Organometallic chemistry. Past, present, and future*

Development of organometallic chemistry in the past half century is reviewed from the author’s personal viewpoint with focus on the establishment of fundamental concepts relevant to catalysis. Further development in the coming century is expected in view of the diversity of transition-metal complexes and their unique properties. IMPACT OF DISCOVERIES Discoveries of ferrocene and Ziegler catalyst brought a new era to organometallic chemistry that had kept a rather low profile before their advent. The sandwich structure of ferrocene revealed after its serendipitous discovery was fascinating and aroused the interest of both theoretical and synthetic chemists. The π bonding theory was established encompassing the previously known Zeise’s complex. The discovery of the Hoechst–Wacker process also served to direct the attention of many people to the potential applicability of palladium-catalyzed organic synthesis. Mechanistic studies of the process revealed the importance of the nucleophilic attack on the π−bonded olefin whose nature is modified by coordination to palladium. Another serendipitous discovery of Ziegler catalyst attracted the great interest of industrial people as well as academic chemists. The active species generated in the Ziegler-type catalyst by the reaction of titanium chlorides and organoaluminum compounds was proposed by Cossee to be a transition-metal alkyl that binds an olefin to initiate the coordination polymerization. The control of the mode of olefin coordination by the stereochemistry around the transition-metal alkyl was proposed as the key feature to produce stereoregular polymers (Scheme 1). However, the paucity of isolated transition-metal alkyls and poor understanding of the nature of the transition-metal alkyls hindered the immediate acceptance of the proposal. Quest for the isolable transition-metal alkyls and attempts to elucidate the reason for instability of the σ-bonded transitionmetal alkyl complexes led to recognition of the role of the ligands to stabilize the transition-metal alkyls and in control of the behavior of these transition-metal complexes. *Lecture presented at the XIX International Conference on Organometallic Chemistry (XIX ICOMC), Shanghai, China, 23–28 July 2000. Other presentations are published in this issue, pp. 205–376. Scheme 1 DEVELOPMENT OF FUNDAMENTAL CONCEPTS In the 1960s, various unique properties of low-valent transition-metal complexes were revealed by pioneers in transition-metal chemistry, including Wilke, Wilkinson, and Vaska. Examination of the chemical properties of novel complexes that were synthesized often serendipitously gave birth to important concepts on the elementary processes taking place on transition-metal complexes. Fundamental concepts such as oxidative addition, reductive elimination, olefin and CO insertion into transition-metal alkyl bonds, together with the reverse of these processes such as β-hydrogen elimination and decarbonylation were established in this period. Also established were the concepts of nucleophilic attack on the molecule such as olefin, CO and allyl ligand coordinated to a transition-metal center. Detailed studies on the mode of attack and control of the stereochemistry successfully led to development of asymmetric synthesis. Design of ligands such as DIOP [(R,R)-2,3-0-isopropylidene-2-3-dihydroxy-1,4-bis(diphenylphosphino)butane] and BINAP [(R)-2,2'-bis(diphenylphosphino)6-6'-dimethyl-1,1'-binaphthyl] led to achievement of catalytic asymmetric synthesis in high optical yields never realized before. On the other hand, studies on the chemistry of transition-metal complexes led to discoveries of quite novel transition-metal complexes having multiple metal-to-carbon bonds (Fischerand Schrocktype complexes). Further studies on the chemistry of these carbene and carbyne type complexes, as well as metallacycle compounds formed by their interaction with olefins, led to the development of the concept and application of olefin metathesis reactions. Application of the concept of ring opening olefin metathesis polymerization has provided the great potentiality of affording new types of polymers, whereas ring-closing olefin metathesis is finding a variety of applications in the synthesis of natural products. Late transition-metal complexes have been found quite suitable as catalysts for organic synthesis, and many synthetic processes have been successfully developed in the past 30 years. The development was accelerated by mutual interaction between organometallic chemists and synthetic organic chemists who were eager to apply the concepts found in organometallic chemistry to new types of organic synthesis. As an example, let me describe a story concerning the development of transition metal-catalyzed cross-coupling processes. In the 1960s and 1970s, we were interested in isolation of transition-metal alkyls and their chemical properties. In a study related to propylene dimerization by a diethylnickel complex having bipyridine ligand the following reaction was found (Scheme 2), and we simply reported the result describing the chemistry without recognizing the application potentiality. Application-minded researchers did not overlook the importance of the finding where coupling of the two organic entities and cleavage of carbon-chloride bond are involved. By combining the concept of transmetallation with organometallic compounds such as Grignard reagents, versatile cross coupling processes have been developed by Tamao, Kumada, and Corriu (Scheme 3). The process was later expanded to synthesis employing alkyl-zinc, boron, aluminum, tin, silicon, and zirconium compounds. Many name reactions useful in organic synthesis have been developed. A. YAMAMOTO © 2001 IUPAC, Pure and Applied Chemistry 73, 205–208 206