Quantitative investigation of the ozonolysis reaction. XIV. A simple carbonium ion stabilization approach to the ozone cleavage of unsymmetrical olefins

Abstract

The direction of ozone cleavage of selected unsymmetrical olefins R 1 R 2 C=CR 3 R 4 to give (a) R 1 R 2 CO + R 3 R 4 C + OO − and (b) R 1 R 2 C + OO − + R 3 R 4 CO has been measured, with particular emphasis on the mode of participation of alkyl substituents in distributing the reaction paths a and b. The interpretation, in terms of substituent effects in stabilizing potential zwitterionic carbo-cations in the transition states corresponding to paths a and b, considers (i) the "hyperconjugative stabilization" of positive sites, where the stabilizing abilities of the substituents are in the order CH 3 > C 2 H 5 > i-C 3 H 7 > tert-C 4 H 9 , and (ii) the participation of inductive effects. The latter appear to be particularly important when the substituents on the two C atoms of the primary ozonide act in the same direction in favoring its electrocyclic cleavage. In the ozonolysis of monosubstituted ethylenes the inductive effects are predominant, i.e. the R—C + HOO − formation is favored by the R groups in the order tert-C 4 H 9 > i-C 3 H 7 > C 2 H 5 > CH 3 , whereas in trans- and cis-1,2-disubstituted ethylenes "hyperconjugation" governs the direction of cleavage. The cleavage of selected 1,1-disubstituted, tri-, and tetrasubstituted ethylenes is also studied.