title: 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.
abstract: A pincer-ligated iridium complex, (PCP)Ir (PCP = κ(3)-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp(3))-O bonds of methyl esters (CH3-O2CR'), methyl tosylate (CH3-OTs), and certain electron-poor methyl aryl ethers (CH3-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)[κ(2)-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(κ(2)-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 Cipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(κ(2)-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.
authors.affiliation: Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey , New Brunswick, New Jersey 08903, United States.
authors.forename: Sabuj
authors.initials: S
authors.lastname: Kundu
authors.forename: Jongwook
authors.initials: J
authors.lastname: Choi
authors.forename: David Y
authors.initials: DY
authors.lastname: Wang
authors.forename: Yuriy
authors.initials: Y
authors.lastname: Choliy
authors.forename: Thomas J
authors.initials: TJ
authors.lastname: Emge
authors.forename: Karsten
authors.initials: K
authors.lastname: Krogh-Jespersen
authors.forename: Alan S
authors.initials: AS
authors.lastname: Goldman