Journal of the American Chemical Society p. 7583 - 7588 (1984)
Update date:2022-08-11
Topics:
Wu, Shune-Long
Tao, Yu-Tai
Saunders, William H.
Possible reasons are examined for the nonadditivity of the effect of successive β-methyl substitution on the rates of elimination reactions of quaternary ammonium salts.The temperature dependences of the deuterium isotope effects in E2 reactions of R1R2NMe2+ show that tunneling is not a significant source of nonadditivity.Neither is a change of gross mechanism or stereochemistry, for studies with C4H9CHDCHDNMe3+ and C4H9(CH3)CHCHDNMe3+ show the reactions to be very predominantly (>88percent) anti-E2 in both cases.Secondary tritium isotope effects with R1R2CHCHTNMe3+ increase, however in the order ethyl (1.108 +/- 0.002), propyl (1.150 +/- 0.015), isobutyl (1.216 +/- 0.012).This result suggests increasing rehybridization at the α-carbon in the transition state and therefore a shift toward less E1cB and more central-E2 character.Since methyl substitution is expected to favor a developing double bond, the much smaller rate-depressing effect of the second β-methyl is accounted for by such a shift in transition-state character.
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