Effect of Cations and Solvents on the SN2 and SN2' Product Distribution in the Reaction of 4-Bromo-3,5,5-trimethylcyclohex-2-enone with Pivalate Salts. 2. Evidence for a Concerted SN2' Reaction on a Covalently Bonded Substrate

Article abstract of DOI:10.1021/jo00134a004

4-Bromoisophorone (1) undergoes nucleophilic displacement with pivalate salts at a faster rate in dipolar aprotic solvents than in protic solvents.In protic solvents only the S_N2' product is obtained, whereas in dipolar aprotic solvents, mixtures of S_N2' and S_N2 products are obtained with the ratio depending on the counterion and the solvent.In protic solvents the order of relative rates of MOPiv based on disappearance of 1 was in the order of increasing size of the counterion, Et4N(+) > K(+) <*> Na(+) > Li(+).In dipolar aprotic solvents the order of relativerates, K(+) <*> Li(+) > Na(+) > Et4N(+), did not show the expected dependence on the size of the counterion.The variation of the product ratio is rationalized in terms of the effect of solvation and ion pairing on the reactivity of OPiv(-), and it appears that an increase in reactivity of OPiv(-) favors displacement at C₄ (S_N2 pathway) more than displacement at C₂ (S_N2' pathway).The results suggest S_N2 and S_N2' attacks occur on a covalently bonded substrate rather than on a preformed tight ion pair.