Analyzing mechanisms in Co(i) redox catalysis using a pattern recognition platform

Redox catalysis has been broadly utilized in electrochemical synthesis due to its kinetic advantages over direct electrolysis. The appropriate choice of redox mediator can avoid electrode passivation and overpotential, which strongly inhibit the efficient activation of substrates in electrolysis. Despite the benefits brought by redox catalysis, establishing the precise nature of substrate activation remains challenging. Herein, we determine that a Co(i) complex bearing two N,N,N-tridentate ligands acts as a competent redox catalyst for the reduction of benzyl bromide substrates. Kinetic studies combining electroanalytical techniques with multivariable linear-regression analysis were conducted, disclosing an outer-sphere electron-transfer mechanism, which occurs in concert with C–Br bond cleavage. Furthermore, we apply a pattern recognition platform to distinguish between mechanisms in the activation of benzyl bromides, found to be dependent on the ligation state of the cobalt(i) center and ligand used.

Through kinetic studies combining electroanalytical techniques with multivariable linear-regression (MLR) analysis, a pattern recognition platform is established to determine the electron-transfer mechanism (inner-sphere or outer-sphere) of an electrochemical reduction of benzyl bromides, mediated by different cobalt complexes.

Regioselective Mono- and Dialkylation of [6,6]-open C60 (CF2 ): Synthetic and Kinetic Aspects

Alkylation of homofullerene [6,6]-C₆₀ (CF₂ )^2- dianion with the set of alkyl halides, RX, was established to demonstrate an effect of RX nature on the conversion, product composition, and regioselectivity. The respective C₆₀ (CF₂ )RH, C₆₀ (CF₂ )R₂ and C₆₀ (CF₂ )R'R'' compounds were obtained in the reaction with sterically unhindered RX, isolated by HPLC and unequivocally characterized. The kinetic studies evidenced S_N 2 mechanism for both alkylation steps, yielding mono- and dialkylated C₆₀ (CF₂ ), respectively, and indicated the negative charge localization at the bridgehead carbon atoms as well as a steric hindrance of the CF₂ moiety likely to be a key factors for the S_N 2 reaction mechanism and observed regioselectivity. The significant difference in the rate constants of the first and the second steps is attributed to the different activation barriers predicted by DFT calculations which makes possible to develop synthetic methods for the regioselective preparation of monoalkylated C₆₀ (CF₂ )RH and heterodialkylated C₆₀ (CF₂ )R'R'' derivatives.

Nature of Salt Effects and Mechanism of Covalent Bond Heterolysis

Data on the influence of neutral salts on the rates of unimolecular heterolyses of organic substrates, obtained mainly by the verdazyl method, are summarized here. It is assumed that heterolysis takes place with consecutive formation of four ion pairs: contact (CIP), cavity-separated (CSIP), one solvent moleculeseparated (SIP) and solvent-separated (SSIP). [Formula: see text]

In the limiting step, the CIP interacts with a solvent cavity and the CSIP is formed, which converts quickly into the SIP and subsequently to the SSIP, which also quickly gives the reaction products. In the transition state, bonds between the molecules solvating the CIP are broken. In the absence of salt, the return from external ion pairs does not have much importance. The verdazyl indicator quickly and quantitatively reacts with the SSIP.

The normal salt effect takes place due to the action of salt on the covalent substrate, which catalyses CIP formation. The special salt effect is caused by the association of the salt with the CIP, which prolongs the lifetime of the intermediate and increases the probability of its contact with a solvent cavity.

The negative special salt effect is caused by association of the salt with the SIP or SSIP, which prolongs the lifetime of the intermediates and increases the probability of their contact with a solvent cavity to return to the covalent substrate. When the salt reacts with the SIP, the salt effect does not depend on the concentration and nature of verdazyl, but such a dependence takes place when the salt reacts with the SSIP.

The site of the action of the salt is determined by the Hard and Soft Acids and Bases (HSAB) principle.

Predicting reduction rates of energetic nitroaromatic compounds using calculated one-electron reduction potentials

The evaluation of new energetic nitroaromatic compounds (NACs) for use in green munitions formulations requires models that can predict their environmental fate. Previously invoked linear free energy relationships (LFER) relating the log of the rate constant for this reaction (log(k)) and one-electron reduction potentials for the NAC (E1NAC) normalized to 0.059 V have been re-evaluated and compared to a new analysis using a (nonlinear) free-energy relationship (FER) based on the Marcus theory of outer-sphere electron transfer. For most reductants, the results are inconsistent with simple rate limitation by an initial, outer-sphere electron transfer, suggesting that the linear correlation between log(k) and E1NAC is best regarded as an empirical model. This correlation was used to calibrate a new quantitative structure-activity relationship (QSAR) using previously reported values of log(k) for nonenergetic NAC reduction by Fe(II) porphyrin and newly reported values of E1NAC determined using density functional theory at the M06-2X/6-311++G(2d,2p) level with the COSMO solvation model. The QSAR was then validated for energetic NACs using newly measured kinetic data for 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (2,4-DNT), and 2,4-dinitroanisole (DNAN). The data show close agreement with the QSAR, supporting its applicability to other energetic NACs.

Free radicals in vicarious C-amination reactions of 1-methyl-4-nitropyrazole

The primary radical anions of substrate in the reactions of vicarious nucleophilic substitution of hydrogen in 1-methyl-4-nitropyrazole with 1,1,1-trimethylhydrazinium iodide and 4-amino-1,2,4-triazole in t-BuOK-DMSO were observed and identified by EPR monitoring. The probable mechanism of the substrate radical anions formation is discussed.