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.

Formation of 5-membered metallacycles at iPrPCPIr by C–H, O–H, and C–CO bond cleavage

“…This reactivity was interpreted to indicate that in PCPIr systems directing groups may actually hinder C-H activation at ortho positions due to steric interaction. Similar 5-membered metallacycle formation, as well as one example of a 4-membered ring, was later observed with tBu PCPIr during the dehydrogenation of ketones [29] and during reactions with anisole derivatives, methyl benzoate, and several acetates [24]. Recently, metallacycle formation by C-C activation of biphenylene was also reported [30].…”

“…Pincer iridium species have been shown to form metallacycles by C-H activation of phosphine alkyl substituents in POCOP [23] and PCP [24] systems, by metalation of a second pincer ligand in less sterically demanding systems [25], as well as during the catalytic cleavage of C-O bonds in substituted alkyl aryl ethers [26]. However, while the use of pincer iridium species has been extensively investigated in alkane dehydrogenation and similar reactions [19,22,27], examples of processes or complexes employing metallacycles are limited.…”

Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights

“…Krogh-Jespersen and Goldman also investigated the C(sp 3 )−O activation reactions of ethers, esters, and tosylates by an Ir−PCP pincer complex 553. Similarly to their C−F activation mentioned above (see section 6.4.2), reactions were shown to occur via an initial C−H bond activation and sequential α-OR migration (OR = OAr, OTs, OAc, or OC(O)Ar) rather than via direct C−O addition (Scheme 60).…”

Photochemical Reactions of Fluorinated Pyridines at Half-Sandwich Rhodium Complexes: Competing Pathways of Reaction

“…A large isotopic effect was reported for an Ir-PCP/4-methoxy-2,3,5,6-tetrafluorotoluene system to form the C–O activated product where neither a direct oxidative addition nor a simple SN 2 mechanism was observed. 28 The deuterated analogue of 2,3,5,6-tetrafluoro-4-methoxypyridine, C 5 NF 4 OCD 3 , was synthesized and characterized by NMR spectroscopy, mass spectrometry, and IR absorption (see the Supporting Information). The irradiation of 1 in hexane with excess of both substituted tetrafluoropyridines d and d –OCD 3 , present in a 1:5:5 ratio (λ >290 nm, 12 h, room temperature) generates a mixture of the metallacycles CpRh(PMe 3 )(κ 2 - C , C - C H 2 OC 5 NF 3 ) ( 5 ) and the deuterated analogue CpRh(PMe 3 )(κ 2 - C , C - C D 2 OC 5 NF 3 ) ( 6 ) with an NMR yield of 30%.…”