Theoretical mechanistic insights into the polar hydrohalogenation of olefins.

Polar hydrohalogenation of olefins, also called electrophilic addition of hydrogen halides to olefins, is an essential, classical, and important organic reaction. Mechanistic and stereoselective insights into the polar hydrohalogenations of nine structurally different distinct olefins were obtained through theoretical investigation via density functional theory (DFT) calculations. The results indicate that, after the formation of π-complexes of hydrogen halides and olefins, all olefins can undergo bimolecular intimate ion-pair syn-addition processes. However, aliphatic olefins can also undergo a pentamolecular concerted anti-addition process through a cyclic proton transferring mechanism, with a fourteen-membered cyclic transition state composed of olefins, two molecules of hydrogen halides, and an acetic acid dimer for the proton transfer. Although unsymmetric aliphatic olefins exhibit an obvious electrostatic intimate ion-pair process in their anti-addition, the process is still concerted with significantly asynchronous characteristics due to a greater hyperconjugation effect. The activation energies of anti-additions are generally lower than those of the corresponding syn-additions, especially for hydrobromination. The relative reaction rate constants of the anti-additions are always larger than those of the corresponding syn-additions on the basis of dynamics treatment according to transition state theory. Thus, in the hydrohalogenations, aliphatic olefins always give anti-adducts as major products, while aromatic olefins produce syn-adducts as major products. The substituent-controlled mechanisms and stereoselectivities are summarized and rationalized. The current investigation provides comprehensive insights into the mechanism and stereoselectivity of hydrohalogenations of olefins.

Direct oxidation of N-ynylsulfonamides into N-sulfonyloxoacetamides with DMSO as a nucleophilic oxidant.

N-Arylethynylsulfonamides are oxidized into N-sulfonyl-2-aryloxoacetamides directly and efficiently with dimethyl sulfoxide (DMSO) as both an oxidant and solvent with microwave assistance. DFT calculations indicate that DMSO nucleophilically attacks the ethylic triple bond and transfers its oxygen atom to the triple bond to form zwitterionic anionic N-sulfonyliminiums to trigger the reaction. Then it nucleophilically attacks the generated iminium intermediates to accomplish the oxidation via the second oxygen atom transfer. The current method provides a straightforward and efficient strategy to transform various N-arylethynylsulfonamides into N-sulfonyl-2-aryloxoacetamides, sulfonyl oxoacetimides, without any other electrophilic activators or oxidants.

Universal thermoelectric effect of Dirac fermions in graphene.

We numerically study the thermoelectric transports of Dirac fermions in graphene in the presence of a strong magnetic field and disorder. We find that the thermoelectric transport coefficients demonstrate universal behavior depending on the ratio between the temperature and the width of the disorder-broadened Landau levels (LLs). The transverse thermoelectric conductivity alpha{xy} reaches a universal quantum value at the center of each LL in the high temperature regime, and it has a linear temperature dependence at low temperatures. The calculated Nernst signal has a peak at the central LL with heights of the order of k{B}/e, and changes sign near other LLs, while the thermopower has an opposite behavior, in good agreement with experimental data. The validity of the generalized Mott relation between the thermoelectric and electrical transport coefficients is verified in a wide range of temperatures.