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

Abstract

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.

Effect of the leaving group solvation on solvolytic behavior of benzhydryl derivatives

“…[4] Generally a lower slope of Hammett plot in comparison to some reference S N 1 reaction has been rationalized in different ways: as an increase in the S N 2 character, as an earlier transition state due to relief of steric strain of tertiary derivatives, or as a neighboring group participation in the rate determining step. [7][8][9][10][11][12][13] Essentially, in all cases effects come from less positive charge on the reaction center in the transition state. According to above consideration, the same phenomena can account for lower s f parameters.…”

Solvolysis of 1,1-Dimethyl-4-Alkenyl Chlorides: Evidence for π-Participation

“…A remote double bond which is appropriately placed toward the reaction center participates in solvolytic reactions . The assistance can result in reduction of the secondary α- and β-deuterium kinetic isotope effects (KIEs), in enhancement of the solvolytic reactivity, in formation of cyclic products, or in lowering the slope of the Hammett ρσ plot, etc. There are many results published supporting allyl and homoallyl participation in solvolysis.…”