The reaction mechanisms as well as substituted effect and solvent effect of the enyne-allenes are investigated by Density Functional Theory (DFT) method and compared with the Myers-Saito and Schmittel reactions. The MyersSaito reaction of non-substituted enyne-allenes is kinetically and thermodynamically favored as compared to the Schmittel reaction; while the concerted [4 + 2] cycloaddition is only 1.32 kcal/mol higher than the C(2)-C(7) cyclization and more exothermic (Delta(R)E = -69.38 kcallmol). For R(1) = CH(3) and t-Bu, the increasing barrier of the C(2)-C(7) cyclization is higher than that for the C(2)-C(6) cyclization because of the steric effect, so the increased barrier of the [4 + 2] cycloaddition is affected by such substituted electron-releasing group. Moreover, the strong steric effect of R, = t-Bu would shift the C(2)-C(7) cyclization to the [4 + 2] cycloaddition. On the other hand, for R(1) = Ph, NH(2), O(-), NO(2), and CN substituents, the barrier of the C(2)-C(6) cyclization would be more diminished than the C(2)-C(7) cyclization due to strong mesomeric effect; the reaction path of C2-C7 cyclization would also shift to the [4 + 2] cycloaddition. The solvation does not lead to significant changes in the potential-energy surface of the reaction except for the more polar surrounding solvent such as dimethyl sulfoxide (DMSO), or water. Copyright (C) 2009 John Wiley & Sons, Ltd. Supporting information may be found in the online version of this article.
