In order to quantify the electrophilic reactivities of common Michael acceptors, we measured the kinetics of the reactions of monoacceptor-substituted ethylenes (HC═CH-Acc, 1) and styrenes (PhCH═CH-Acc, 2) with pyridinium ylides 3, sulfonium ylide 4, and sulfonyl-substituted chloromethyl anion 5. Substitution of the 57 measured second-order rate constants (log k) and the previously reported nucleophile-specific parameters N and s for 3-5 into the correlation log k = s(E + N) allowed us to calculate 15 new empirical electrophilicity parameters E for Michael acceptors 1 and 2. The use of the same parameters s, N, and E for these different types of reactions shows that all reactions proceed via a common rate-determining step, the nucleophilic attack of 3-5 at the Michael acceptors with formation of acyclic intermediates, which subsequently cyclize to give tetrahydroindolizines (stepwise 1,3-dipolar cycloadditions with 3) and cyclopropanes (with 4 and 5), respectively. The electrophilicity parameters E thus determined can be used to calculate the rates of the reactions of Michael acceptors 1 and 2 with any nucleophile of known N and s. DFT calculations were performed to confirm the suggested reaction mechanisms and to elucidate the origin of the electrophilic reactivities. While electrophilicities E correlate poorly with the LUMO energies and with Parr's electrophilicity index ω, good correlations were found between the experimentally observed electrophilic reactivities of 44 Michael acceptors and their calculated methyl anion affinities, particularly when solvation by dimethyl sulfoxide was taken into account by applying the SMD continuum solvation model. Because of the large structural variety of Michael acceptors considered for these correlations, which cover a reactivity range of 17 orders of magnitude, we consider the calculation of methyl anion affinities to be the method of choice for a rapid estimate of electrophilic reactivities.

Radical fluorination has been known for a long time, but synthetic applications were severely limited by the hazardous nature of the first generation of reagents such as F2 and the strongly electrophilic nature of the second generation of reagents such as N-fluorobenzenesulfonimide (NFSI) and Selecfluor®. Here, we report the preparation, use and properties of N-fluoro-N-arylsulfonamides (NFASs), a class of fluorinating reagents suitable for radical fluorination under mild conditions. Their N–F bond dissociation energies (BDE) are 30–45 kJ mol−1 lower than the N–F BDE of the reagents of the second generation. This favors clean radical fluorination processes over undesired side reactions. The utility of NFASs is demonstrated by a metal-free radical hydrofluorination of alkenes including an efficient remote C–H fluorination via a 1,5-hydrogen atom transfer. NFASs have the potential to become the reagents of choice in many radical fluorination processes.

The kinetics of epoxide formation by Darzens condensation of aliphatic ketones 1 with arylsulfonyl-substituted chloromethyl anions 2 (ArSOCHCl) have been determined photometrically in DMSO solution at 20 °C. The reactions proceed via nucleophilic attack of the carbanions at the carbonyl group to give intermediate halohydrin anions 4, which subsequently cyclize with formation of the oxiranes 3. Protonation of the reaction mixture obtained in THF solution at low temperature allowed the intermediates to be trapped and the corresponding halohydrins 4-H to be isolated. Crossover experiments, i.e., deprotonation of the halohydrins 4-H in the presence of a trapping reagent for the regenerated arylsulfonyl-substituted chloromethyl anions 2, provided the relative rates of backward ( k) and ring closure ( k) reactions of the intermediates. Combination of the kinetic data ( k) with the splitting ratio ( k/ k) gave the second-order rate constants k for the attack of the carbanions 2 at the ketones 1. These k values and the previously reported reactivity parameters N and s for the arylsulfonyl-substituted chloromethyl anions 2 allowed us to use the linear free energy relationship log k(20 °C) = s( N + E) for deriving the electrophilicity parameters E of the ketones 1 and thus predict potential nucleophilic reaction partners. Density functional theory calculations of the intrinsic reaction pathways showed that the reactions of the ketones 1 with the chloromethyl anions 2 yield two rotational isomers of the intermediate halohydrin anions 4, only one of which can cyclize while the other undergoes retroaddition because the barrier for rotation is higher than that for reversal to the reactants 1 and 2. The electrophilicity parameters E correlate moderately with the lowest unoccupied molecular orbital energies of the carbonyl groups, very poorly with Parr's electrophilicity indices, and best with the methyl anion affinities calculated for DMSO solution.

Protein−water interactions have widespread effects on protein structure and dynamics. As such, the function of many biomacromolecules can be directly related to the presence and exchange of water molecules. While the presence of structural water sites can be easily detected by Xray crystallography, the dynamics within functional water− protein network architectures is largely elusive. Here we use solid-state NMR relaxation dispersion measurements with a focus on those active-site residues in the enzyme human carbonic anhydrase II (hCAII) that constitute the evolutionarily conserved water pocket, key for CAs' enzymatic catalysis. Together with chemical shifts, peak broadening, and results of molecular dynamics (MD) and DFT shift calculations, the relaxation dispersion data suggest the presence of a widespread fast μs-time-scale dynamics in the pocket throughout the protein−water network. This process is abrogated in the presence of an inhibitor which partially disrupts the network. The time scale of the protein−water pocket motion coincides both with the estimated residence time of Zn-bound water/OH − in the pocket showing the longest lifetimes in earlier magnetic relaxation dispersion experiments as well as with the rate-limiting step of catalytic turnover. As such, the reorganization of the water pocket:enzyme architecture might constitute an element of importance for enzymatic activity of this and possibly other proteins.

Kinetics of the reactions of aryldiazomethanes (ArCHN2) with benzhydrylium ions (Ar2CH+) have been measured photometrically in dichloromethane. The resulting second-order rate constants correlate linearly with the electrophilicities E of the benzhydrylium ions which allowed us to use the correlation lg k = s N(N + E) (eq 1) for determining the nucleophile-specific parameters N and s N of the diazo compounds. UV–vis spectroscopy was analogously employed to measure the rates of the 1,3-dipolar cycloadditions of these aryldiazomethanes with acceptor-substituted ethylenes of known electrophilicities E. The measured rate constants for the reactions of the diazoalkanes with highly electrophilic Michael acceptors (E > −11, for example 2-benzylidene Meldrum’s acid or 1,1-bis­(phenylsulfonyl)­ethylene) agreed with those calculated by eq 1 from the one-bond nucleophilicities N and s N of the diazo compounds and the one-bond electrophilicities of the dipolarophiles, indicating that the incremental approach of eq 1 may also be applied to predict the rates of highly asynchronous cycloadditions. Weaker electrophiles, e.g., methyl acrylate, react faster than calculated from E, N, and s N, and the ratio of experimental to calculated rate constants was suggested to be a measure for the energy of concert ΔG ‡ concert = RT ln­(k 2 exptl/k 2 calcd). Quantum chemical calculations indicated that all products isolated from the reactions of the aryldiazomethanes with acceptor substituted ethylenes (Δ2-pyrazolines, cyclopropanes, and substituted ethylenes) arise from intermediate Δ1-pyrazolines, which are formed through concerted 1,3-dipolar cycloadditions with transition states, in which the C–N bond formation lags behind the C–C bond formation. The Gibbs activation energies for these cycloadditions calculated at the PCM­(UA0,CH2Cl2)/(U)­B3LYP-D3/6-31+G­(d,p) level of theory agree within 5 kJ mol–1 with the experimental numbers showing the suitability of the applied polarizable continuum model (PCM) for considering solvation.