Rapid C-H bond activation by a monocopper(III)-hydroxide complex

One-electron oxidation of the tetragonal Cu(II) complex [Bu(4)N][LCuOH] at -80 °C generated the reactive intermediate LCuOH, which was shown to be a Cu(III) complex on the basis of spectroscopy and theory (L = N,N'-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide). The complex LCuOH reacts with dihydroanthracene to yield anthracene and the Cu(II) complex LCu(OH(2)). Kinetic studies showed that the reaction occurs via H-atom abstraction via a second-order rate law at high rates (cf. k = 1.1(1) M(-1) s(-1) at -80 °C, ΔH(?) = 5.4(2) kcal mol(-1), ΔS(?) = -30(2) eu) and with very large kinetic isotope effects (cf. k(H)/k(D) = 44 at -70 °C). The findings suggest that a Cu(III)-OH moiety is a viable reactant in oxidation catalysis.     

Ru3(CO)12-catalyzed silylation of benzylic C-H bonds in arylpyridines and arylpyrazoles with hydrosilanes via C-H bond cleavage

Ruthenium-catalyzed silylation of sp3 C-H bonds at a benzylic position with hydrosilanes gave benzylsilanes. For this silylation reaction, Ru3(CO)12 complex showed high catalytic activity. This silylation proceeded at the methyl C-H bond selectively. For this silylation reaction, pyridyl and pyrazolyl groups, and the imino group in hydrazones, can function as a directing group. Several hydrosilanes involving triethyl-, dimethylphenyl-, tert-butyldimethyl-, and triphenylsilanes can be used as a silylating reagent. Coordination of an sp2 nitrogen atom to the ruthenium complex is important for achieving this silylation reaction.     

Intermolecular arene C-H activation by nickel(II)

Intermolecular arene C-H activation mediated by a divalent nickel species under extremely mild conditions is described. The reactions of either Ni(COD)2 with H[N(o-C6H4PR2)2] (H[R-PNP]; R = iPr, Cy) in benzene at room temperature or [R-PNP]NiCl with LiBHEt3 in THF at -35 degrees C produced the corresponding [R-PNP]NiH in high yield. Addition of 1 equiv of B(C6F5)3 to a benzene solution of [R-PNP]NiH at room temperature led to the formation of a mixture that contains [R-PNP]NiPh and [R-PNP]Ni(C6F5), both of which are proposed to evolve from zwitterionic [R-PNP]Ni(mu-H)B(C6F5)3. In contrast, the reaction of [R-PNP]NiH with AlMe3 in benzene at room temperature afforded exclusively the corresponding [R-PNP]NiPh. Similar results were also observed for intermolecular toluene and m-xylene C-H activation by [R-PNP]NiH. A parallel study involving [R-PNP]NiMe (R = Ph, iPr, Cy) on the reactivity of intermolecular arene activation reveals the significance of pi basicity of Ni(II) in these molecules. The remarkable reactivity of inexpensive Ni(II) species established in this study is attractive, particularly from an economic viewpoint, as compared to the current alternatives of 4d and 5d metals.     

Synthesis of a beta-diiminate iridium tetrahydride for arene C-H bond activation

The preparation of the iridium tetrahydride ((iPr)BDI)IrH(4)[BDI = ArNC(Me)CH(Me)CNAr, Ar = 2,6-(i)Pr(2)C(6)H(3)] and its activity in the catalytic C-H activation of arenes is described.     

A general method for copper-catalyzed arylation of arene C-H bonds

A general method for copper-catalyzed arylation of sp (2) C-H bonds with p K a's below 35 has been developed. The method employs aryl halide as the coupling partner, lithium alkoxide or K 3PO 4 base, and DMF, DMPU, or mixed DMF/xylenes solvent. A variety of electron-rich and electron-poor heterocycles such as azoles, caffeine, thiophenes, benzofuran, pyridine oxides, pyridazine, and pyrimidine can be arylated. Furthermore, electron-poor arenes possessing at least two electron-withdrawing groups on a benzene ring can also be arylated. Two arylcopper-phenanthroline complex intermediates were independently synthesized.     

An efficient Pd(II)-based catalyst system for carboxylation of aromatic C-H bond by addition of a phosphenium salt

Addition of a phosphenium dramatically improved the reaction yields in the carboxylation of arenes by formic acid catalyzed by Pd(II). Control experiments revealed that the majority of the phosphenium triflate was converted to a mixed anhydride of phosphonic acid and formic acid (7), which however did not substitute for the phosphenium to improve the reaction yield.     

Insertion of singlet chlorocarbenes across C-H bonds in alkanes: Evidence for two phase mechanism

Transition states for the insertion reactions of singlet mono and dichlorocarbenes (¹CHCl and ¹CCl₂) into C-H bonds of alkanes (methane, ethane, propane and n-butane) have been investigated at MP2 and DFT levels with 6–31g (d, p) basis set. The p _π of ¹CHCl and ¹CCl₂ may interact with alkane’s filled fragment orbital of either σ or π symmetry. So chlorocarbenes insertion reactions have been investigated for both (σ/π) approaches. The σ approach has been adjudicated to be the minimum energy path over the π approach both at the MP2 and DFT levels. Mulliken, NPA and ESP derived charge analyses have been carried out along the minimal energy reaction path using the IRC method for ¹CHCl and ¹CCl₂ insertions into the primary and secondary C-H bonds of propane. The occurrence of TSs either in the electrophilic or nucleophilic phase has been identified through NBO charge analyses in addition to the net charge flow from alkane to the carbene moiety. Dedicated to our senior colleague Dr V Sethuraman on the occasion of his retirement from collegiate service     

Ru(II)-catalyzed C-H bond activation for the synthesis of substituted isoquinolinium salts from benzaldehydes, amines, and alkynes

An efficient method for the synthesis of substituted isoquinolinium salts from benzaldehydes, amines, and alkynes via ruthenium-catalyzed C-H bond activation and annulation in one pot is described.     

Reversible formal insertion of CO2 into a remote C-H bond of a ligand in a Ru(II) complex at room temperature

Here we report a reversible formal insertion of CO(2) into a remote C-H bond of the diazafluorenide ligand (L(-)) in a Ru(II) complex which occurs at ambient temperature.     

Unexpected C-H activation of Ru(II)-dithiomaltol complexes upon oxidation

Thione-substituted derivatives of maltol are of interest in several applications of metal-based drugs. In order to investigate the effect of the oxygenation on such thione chelates, Ru complexes of 3-hydroxy-2-methyl-4-thiopyrone (thiomaltol or Htma) and 3-hydroxy-2-methyl-4H-thiopyran-4-thione (dithiomaltol or Httma), [Ru(bpy)2(tma)](+), 1, and [Ru(bpy) 2(ttma)] (+), 2, were synthesized as diamagnetic PF6(-) salts. Peroxidation of 2 unexpectedly generated products of C-H activation at its pendant methyl group; an air-stable aldehyde [Ru(bpy)2(ttma-aldehyde)](+), 4, was the major product. In addition, an intermediate oxidation product [Ru(bpy) 2(ttma-alcohol)](PF6), 3, was characterized. Both 3 and 4 are also formed by reaction of 2 with outersphere oxidants (e.g., Na2IrCl6) and by bulk electrolysis under anaerobic conditions. Similar oxidations of the analogous [Ru(bpy)2(ettma)](+), 2' , complex (3-hydroxy-2-ethyl-4H-thiopyran-4-thione; ethyl dithiomaltol or Hettma) formed the corresponding ketone, [Ru(bpy)2(ettma-ketone)](PF6), 4', by oxidation at the same position adjacent to the conjugated ring. The structures of the aldehyde 4 and starting materials 1 and 2 have been confirmed by X-ray crystallography, and all complexes have been characterized by UV-vis, (1)H NMR, and IR spectroscopies. Initial mechanistic investigations are discussed.     

Catalytic addition of C-H bonds to multiple bonds with the aid of ruthenium complexes

Ruthenium complexes, e.g., RuH2(CO)(PPh3)3, have been found to catalyze the direct addition of ortho carbon-hydrogen bonds of aromatic ketones to olefins and acetylenes with high efficiency and selectivity. The C-H/olefin coupling reaction is applicable to not only C-H bonds in aromatic ketones but also to those in a,b-enones and aro-matic esters. Catalytic addition of ortho carbon-hydrogen bonds of aromatic imines to olefins is found to be catalyzed by Ru3(CO)12.     

Rhodium(III)-catalyzed arene and alkene C-H bond functionalization leading to indoles and pyrroles

Recently, the rhodium(III)-complex [Cp*RhCl(2)](2) 1 has provided exciting opportunities for the efficient synthesis of aromatic heterocycles based on a rhodium-catalyzed C-H bond functionalization event. In the present report, the use of complexes 1 and its dicationic analogue [Cp*Rh(MeCN)(3)][SbF(6)](2) 2 have been employed in the formation of indoles via the oxidative annulation of acetanilides with internal alkynes. The optimized reaction conditions allow for molecular oxygen to be used as the terminal oxidant in this process, and the reaction may be carried out under mild temperatures (60 °C). These conditions have resulted in an expanded compatibility of the reaction to include a range of new internal alkynes bearing synthetically useful functional groups in moderate to excellent yields. The applicability of the method is exemplified in an efficient synthesis of paullone 3, a tetracyclic indole derivative with established biological activity. A mechanistic investigation of the reaction, employing deuterium labeling experiments and kinetic analysis, has provided insight into issues of reactivity for both coupling partners as well as aided in the development of conditions for improved regioselectivity with respect to meta-substituted acetanilides. This reaction class has also been extended to include the synthesis of pyrroles. Catalyst 2 efficiently couples substituted enamides with internal alkynes at room temperature to form trisubstituted pyrroles in good to excellent yields. The high functional group compatibility of this reaction enables the elaboration of the pyrrole products into a variety of differentially substituted pyrroles.     

Diastereoselective formation of metallamacrocyclic (arene)Ru(II) and CpRh(III) complexes

The reaction of [(arene)RuCl(2)](2) (arene = cymene, 1,3,5-C(6)H(3)Me(3)) and [CpRhCl(2)](2) half-sandwich complexes with tridentate heterocyclic ligands in the presence of base has been investigated. In all cases, the chloro-ligands were substituted to give metallacyclic products with ring sizes between 4 and 18 atoms. The cyclization occurs in a highly diastereoselective fashion with chiral recognition between the different metal fragments. The complexes were comprehensively characterized by elemental analysis, NMR spectroscopy, and single crystal X-ray crystallography. For 2-hydroxy-nicotinic acid and 2-amino-nicotinic acid, dinuclear structures were obtained (15-17) whereas for 2,3-dihydroxyquinoline, 2,3-dihydroxyquinoxaline, and 6-methyl-2,3-phenazinediol, trimeric assemblies were found (19-22), and for 4-imidazolecarboxylic acid, a tetrameric assembly (18) was found.     

Activation of C-H / H-H bonds by rhodium(II) porphyrin bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(II) bimetalloradical with H(2), and with the methyl C-H bonds for a series of substrates CH(3)R (R = H, CH(3), OH, C(6)H(5)) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M*...X...Y*...M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C-H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (DeltaG++) decreases regularly as DeltaG degrees becomes more favorable. An absolute Rh-H bond dissociation enthalpy of 61.1 +/- 0.4 kcal mol(-1) is directly determined, and the derived Rh-CH(2)R BDE values increase regularly with the increase in the C-H BDE.     

Copper(II)-catalysed addition of O-H bonds to norbornene

Cu(OTf)2 is an inexpensive, air- and moisture-stable catalyst for the O-H addition of aliphatic and aromatic acids and alcohols to norbornene.     

Pt(II) as a proton shuttle during C-H bond activation in the Shilov process

The C-H activation in Shilov's system on cis- and trans-PtCl(2)(H(2)O)(CH(4)) was investigated by ab initio molecular dynamics in water. Simulations revealed an easy C-H bond cleavage forming a transient 5-coordinated species Pt(H)Cl(2)(H(2)O)(CH(3)) that spontaneously releases a proton to the bulk solution.