Cleavage of ether, ester, and tosylate C(sp³)-O bonds by an iridium complex, initiated by oxidative addition of C-H bonds. Experimental and computational studies

A pincer-ligated iridium complex, (PCP)Ir (PCP = κ³-C ₆H₃-2,6-[CH₂P(t-Bu)₂]₂), is found to undergo oxidative addition of C(sp³)-O bonds of methyl esters (CH₃-O₂CR′), methyl tosylate (CH ₃-OTs), and certain electron-poor methyl aryl ethers (CH ₃-OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C-H bonds followed by oxygenate migration, rather than by direct C-O addition. Thus, methyl aryl ethers react via addition of the methoxy C-H bond, followed by α-aryloxide migration to give cis-(PCP)Ir(H)(CH₂)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH₃)(OAr). Methyl acetate undergoes C-H bond addition at the carbomethoxy group to give (PCP)Ir(H) [κ²-CH₂OC(O)Me] which then affords (PCP-CH ₂)Ir(H)(κ²-O₂CMe) (6-Me) in which the methoxy C-O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP C_ipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH₃)(κ²-O₂CR), the net product of methoxy group C-O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO₂CMe or ROTs, where R possesses β-C-H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O₂CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C-O bond oxidative additions, these reactions also proceed via initial activation of a C-H bond; in this case, C-H addition at the β-position is followed by β-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the β-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C-O bond is cleaved with no direct participation by the iridium center.