C−H Bond Activation Reactions of Ethers That Generate Iridium Carbenes

摘要

Tnwo important objectives in organometal-nlic chemistry are to understand C Hnbond activation reactions mediated by tran-nsition metal compounds and then to developnefficient ways of functionalizing the result-ning products. A particularly ambitious goal isnthe generation of metal carbenes from sim-nple organic molecules; the synthetic chem-nist can then take advantage of the almostnunlimited reactivity of this metal organicnfunctionality. This goal remains very diffi-ncult indeed with saturated hydrocarbons, butnit is considerably more facile for molecules that possess a heteroatom (such as ethers), because coordination of thenheteroatom to the metal renders the ensuing C H activation an intramolecular reaction.nIn this Account, we focus on the activation reaction of different types of unstrained ethers, both aliphatic and hemi-naromatic, by (mostly) iridium compounds. We emphasize our recent results with the TpMe2nIr(C6H5)2(N2)(1·N2) com-nplex (where TpMe2ndenotes hydrotris(3,5-dimethylpyrazolyl)borate). Most of the reactivity observed with this system,nand with related electronically unsaturated iridium species, starts with a C H activation reaction, which is then fol-nlowed by reversible -hydrogen elimination. An -C H bond is, in every instance, broken first; when there is a choice,ncleavage of the stronger terminal Csp3 H bonds is always preferred over the weaker internal Csp3 H (methylene) bondsnof the ether. Nevertheless, competitive reactions of the unsaturated [TpMe2nIr(C6H5)2] iridium intermediate with ethersnthat contain Csp3 H and Csp2 H bonds are also discussed. We present theoretical evidence for a σ-complex-assistednmetathesis mechanism (σ-CAM), although for other systems oxidative addition and reductive elimination events cannbe effective reaction pathways. We also show that additional unusual chemical transformations may occur, depend-ning on the nature of the ether, and can result in C O and C C bond-breaking and bond-forming reactions, leadingnto the formation of more elaborate molecules.nAlthough the possibility of extending these results to saturated hydrocarbons appears to be limited for this iri-ndium system, the findings described in this Account are of fundamental importance for various facets of C H bondnactivation chemistry, and with suitable modifications of the ancillary ligands, they could be even broader in scope. Wenfurther discuss experimental and theoretical studies on unusual alkene-to-alkylidene equilibria for some of the prod-nucts obtained in the reactions of iridium complex 1·N2 with alkyl aryl ethers. The rearrangement involves reversiblen- and u0002-hydrogen eliminations, with a rate-determining metal inversion step (supported by theoretical calcula-ntions); the alkylidene is always favored thermodynamically over the alkene. This startling result contrasts with the ener-ngetically unfavorable isomerization of free ethene to ethylidene (by about 80 kcal moln1n), showing that the tautomerismnequilibrium can be directed toward one product or the other by a judicious choice of the transition metal complex.