A DFT study on the mechanisms for the cycloaddition reactions between 1-aza-2-azoniaallene cations and acetylenes

Diels–Alder Reactions of α-Cyano α,β-Unsaturated Ketones with 2-Methyl-1,3-Butadiene: DFT Study of Mechanism, Reactivity and Regioselectivity

The mechanism, reactivity and regioselectivity of Diels–Alder cycloaddition reactions of 2-methyl-1,3-butadiene with four dienophiles, ( E)-2-benzylidene-3-oxobutanenitrile, ( E)-2-(furan-2-ylmethylene)-3-oxopentanenitrile, ( E)-2-benzylidene-4,4-dimethyl-3-oxopentanenitrile, and (E)-2-benzoyl-3-(furan-2-yl) acrylonitrile, have been studied by means of several theoretical approaches. The regioselectivity of these reactions has been confirmed by their activation energies, the Gazquez–Mendez rule, Houk's rule and global descriptors of the products. All the studied reactions have normal electron demand character and a polar nature. All calculations were done at the DFT-B3LYP/6-31G(d) level of theory.

More on diphosphadithiatetrazocines and the importance of being bonded

The cross-ring sulfur-sulfur bond for seven R'RP(NSN)2PRR' molecules 1a (R = R' = Me), 1b (R = R' = Ph), 1c (R = R' = Et), 1d (R = Cl, R' = CCl3), 1e (R = R'= Cl), 1f (R = R' = F), and 1g (R = R' = H) has been scrutinized by a topology analysis for a bond descriptor based on the kinetic energy density, supported by a fragment-based bond energy analysis. Contrary to a regular disulfide bond, the cross-ring connection is only a secondary electron-sharing bond, about half as strong as a common S-S linkage. The regular disulfide bond itself is best described as a charge-shift bond. These analyses are based on results obtained from B3PW91/def2-TZVP density functional calculations.