Bond Cleavage Process Research Articles

Nickel- and Photoredox-Catalyzed Cross-Coupling Reactions of Aryl Halides with 4-Alkyl-1,4-dihydropyridines as Formal Nucleophilic Alkylation Reagents.

A combination of nickel and photoredox catalysts promoted novel cross-coupling reactions of aryl halides with 4-alkyl-1,4-dihydropyridines. 4-Alkyl-1,4-dihydropyridines act as formal nucleophilic alkylation reagents through a photoredox-catalyzed carbon-carbon (C-C) bond-cleavage process. The present strategy provides an alternative to classical carbon-centered nucleophiles, such as organometallic reagents.

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

The experimental results on detection and identification of intermediate radicals and molecular products from gas-phase pyrolysis of cinnamyl alcohol (CnA), the simplest non-phenolic lignin model compound, over the temperature range of 400–800°C are reported. The low temperature matrix isolation – electron paramagnetic resonance (LTMI-EPR) experiments along with the theoretical calculations, provided evidences on the generation of the intermediate carbon and oxygen centered as well as oxygen-linked, conjugated radicals. A mechanistic analysis is performed based on density functional theory to explain formation of the major products from CnA pyrolysis; cinnamaldehyde, indene, styrene, benzaldehyde, 1-propynyl benzene, and 2-propenyl benzene. The evaluated bond dissociation patterns and unimolecular decomposition pathways involve dehydrogenation, dehydration, 1,3-sigmatropic H-migration, 1,2-hydrogen shift, CO and CC bond cleavage processes.

ChemInform Abstract: Copper‐Catalyzed and Iodide‐Promoted Aerobic C—C Bond Cleavage/C—N Bond Formation Toward the Synthesis of Amides.

A copper-catalyzed and iodide-promoted aerobic C–C bond cleavage/C–N bond formation reaction between ketone and simple amine was developed toward the synthesis of amides, which are useful synthetic intermediates in organic synthesis and important skeletons of biologically active molecules. Notably, the reaction conditions are very mild, and preliminary mechanistic investigations indicate that molecular oxygen might be involved in the C–C bond cleavage process.

The Nature of the Co-C Bond Cleavage Processes in Methylcob(II)Alamin and Adenosylcob(III)Alamin

The Nature of the Co-C Bond Cleavage Processes in Methylcob(II)Alamin and Adenosylcob(III)Alamin

A DFT Study on the Conversion of Aryl Iodides to Alkyl Iodides: Reductive Elimination of R-I from Alkylpalladium Iodide Complexes with Accessible β-Hydrogens.

DFT calculations have been performed on the palladium-catalyzed carboiodination reaction. The reaction involves oxidative addition, alkyne insertion, C-N bond cleavage, and reductive elimination. For the alkylpalladium iodide intermediate, LiOtBu stabilizes the intermediate in non-polar solvents, thus promoting reductive elimination and preventing β-hydride elimination. The C-N bond cleavage process was explored and the computations show that PPh3 is not bound to the Pd center during this step. Experimentally, it was demonstrated that LiOtBu is not necessary for the oxidative addition, alkyne insertion, or C-N bond cleavage steps, lending support to the conclusions from the DFT calculations. The turnover-limiting steps were found to be C-N bond cleavage and reductive elimination, whereas oxidative addition, alkyne insertion, and formation of the indole ring provide the driving force for the reaction.

Dissociative electron transfer in polychlorinated aromatics. Reduction potentials from convolution analysis and quantum chemical calculations.

Formal potentials of the first reduction leading to dechlorination in dimethylformamide were obtained from convolution analysis of voltammetric data and confirmed by quantum chemical calculations for a series of polychlorinated benzenes: hexachlorobenzene (-2.02 V vs. Fc(+)/Fc), pentachloroanisole (-2.14 V), and 2,4-dichlorophenoxy- and 2,4,5-trichlorophenoxyacetic acids (-2.35 V and -2.34 V, respectively). The key parameters required to calculate the reduction potential, electron affinity and/or C-Cl bond dissociation energy, were computed at both DFT-D and CCSD(T)-F12 levels. Comparison of the obtained gas-phase energies and redox potentials with experiment enabled us to verify the relative energetics and the performance of various implicit solvent models. Good agreement with the experiment was achieved for redox potentials computed at the DFT-D level, but only for the stepwise mechanism owing to the error compensation. For the concerted electron transfer/C-Cl bond cleavage process, the application of a high level coupled cluster method is required. Quantum chemical calculations have also demonstrated the significant role of the π*ring and σ*C-Cl orbital mixing. It brings about the stabilisation of the non-planar, C2v-symmetric C6Cl6˙(-) radical anion, explains the experimentally observed low energy barrier and the transfer coefficient close to 0.5 for C6Cl5OCH3 in an electron transfer process followed by immediate C-Cl bond cleavage in solution, and an increase in the probability of dechlorination of di- and trichlorophenoxyacetic acids due to substantial population of the vibrational excited states corresponding to the out-of-plane C-Cl bending at ambient temperatures.

ChemInform Abstract: Metal‐Organic Cooperative Catalysis in C—C Bond Activation

AbstractReview: 40 refs.

Intramolecular Halogen Transfer via Halonium Ion Intermediates in the Gas Phase

The fragmentation of halogen-substituted protonated amines and quaternary ammonium ions (R(1)R(2)R(3)N(+)CH2(CH2)nX, where X = F, Cl, Br, I, n = 1, 2, 3, 4) was studied by electrospray ionization tandem mass spectrometry. A characteristic fragment ion (R(1)R(2)R(3)N(+)X) resulting from halogen transfer was observed in collision-induced dissociation. A new mechanism for the intramolecular halogen transfer was proposed that involves a reactive intermediate, [amine/halonium ion]. A potential energy surface scan using DFT calculation for CH2-N bond cleavage process of protonated 2-bromo-N,N-dimethylethanamine supports the formation of this intermediate. The bromonium ion intermediate-involved halogen transfer mechanism is supported by an examination of the ion/molecule reaction between isolated ethylenebromonium ion and triethylamine, which generates the N-bromo-N,N,N-triethylammonium cation. For other halogens, Cl and I also can be involved in similar intramolecular halogen transfer, but F cannot be involved. With the elongation of the carbon chain between the halogen (bromine as a representative example) and amine, the migration ability of halogen decreases. When the carbon chain contains two or three CH2 units (n = 1, 2), formal bromine cation transfer can take place, and the transfer is easier when n = 1. When the carbon chain contains four or five CH2 units (n = 3, 4), formal bromine cation transfer does not occur, probably because the five- and six-membered cyclic bromonium ions are very stable and do not donate the bromine to the amine.

ChemInform Abstract: Ionic Liquid‐Stabilized Nanoparticles as Catalysts for the Conversion of Biomass

AbstractReview: 102 refs.

Enhanced Squaraine Rotaxane Endoperoxide Chemiluminescence in Acidic Alcohols

Squaraine rotaxane endoperoxides (SREPs) are storable chemiluminescent compounds that undergo a clean cycloreversion reaction that releases singlet oxygen and emits near-infrared light when warmed to body temperature. This study examined the effect of solvent on SREP chemiluminescence intensity and found that acidic alcohols, such as 2,2,2-trifluoroethanol, α-(trifluoromethyl)benzyl alcohol, and 1,1,1,3,3,3-hexafluoroisopropanol, greatly increased chemiluminescence. In contrast, aprotic solvents, such as trifluoroethylmethyl ether, had no effect. The interlocked rotaxane structure was necessary as no chemiluminescence was observed when the experiments were conducted with samples containing a mixture of the two non-interlocked components (squaraine thread and macrocycle endoperoxide). Spectroscopic analyses of the enhanced SREP chemiluminescent reactions showed a mixture of products. In addition to the expected squaraine rotaxane product caused by cycloreversion of the endoperoxide, a diol derivative was isolated. The results are consistent with an endoperoxide O–O bond cleavage process that is promoted by the hydrogen bonding solvent and produces light emission from a squaraine excited state.

C–O Bond Activation by a Tantalum‐Bonded Pincer Ligand – Ligand Modification Effects on the Selectivity of C–H Bond Cleavage Processes

AbstractThe tantalum complexes [TaCp*Me{(OCH2)(OCHCO2)py‐κ4‐N,O,O,O}]2 (2, Cp* = η5‐C5Me5, py = pyridyl) and [TaCp*Me{(OCH2)(OCHCONAr)py‐κ4‐N,O,O,O}]2 (3, Ar = p‐tolyl) have been synthesized by the reactions of [TaCp*Me{(OCH2)(OCH)py‐κ3‐N,O,O}] (1) with CO2 and p‐tolyl isocyanate, respectively. The reaction of 3 with an excess of water renders the corresponding tantalum hydroxide [TaCp*Me(OH){(OCH2)(OCHCONHAr)py‐κ3‐N,O,O}] (4). Compound 1 reacts with pyridyl triflate (pyOTf) to afford the new pyridyl‐functionalized pincer derivative [TaCp*Me{(OCH2)(OCHpy)py‐κ3‐N,O,O}(OTf)] (5). Compound 5 reacts with water to yield the aqua adduct [TaCp*Me(OH2){(OCH2)(OCHpy)py‐κ3‐N,O,O}]OTf (6). The reaction of 5 with LiN(SiMe3)2 gives rise to the new tantalum compound [TaCp*Me{(OCH2)(OCpy)py‐κ3‐N,O,O}] (7). Complex 7 reacts with pyOTf to give the dipyridyl‐functionalized pincer derivative [TaCp*Me{(OCH2)(OCpy2)py‐κ3‐N,O,O}(OTf)] (8). The molecular structures of complexes 2, 4 and 6 have been established by single‐crystal X‐ray diffraction studies.

Tunable arylative cyclization of 1,6-enynes triggered by rhodium(III)-catalyzed C-H activation.

Two tunable arylative cyclizations of cyclohexadienone-containing 1,6-enynes are reported via rhodium(III)-catalyzed C-H activation of O-substituted N-hydroxybenzamides. The use of different O substituents, i.e. O-Piv and O-Me, on the directing group allows the formation of either tetracyclic isoquinolones through an Ⓝ-Michael addition process or hydrobenzofurans through a Ⓒ-Michael addition process. Mechanistic investigations of these two cascade reactions clearly indicated that the C-H bond cleavage process was involved in the turnover-limiting step. Furthermore, the cyclization products could be subjected to various transformations for elaborating the pharmaceutically and synthetically valuable potential. This is the first example of a rhodium(III)-catalyzed arylative cyclization reaction of 1,6-enynes, and the results extend the application realm of Cp*Rh(III)-catalyzed C-H activation cascade reactions.

Sc(OTf)3-catalyzed transfer diazenylation of 1,3-dicarbonyls with triazenes via N-N bond cleavage.

A new and efficient method for diazenylation reactions was developed with a Sc(OTf)3-catalyzed nitrogen-nitrogen bond cleavage process with triazenes. The transfer diazenylation reactions accommodate a diverse range of active methylene substrates including simple ketones to give aliphatic azo compounds that are of significant potential as azo prodrugs in high yields under mild conditions.

Intramolecular C-H/O-H bond cleavage with water and alcohol using a phosphine-free ruthenium carbene NCN pincer complex.

Transition metal complexes that exhibit metal-ligand cooperative reactivity could be suitable candidates for applications in water splitting. Ideally, the ligands around the metal should not contain oxidizable donor atoms, such as phosphines. With this goal in mind, we report new phosphine-free ruthenium NCN pincer complexes with a central N-heterocyclic carbene donor and methylpyridyl N-donors. Reaction with base generates a neutral, dearomatized alkoxo-amido complex, which has been structurally and spectroscopically characterized. The tert-butoxide ligand facilitates regioselective, intramolecular proton transfer through a CH/OH bond cleavage process occurring at room temperature. Kinetic and thermodynamic data have been obtained by VT NMR experiments; DFT calculations support the observed behavior. Isolation and structural characterization of a doubly dearomatized phosphine complex also strongly supports our mechanistic proposal. The alkoxo-amido complex reacts with water to form a dearomatized ruthenium hydroxide complex, a first step towards phosphine-free metal-ligand cooperative water splitting.

Understanding intrinsically irreversible, non-Nernstian, two-electron redox processes: a combined experimental and computational study of the electrochemical activation of platinum(IV) antitumor prodrugs.

Six-coordinate Pt(IV)-complexes are prominent prodrug candidates for the treatment of various cancers where, upon two-electron reduction and loss of two axial ligands, they form more familiar, pharmacologically active four-coordinate Pt(II) drugs. A series of electrochemical experiments coupled with extensive density functional calculations has been employed to elucidate the mechanism for the two-electron reduction of Pt(IV)(NH3)2Cl2L2 to Pt(II)(NH3)2Cl2 (L = CH3COO(-), 1; L = CHCl2COO(-), 2; L = Cl(-), 3). A reliable estimate for the normal reduction potential E(o) is derived for the electrochemically irreversible Pt(IV) reduction and is compared directly to the quantum chemically calculated reduction potentials. The process of electron transfer and Pt-L bond cleavage is found to occur in a stepwise fashion, suggesting that a metastable six-coordinate Pt(III) intermediate is formed upon addition of a single electron, and the loss of both axial ligands is associated with the second electron transfer. The quantum chemically calculated reduction potentials are in excellent agreement with experimentally determined values that are notably more positive than peak potentials reported previously for 1-3.

Ceric Ammonium Nitrate (CAN) Catalyzed Modification of Ketones via Two C–C Bond Cleavages with the Retention of the Oxo-Group

A simple ceric ammonium nitrate (CAN) catalyzed functionalization of ketones through double C-C bond cleavage strategy has been disclosed. This reaction provides a mild, practical method toward carbamoyl azides, which are versatile intermediates and building blocks in organic synthesis. Based on relevant mechanistic studies, a unique and plausible C-C bond and N-O bond cleavage process is proposed, where the oxyamination intermediate plays an important role in this reaction.

Base‐Catalyzed NN Bond Cleavage of Hydrazones: Synthesis of α‐Amino Ketones

An efficient Cs2CO3-promoted synthesis of α-amino ketones using hydrazines, aldehydes, and α-haloketones as starting materials through a cascade condensation/nucleophilic substitution/N-N bond cleavage route is developed. The carbonyl group plays a key role in this novel N-N bond cleavage process.

Octakis(tert-butoxo)dicerium(IV) [Ce2(OtBu)8]: Synthesis, Characterization, Decomposition, and Reactivity

An advanced synthesis for the homometallic derivative [Ce2(O(t)Bu)8] (1) starting from [Ce(O(t)Bu)2{N(SiMe3)2}2] was developed. Structural characterization of a cerium(IV) complex and its decomposition products confirmed the coexistence of both ether elimination and Ce-O bond cleavage processes, which lead to the formation of [Ce3O(O(t)Bu)10] and [Ce3(O(t)Bu)11] (2) derivatives, respectively. Variable-temperature NMR spectroscopy under strict exclusion of moisture enabled insight into the decomposition processes in noncoordinating solvents and at elevated temperature. In addition, structural analysis of the heterovalent 2 and of two new complexes of the general formula [Ce2(O(t)Bu)8(L)] [L = HO(t)Bu (3), OCPh2 (4)] is described.

Mechanism of N–H Bond Cleavage of Aniline by a Dearomatized PNP-Pincer Type Phosphaalkene Complex of Iridium(I)

Detailed mechanistic investigations using kinetic and theoretical methods have been conducted for deprotonative N–H bond cleavage of p-YC6H4NH2 (Y = H, MeO, Me, Cl, Br, NO2) by [K(18-crown-6)][Ir(Cl)(PPEP*)] (1a) bearing a dearomatized PNP-pincer type phosphaalkene ligand (PPEP*) to afford [Ir(NHC6H4Y)(PPEP)] (2) with an aromatized ligand (PPEP). While 1a is in equilibrium with [K(18-crown-6)]Cl (3) and [Ir(PPEP*)] (4) in solution, the N–H bond cleavage proceeds via association of 1a with aniline, where the coordination of aniline to iridium is insignificant; instead, aniline is associated with PPEP* by hydrogen bonding. In contrast, the N–H bond cleavage of ammonia proceeds via the pentacoordinate intermediate [Ir(Cl)(NH3)(PPEP*)]. The difference between the N–H bond cleavage processes of aniline and ammonia is examined by DFT calculations.

ChemInform Abstract: Iron‐Catalyzed Three‐Component Reaction: Multiple C—C Bond Cleavages and Reorganizations.

AbstractA novel iron‐catalyzed C‐C bond cleavage and reorganization process is developed consisting of a three component reaction of crotonate derived phosphorous ylides (I), methyl diazoacetate (II) and a variety of α‐bromoacetophenones (III).