meta-position. The ortho C-H activation issuppressed when a directing group is attached due to the steric repulsion between theortho substituent group and the phebox skeleton. All the conclusions accord with theexperimental reports. The calculation results could explain the experimentalphenomena and reveal the essence of catalytic reaction very well.These results not only have reference value for the research and design of newhigh efficient C-H activation catalyst, but also provide theoretical reference for Pdand Ir complexes as catalysts for other types of reactions.