Rh-catalyzed ortho-selective C-H borylation of N-functionalized arenes with silica-supported bridgehead monophosphine ligands

Supported phosphine-Rh systems, prepared in situ from silica-supported bridgehead monophosphines and [Rh(OH)(cod)](2), have enabled ortho-selective C-H borylation for a range of arenes containing nitrogen-based directing groups. The regioselectivity was excellent with various N-directing groups, including saturated and unsaturated N-heterocycles, tert-aminoalkyl groups, and imine-type C-N double bonds. The reaction showed significant tolerance toward steric repulsion around the reacting C-H bond. This Rh catalysis complements the Ir-catalyzed ortho-borylation, which is effective for arenes with oxygen-based directing groups.     

Intermolecular and intramolecular, platinum-catalyzed, acceptorless dehydrogenative coupling of hydrosilanes with aryl and aliphatic methyl C-H bonds

Intermolecular acceptorless dehydrogenative coupling of silanes with arene C-H bonds and intramolecular coupling of silanes with aryl and alkyl C-H bonds occur in good yield in the presence of 5 mol % of TpMe2PtMe2H (TpMe2 = hydridotris(3,5-dimethylpyrazolyl)borate) and related platinum(IV) complexes. The intermolecular reactions of arenes occurred with both trialkyl and dialkylaryl silanes. Intramolecular reactions of dialkylsilylalkylarenes occurred at aryl C-H bonds, and reactions of tributylsilane or dibutylphenylsilane occurred intramolecularly at the aliphatic, primary C-H bond. The reactions of arenes occurred preferentially at the least sterically hindered C-H bonds and preferentially with more electron-poor arenes. Crossover experiments and the lack of reactivity of the arylsilanes with H2 imply that the dehydrogenative silylation of arenes can be irreversible, even in a closed reaction vessel.     

Hydrofluoroarylation of alkynes with fluoroarenes

A combination of Ni(cod)(2) and PCyp(3) is found to be an effective catalyst for chemoselective activation of the C-H bond of fluoroarenes over C-F bonds followed by insertion of alkynes to allow direct alkenylation of the electron-deficient arenes. The characteristics of the reactions are: a C-H bond ortho to a fluorine substituent is selectively activated; the reactivity of fluorobenzenes is roughly proportional to the number of fluorine atoms. The reaction conditions tolerate a broad range of both alkynes and fluoroarenes containing both electron-withdrawing and -donating groups, thus allowing efficient synthesis of a variety of substituted ethenes containing a fluoroaryl motif in high regio- and stereoselective manners. Mechanistic studies including both labeling experiments and stoichiometric reactions reveal that oxidative addition of C-H bonds in fluoroarenes to nickel(0) is kinetically highly facile whereas that of C-F bonds is thermodynamically favoured.     

Pd(OAc)(2)-catalyzed oxidative C-H/C-H cross-coupling of electron-deficient polyfluoroarenes with simple arenes

Pd(OAc)(2)-catalyzed intermolecular C-H/C-H cross-coupling reactions between electron-deficient polyfluoroarenes and simple arenes for the synthesis of fluorinated biaryls have been developed. Deuterium-labeling experiments suggested that C-H bond cleavage of the simple arenes rather than the polyfluoroarenes is involved in the rate-limiting step.     

Synthesis of masked haloareneboronic acids via iridium-catalyzed aromatic C-H borylation with 1,8-naphthalenediaminatoborane (danBH)

“Masked” areneboronic acids have been prepared by Ir-catalyzed C-H borylation of arenes. A [Ir(OMe)(cod)]₂ complex with a DPPE ligand showed the highest catalytic activity in the C-H borylation of benzene at 80 °C. The reaction system can be applied to substituted arenes, including halogen-substituted arenes. 1,3-Dihalobenzenes undergo the C-H borylation at their 5-positions in a regioselective fashion, affording 3,5-dihaloareneboronic acid derivatives, which serve as useful coupling modules for the convergent dendrimer synthesis.     

Copper-mediated C-H bond arylation of arenes with arylboronic acids

A new copper-mediated cross-coupling of arenes and arylboronic acids is described. Under the influence of Cu(OCOCF 3) 2, the C-H bond arylation of electron-rich arenes with arylboronic acids takes place to afford a range of biaryls in good yields. The reaction is selective for cross-coupling; no homocoupling product arising from arenes or arylboronic acids is detected. Multiple C-H bond arylation is possible with indoles and pyrroles furnishing interesting extended pi-systems.     

Double-fold C-H oxygenation of arenes using PyrDipSi: a general and efficient traceless/modifiable silicon-tethered directing group

The efficient Pd-catalyzed double-fold C-H oxygenation of arenes into resorcinols using the newly developed 2-pyrimidyldiisopropylsilyl (PyrDipSi) directing group is described. Its use allows for the sequential introduction of OAc and OPiv groups in a one-pot manner to produce orthogonally protected resorcinol derivatives. The PyrDipSi group is superior to the previously developed 2-pyridyldiisopropylsilyl (PyDipSi) group, as it is efficient for monooxygenation of ortho-substituted arenes. Notably, the PyrDipSi group can be easily installed into arene molecules and can be easily removed or modified after the oxygenation reaction.     

Transition metal-catalyzed arylation of unactivated C(sp3)-H bonds

Transition-metal-catalyzed C-H bond arylation has recently emerged as a powerful tool for the functionalization of organic molecules that may complement or even replace traditional catalytic cross-couplings. While many efforts have focused on the arylation of arenes and heteroarenes in the past two decades, less studies have been devoted to the arylation of nonacidic C-H bonds of alkyl groups. This tutorial review highlights recent work in this active area.     

Rh-Catalyzed Intermolecular Carbenoid Functionalization of Aromatic C-H Bonds by α-Diazomalonates

A Rh-catalyzed intermolecular coupling of diazomalonates with arene C-H bonds is reported. The reaction is initiated by electrophilic C-H activation, which is followed by coupling of the arylrhodium(III) complex with the diazomalonate. In most cases, arenes with oximes, carboxylic acids, and amines as directing groups cross-couple with diazomalonates with excellent regioselectivities and functional group tolerance, and thus, this reaction offers a new route to α-aryl carbonyl compounds for specific applications.     

Pd(II)-catalyzed para-selective C-H arylation of monosubstituted arenes

Pd-catalyzed highly para-selective C-H arylation of monosubstituted arenes (including toluene) is developed for the first time using an F(+) reagent as a bystanding oxidant. This finding provides a new retrosynthetic disconnection for para-substituted biaryl synthesis via C-H/C-H cross-coupling.     

Rhodium-catalyzed C-H activation and conjugate addition under mild conditions

An efficient rhodium(III)-catalyzed C-H activation and subsequent conjugate addition was achieved under mild conditions. The reaction utilized inert arenes to replace stoichiometric organometallic reagents and can tolerate various functional groups as well as air and water.     

Regioselectivity of the borylation of alkanes and arenes

The borylation of alkanes and arenes has become some of the most practical C-H bond functionalization chemistry. Most striking is the high regioselectivity of these reactions. Rhodium and ruthenium complexes catalyze with exquisite selectivity the borylation of methyl C-H bonds over methylene or methine C-H bonds. Iridium complexes catalyze, with high steric control, the borylation of one aromatic C-H bond over another. In contrast, iridium-catalyzed borylation of heteroaromatic C-H bonds is more controlled by electronic effects. Detailed information on these selectivities and mechanistic information on the origins of this regioselectivity will be described in this critical review (95 references).     

Are C-H groups significant hydrogen bonding sites in anion receptors? Benzene complexes with Cl-, NO3-, and ClO4-

Theoretical calculations,examination of crystallographic data, and experimental binding energies suggest that even in the absence of electron-withdrawing substituents, simple arenes such as benzene form hydrogen bonds with anions that can exceed 50% of the strength of those formed by O-H and N-H groups. Thus, when present in a receptor, even moderately acidic C-H groups can significantly enhance the anion binding affinity and they should be considered as additional binding sites within the host cavity.     

Oxidation-resistant, sterically demanding phenanthrolines as supporting ligands for copper(I) nitrene transfer catalysts

New 1,10-phenanthroline ligands have been synthesized with C6F5- or 2,4,6-(CF3)3C6H2- groups in the 2- and 9-positions; a cationic copper(I) complex of the latter catalyses nitrene transfer to the C-H bonds of electron-rich arenes.     

Halobenzenes and Ir(I): kinetic C-H oxidative addition and thermodynamic C-Hal oxidative addition

A (PNP)Ir fragment undergoes facile, room-temperature oxidative addition of C-H bonds in arenes and haloarenes in preference to aromatic carbon-halogen bonds. This preference, however, is determined to be kinetic in nature. Oxidative addition of C-Cl and C-Br is preferred thermodynamically. The products of the C-Cl or C-Br oxidative addition are separated from the C-H oxidative addition products by a high activation barrier and are only accessible at >100 degrees C. Of the C-H oxidative addition products of chlorobenzene, the isomer with the o-ClC6H4 ligand has the lowest energy.     

Direct C-H arylation of (hetero)arenes with aryl iodides via rhodium catalysis

A new method for the catalytic C-H arylation of heteroarenes and arenes that manifests high activity paired with reasonably broad scope was developed. Under the catalytic influence of RhCl(CO){P[OCH(CF3)2]3}2 and Ag2CO3, the direct C-H arylation of heteroarenes/arenes with aryl/heteroaryl iodides took place to afford a range of biaryls in good to excellent yields with high regioselectivity. Thiophenes, furans, pyrroles, indoles, and alkoxybenzenes are applicable in this arylation protocol.     

Novel neutral guest recognition and interpenetrated complex formation from pillar[5]arenes

Simple alkyl-substituted pillar[5]arenes can form stable interpenetrated complexes with neutral bis(imidazole) guests utilizing multiple C-H···O(N) hydrogen bond and C-H···π interactions.     

Copper-catalyzed cyanation of arenes using benzyl nitrile as a cyanide anion surrogate

The copper-catalyzed cyanation of arenes using benzyl nitrile as a cyanide anion surrogate furnishes aromatic nitriles in moderate to good yields. The cascade process involves a copper-catalyzed aerobic C-H oxidation, a retro-cyanohydrination, and a copper-catalyzed aerobic oxidative C-H functionalization.     

Chelation-assisted rhodium hydride-catalyzed regioselective H/D exchange in arenes

A series of air stable rhodium(III) hydride complexes are synthesized via cyclometalation of functionalized arenes, and are active catalysts for regioselective H/D exchange in various arenes via chelation-assisted C-H activation in acetone-d₆.     

C-H arylation of unactivated arenes with aryl halides catalyzed by cobalt porphyrin

A general procedure for cobalt-catalyzed direct C-H arylation of unactivated arenes has been discovered. This method employs aryl halides as the direct coupling partners with arenes without using any Grignard-type reagents. This catalysis opens a new methodology for the preparation of symmetrical as well as unsymmetrical biaryls in a user-friendly approach.