Copper-Catalyzed Oxidative Dehydrogenative Carboxylation of Unactivated Alkanes to Allylic Esters via Alkenes

We report copper-catalyzed oxidative dehydrogenative carboxyl­ation (ODC) of unactivated alkanes with various substituted benzoic acids to produce the corresponding allylic esters. Spectroscopic studies (EPR, UV–vis) revealed that the resting state of the catalyst is [(BPI)­Cu­(O₂CPh)] (<b>1-O</b>_<b>2</b><b>CPh</b>), formed from [(BPI)­Cu­(PPh₃)₂], oxidant, and benzoic acid. Catalytic and stoichiometric reactions of <b>1-O</b>_<b>2</b><b>CPh</b> with alkyl radicals and radical probes imply that C–H bond cleavage occurs by a <i>tert</i>-butoxy radical. In addition, the deuterium kinetic isotope effect from reactions of cyclo­hexane and <i>d</i>₁₂-cyclo­hexane in separate vessels showed that the turnover-limiting step for the ODC of cyclo­hexane is C–H bond cleavage. To understand the origin of the difference in products formed from copper-catalyzed amid­ation and copper-catalyzed ODC, reactions of an alkyl radical with a series of copper–carboxylate, copper–amidate, and copper–imidate complexes were performed. The results of competition experiments revealed that the relative rate of reaction of alkyl radicals with the copper complexes follows the trend Cu­(II)–amidate > Cu­(II)–imidate > Cu­(II)–benzoate. Consistent with this trend, Cu­(II)–amidates and Cu­(II)–benzoates containing more electron-rich aryl groups on the benzamidate and benzoate react faster with the alkyl radical than do those with more electron-poor aryl groups on these ligands to produce the corresponding products. These data on the ODC of cyclo­hexane led to preliminary investig­ation of copper-catalyzed oxidative dehydrogenative amin­ation of cyclo­hexane to generate a mixture of <i>N</i>-alkyl and <i>N</i>-allylic products

Copper-Catalyzed Oxidative Dehydrogenative Carboxylation of Unactivated Alkanes to Allylic Esters via Alkenes

We report copper-catalyzed oxidative dehydrogenative carboxyl­ation (ODC) of unactivated alkanes with various substituted benzoic acids to produce the corresponding allylic esters. Spectroscopic studies (EPR, UV–vis) revealed that the resting state of the catalyst is [(BPI)­Cu­(O₂CPh)] (<b>1-O</b>_<b>2</b><b>CPh</b>), formed from [(BPI)­Cu­(PPh₃)₂], oxidant, and benzoic acid. Catalytic and stoichiometric reactions of <b>1-O</b>_<b>2</b><b>CPh</b> with alkyl radicals and radical probes imply that C–H bond cleavage occurs by a <i>tert</i>-butoxy radical. In addition, the deuterium kinetic isotope effect from reactions of cyclo­hexane and <i>d</i>₁₂-cyclo­hexane in separate vessels showed that the turnover-limiting step for the ODC of cyclo­hexane is C–H bond cleavage. To understand the origin of the difference in products formed from copper-catalyzed amid­ation and copper-catalyzed ODC, reactions of an alkyl radical with a series of copper–carboxylate, copper–amidate, and copper–imidate complexes were performed. The results of competition experiments revealed that the relative rate of reaction of alkyl radicals with the copper complexes follows the trend Cu­(II)–amidate > Cu­(II)–imidate > Cu­(II)–benzoate. Consistent with this trend, Cu­(II)–amidates and Cu­(II)–benzoates containing more electron-rich aryl groups on the benzamidate and benzoate react faster with the alkyl radical than do those with more electron-poor aryl groups on these ligands to produce the corresponding products. These data on the ODC of cyclo­hexane led to preliminary investig­ation of copper-catalyzed oxidative dehydrogenative amin­ation of cyclo­hexane to generate a mixture of <i>N</i>-alkyl and <i>N</i>-allylic products

Copper-Catalyzed Intermolecular Amidation and Imidation of Unactivated Alkanes

We report a set of rare copper-catalyzed reactions of alkanes with simple amides, sulfonamides, and imides (i.e., benzamides, tosylamides, carbamates, and phthalimide) to form the corresponding <i>N</i>-alkyl products. The reactions lead to functionalization at secondary C–H bonds over tertiary C–H bonds and even occur at primary C–H bonds. [(phen)­Cu­(phth)] (<b>1-phth</b>) and [(phen)­Cu­(phth)₂] (<b>1-phth</b>_<b>2</b>), which are potential intermediates in the reaction, have been isolated and fully characterized. The stoichiometric reactions of <b>1-phth</b> and <b>1-phth</b>_<b>2</b> with alkanes, alkyl radicals, and radical probes were investigated to elucidate the mechanism of the amidation. The catalytic and stoichiometric reactions require both copper and <i>t</i>BuOO<i>t</i>Bu for the generation of <i>N</i>-alkyl product. Neither <b>1-phth</b> nor <b>1-phth</b>_<b>2</b> reacted with excess cyclohexane at 100 °C without <i>t</i>BuOO<i>t</i>Bu. However, the reactions of <b>1-phth</b> and <b>1-phth</b>_<b>2</b> with <i>t</i>BuOO<i>t</i>Bu afforded <i>N</i>-cyclohexylphthalimide (Cy-phth), <i>N</i>-methylphthalimide, and <i>tert</i>-butoxycyclohexane (Cy-O<i>t</i>Bu) in approximate ratios of 70:20:30, respectively. Reactions with radical traps support the intermediacy of a <i>tert</i>-butoxy radical, which forms an alkyl radical intermediate. The intermediacy of an alkyl radical was evidenced by the catalytic reaction of cyclohexane with benzamide in the presence of CBr₄, which formed exclusively bromocyclohexane. Furthermore, stoichiometric reactions of [(phen)­Cu­(phth)₂] with <i>t</i>BuOO<i>t</i>Bu and (Ph­(Me)₂CO)₂ at 100 °C without cyclohexane afforded <i>N</i>-methylphthalimide (Me-phth) from β-Me scission of the alkoxy radicals to form a methyl radical. Separate reactions of cyclohexane and <i>d</i>₁₂-cyclohexane with benzamide showed that the turnover-limiting step in the catalytic reaction is the C–H cleavage of cyclohexane by a <i>tert</i>-butoxy radical. These mechanistic data imply that the <i>tert</i>-butoxy radical reacts with the C–H bonds of alkanes, and the subsequent alkyl radical combines with <b>1-phth</b>_<b>2</b> to form the corresponding <i>N</i>-alkyl imide product

3d Early Transition Metal Complexes Supported by a New Sterically Demanding Aryloxide Ligand

The bulky aryloxide 2,6-bis­(diphenylmethyl)-4-<i>tert</i>-butylphenol [HOAr^tBu] (<b>1</b>) can be synthesized from 4-<i>tert</i>-butylphenol and benzhydrol in solvent-free conditions and obtained pure in 91% yield. Deprotonation of HOAr^tBu is accomplished with M­(N­(SiMe₃)₂) (M = Na, Li), yielding the corresponding salts of the aryloxide [MOAr^tBu] (M⁺ = Na (<b>2</b>), Li­(<b>3</b>)) in 83% and 73% yield, respectively. Facile salt formation of the aryloxide ligand allows for transmetalation to a variety of metal halides. Through transmetalation reactions involving two aryloxides, mononuclear complexes of the type [M′(OAr^tBu)₂Cl­(THF)₂] (M′ = Sc (<b>4</b>), V (<b>5</b>), Cr (<b>6</b>), Ti (<b>7</b>)) can be prepared from the corresponding metal halide precursor MCl₃(THF)₃. Additionally, two aryloxides can be coordinated to Ti­(IV) via a protonolysis route of Ti­(NMe₂)₂Cl₂ and 2 equiv of HOAr^tBu to yield [Ti­(OAr^tBu)₂Cl₂(NHMe₂)] (<b>8</b>) in 72% isolated yield. Single-crystal X-ray diffraction studies of <b>1</b>,<b> 2</b>, and the 3d metal complexes <b>5</b>–<b>8</b> clearly show the steric demand of the bulky ligand, whereas in transition metal complexes we do not observe the formation of mononuclear tris-aryloxide complexes

Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring Metal–Arene Interactions and Their Reactivity Toward Nitrous Oxide

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U­(III) complexes were prepared that are masked by a metal–arene interaction via <i>δ</i>-backbonding. Depending on solvent and uranium starting material, both a tetrahydrofuran (THF)-bound and Lewis-base-free U­(III) precursor can easily be prepared on the multigram scale. The reaction of these trivalent uranium species with nitrous oxide, N₂O, was studied and an X-ray diffraction (XRD) study on single crystals of the product revealed the formation of a five-coordinate U­(V) oxo complex with two different molecular geometries, namely, square pyramidal and trigonal bipyramidal

Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring Metal–Arene Interactions and Their Reactivity Toward Nitrous Oxide

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U­(III) complexes were prepared that are masked by a metal–arene interaction via <i>δ</i>-backbonding. Depending on solvent and uranium starting material, both a tetrahydrofuran (THF)-bound and Lewis-base-free U­(III) precursor can easily be prepared on the multigram scale. The reaction of these trivalent uranium species with nitrous oxide, N₂O, was studied and an X-ray diffraction (XRD) study on single crystals of the product revealed the formation of a five-coordinate U­(V) oxo complex with two different molecular geometries, namely, square pyramidal and trigonal bipyramidal

Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring Metal–Arene Interactions and Their Reactivity Toward Nitrous Oxide

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U­(III) complexes were prepared that are masked by a metal–arene interaction via <i>δ</i>-backbonding. Depending on solvent and uranium starting material, both a tetrahydrofuran (THF)-bound and Lewis-base-free U­(III) precursor can easily be prepared on the multigram scale. The reaction of these trivalent uranium species with nitrous oxide, N₂O, was studied and an X-ray diffraction (XRD) study on single crystals of the product revealed the formation of a five-coordinate U­(V) oxo complex with two different molecular geometries, namely, square pyramidal and trigonal bipyramidal

Uranium(III) Complexes with Bulky Aryloxide Ligands Featuring Metal–Arene Interactions and Their Reactivity Toward Nitrous Oxide

We report the synthesis and use of an easy-to-prepare, bulky, and robust aryloxide ligand starting from inexpensive precursor materials. Based on this aryloxide ligand, two reactive, coordinatively unsaturated U­(III) complexes were prepared that are masked by a metal–arene interaction via <i>δ</i>-backbonding. Depending on solvent and uranium starting material, both a tetrahydrofuran (THF)-bound and Lewis-base-free U­(III) precursor can easily be prepared on the multigram scale. The reaction of these trivalent uranium species with nitrous oxide, N₂O, was studied and an X-ray diffraction (XRD) study on single crystals of the product revealed the formation of a five-coordinate U­(V) oxo complex with two different molecular geometries, namely, square pyramidal and trigonal bipyramidal

A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)­FeCl₂] (<b>1</b>) (PNP = <i>N</i>[2-P­(CHMe₂)₂-4-methylphenyl]₂^–), prepared from one-electron oxidation of [(PNP)­FeCl] with ClCPh₃, displays an unexpected <i>S</i> = 3/2 to <i>S</i> = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex <b>1</b> to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of <i>U</i>_eff = 32–36 cm^–1 (47–52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and Δ<i>E</i>_Q values for <b>1</b>, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of <b>1</b> reveals the <i>S</i> = 3/2 to be likely the ground state with the <i>S</i> = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of <b>1</b> to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of <b>1</b> collected between 30 and 300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the <i>S</i> = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d­(<i>z</i>²) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet (⁴B) and the sextet (⁶A) states are close in energy

Addition of Si–H and B–H Bonds and Redox Reactivity Involving Low-Coordinate Nitrido–Vanadium Complexes

In this study we enumerate the reactivity for two molecular vanadium nitrido complexes of [(nacnac)­VN­(X)] formulation [nacnac = (Ar)­NC­(Me)­CHC­(Me)­(Ar)⁻, Ar = 2,6-(CHMe₂)₂C₆H₃); X^– = OAr (<b>1</b>) and N­(4-Me-C₆H₄)₂ (Ntolyl₂) (<b>2</b>)]. Density functional theory calculations and reactivity studies indicate the nitride motif to have nucleophilic character, but where the nitrogen atom can serve as a conduit for electron transfer, thus allowing the reduction of the vanadium­(V) metal ion with concurrent oxidation of the incoming substrate. Silane, H₂SiPh₂, readily converts the nitride ligand in <b>1</b> into a primary silyl–amide functionality with concomitant two-electron reduction at the vanadium center to form the complex [(nacnac)­V­{N­(H)­SiHPh₂}­(OAr)] (<b>3</b>). Likewise, addition of the B–H bond in pinacolborane to the nitride moiety in <b>2</b> results in formation of the boryl–amide complex [(nacnac)­V­{N­(H)­B­(pinacol)}­(Ntolyl₂)] (<b>4</b>). In addition to spectroscopic data, complexes <b>3</b> and <b>4</b> were also elucidated structurally by single-crystal X-ray diffraction analysis. One-electron reduction of <b>1</b> with 0.5% Na/Hg on a preparative scale allowed for the isolation and structural determination of an asymmetric bimolecular nitride radical anion complex having formula [Na]₂[(nacnac)­V­(N)­(OAr)]₂ (<b>5</b>), in addition to room-temperature solution X-band electron paramagnetic resonance spectroscopic studies