The reaction of grignard reagents with 1,3-dioxolanes. Influence of solvation; kinetics and mechanism

The reaction of 2-ethyl-1,3-dioxolane (D) with ethylmagnesium bromide, solvated by one equivalent of a basic solvent (E) (diethyl ether, diisopropyl ether and perdeutero-tetrahydrofuran) was studied in benzene at 40.5°. Ring-cleavage of the dioxolane occurs with first-order kinetics in two different coordination complexes, i.e. EtMgBr·E·D and EtMgBr·2D. When the Grignard reagent is in excess, an additional second-order reaction takes place between the complex EtMgBr·E·D and the monosolvated Grignard reagent. The rate constants are related to the basicities of the different ethers, E, used. The results confirm that the reaction proceeds via a transition state with oxocarbonium ion character. In agreement with this concept the reaction rates decrease with increasing basicity of the complexing ethers as the Lewis acidity of the attacking Grignard reagent is lowered.