Transition Regions in the Cope Rearrangement of 1,5-Hexadiene and Its Cyano Derivatives

Substituent effects on the mechanism of the Cope rearrangement were studied by computing C_2h (C_s) cuts through the potential energy surface (PES) for the reaction of 1,5-hexadiene and its di-, tri-, and tetracyano derivatives at the (U)B3LYP/6-31G* and (U)BPW91/6-31G* levels. The stabilization of substituted structures along the cuts is discussed in terms of the energies of isodesmic formation from allyl radicals and acetonitrile molecules. Cyano groups at C1, C3, and C5 provide a nearly additive stabilization of each point along the C_s cut even though their influence on the geometry is competitive. Evaluation of the density of effectively unpaired electrons at various geometries indicates that the radical character of a transition state (TS) is not altered by radical stabilizing substituents as such but depends solely on the interallylic bond length. Although the UB3LYP diyl intermediate for the parent compound is plausible when compared to the lowest triplet PES, neither MRPT2 at the UB3LYP geometries nor UDFT with the original Becke exchange (B) predict any intermediates. Similarly, for each of the three substituted compounds, the most believable (U)BPW91 model gives either one TS or one intermediate. Derivative 1,5-hexadienes with cyano groups at C1, C3, C5 or at C1, C3, C4, C6 rearrange by the same mechanism as the parent, i.e., through a mostly aromatic TS. However, 2,5-dicyano-1,5-hexadiene is found to react through an intermediate. It is suggested that B- rather than B3-type functionals should be used for sigmatropic rearrangements to avoid spurious stationary points.