[3+2] cycloaddition reaction of N-methyl-C-(2-furyl) nitrone with
maleimide derivatives: mechanistic study and selectivity, solvent and
temperature effects
Abstract
The [3+2] cycloaddition (32CA) reactions of N-methyl-C-(2-furyl)
nitrone with maleimide derivatives have been studied in gas phase,
ethanol and acetonitrile solvents within the Molecular Electron Density
Theory (MEDT) framework at the B3LYP-D3/6-31G(d) level. Topological
analysis classifies the nitrone as a zwitter-ionic (zw-) three atom
component (TAC) associated with high energy barrier, while the high
global electron density transfer (GEDT) decreases the requested energy
cost for the rupture of the olefinic double bond of the maleimide
derivatives and consequently decreases the activation barrier.
Computations show that these 32CA reactions follow a one-step mechanism
with highly asynchronous bond formation, while no new covalent bonds are
formed at the transition states (TSs). Moreover, strongly repulsive
non-covalent interactions at the endo TSs are found, presumably leading
to their energetic destabilization relative to the exo ones. The
predicted exo-selectivity agrees well with the experimental findings.
Inclusion of solvent effects increases the activation energy, in
particular for the endo TSs. Also, the influence of temperature was
investigated at 289.15 K, 298.15 K and 393.15 K.