The reaction mechanisms of [3 + 2]
cycloaddition (32CA) between
two N-substituted phenylnitrones (NPPN and NtBPN) and styrene (STY)
in the presence of a chromium tricarbonyl complex (Cr(CO)3) have been studied within the framework of molecular electron density
theory at the MPWB1K/6-311G(d,p) level. Activation energy analysis
reveals that these 32CA reactions take place with high activation
barriers due to their non-polar character and with the exo/ortho approach as a favorable reaction path. The
activation energy of TS1 series (NPPN) is about 5–6 kcal/mol
lower than that of the TS2 series (NtBPN), due to the steric hindrance
of tert-butyl at the TS2 structures. Coordination
of chromium tricarbonyl to each reactant to form the corresponding
complex slightly increases the nucleophilic and electrophilic character
of NPPN:Cr, NtBPN:Cr, and STY:Cr, which can be considered a strong nucleophile and electrophile able
to participate in a polar 32CA reaction. This polar character in the
presence of the chromium tricarbonyl complex was verified by the higher
(up to 0.20e) values of global electron density transfer for Cr(CO)3-coordinated transition states. Electron localization function
analysis along the exo/ortho reaction
path reveals the non-concerted mechanism in the formation of a new
bond with the initial appearance of a C–C single bond followed
by the O–C one.