Aromatic versus diradical character in the transition states of the Cope rearrangements of 1,5-hexadiene and its cyano derivatives.

Using geometries optimized at the (U)B3LYP/6-31G(d) level, ab initio modern valence bond calculations are used to quantify directly the relative importance of aromatic, diradical, and bisallyl contributions to the transition states of gas-phase Cope rearrangements of 1,5-hexadiene and various cyano derivatives. The main effect on the character of the transition state of substituting radical-stabilizing cyano groups is found to be indirect, via the geometry, with shorter interallyl separations favoring diradical character and larger ones favoring aromaticity. In the case of the parent Cope reaction, the weights of the aromatic and diradical components are comparable at the transition state. We find that bisallyl character only becomes important for transition states with rather large interallyl