PDF《Pure Appl. Chem., Vol. 73, No. 2,》

2001 IUPAC, Pure and Applied Chemistry 73, 209C220

210 Table

1 Selected landmark advances in organotransition metal chemistry. (a) New types of compounds, in many cases characterized by novel ligands and, in some cases, unprecedented metal-ligand bonding interactions. Early examples, notably metal carbonyls and olefin complexes, have been supplemented by the discovery since

1950 of a rich array of metal- locenes, metal arene complexes, transition-metal hydrides, alkyls, carbene (alkylidene) and car- byne (alkylidyne) complexes, and, most recently, agostic metal complexes containing molecu- lar H2 and aliphatic CCH ligands. (b) A variety of basic reactions, summarized in Scheme 1, that constitute the elementary building blocks of organometallic reaction chemistry [7]. These encompass conventional metalCligand dissociation-association processes, as well as homolytic metalCligand dissociation, distinctive for metalCalkyl complexes and reflecting the characteristic weakness of metalCalkyl bonds. Other characteristic elementary processes include dissociative reactions of saturated molecules with metal centers, and insertion of unsaturated molecules into metalCligand bonds (migratory inser- tion reactions). (c) Organic reactions, such as hydroformylation, hydrogenation, polymerization, oxidation, hydro- cyanation, hydrosilylation, and metathesis of olefins, catalyzed by metal complexes, through organometallic pathways. Much of the importance and impact of organometallic chemistry dur- ing the past several decades is attributable to such processes. PATHWAYS OF ORGANOMETALLIC CATALYSIS Recognition of the basic reactions in Scheme

1 was followed almost immediately by appreciation of their potential role as component steps in the pathways of an extensive array of catalytic processes in organic chemistry. The mechanistic scheme of Fig. 1, essentially that proposed by Heck and Breslow [8] in

1961 to depict the pathway(s) of cobalt carbonyl-catalyzed hydroformylation (eq. 1), is prototypical. While ?

2001 IUPAC, Pure and Applied Chemistry 73, 209C220 Organometallic chemistry at the threshold of a new millennium

211 Scheme

1 details of the mechanisms still await substantiation and the possible involvement of additional pathways still cannot be excluded, the essential features of this interpretation have withstood the test of time and analogous mechanistic schemes, involving combinations of the steps in Scheme 1, have been proposed, and come to be accepted, for most of the other catalytic reactions exemplified by those in Table 1. RCH=CH2 + H2 + C........