Protonation of Some Cyclopropylarenes In the Gas-phase

Gas-phase basicities were detd. by ion cyclotron resonance for cyclopropylbenzenes (I; R = H; 2-, 3-, 4-Me); RC6H4CR1:CH2 (R = 2-, 3-, 4-Me; R1 = H, Me); 2-, 3-, and 4-methylbiphenyl; and cis- and trans-β-methylstyrene. I are protonated on the 3-membered ring. The cyclopropyl group in I (R = H) adopts a bisected conformation and thus tolerates the steric perturbation of a 2-Me group more easily than the double bond in α-methylstyrene, which adopts a coplanar conformation.