Oxygen‐Promoted C?H Bond Activation at Palladium

[Pd(P(Ar)(tBu)₂)₂] ( 1 , Ar=naphthyl) reacts with molecular oxygen to form Pd^II hydroxide dimers in which the naphthyl ring is cyclometalated and one equivalent of phosphine per palladium atom is released. This reaction involves the cleavage of both C H and O O bonds, two transformations central to catalytic aerobic oxidizations of hydrocarbons. Observations at low temperature suggest the initial formation of a superoxo complex, which then generates a peroxo complex prior to the C H activation step. A transition state for energetically viable C H activation across a Pd peroxo bond was located computationally.     

Selective C?F Bond Activation of Tetrafluorobenzenes by Nickel(0) with a Nitrogen Donor Analogous to N‐Heterocyclic Carbenes

N, not NHC : A neutral, basic, strong σ‐donor nitrogen ancillary ligand with properties analogous to those of N‐heterocyclic carbenes (NHCs) was developed to aid in the oxidative additions of challenging substrates to late transition metals. Selective, room‐temperature C? F bond activation was observed with hexa‐, penta‐, and all three isomers of tetrafluorobenzene using a nickel(0) source in the presence of this donor.

     

Synthesis of Phenanthridinones from N‐Methoxybenzamides and Aryltriethoxysilanes through RhIII‐Catalyzed C?H and N?H Bond Activation

An efficient method for the one‐pot synthesis of substituted phenanthridinone derivatives from N‐methoxybenzamides and aryltriethoxysilanes through rhodium‐catalyzed dual C? H bond activation and annulation reactions is described. A double‐cycle mechanism is proposed to account for this catalytic reaction. In addition, isotope‐labeling studies were performed to understand the intimate mechanism of the reaction.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

C?H Bond Activation/Borylation of Furans and Thiophenes Catalyzed by a Half‐Sandwich Iron N‐Heterocyclic Carbene Complex

A coordinatively unsaturated iron‐methyl complex having an N‐heterocyclic carbene ligand, [Cp*Fe(L^Me)Me] ( 1 ; Cp*=η⁵‐C₅Me₅, L^Me=1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene), is synthesized from the reaction of [Cp*Fe(TMEDA)Cl] (TMEDA=N,N,N′,N′‐tetramethylethylenediamine) with methyllithium and L^Me. Complex 1 is found to activate the C? H bonds of furan, thiophene, and benzene, giving rise to aryl complexes, [Cp*Fe(L^Me)(aryl)] (aryl=2‐furyl ( 2 ), 2‐thienyl ( 3 ), phenyl ( 4 )). The C? H bond cleavage reactions are applied to the dehydrogenative coupling of furans or thiophenes with pinacolborane (HBpin) in the presence of tert‐butylethylene and a catalytic amount of 1 (10 mol % to HBpin). The borylation of the furan/thiophene or 2‐substituted furans/thiophenes occurs exclusively at the 2‐ or 5‐positions, respectively, whereas that of 3‐substituted furans/thiophenes takes place mainly at the 5‐position and gives a mixture of regioisomers. Treatment of 2 with 2 equiv of HBpin results in the quantitative formation of 2‐boryl‐furan and the borohydride complex [Cp*Fe(L^Me)(H₂Bpin)] ( 5 ). Heating a solution of 5 in the presence of tert‐butylethylene led to the formation of an alkyl complex [Cp*Fe(L^Me)CH₂CH₂tBu] ( 6 ), which was found to cleave the C? H bond of furan to produce 2 . On the basis of these results, a possible catalytic cycle is proposed.     

Carbene–Transition Metal Complexes Formed by Double C?H Bond Activation

The activation of a single sp³ C? H bond of alkanes and their derivatives by electron‐rich transition metal complexes has been a topic of interest since the landmark work by Bergman and Graham in 1982. Ten years later, it was shown that compounds of 5d elements, such as osmium and iridium, even enable a double α‐C? H bond activation of alkane or cycloalkane derivatives containing an OR or NR₂ functional group, thus opening up a new route to obtain Fischer‐type transition metal carbene complexes. Subsequent work focused in particular on the conversion of methyl alkyl and methyl aryl ethers into bound oxocarbenes and also of dimethyl amines to bound aminocarbenes. In the context of this work, it was recently shown that square‐planar oxocarbene–iridium(I) complexes prepared in this way exhibit an unusual mode of reactivity: They react with CO₂, CS₂, COS, PhNCO, and PhNCS by an atom‐ or group‐transfer metathesis, which has no precedent. Organic azides RN₃ and N₂O behave similarly. Recent results confirm that this novel type of metathesis can be made catalytic, thus offering a novel possibility for C? H bond functionalization.     

Autocatalytic Intermolecular versus Intramolecular Deprotonation in C?H Bond Activation of Functionalized Arenes by Ruthenium(II) or Palladium(II) Complexes

The activation of the C? H bond of 1‐phenylpyrazole ( 2 ) and 2‐phenyl‐2‐oxazoline ( 3 ) by [Ru(OAc)₂(p‐cymene)] is an autocatalytic process catalyzed by the co‐product HOAc. The reactions are indeed faster in the presence of acetic acid and water but slower in the presence of a base K₂CO₃. A reactivity order is established in the absence of additives: 2‐phenylpyridine>2‐phenyl‐2‐oxazoline>1‐phenylpyrazole (at RT). The accelerating effect of added acetate ions reveals an intermolecular deprotonation after C? H bond activation by a cationic Ru^II center (S_E3 mechanism). The reactions of 1‐phenylpyrazole and 2‐phenyl‐2‐oxazoline first lead to the neutral cyclometalated complexes A₂ and A₃ ligated by one acetate. The latter dissociate to the cationic complexes B₂ ⁺ and B₃ ⁺ , respectively, and acetate. A slow incorporation of one or two D atoms into 2 , 3 , and 2‐phenylpyridine ( 1 ) was observed in the presence of deuterated acetic acid. The “reversibility” of the C? H bond activation/deprotonation takes place from the cationic complexes B _n ⁺ (n=1–3). They are also involved in oxidative additions to PhI, which are rate‐determining and lead to the mono‐ and bis‐phenylated products at high temperatures. A general mechanism is proposed for the arylation of arenes 1–3 catalyzed by [Ru(OAc)₂(p‐cymene)]. In contrast, the reaction of Pd(OAc)₂ with 2‐phenylpyridine ( 1 ), is much faster: Pd(OAc)₂>[Ru(OAc)₂(p‐cymene)]. Since the kinetics is not affected by added acetates, the reaction proceeds through a CMD mechanism assisted by a ligated acetate (intramolecular process) and is irreversible. A bis‐cyclometalated Pd^II^Pd^II dimer D′₁ is formed whose bielectronic electrochemical oxidation leads to a [Pd^III^Pd^III]²⁺ dimer, in agreement with the result of a reported chemical oxidation used in arene functionalizations catalyzed by Pd(OAc)₂.     

Silver‐Catalyzed Oxidative Activation of Benzylic C?H Bonds for the Synthesis of Difluoromethylated Arenes

A mild and catalytic method to form difluoromethylated arenes through the activation of benzylic C H bonds has been developed. Utilizing AgNO₃ as the catalyst, various arenes with diverse functional groups undergo activation/fluorination of benzylic C H bonds with commercially available Selectfluor reagent as a source of fluorine in aqueous solution. The reaction is operationally simple and amenable to gram‐scale synthesis.     

The Chemistry of C?H Bond Activation on Diamond

The surface of hydrogen‐terminated diamond resembles a solid hydrocarbon substrate. Interestingly, the C? H bonds on the diamond surface are not as unreactive as that of saturated hydrocarbon molecules owing to its unique surface electronic properties. The invention of C? H bond activation and C? C coupling reactions on the diamond surface allows chemists to develop powerful chemical transistors, biosensors, and photovoltaic cells on the diamond platform.     

Beyond Directing Groups: Transition‐Metal‐Catalyzed C?H Activation of Simple Arenes

The use of coordinating moieties as directing groups for the functionalization of aromatic C? H bonds has become an established tool to enhance reactivity and induce regioselectivity. Nevertheless, with regard to the synthetic applicability of C? H activation, there is a growing interest in transformations in which the directing group can be fully abandoned, thus allowing the direct functionalization of simple benzene derivatives. However, this approach requires the disclosure of new strategies to achieve reactivity and to control selectivity. In this review, recent advances in the emerging field of non‐chelate‐assisted C? H activation are discussed, highlighting some of the most intriguing and inspiring examples of induction of reactivity and selectivity.