Cross‐Coupling of Organolithium with Ethers or Aryl Ammonium Salts by C−O or C−N Bond Cleavage

Various aryl‐, alkenyl‐, and/or alkyllithium species reacted smoothly with aryl and/or benzyl ethers with cleavage of the inert C?O bond to afford cross‐coupled products, catalyzed by commercially available [Ni(cod)₂] (cod=1,5‐cyclooctadiene) catalysts with N‐heterocyclic carbene (NHC) ligands. Furthermore, the coupling reaction between the aryllithium compounds and aryl ammonium salts proceeded under mild conditions with C?N bond cleavage in the presence of a [Pd(PPh₃)₂Cl₂] catalyst. These methods enable selective sequential functionalizations of arenes having both C?N and C?O bonds in one pot.     

Facile P−C/C−H Bond‐Cleavage Reactivity of Nickel Bis(diphosphine) Complexes

Unusual cleavage of P?C and C?H bonds of the P₂N₂ ligand, in heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes under mild conditions, results in the formation of an iminium formyl nickelate featuring a C,P,P‐tridentate coordination mode. The structures of both the heteroleptic [Ni(P₂N₂)(diphosphine)]²⁺ complexes and the resulting iminium formyl nickelate have been characterized by NMR spectroscopy and single‐crystal X‐ray diffraction analysis. Density functional theory (DFT) calculations were employed to investigate the mechanism of the P?C/C?H bond cleavage, which involves C?H bond cleavage, hydride rotation, Ni?C/P?H bond formation, and P?C bond cleavage.     

Photoinduced Cobalt(III)−Trifluoromethyl Bond Activation Enables Arene C−H Trifluoromethylation

Visible‐light capture activates a thermodynamically inert Co^III−CF₃ bond for direct C−H trifluoromethylation of arenes and heteroarenes. New trifluoromethylcobalt(III) complexes supported by a redox‐active [OCO] pincer ligand were prepared. Coordinating solvents, such as MeCN, afford green, quasi‐octahedral [(^SOCO)Co^III(CF₃)(MeCN)₂] ( 2 ), but in non‐coordinating solvents the complex is red, square pyramidal [(^SOCO)Co^III(CF₃)(MeCN)] ( 3 ). Both are thermally stable, and 2 is stable in light. But exposure of 3 to low‐energy light results in facile homolysis of the Co^III−CF₃ bond, releasing ^.CF₃ radical, which is efficiently trapped by TEMPO^. or (hetero)arenes. The homolytic aromatic substitution reactions do not require a sacrificial or substrate‐derived oxidant because the Co^II by‐product of Co^III−CF₃ homolysis produces H₂. The photophysical properties of 2 and 3 provide a rationale for the disparate light stability.     

Stereoselective Palladium‐Catalyzed C−F Bond Alkynylation of Tetrasubstituted gem‐Difluoroalkenes

A stereoselective Pd(PPh₃)₄‐catalyzed C?F bond alkynylation of tetrasubstituted gem‐difluoroalkenes with terminal alkynes has been developed. This method gives access to a great variety of conjugated monofluoroenynes bearing a tetrasubstituted alkene moiety with well‐defined stereochemistry. Chelation‐assisted oxidative addition of Pd to the C?F bond is proposed to account for the high level of stereocontrol. An X‐ray crystal structure of a key monofluorovinyl Pd^II intermediate has been obtained for the first time as evidence for the proposed mechanism.     

Site‐Selective C−S Bond Formation at C−Br over C−OTf and C−Cl Enabled by an Air‐Stable,Easily Recoverable,and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging Pd^I catalysis to carbon–heteroatom, that is, C?S bond formation. While Pd⁰‐catalyzed protocols may suffer from the formation of poisonous sulfide‐bound off‐cycle intermediates and lack of selectivity, the mechanistically diverse Pd^I catalysis concept circumvents these challenges and allows for C?S bond formation (S–aryl and S–alkyl) of a wide range of aryl halides. Site‐selective thiolations of C?Br sites in the presence of C?Cl and C?OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X‐ray, and reactivity data support dinuclear Pd^I catalysis to be operative. Contrary to air‐sensitive Pd⁰, the active Pd^I species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.     

Novel P‐Stereogenic PCP Pincer‐Aryl Ruthenium(II) Complexes and Their Use in the Asymmetric Hydrogen Transfer Reaction of Acetophenone

Achiral P‐donor pincer‐aryl ruthenium complexes ([RuCl(PCP)(PPh₃)]) 4c , d were synthesized via transcyclometalation reactions by mixing equivalent amounts of [1,3‐phenylenebis(methylene)]bis[diisopropylphosphine] ( 2c ) or [1,3‐phenylenebis(methylene)]bis[diphenylphosphine] ( 2d ) and the N‐donor pincer‐aryl complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 2). The same synthetic procedure was successfully applied for the preparation of novel chiral P‐donor pincer‐aryl ruthenium complexes [RuCl(P*CP*)(PPh₃)] 4a , b by reacting P‐stereogenic pincer‐arenes (S,S)‐[1,3‐phenylenebis(methylene)]bis[(alkyl)(phenyl)phosphines] 2a , b (alkyl=ⁱPr or ^tBu, P*CHP*) and the complex [RuCl{2,6‐(Me₂NCH₂)₂C₆H₃}(PPh₃)], ( 3 ; Scheme 3). The crystal structures of achiral [RuCl(equation/tex2gif-sup-3.gifPCP)(PPh₃)] 4c and of chiral (S,S)‐[RuCl(equation/tex2gif-sup-6.gifPCP)(PPh₃)] 4a were determined by X‐ray diffraction (Fig. 3). Achiral [RuCl(PCP)(PPh₃)] complexes and chiral [RuCl(P*CP*)(PPh₃)] complexes were tested as catalyst in the H‐transfer reduction of acetophenone with propan‐2‐ol. With the chiral complexes, a modest enantioselectivity was obtained.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

An NHC–Silyl–NHC Pincer Ligand for the Oxidative Addition of C−H,N−H,and O−H Bonds to Cobalt(I) Complexes

N‐Heterocyclic carbene based pincer ligands bearing a central silyl donor, [CSiC]⁻, have been envisioned as a class of strongly σ‐donating ligands that can be used for synthesizing electron‐rich transition‐metal complexes for the activation of inert bonds. However, this type of pincer ligand and complexes thereof have remained elusive owing to their challenging synthesis. We herein describe the first synthesis of a CSiC pincer ligand scaffold through the coupling of a silyl–NHC chelate with a benzyl–NHC chelate induced by one‐electron oxidation in the coordination sphere of a cobalt complex. The monoanionic CSiC ligand stabilizes the Co^I dinitrogen complex [(CSiC)Co(N₂)] with an unusual coordination geometry and enables the challenging oxidative addition of E−H bonds (E=C, N, O) to Co^I to form Co^III complexes. The structure and reactivity of the cobalt(I) complex are ascribed to the unique electronic properties of the CSiC pincer ligand, which provides a strong trans effect and pronounced σ‐donation.     

Interconversion of Molybdenum Imido and Amido Complexes by Proton‐Coupled Electron Transfer

Interconversion of the molybdenum amido [(^PhTpy)(PPh₂Me)₂Mo(NHtBuAr)][BArF²⁴] (^PhTpy=4′‐Ph‐2,2′,6′,2“‐terpyridine; tBuAr=4‐tert‐butyl‐C₆H₄; ArF²⁴=(C₆H₃‐3,5‐(CF₃)₂)₄) and imido [(^PhTpy)(PPh₂Me)₂Mo(NtBuAr)][BArF²⁴] complexes has been accomplished by proton‐coupled electron transfer. The 2,4,6‐tri‐tert‐butylphenoxyl radical was used as an oxidant and the non‐classical ammine complex [(^PhTpy)(PPh₂Me)₂Mo(NH₃)][BArF²⁴] as the reductant. The N?H bond dissociation free energy (BDFE) of the amido N?H bond formed and cleaved in the sequence was experimentally bracketed between 45.8 and 52.3 kcal mol^?1, in agreement with a DFT‐computed value of 48 kcal mol^?1. The N?H BDFE in combination with electrochemical data eliminate proton transfer as the first step in the N?H bond‐forming sequence and favor initial electron transfer or concerted pathways.     

C−H Activation from Iron(II)‐Nitroxido Complexes

The reaction of nitroxyl radicals TEMPO (2,2′,6,6′‐tetramethylpiperidinyloxyl) and AZADO (2‐azaadamantane‐N‐oxyl) with an iron(I) synthon affords iron(II)‐nitroxido complexes (^ArL)Fe(κ¹‐TEMPO) and (^ArL)Fe(κ²‐N,O‐AZADO) (^ArL=1,9‐(2,4,6‐Ph₃C₆H₂)₂‐5‐mesityldipyrromethene). Both high‐spin iron(II)‐nitroxido species are stable in the absence of weak C−H bonds, but decay via N−O bond homolysis to ferrous or ferric iron hydroxides in the presence of 1,4‐cyclohexadiene. Whereas (^ArL)Fe(κ¹‐TEMPO) reacts to give a diferrous hydroxide [(^ArL)Fe]₂(μ‐OH)₂, the reaction of four‐coordinate (^ArL)Fe(κ²‐N,O‐AZADO) with hydrogen atom donors yields ferric hydroxide (^ArL)Fe(OH)(AZAD). Mechanistic experiments reveal saturation behavior in C−H substrate and are consistent with rate‐determining hydrogen atom transfer.     

Transition‐Metal Complexes of Tetrylones [(CO)5W‐E(PPh3)2] and Tetrylenes [(CO)5W‐NHE] (E=C–Pb): A Theoretical Study

Quantum chemical calculations at the BP86/TZVPP//BP86/SVP level are performed for the tetrylone complexes [W(CO)₅‐E(PPh₃)₂] ( W‐1 E ) and the tetrylene complexes [W(CO)₅‐NHE] ( W‐2 E ) with E=C–Pb. The bonding is analyzed using charge and energy decomposition methods. The carbone ligand C(PPh₃) is bonded head‐on to the metal in W‐1 C , but the tetrylone ligands E(PPh₃)₂ are bonded side‐on in the heavier homologues W‐1 Si to W‐1 Pb . The W? E bond dissociation energies (BDEs) increase from the lighter to the heavier homologues ( W‐1 C : D_e=25.1 kcal mol^?1; W‐1 Pb : D_e=44.6 kcal mol^?1). The W(CO)₅←C(PPh₃)₂ donation in W‐1 C comes from the σ lone‐pair orbital of C(PPh₃)₂, whereas the W(CO)₅←E(PPh₃)₂ donation in the side‐on bonded complexes with E=Si–Pb arises from the π lone‐pair orbital of E(PPh₃)₂ (the HOMO of the free ligand). The π‐HOMO energy level rises continuously for the heavier homologues, and the hybridization has greater p character, making the heavier tetrylones stronger donors than the lighter systems, because tetrylones have two lone‐pair orbitals available for donation. Energy decomposition analysis (EDA) in conjunction with natural orbital for chemical valence (NOCV) suggests that the W? E BDE trend in W‐1 E comes from the increase in W(CO)₅←E(PPh₃)₂ donation and from stronger electrostatic attraction, and that the E(PPh₃)₂ ligands are strong σ‐donors and weak π‐donors. The NHE ligands in the W‐2 E complexes are bonded end‐on for E=C, Si, and Ge, but side‐on for E=Sn and Pb. The W? E BDE trend is opposite to that of the W‐1 E complexes. The NHE ligands are strong σ‐donors and weak π‐acceptors. The observed trend arises because the hybridization of the donor orbital at atom E in W‐2 E has much greater s character than that in W‐1 E , and even increases for heavier atoms, because the tetrylenes have only one lone‐pair orbital available for donation. In addition, the W? E bonds of the heavier systems W‐2 E are strongly polarized toward atom E, so the electrostatic attraction with the tungsten atom is weak. The BDEs calculated for the W? E bonds in W‐1 E , W‐2 E and the less bulky tetrylone complexes [W(CO)₅‐E(PH₃)₂] ( W‐3 E ) show that the effect of bulky ligands may obscure the intrinsic W? E bond strength.     

From As‐Zincoarsasilene (LZn‐As=SiL′) to Arsaethynolato (As≡C−O) and Arsaketenylido (O=C=As) Zinc Complexes

The reactivity of the As‐zincosilaarsene LZn?As=SiL′ A (L=[CH(CMeNDipp)₂]^?, Dipp=2,6‐ⁱPr₂C₆H₃, L′=[{C(H)N(2,6‐ⁱPr₂‐C₆H₃)}₂]^2?) towards small molecules was investigated. Due to the pronounced zwitterionic character of the Si=As bond of A , it undergoes addition reactions with H₂O and NH₃, forming LZnAs(H)SiOH(L′) 1 and LZnAs(H)SiNH₂(L′) 2 . Oxygenation of A with N₂O at ?60 °C furnishes the deep blue 1,2‐disiloxydiarsene, [LZnOSi(L′)As]₂ 4 , presumably via dimerization of the arsinidene intermediate LZnOSi(L′)As 3 . Oxygenation of A with CO₂ leads to the monomeric arsaethynolato siloxido zinc complex LZnOSi(L′)(OC≡As) 5 , essentially trapping the intermediary arsinidene 3 with liberated CO following initial oxidation of the Si=As bond. DFT calculations confirm the ambident coordination mode of the anionic [AsCO] ligand in solution, with the O‐arsaethynolato [As≡C?O]^.? in 5 , and the As‐arsaketenylido ligand mode [O=C=As]^? present in LZnO?Si(L′)(?As=C=O) 5′ akin to the analogous phosphorus system, [PCO]^?.     

Reactive Dimerization of an N‐Heterocyclic Plumbylene: C−H Activation with PbII

The N‐heterocyclic plumbylene [Fe{(η⁵‐C₅H₄)NSiMe₃}₂Pb:] is in equilibrium with an unprecedented dimer in solution, whose formation involves the cleavage of a strong C?H bond and concomitant formation of a Pb?C and an N?H bond. According to a mechanistic DFT assessment, dimer formation does not involve direct Pb^II insertion into a cyclopentadienyl C?H bond, but is best described as an electrophilic substitution. The bulkier plumbylene [Fe{(η⁵‐C₅H₄)NSitBuMe₂}₂Pb:] shows no dimerization, but compensates its electrophilicity by the formation of an intramolecular Fe?Pb bond.     

A Cobalt(I) Pincer Complex with an η2‐Caryl−H Agostic Bond: Facile C−H Bond Cleavage through Deprotonation,Radical Abstraction,and Oxidative Addition

The synthesis and reactivity of a Co^I pincer complex [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ featuring an η²‐ C_aryl?H agostic bond is described. This complex was obtained by protonation of the Co^I complex [Co(PCP^NMe‐iPr)(CO)₂]. The Co^III hydride complex [Co(PCP^NMe‐iPr)(CNtBu)₂(H)]⁺ was obtained upon protonation of [Co(PCP^NMe‐iPr)(CNtBu)₂]. Three ways to cleave the agostic C?H bond are presented. First, owing to the acidity of the agostic proton, treatment with pyridine results in facile deprotonation (C?H bond cleavage) and reformation of [Co(PCP^NMe‐iPr)(CO)₂]. Second, C?H bond cleavage is achieved upon exposure of [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ to oxygen or TEMPO to yield the paramagnetic Co^II PCP complex [Co(PCP^NMe‐iPr)(CO)₂]⁺. Finally, replacement of one CO ligand in [Co(?³P,CH,P‐P(CH)P^NMe‐iPr)(CO)₂]⁺ by CNtBu promotes the rapid oxidative addition of the agostic η²‐C_aryl?H bond to give two isomeric hydride complexes of the type [Co(PCP^NMe‐iPr)(CNtBu)(CO)(H)]⁺.     

Unprecedented Reaction Pathway of Sterically Crowded Calcium Complexes: Sequential C−N Bond Cleavage Reactions Induced by C−H Bond Activations

Five bis(quinolylmethyl)‐(1H ‐indolylmethyl)amine (BQIA) compounds, that is, {(quinol‐8‐yl‐CH₂)₂NCH₂(3‐Br‐1H ‐indol‐2‐yl)} ( L¹H ) and {[(8‐R³‐quinol‐2‐yl)CH₂]₂NCH(R²)[3‐R¹‐1H ‐indol‐2‐yl]} ( L^2–5H ) ( L²H : R¹=Br, R²=H, R³=H; L³H : R¹=Br, R²=H, R³=i Pr; L⁴H : R¹=H, R²=CH₃, R³=i Pr; L⁵H : R¹=H, R²=n Bu, R³=i Pr) were synthesized and used to prepare calcium complexes. The reactions of L^1–5H with silylamido calcium precursors (Ca[N(SiMe₂R)₂]₂(THF)₂, R=Me or H) at room temperature gave heteroleptic products ( L^{1, 2} )CaN(SiMe₃)₂ ( 1 , 2 ), ( L^{3, 4} )CaN(SiHMe₂)₂ ( 3 a , 4 a ) and homoleptic complexes ( L^{3, 5} )₂Ca ( D3 , D5 ). NMR and X‐ray analyses proved that these calcium complexes were stabilized through Ca⋅⋅⋅C−Si, Ca⋅⋅⋅H−Si or Ca⋅⋅⋅H−C agostic interactions. Unexpectedly, calcium complexes (( L^3–5 )CaN(SiMe₃)₂) bearing more sterically encumbered ligands of the same type were extremely unstable and underwent C−N bond cleavage processes as a consequence of intramolecular C−H bond activation, leading to the exclusive formation of (E )‐1,2‐bis(8‐isopropylquinol‐2‐yl)ethane.     

Copper‐Catalyzed Oxidative C−H Bond Functionalization of N‐Allylbenzamide for Regioselective C−N and C−O Bond Formation

Copper‐catalyzed oxidative couplings of N‐allylbenzamides for C?N and C?O bond formations have been developed through C?H bond functionalization. To demonstrate the utility of this approach, it was applied to the synthesis of β‐aminoimides and imides. To the best of our knowledge, these are the first examples in which different classes of N‐containing compounds have been directly prepared from the readily available N‐allylbenzamides using an inexpensive catalyst/oxidant/base (CuSO₄/TBHP/Cs₂CO₃) system.     

Calcium‐Catalyzed Arene C−H Bond Activation by Low‐Valent AlI

The low‐valent ß‐diketiminate complex (^DIPPBDI)Al is stable in benzene but addition of catalytic quantities of [(^DIPPBDI)CaH]₂ at 20 °C led to (^DIPPBDI)Al(Ph)H (^DIPPBDI=CH[C(CH₃)N‐DIPP]₂, DIPP=2,6‐diisopropylphenyl). Similar Ca‐catalyzed C?H bond activation is demonstrated for toluene or p‐xylene. For toluene a remarkable selectivity for meta‐functionalization has been observed. Reaction of (^DIPPBDI)Al(m‐tolyl)H with I₂ gave m‐tolyl iodide, H₂ and (^DIPPBDI)AlI₂ which was recycled to (^DIPPBDI)Al. Attempts to catalyze this reaction with Mg or Zn hydride catalysts failed. Instead, the highly stable complexes (^DIPPBDI)Al(H)M(^DIPPBDI) (M=Mg, Zn) were formed. DFT calculations on the Ca hydride catalyzed arene alumination suggest that a similar but more loosely bound complex is formed: (^DIPPBDI)Al(H)Ca(^DIPPBDI). This is in equilibrium with the hydride bridged complex (^DIPPBDI)Al(μ‐H)Ca(^DIPPBDI) which shows strongly increased electron density at Al. The combination of Ca‐arene bonding and a highly nucleophilic Al center are key to facile C?H bond activation.     

Catalytic C−H Borylation Using Iron Complexes Bearing 4,5,6,7‐Tetrahydroisoindol‐2‐ide‐Based PNP‐Type Pincer Ligand

Catalytic C?H borylation has been reported using newly designed iron complexes bearing a 4,5,6,7‐tetrahydroisoindol‐2‐ide‐based PNP pincer ligand. The reaction tolerated various five‐membered heteroarenes, such as pyrrole derivatives, as well as six‐membered aromatic compounds, such as toluene. Successful examples of the iron‐catalyzed sp³ C?H borylation of anisole derivatives were also presented.     

Site‐Selective C−S Bond Formation at C−Br over C−OTf and C−Cl Enabled by an Air‐Stable,Easily Recoverable,and Recyclable Palladium(I) Catalyst

This report widens the repertoire of emerging Pd^I catalysis to carbon–heteroatom, that is, C−S bond formation. While Pd⁰‐catalyzed protocols may suffer from the formation of poisonous sulfide‐bound off‐cycle intermediates and lack of selectivity, the mechanistically diverse Pd^I catalysis concept circumvents these challenges and allows for C−S bond formation (S–aryl and S–alkyl) of a wide range of aryl halides. Site‐selective thiolations of C−Br sites in the presence of C−Cl and C−OTf were achieved in a general and a priori predictable fashion. Computational, spectroscopic, X‐ray, and reactivity data support dinuclear Pd^I catalysis to be operative. Contrary to air‐sensitive Pd⁰, the active Pd^I species was easily recovered in the open atmosphere and subjected to multiple rounds of recycling.     

Carbon Monoxide Activation by a Molecular Aluminium Imide: C−O Bond Cleavage and C−C Bond Formation

Anionic molecular imide complexes of aluminium are accessible via a rational synthetic approach involving the reactions of organo azides with a potassium aluminyl reagent. In the case of K₂[( NON )Al(NDipp)]₂ ( NON =4,5‐bis(2,6‐diisopropylanilido)‐2,7‐di‐tert‐butyl‐9,9‐dimethyl‐xanthene; Dipp=2,6‐diisopropylphenyl) structural characterization by X‐ray crystallography reveals a short Al?N distance, which is thought primarily to be due to the low coordinate nature of the nitrogen centre. The Al?N unit is highly polar, and capable of the activation of relatively inert chemical bonds, such as those found in dihydrogen and carbon monoxide. In the case of CO, uptake of two molecules of the substrate leads to C?C coupling and C≡O bond cleavage. Thermodynamically, this is driven, at least in part, by Al?O bond formation. Mechanistically, a combination of quantum chemical and experimental observations suggests that the reaction proceeds via exchange of the NR and O substituents through intermediates featuring an aluminium‐bound isocyanate fragment.