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B3LYP/6-31G* calculations have been performed on the chair Cope rearrangements of a wide variety of 1,5-hexadienes, substituted with cyano or with vinyl groups. In agreement with experimental data from the study of phenyl substituent effects, cyano and vinyl groups at C(2) and C(5) are found to provide cooperative lowering of the activation enthalpy, as are substituents at C(1), C(3), C(4), and C(6). In contrast, the stabilization of the transition structure by substituents at C(2) and C(4) or at C(1), C(3), and C(5) is predicted to be competitive, rather than cooperative. These findings are consistent with what Doering has termed a chameleonic transition state for the Cope rearrangement, one in which the relative importance of the cyclohexane-1,4-diyl and bis-allyl radical resonance contributors can be altered by substituents, depending on the carbons to which the substituents are attached. The computed bond lengths in the transition structures and the calculated and experimentally observed kinetic isoto...

The potential energy surfaces for the thermal reactions of bicyclo[3.2.0]hept-2-ene and norbornene have been explored with density functional theory at the Becke3LYP/6-31G* level. Both concerted and diradical pathways for the retro-Diels−Alder reaction of norbornene have been examined, and the activation parameters and 13C primary kinetic isotope effects predicted for the concerted pathway are in excellent agreement with experimental data. The concerted mechanism is favored over the lowest energy stepwise diradical route by 12.4 kcal/mol. For the orbital symmetry-allowed suprafacial-inversion (si) pathway of the [1,3] sigmatropic rearrangement of bicyclo[3.2.0]hept-2-ene to form norbornene, a mechanism involving a transition state which leads to a broad diradical plateau on the potential energy surface is predicted. Implications of these surfaces, which differ substantially from those obtained by semiempirical calculations, are also discussed.

The mechanisms of the degenerate [5,5] sigmatropic rearrangements of 5,5a,10,10a-tetrahydroheptalene and (Z,Z)-1,3,7,9-decatetraene were explored with restricted and unrestricted Becke3LYP/6-31G* hybrid HF-DFT calculations. The restricted DFT wavefunctions for the synchronous, concerted transition structures for these formally Woodward−Hoffmann allowed 10-electron rearrangements are unstable with respect to unrestricted wavefunctions. A stepwise diradical mechanism is predicted for both reactions. The 9 kcal/mol preference for the [5s,5s] mechanism over the [5a,5a] pathway in the rearrangement of decatetraene results primarily from the geometric distortions that the system must adopt in the [5a,5a] transition structure. The geometric and electronic characteristics of the [5,5] and [3,3] (Cope) rearrangements are compared.

The (2+2) cycloadditions of SO3 to ethene, propene, and 2-methylpropene were investigated with Hartree−Fock, MP2, QCISD(T), and hybrid Hartree−Fock/density functional theory (HF-DFT) methods. Exper...

Experimental activation parameters for the intramolecular Diels−Alder reactions of 1,3,8-nonatriene, 1,3,9-decatriene, and 1,3,10-undecatriene have been measured, and the Monte Carlo-Jumping between Wells/Stochastic Dynamics [MC(JBW)/SD] method, which gives relative free energies of activation, was tested as a means to predict stereoselectivities. The predictions are compared to experimental results, and to predictions from quantum and molecular mechanics methods.