ADDITION-REACTIONS OF DIARYLCARBENIUM IONS TO 2-METHYL-1-PENTENE - KINETIC METHOD AND REACTION-MECHANISM

A kinetic method that allows the determination of reactivities of carbenium ions toward alkenes is described: Diarylmethyl chlorides (1) are completely ionized by BC13 in CH2C12 to give colored solutions of diarylcarbenium (2) tetrachloroborates, which show conductivity. Upon addition of the model alkene 3 (2-methyl-l-pentene) conductance and absorbance disappear due to the formation of the covalent and colorless adducts 5 (Ar2CH+-BCl4-+H2C=C(CH3)C3H7 Ar2CHCH2C(CH3)(C3H7)Cl+BC13). The second-order reactions are linear with respect to the concentration of carbenium ions and of the alkene. Free and paired ions exhibit identical reactivity. The attack of the carbenium ions 2 on the alkene 3 is usually rate-determining, but in the case of the highly stabilized 2-OCH3,OCH3, a small degree of retroaddition can be detected. Variation of the para substituents, X and Y, in the carbenium ions 2 alters AH* while AS* remains unaffected. Variation of solvent polarity has a very small effect on the addition rates (KCH3NO2/KCHCl3=5). Kinetic isotope effects (kH/kD « 0.8) and rate equilibrium relationships indicate a late transition state for the reaction of 2 with 3. In some cases, the rates of ionization of the diarylmethyl chlorides 1 can be measured. Solvolytic studies on the adducts 5 allow the derivation of the energy of the intermediates 4. Complete energy profiles for the multistep reaction sequence (1) are elaborated. © 1990, American Chemical Society. All rights reserved.