Secondary hydrogen isotope effects—IX

Abstract

Abstract Methyl and methyl-d 3 substituted cyclopropylcarbinyl and cyclobutyl derivatives (I–IV) were prepared and their solvolysis rates measured. It was found that methyl substitution on the ring in cyclopropylcarbinyl derivatives (I, III) produces small but significantly larger rate enhancements than an analogous phenyl substitution. Methyl deuteration has no significant effect on the solvolysis rates. Methyl substitution at the carbinyl position (IV) accelerates the solvolysis rate by a factor of 2. 10 3 to 4. 10 3 which is less than usually observed (10 4 –10 6 ). 1-Methyl-d 3 -cyclobutyl methanesulfonate (IIb-D) and 1-cyclopropylethyl-2-d 3 chloride (IVc-D) display upon solvolysis in 96% ethanol only one fourth (k H /k D = 1·09) resp. one half (k H /k D = 1·19) of the kinetic secondary β-deuterium isotope effect usually observed for a CD 3 -group (1·30–1·40). The results are discussed in terms of transition states and carbonium ion intermediates related to bicylobutonium ions (V). The reduced magnitude of the secondary deuterium isotope effects is correlated with small methyl- and phenyl-substitution rate effects. The hypothesis is advanced that secondary β-deuterium isotope effects might be a criterion for charge delocalization such as occurs in the formation of non-classical carbonium ions.