Transition metal catalysis exemplified by some rhodium-promoted reactions of olefins

Abstract

A variety of organic reactions involving olefins, CO, and Hz are catalyzed by solutions of transition metal compounds. Catalysis may be attributed to the ability of the metal ions to coordinate with reactant molecules, thereby both orienting them and reducing the energy barrier for bond making and breaking.Notably, the C-H bond of coordinated molecules when appropriately located with respect to the transition metal is unusually labile. Also, auxiliary ligands, although not ultimately incorporated in the product, may be critically important for catalysis. The rhodium-catalyzed syntheses discussed here (ethylene 4 butene, ethylene + butadiene 4 1,4-hexadienej and 1-butene + isomeric linear butenes) each comprise a characterizing sequence of reversible reaction steps. These steps, which are common to all three syntheses, are: (1) coordination of olefin with Rh(1) or Rh(II1); ( 2 ) oxidative addition of HC1 to Rh(1) to form a Rh(II1) hydride; and (3) addition of coordinated hydride or alkyl to coordinated olefin to give, respectively, a coordinated alkyl or coordinated higher alkyl. In each synthesis system many combinations of these three steps are conceivable. The ability of the catalysts to produce clean products through thermodynamic or kinetic control is discussed for each synthesis.The discoveries of ferrocene and Ziegler catalysts in 1951 awakened interest in the organic chemistry of transition metals. During the fifties most attention was given to the development of these two discoveries. But research of broader implications was also undertaken, including, notably, some very important work characterizing transition metal alkyls, olefin complexes, and related compounds by a group associated with Joseph Chatt. Around 1960 some of the newly organized chemistry was used to explain the cobalt carbonyl catalyzed synthesis of aldehydes from olefins, CO, and Hz. Since then, accelerated by information uncovered by such analytic techniques as infrared, nmr, chromatography, and mass spectroscopy, organotransition metal chemistry has grown expansively. One reason for the interest which led to this growth is that a variety of syntheses, sometimes impressively selective, involving olefins, acetylenes, CO, and Hz are catalyzed by transition metal compounds. These syntheses proceed through, and are understandable in terms of, the chemistry of organometallics, and it is expected (or, at least, hoped) that through increased understanding, catalysts may be designed whose specificity and efficiency will approach those of enzymes.We will be concerned here almost exclusively with the mechanism of reactions of olefins that are catalyzed by compounds of Rh. Focusing on Rh catalysis is not as restrictive as it might appear, because if a compound of one transition metal is effective, generally compounds of other metals can be found which are also catalytically active. Catalysts based on four or more other metals have already been reported for each of the rhodium-promoted reactions described here. Principles of Transition Metal Catalys...

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