A computational study into the mechanism and selectivities of the reaction of 1, 1-diaryl-2-isopropylidene-3-methylenecyclopropane (DIMCP) with C, N-diarylnitrone
摘要
The site- and regioselectivities of the [3 + 2] cycloaddition (32CA) reactions of C, N-diarylnitrone [B2] with 1, 1-diaryl-2-isopropylidene-3-methylenecyclopropane (DIMCP) [B1] and subsequent rearrangement have been studied using unrestricted density functional theory (UDFT) at the UB3LYP/6-311G (d, p) level of theory. 1, 1-Diaryl-2-isopropylidene-3-methylenecyclopropane possesses two exocyclic olefinic bonds (reactive centers): one unsubstituted and the other dimethyl-substituted. The preferred pathway with the lowest activation barrier occurs by the addition of B2 across the unsubstituted exocyclic bond of B1 to afford two regioisomeric products P4A and intermediate INT1A, where the generated INT1A proceeds through rearrangement to afford the experimentally observed products P2A and P3A. The rearrangement process was triggered by the concerted homolytic cleavage of the isoxazolidine N–O bond and the cyclopropane C–C (Ar1)2 bond. Electron-donating groups (EDGs) at the para position of Ar1 and Ar2 substituted on B1 and B2 increase the activation barrier, while electron-withdrawing groups (EWGs) decrease the activation barrier. EDGs and EWGs at the meta position of Ar1 and Ar2 decrease the activation barrier. The rate constant for the preferred pathway (formation of intermediate, INT1A) in the 32CA of B1 (Ar1 = Ph) and B2 (Ar2 = Ph) in benzene is 2.98 × 10–6 s−1, which is about 126 times faster than the non-competing channel, leading to the formation of P4A (2.35 × 10–8 s−1) through TS2A. Results from the analysis of the global reactivity indices at the ground state of the reactants show that B1 (Ar1 = Ph) acts as the nucleophile with chemical potential of −3.73 eV and B2 (Ar2 = Ph) acts as the electrophile with chemical potential of −3.83 eV. GEDT analysis infers that the 32CA reaction between B1 and B2 is one of a nonpolar character.