Intramolecular Diels-Alder reaction in enyne-allenes: A computational investigation and comparison with the Myers-Saito and Schmittel reactions

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 Myers-Saito 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 C₂-C₇ cyclization and more exothermic (δ_RE = -69.38 kcal/mol). For R1=CH₃ and t-Bu, the increasing barrier of the C₂-C ₇ cyclization is higher than that for the C₂-C₆ 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 R₁ = t-Bu would shift the C₂-C₇ cyclization to the [4 + 2] cycloaddition. On the other hand, for R₁ =Ph, NH₂, O^-, NO ₂, and CN substituents, the barrier of the C₂-C ₆ cyclization would be more diminished than the C₂-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.