Activation of Alkanes with Organotransition Metal Complexes

Abstract

Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H → R-M-H). This is a dihydride having the formula Cp^*(L)lrH₂, where Cp^* and L are abbreviations for the ligands (CH₃)₅C₅ and (CH₃)₃P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H₂, generating the reactive intermediate Cp^*lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp^*(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp^*lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules—alkyl halides—by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp^*lrL. Oxidative addition of the corresponding rhodium complexes Cp^*RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20°C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds—notably the need for electron-rich metals and the proximity of reacting centers.