Overview and Principles

Because of the favorable rates for reductive eliminations involving a hydride ligand noted in the introductory section, examples of complexes that undergo reductive elimination to form the C-H bonds in alkanes and arenes span the transition series. Thermally induced reductive eliminations to form dihydrogen (or dihydrogen complexes) from dihydride complexes can also be rapid, but this reaction occurs less frequently because the oxidative addition of dihydrogen is typically favored thermodynamically. Reductive elimination to form a C-H bond is the last step of many catalytic reactions, such as the hydrogenation and hydroformylation of olefins. 

Much information has been gained on the mechanism of C-H bond-forming reductive elimination (see Equation 8.9). In addition to creating an understanding of C-H bond formation, this information has been used to understand the mechanism of the opposite reaction, the oxidative addition of C-H bonds. Because reductive eliminations of alkanes are faster from three- and five-coordinate species than from four- and six-coordinate species, square planar and octahedral complexes often dissociate or associate a dative ligand prior to reductive elimination. However, elimination to form a C-H bond from a four- or six-coordinate complex can also be fast enough that it occurs directly from the alkylmetal-hydride complexes prior to ligand dissociation. 

As shown in Equation 8.10, reductive eliminations of alkanes generate cr-complexes as the first-formed product, and the alkane dissociates or is displaced by an incoming ligand by an associative process to form the final products. This a-complex is often formed reversibly in these cases, dissociation or displacement of the coordinated alkane is the irreversible step. Kinetic isotope effects and isotopic exchanges have revealed the intermediacy 

Reductive eliminations of methane, toluene, and cyclohexane (Equations 8.11-8.13) occur from (PPh3)jPt(Me)(H), - (PMe3),Ru(H)(CHjPh), and Cp Lr(PMe)3(C,H )H without prior dissociation of ligand. The absence of ligand dissociation prior to reductive elimination was shown in these cases by a zero-order dependence of the reaction rates on the concentration of added phosphine. The rates for these elimination reactions vary dramatically. The platinum complex reacts at -25 °C, the ruthenium complex at 85 °C, and the iridium complex at 135 °C. Although one could rationalize these relative rates in many ways, the platinum compound likely reacts faster than the iridium complex because it is less electron rich, while the ruthenium complex likely reacts faster than the iridium complex because it contains a second-row metal center. 

Otiier reductive eliminations that form C-H bonds do occur after initial dissociation of a ligand. For example, the elimination of carborane in Equation 8.14, - the elimination of ketone in Equation 8.15, and the elimination of aldehyde in Equation 8.16 aU occur after dissociation of a phosphine ligand. In contrast, die reductive elimination of alkane from the zirconocene alkyl hydride complex in Equation 8.17 occurs after association of ligand.  

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Pfluger P. and D.R. Dietrich (2001). Pharmaceuticals in the Environment—An Overview and Principle Considerations. In K. Kiimmerer (Ed.). Pharmaceuticals in the Environment Sources, Fate, Effects and Risks. Springer, New York, pp. 11-17. 

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