Trapping-mediated dissociative chemisorption of cycloalkanes on Ru(001) and Ir(111): influence of ring strain and molecular geometry on the activation of C-C and C-H bonds.

We have measured the initial probabilities of dissociative chemisorption of perhydrido and perdeutero cycloalkane isotopomers on the hexagonally close-packed Ru(001) and Ir(111) single-crystalline surfaces for surface temperatures between 250 and 1100 K. Kinetic parameters (activation barrier and preexponential factor) describing the initial, rate-limiting C-H or C-C bond cleavage reactions were quantified for each cycloalkane isotopomer on each surface. Determination of the dominant initial reaction mechanism as either initial C-C or C-H bond cleavage was judged by the presence or absence of a kinetic isotope effect between the activation barriers for each cycloalkane isotopomer pair, and also by comparison with other relevant alkane activation barriers. On the Ir(111) surface, the dissociative chemisorption of cyclobutane, cyclopentane, and cyclohexane occurs via two different reaction pathways: initial C-C bond cleavage dominates on Ir(111) at high temperature (T > approximately 600 K), while at low temperature (T < approximately 400 K), initial C-H bond cleavage dominates. On the Ru(001) surface, dissociative chemisorption of cyclopentane occurs via initial C-C bond cleavage over the entire temperature range studied, whereas dissociative chemisorption of both cyclohexane and cyclooctane occurs via initial C-H bond cleavage. Comparison of the cycloalkane C-C bond activation barriers measured here with those reported previously in the literature qualitatively suggests that the difference in ring-strain energies between the initial state and the transition state for ring-opening C-C bond cleavage effectively lowers or raises the activation barrier for dissociative chemisorption via C-C bond cleavage, depending on whether the transition state is less or more strained than the initial state. Moreover, steric arguments and metal-carbon bond strength arguments have been evoked to explain the observed trend of decreasing C-H bond activation barrier with decreasing cycloalkane ring size.     

Palladium-catalyzed synthesis of carbazoles from N-(2-halophenyl)-2,6-diisopropylanilines via C-C cleavage

The synthesis of 1-isopropyl-substituted carbazoles by the palladium-catalyzed dealkylative cyclization of N-(2-halophenyl)-2,6-diisopropylanilines is described. The reaction involves intramolecular C-C bond formation, coupled with the cleavage of a C-X bond and a C-C bond, and is proposed to proceed through the formation of a dearomatized intermediate.     

Palladium-catalyzed transformation of cyclobutanone O-benzoyloximes to nitriles via C-C bond cleavage

Palladium-catalyzed transformation of cyclobutanone O-benzoyloximes to a variety of nitriles is described. The reaction may proceed via two important steps, that is, (i) oxidative addition of the N-O bond of oximes to Pd(0) to give a cyclobutylideneaminopalladium(II) species and (ii) beta-carbon elimination of this species to afford a reactive alkylpalladium species. The kind of products is very dependent on the nature of substituents on the cyclobutane ring. The direction of the C-C bond cleavage is controlled by the kind of ligand employed. The sequential reaction composed of the C-C bond cleavage and the subsequent intra- and intermolecular C-C bond formations via the corresponding alkylpalladium species is also demonstrated. For example, an oxime having an alkynyl moiety at a suitable position reacts with a variety of alkenes to afford nitriles bearing dienylcyclopentane moiety in moderate to good yields.     

Cleavage of carbon-carbon bonds of diphenylacetylene and its derivatives via photolysis of Pt complexes: tuning the C-C bond formation energy toward selective C-C bond activation

Carbon-carbon bond activation of diphenylacetylene and several substituted derivatives has been achieved via photolysis and studied. Pt0-acetylene complexes with eta2-coordination of the alkyne, along with the corresponding PtII C-C activated photolysis products, have been synthesized and characterized, including X-ray crystal structural analysis. While the C-C cleavage reaction occurs readily under photochemical conditions, thermal activation of the C-C bonds or formation of PtII complexes was not observed. However, the reverse reaction, C-C reductive coupling (PtII --> Pt0), did occur under thermal conditions, allowing the determination of the energy barriers for C-C bond formation from the different PtII complexes. For the reaction (dtbpe)Pt(-Ph)(-CCPh) (2) --> (dtbpe)Pt(eta2-PhCCPh) (1), DeltaG was 32.03(3) kcal/mol. In comparison, the energy barrier for the C-C bond formation in an electron-deficient system, that is, (dtbpe)Pt(C6F5)(CCC6F5) (6) --> (dtbpe)Pt(eta2-bis(pentafluorophenyl)acetylene) (5), was found to be 47.30 kcal/mol. The energy barrier for C-C bond formation was able to be tuned by electronically modifying the substrate with electron-withdrawing or electron-donating groups. Upon cleavage of the C-C bond in (dtbpe)Pt(eta2-(p-fluorophenyl-p-tolylacetylene) (9), both (dtbpe)Pt(p-fluorophenyl)(p-tolylacetylide) (10) and (dtbpe)Pt(p-tolyl)(p-fluorophenylacetylide) (11) were obtained. Kinetic studies of the reverse reaction confirmed that 10 was more stable toward the reductive coupling [the term "reductive coupling" is defined as the formation of (dtbpe)Pt(eta2-acetylene) complex from the PtII complex] than 11 by 1.22 kcal/mol, under the assumption that the transition-state energies are the same for the two pathways. The product ratio for 10 and 11 was 55:45, showing that the electron-deficient C-C bond is only slightly preferentially cleaved.     

Transition metal-catalyzed carbon-carbon bond activation

This tutorial review deals with recent developments in the activation of C-C bonds in organic molecules that have been catalyzed by transition metal complexes. Many chemists have devised a variety of strategies for C-C bond activation and significant progress has been made in this field over the past few decades. However, there remain only a few examples of the catalytic activation of C-C bonds, in spite of the potential use in organic synthesis, and most of the previously published reviews have dwelt mainly on the stoichiometric reactions. Consequently, this review will focus mainly on the catalytic reaction of C-C bond cleavage by homogeneous transition metal catalysts. The contents include cleavage of C-C bonds in strained and unstrained molecules, and cleavage of multiple C-C bonds such as C[triple bond]C triple bonds in alkynes. Multiple bond metathesis and heterogeneous systems are beyond the scope of this review, though they are also fascinating areas of C-C bond activation. In this review, the strategies and tactics for C-C bond activation will be explained.     

Carbon-carbon bond cleavage of diynes through the hydroamination with transition metal catalysts

The C-C bond cleavage of terminal and internal diynes takes place readily in the presence of catalytic amounts of Ru3(CO)12 or Pd(NO3)2 and of 2-aminophenol, giving the corresponding benzoxazoles and ketones in good to high yields. There are two different modes of the bond cleavage: (a) an alkyne C-C triple bond is cleaved, and (b) the C-C single bond between the two alkyne groups is cleaved.     

A facile synthesis of indolo[3,2,1-jk]carbazoles via palladium-catalyzed intramolecular cyclization

A new efficient synthesis of indolo[3,2,1-jk]carbazoles by the palladium-catalyzed cyclization of N-(2-bromoaryl)carbazoles is described. The reaction involves intramolecular C-C bond formation, coupled with the cleavage of a C-X bond and a C-H bond on carbazole ring. Substitutions on N-aryl core with either electron-donating or electron-withdrawing groups are introduced, and different reaction factors for cyclization are evaluated.     

Photochemistry of 2-(1-naphthyl)-2H-azirines in matrixes and in solutions: wavelength-dependent C-C and C-N bond cleavage of the azirine ring

The photochemistry of 3-methyl-2-(1-naphthyl)-2H-azirine (1a) was investigated by the direct observation of reactive intermediates in matrixes at 10 K and by the characterization of reaction products in solutions. As already reported, the photolysis of the azirine 1a with the short-wavelength light (>300 nm) caused the C-C bond cleavage of the 2H-azirine ring to produce the nitrile ylide 2. However, the products derived from the C-N bond cleavage were exclusively obtained in the irradiation of 1a with the long-wavelength light (366 nm) both in matrixes and in solutions. When 1a was irradiated in the presence of O(2) with the long-wavelength light, acetonitrile oxide (6) was produced through the capture of the biradical 4 generated by the C-N bond cleavage of 1a with O(2). An introduction of a nitro group into the naphthyl ring of 1a resulted in an acceleration of the decomposition in the long-wavelength irradiation and an extension of the wavelength region where the products derived from the C-N bond cleavage were selectively obtained. On the basis of molecular orbital calculations with the INDO/S method, the reason for the wavelength-dependent selective C-C and C-N bond cleavage of the azirine ring of 1a is discussed.     

Cu(Ⅱ)-catalyzed domino reaction of 2-halobenzamide and arylmethanamine to construct 2-aryl quinazolinone

A novel method for achieving a copper(II)-catalyzed domino reaction for the construction of 2-aryl quinazolinones,without the addition of any ligand and additive,has been developed.The domino reaction achieved N-(a-substituted)benzylation,benzylic C–H amidation,and C–C(or C–H) bond cleavage.     

Oxidative fragmentation of oxiranes to nitriles with hypervalent iodine(V) reagents in aqueous ammonia

Oxiranes undergo oxidative fragmentation when treated with hypervalent iodine(V) reagents particularly o-iodoxybenzoic acid in aqueous ammonia to give nitriles. The reaction goes via the formation of 1, 2-amino alcohols as intermediates followed by C-C bond cleavage.     

Ir(I)-catalyzed enantioselective secondary sp3 C-H bond activation of 2-(alkylamino)pyridines with alkenes

A cationic Ir(I)-tolBINAP complex catalyzed an enantioselective C-C bond formation initiated by secondary sp(3) C-H bond cleavage adjacent to a nitrogen atom. The reaction of 2-(alkylamino)pyridines with various alkenes gave chiral amines in good yields with high enantiomeric excesses.     

Ni-catalyzed, ZnCl(2)-assisted domino coupling of enones, alkynes, and alkenes

A Ni(0)/ZnCl(2) system effectively promotes the coupling of enones and alkene-tethered alkynes. In the reaction with 1,6-enynes, the oxidative cyclization of Ni(0) species on enones across the alkyne part followed by ZnCl(2)-promoted cleavage generates alkenylnickel intermediates. Subsequent migratory insertion of the tethered alkene occurs with 5-exo-cyclization. When the resulting sigma-alkylnickel intermediates have beta-hydrogen atoms, the reaction terminates by beta-hydrogen elimination to provide cyclopentane derivatives. On the other hand, a sigma-alkylnickel intermediate that does not have beta-hydrogen atoms undergoes the insertion of a second alkene unit to cause a domino effect via a three-fold C-C bond formation process with and without the cleavage of one C-C bond.     

Enzyme catalyzed hydroxymethylation of aromatic aldehydes with formaldehyde. Synthesis of hydroxyacetophenones and (S)-benzoins

Benzaldehyde lyase from the Pseudomonas Fluorescens catalyzed reaction of aromatic aldehydes with formaldehyde providing 2-hydroxy-1-arylethan-1-one in high yields via an acyloin linkage. Kinetic resolution of rac-benzoins with formaldehyde providing (S)-benzoins and 2-hydroxy-1-arylethan-1-one via C-C bond cleavage and a bond formation reaction.     

Sequential coupling/desilylation-coupling/cyclization in a single pot under Pd/C-Cu catalysis: synthesis of 2-(hetero)aryl indoles

A new one-pot synthesis of 2-(hetero)aryl indoles via sequential C-C coupling followed by C-Si bond cleavage and a subsequent tandem C-C/C-N bond forming reaction is described. A variety of functionalized indole derivatives were prepared by conducting this four step reaction under Pd/C-Cu catalysis. The methodology involved coupling of (trimethylsilyl)acetylene with iodoarenes in the presence of 10% Pd/C-CuI-PPh(3) and triethylamine in MeOH, followed by treating the reaction mixture with K(2)CO(3) in aqueous MeOH, and finally coupling with o-iodoanilides. The single crystal X-ray data of a synthesized indole derivative is presented. Application of the methodology, in vitro pharmacological properties of the synthesized compound, along with a docking study is described.     

Catalytic C-C bond cleavage and C-Si bond formation in the reaction of RCN with Et3SiH promoted by an iron complex

Catalytic C-C bond cleavage of acetonitrile and C-Si bond formation have been attained in the photoreaction of MeCN with Et3SiH in the presence of an iron complex, Cp(CO)2FeMe. This catalytic system can be applied for arylnitrile C-C bond cleavage.     

Novel families of three-component reversible redox cycles involving cage deformation via intramolecular redox reaction: tetrathiolate-bridged dinuclear molybda- and tungstacarboranes

The synthesis and structural analysis of two novel families of three-component reversible redox cycles [(C(2)B(9)H(11))M(mu-SPh)(2)](2)(n)PPN(n) (M = Mo, n = 2-, 2; 1-, 3; 0, 4; and M = W, n = 2-, 6; 1-, 7; 0, 8), where the cleavage and re-formation of the carborane cage C-C bond is observed during the redox reaction, are reported. Electronic saturation of the metal center (18e center) and the lack of bulky substituents on the carborane cage suggest that the deformed carborane cages in 2.PPN(2), 6.PPN(2), and 7.PPN invoke a new kind of deformed cage ("semicloso" framework). The XPS results show that the unprecedented competition for electron density between the metal center and the carborane cage is involved in the cleavage and formation of the carborane C-C bond.     

Direct observation of the oxidative addition of the aryl carbon-oxygen bond to a ruthenium complex and consideration of the relative reactivity between aryl carbon-oxygen and aryl carbon-hydrogen bonds

When RuH2(CO)(PPh3)3 was reacted with 2,2-dimethyl-1-(2-p-tolylphenyl)propan-1-one (2), the ruthenium-aryloxy complex 3 was obtained in 76% yield. The structure of this complex was determined from 1H and 31P NMR and X-ray data. Complex 3 showed the catalytic activity for the coupling of 2 with the phenylboronate 4. The 1H and 31P NMR studies of the reaction of Ru(CO)(PPh3)3 with o-aryloxy pivalophenone revealed that the C-H bond cleavage is a kinetically favorable process but the C-O bond cleavage is a thermodynamic one. The reaction of 2'-methoxyacetophenone with vinylsilane and organoboronate resulted in chemoselective C-C bond formation.     

Skeletal reorganization of enynes catalyzed by InCl3

[reaction: see text] The skeletal reorganization of enynes is achieved by the presence of InCl(3) as the catalyst. The reaction of enynes having a terminal acetylenic moiety proceeds in a stereospecific manner to give 1-vinylcycloalkenes. The reaction of enynes containing an alkyl group on the acetylenic terminal carbon resulted in a new type of skeletal reorganization to give 1-allylcycloalkenes, formation of which involves a double cleavage of the C-C double bond and the triple bond.     

Organocatalytic asymmetric direct alkylation of alpha-diazoester via C-H bond cleavage

A new variant of phosphoric acid-catalyzed C-C bond forming reaction, direct alkylation of alpha-diazoester, via C-H bond cleavage is presented. The resulting products, beta-amino-alpha-diazoesters, are highly functionalized and useful synthetic precursors for various types of beta-amino acids.     

Application of Rh(I)-catalyzed C-H bond activation to the ring opening of 2-cycloalkenones in the presence of amines

[reaction: see text]. Herein described is the application of the Rh(I)-catalyzed C-H bond activation to the ring-opening of 2-cycloalkenones in the presence of cyclohexylamine. This reaction includes the C-C double bond cleavage of 2-cycloalkenones through the conjugate addition of cyclohexylamine followed by the retro-Mannich-type fragmentation. The resulting ring-opened intermediates subsequently underwent either chelation-assisted hydroacylation to afford a ring-opened dicarbonyl compound or beta-alkylation via a ring contraction.