Borderline between E1cB and E2 mechanisms. Chlorine isotope effects in base-promoted elimination Reactions.
The chlorine leaving group isotope effect has been measured for the base-promoted elimination reaction of 1-(2-chloro-2-propyl)indene (1-Cl) in methanol at 30 degrees C: k(35)/k(37) = 1.0086 +/- 0.0007 with methoxide as the base and k(35)/k(37) = 1.0101 +/- 0.0001 with triethylamine (TEA) as the base. These very large chlorine isotope effects combined with large kinetic deuterium isotope effects of 7.1 and 8.4, respectively, are consistent not with the irreversible E1cB mechanism proposed previously (J. Am. Chem. Soc. 1977, 99, 7926) but with the E2 mechanism with transition states having large amounts of hydron transfer and very extensive cleavage of the bond to chlorine.
Jia
Zhi Sheng
ZS
Institute of Chemistry, University of Uppsala, P.O. Box 531, SE-751 21 Uppsala, Sweden.
Rudziński
Juliusz
J
Paneth
Piotr
P
Thibblin
Alf
A