Dienophile Twisting and Substituent Effects Influence Reaction Rates of

e TS is reminiscent of that observed for the intermolecular cycloadditions involving cyclopentadiene. What distinguishes this intramolecular cycloaddition from typical cycloadditions is that nonparallel formation of two bonds forces the terminal end of the vinyl group to twist with respect to the internal terminus, destabilizing the # bond of the dienophile. The degree of twisting is quantified by measuring the cis dihedral angle containing either the exo substitutent ( ) x twist) or the endo substituent ( ) n twist). One consequence of the cycloaddition geometry is that the exo substituent must twist significantly more than the endo (61 versus 10). In contrast, no twisting of the dienophile occurs in the intermolecular TS between cyclopentadiene and ethylene ( ) ) 0). The reluctance to twist in the intramolecular case causes the exo substituent to be very close to a methylene H (see Figure 1). Results for the substituted vinylcyclohexadienes are compiled in Table 1. Entries 1-4 ar