reactions, which had opened up a whole new field of asymmetric catalysis with transition metals [86]. W. S. Knowles shared 2001 Nobel prize in chemistry with K. B. Sharples and R. Noyori, who promoted the research upsurge of asymmetric catalysis. Moreover, Chauvin, Grubbs, and Schrock won the 2005 Nobel prize in recognition of their outstanding contributions in transition metal‐mediated metathesis of olefins.
Based on the advances of methodology study and ligand design, transition metal catalysis has become one of the important means for synthetic chemists to construct more complex new substances in this century. The current pursuit is to selectively construct multiple covalent bonds in one reaction synchronously by transition metal catalysis. To achieve this goal, transition metal catalyst has been employed to selectively activate some inert covalent bonds. The most famous example – transition metal‐mediated C—H bond activation – became the focus of chemists. This process could afford a carbon–metal bond directly, which could be used as a powerful nucleophile in further transformations. In modern organometallic chemistry, multistep elementary reactions in series have been extensively studied, which could afford a battery of new covalent bonds through one catalytic cycle. Synthetic efficiency in organometallic chemistry has become the focus of attention. Hereon, transition metal catalysis with higher turnover numbers was pursued to further improve the economy and environmental protection. Current research on transition metal catalysis is also devoted to improving the accuracy of synthesis, aiming at achieving specific functional group transformation in the exact location. To achieve these goals, the design of transition metal catalysis becomes more complex, and the requirements for suitable ligands are higher. It is necessary to design the corresponding ligands manually according to aspects of structure, electronic properties, steric effect, and coordination ability. These auxiliary designs also make the catalytic cycle with transition metal lengthier; meanwhile, the possibility of side reactions increases. Therefore, mechanistic studies for transition metal catalysis became more and more important, which were helpful for design of new catalysis, enhanced efficiency, increased selectivity, improved turnover number, and accurate synthesis.
1.2 Using Computational Tool to Study the Organometallic Chemistry Mechanism
Transition metal catalysis is one of the most powerful tools for the construction of new organic materials, whose development trend is more efficient as well as more complex. Therefore, studying the mechanism of organometallic catalysis has become even more essential, and has proved to be the basis for the design of new ligands, catalysts, and reactions.
1.2.1 Mechanism of Transition Metal Catalysis
Generally, reaction mechanism could be considered to be all elementary reactions used to describe a chemical change passing in a reaction. It is to decompose a complex reaction into several elementary reactions and then combine them according to certain rules, so as to expound the internal relations of complex reactions and the internal relations between total reactions and elementary reactions. The rate of chemical reaction is closely related to the specific pathways through which the reaction takes place.
To study the law of chemical reaction rate and find out the intrinsic causes of various chemical reaction rates, synthetic chemists must explore the reaction mechanism and find out the key to determine the reaction rate, so as to control the chemical reaction rate more effectively. As shown in Scheme 1.4, traditional research methods for reaction mechanism include: (i) determining the important intermediate or decisive step of a reaction by isotope tracing, (ii) determining the effect of different factors (e.g. reaction temperature, solvent, substituent effect, etc.) on reaction rate and selectivity by competitive test, (iii) studying the relationship between the reaction rate and the concentration of reactants and catalysts obtaining by kinetic experiments, and (iv) characterizing and tracking intermediates by instrumental analysis. However, these methods are often macroscopic observation of the average state of many molecules, which cannot watch a process of the transformation for one molecule from a micro‐perspective. Fortunately, theoretical calculations based on first principles have become one of the important means to study the reaction mechanism with the development of software and the improvement of hardware computing capability in recent several decades. Through theoretical calculation and simulation, the transformation of one molecule in reaction process can be “watched” more clearly from the microscopic point of view. Actually, theoretical calculation can be considered to be a special kind of microscope, which can see the geometrical structure, electronic structure, spectrum, and dynamic process at atomic level, and is helpful for chemists to understand the real reaction mechanism.
Scheme 1.4 Revealing the reaction mechanism of organometallic catalysis.
The combination of theoretical and experimental techniques could not only greatly improve the efficiency of reaction and yield of product, but also uncover the factors that control the selectivity of product more clearly. The promotion of theoretical study to experimental investigation could be summarized into “3D,” i.e. description, design, and direction. Based on the data obtained from experimental technique, detailed description for the mechanism of organometallic catalysis could be fulfilled using theoretical calculations. Based on the results of computations, the mechanisms could be verified by the designed experiment. To put in a nutshell, theoretical calculations could play a critical role in the direction of transition‐metal‐organic synthesis.
1.2.2 Mechanistic Study of Transition Metal Catalysis by Theoretical Methods
Quantum chemical computation based on first principle provides a powerful tool for the mechanistic study of transition metal catalysis. Since the whole content of this book is to discuss the theoretical calculation‐based study for the mechanism of transition metal catalysis, we will give only a few examples to show how to study the reaction mechanism by theoretical calculations.
Generally, mechanism research of transition metal catalysis initially faces a series of studies involving the molecular structure and electronic states. As an example, (Xantphos)Pd(CH2NBn2)+ is an important precursor for aminomethylation reactions, the geometric structure of which has been confirmed by X‐ray analysis [87]. However, why this complex could be formed and the electronic properties of this complex still remained unclear. As shown in Scheme 1.5, in resonance structure 1‐1, the Pd—C bond is a normal single bond, and Pd—N is a coordination bond. The formal positive charge is localized on palladium, and the formal oxidation state of palladium is +2. Alternatively, the iminium moiety acts as a monodentate ligand coordinated with Pd(0) in resonance structure 1‐2, and the formal positive charge is mainly localized on the iminium moiety. The real structure of this complex would be a mixture of resonance structures 1‐1 and 1‐2. On the other hand, the bond orders of Pd—C, Pd—N, and C—N are determined to be 0.322, 0.135, and 0.965, respectively, which indicate that the Pd—C and Pd—N bonds are very weak. More importantly, these data support that the C—N is a double bond and 1‐2 is most likely to be the main structure of this complex. Further frontier molecular orbital studies also supported this point.
Scheme 1.5 The resonance structures of (Xantphos)Pd(CH2NBn2)+.
To summarize, computational organometallic chemistry focuses on some of the stationary points
