Computational Methods in Organometallic Catalysis. Yu LanЧитать онлайн книгу.
of the structural study of organometallic complexes. Two years later, Werner proposed structural theory of organometallic complexes involving the tetrahedral, octahedral, square planar, etc. which won him the Nobel prize in chemistry in 1913 [71]. In 1919, Cr(C6H6)2 was prepared by Hein using MgPhBr to react with CrCl3 [72]. However, the sandwich‐like structure of this complex was proved by Fischer 36 years later. In 1951, Fe(C5H5)2 had been synthesized by Kealy and Pauson individually [73]. The sandwich‐like structure of that complex was confirmed by G. Wilkinson the following year, which aroused chemists' enthusiasm for the study of transition metal organic compounds. In 1964, tungsten carbene complex was reported by Fischer, who shared 1973 Nobel prize in chemistry with G. Wilkinson [74]. By the 1950s, with the appearance of representational methods, involving X‐ray crystallography, infrared spectrum, and nuclear magnetic resonance spectrum, means of characterizing transition metal compounds were becoming more and more mature. Therefore, organometallic chemistry became an independent discipline.
From the middle of the twentieth century, organometallic compounds were gradually considered as a catalyst in organic reactions. In 1953, Ziegler and Natta found that TiCl4/AlEt3 could promote atmospheric polymerization of olefins, which helped them share 1963 Nobel prize in chemistry [75, 76]. In 1959, allylic palladium was prepared by Smidt and Hafner, which was the beginning of π‐allyl metal chemistry [77]. The same year, Shaw and Ruddick reported an elementary reaction of oxidative addition [78]. In 1974, Wilkinson reported another elementary reaction of β‐hydride elimination [79]. Those works led to a series of following mechanistic studies for organometallic reactions. In 1972, Heck and Nolley reported a palladium‐catalyzed coupling reaction between aryl halides and olefins, which was named Heck reaction [80]. Meanwhile, a series of palladium‐catalyzed cross‐coupling reaction, including Kumada coupling with Grignard reagent [81], Suzuki coupling with aryl borane [82], Negishi coupling with organo zinc [83], Stille coupling with aryl tin [84], and Sonogashira coupling with alkynyl copper [85], were reported. Those reactions made transition metal‐catalyzed cross‐coupling reactions one of the most important ways to construct new C—C covalent bonds in synthetic chemistry. Therefore, R. F. Heck, E. Negishi, and A. Suzuki won the 2010 Nobel prize in chemistry. Also in 1971, W. S. Knowles applied chiral bisphosphine ligands as ligand in rhodium‐catalyzed hydrogenation 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
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