Arene Activation at Iridium Facilitates C–O Bond Cleavage of Aryl Ethers

An arene activation strategy for the selective disassembly of aryl ethers is reported. A variety of aryl ethers readily bind an electrophilic pentamethylcyclopentadienyl iridium center by η⁶-arene coordination, generating complexes that are activated toward hydrolysis and cleavage of the Ar–OR bond (R = Me, Et, Ph). Hydrolysis occurs rapidly at room temperature in aqueous pH 7 phosphate buffer (or upon modest heating under acidic conditions), releasing alcohol while forming cyclohexadienyl-one products. Under strongly acidic conditions, protonation of the dienyl-one followed by substitution with starting aryl ether completes a hydrolysis cycle. Mechanistic studies suggest that the key hydrolysis step proceeds via nucleophilic attack at the ipso position of the arene (S_NAr mechanism). The observed mechanism is considered in the context of lignocellulosic biomass conversion

Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum(IV) in Competition with Methyl Group Transfer To Form Dimethylplatinum(IV)

Reaction of molecular oxygen with the methylplatinum­(II) complex K­[(κ²-NNO)­Pt^II(CH₃)­(OH)] (<b>3</b>; NNO = bis­(3,5-dimethylpyrazol-1-yl)­acetate) in D₂O results in oxidation to (κ³-NNO)­Pt^IV(CH₃)­(OD)₂ along with competitive methyl transfer to produce (κ³-NNO)­Pt^IV(CH₃)₂(OD). Methyl transfer is favored under more alkaline conditions and at higher temperatures. Mechanistic studies are consistent with the direct reaction of <b>3</b> with molecular oxygen as a common first reaction step on the path to both products

Photolysis of Pincer-Ligated Pd^II–Me Complexes in the Presence of Molecular Oxygen

The reactions of ^{<i>t</i>‑Bu}PNP and ^{<i>t</i>‑Bu}PCP Pd^II–Me complexes (^{<i>t</i>‑Bu}PNP = 2,6-bis­[(di-<i>tert</i>-butylphosphino)­methyl]­pyridine and ^{<i>t</i>‑Bu}PCP = 2,6-bis­[(di-<i>tert</i>-butylphosphino)­methyl]­phenyl) with O₂ are described and compared with the reported O₂ reactivity of related Pd^II–Me complexes. [(^{<i>t</i>‑Bu}PNP)­PdMe]Cl was found to react with O₂ upon photolysis resulting in oxidation of the pincer ligand backbone to produce a (^{<i>t</i>‑Bu}PNO)­PdCl complex. In contrast, photolysis of (^{<i>t</i>‑Bu}PCP)­PdMe with O₂ resulted in oxidation of the Pd–Me group to form (^{<i>t</i>‑Bu}PCP)­PdOCO₂H. Isotopic labeling, radical initiators, and solvent studies were used to gain insight into the mechanisms of these unusual reactions of late metal alkyls with molecular oxygen

Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum(IV) in Competition with Methyl Group Transfer To Form Dimethylplatinum(IV)

Reaction of molecular oxygen with the methylplatinum­(II) complex K­[(κ²-NNO)­Pt^II(CH₃)­(OH)] (<b>3</b>; NNO = bis­(3,5-dimethylpyrazol-1-yl)­acetate) in D₂O results in oxidation to (κ³-NNO)­Pt^IV(CH₃)­(OD)₂ along with competitive methyl transfer to produce (κ³-NNO)­Pt^IV(CH₃)₂(OD). Methyl transfer is favored under more alkaline conditions and at higher temperatures. Mechanistic studies are consistent with the direct reaction of <b>3</b> with molecular oxygen as a common first reaction step on the path to both products

Direct Formation of Carbon(sp³)–Heteroatom Bonds from Rh^III To Produce Methyl Iodide, Thioethers, and Alkylamines

Thermolysis of the Rh^III–Me complex (DPEphos)­RhMeI₂ (<b>1</b>) results in reductive elimination of MeI. Mechanistic studies are consistent with S_N2 attack by I⁻ at the Rh^III–Me group via <i>two separate</i> competing paths. Addition of sulfur and nitrogen nucleophiles allows effective competition and formation of C­(sp³)–S and C­(sp³)–N coupled products in high yields. C­(sp³)–N bond formation is second-order in amine, consistent with amine substitution of iodide at the metal followed by nucleophilic attack at carbon by a second amine

Arene Activation at Iridium Facilitates C–O Bond Cleavage of Aryl Ethers

An arene activation strategy for the selective disassembly of aryl ethers is reported. A variety of aryl ethers readily bind an electrophilic pentamethylcyclopentadienyl iridium center by η⁶-arene coordination, generating complexes that are activated toward hydrolysis and cleavage of the Ar–OR bond (R = Me, Et, Ph). Hydrolysis occurs rapidly at room temperature in aqueous pH 7 phosphate buffer (or upon modest heating under acidic conditions), releasing alcohol while forming cyclohexadienyl-one products. Under strongly acidic conditions, protonation of the dienyl-one followed by substitution with starting aryl ether completes a hydrolysis cycle. Mechanistic studies suggest that the key hydrolysis step proceeds via nucleophilic attack at the ipso position of the arene (S_NAr mechanism). The observed mechanism is considered in the context of lignocellulosic biomass conversion

Alkane Dehydrogenation by C–H Activation at Iridium(III)

Stoichiometric alkane dehydrogenation utilizing an Ir^III pincer complex, (^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>), has been described. The reaction between <b>1a</b> and octane resulted in quantitative formation of (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>3a</b>) and octene. At early reaction times 1-octene is the major product, indicative of terminal C–H activation by <b>1a</b>. In contrast to prior reports of alkane dehydrogenation with Ir, C–H bond activation occurs at Ir^III and the dehydrogenation is not inhibited by nitrogen, olefin, or water

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems

Comparing Square-Planar Rh^I and Ir^I: Metal–Ligand Proton Tautomerism, Fluxionality, and Reactivity

A series of low-valent square-planar Rh complexes [LHRh][X] (X = PF6, Br, Cl, I) bearing two protic imidazolyl phosphines (one κ2) and a CO ligand were synthesized and fully characterized. A comparison of the CO stretching frequencies with those of the previously reported [LHIr][X] complexes indicates a much lower electron density at the Rh centers. This lower electron density at Rh results in a lower propensity to undergo ligand to metal proton transfer, and in contrast to observations with Ir, the [LHRh][X] complexes (X = Cl, Br, I) do not equilibrate with their metal-protonated congeners. Furthermore, the weaker bond strengths of Rh complexes compared to Ir lead to an increased degree of fluxionality in the former, along with a difference in reactivity with hydrogen (H2) and iodine (I2)

Regeneration of an Iridium(III) Complex Active for Alkane Dehydrogenation Using Molecular Oxygen

(^<i>dm</i>Phebox)­Ir­(OAc)₂(OH₂) (<b>1a</b>) has previously been found to promote stoichiometric alkane dehydrogenation, affording alkene, acetic acid, and the corresponding Ir hydride complex (^<i>dm</i>Phebox)­Ir­(OAc)­(H) (<b>2a</b>) in high yield. In this study, we describe the use of oxygen to quantitatively regenerate <b>1a</b> from <b>2a</b> and HOAc. Distinct reaction intermediates are observed during the conversion of <b>2a</b> to <b>1a</b>, depending upon the presence or absence of 1 equiv of acetic acid, highlighting potential mechanistic implications regarding alkane dehydrogenation catalysis and the use of oxygen as an oxidant in such systems