Hammett ρ+ Values as Kinetic Evidence for the Concerted Biomimetic Bicyclization Mechanism

Chlorides 1 (1-aryl-1-chloro-5,9-dimethyl-5,9-undecadienes) with various phenyl substituents were prepared (Y = p-OCH3, p-CH3, H, p-Br, and m-Br), and solvolysis rates were measured in 80% (v/v) aqueous ethanol and in 97% (wt/wt) aqueous 2,2,2-trifluoroethanol. The Hammett ρ+ values obtained are −1.5 and −1.8, respectively, indicating the concerted bicyclization. Comparison with ρ+ values that correspond to solvolysis of the benzylic squalene derivatives 3 (ρ+ = −1.8 and −1.6 in the same solvents) leads to the conclusion of the concerted bicyclization in 3 and may also suggest concerted bicyclization under biomimetic conditions in the natural precursor, 2,3-epoxysqualene, as well.