Reactions of nucleophilic cyclopropanes: Kinetics and mechanisms

The kinetics of cleavage of phenylcyclopropanone acetals, 2-phenyl-4,7-dioxaspiro (2.4) heptane (3) and 1,1-dimethoxy-2-phenyl cyclopropane (2) in aqueous sulfuric acid (57-77 wt %) have been investigated. The observed acidity dependence (X), enthalpy of activation ($\Delta$H$\sp{\ddagger}$ = 17.6 Kcal/mol), entropy of activation)$\Delta$S$\sp{\ddagger}$ = $-10.4$ eu) and deuterium solvent isotope effect (k$\sb{\rm H\sb3 O}\sp{+}$/k$\sb{\rm D\sb3 O}\sp{+}$ = 2.6) for the cleavage of 3 are consistent with a A-S$\sb{\rm E}$2 reaction mechanism which involves rate-determining protonation of the cyclopropane ring. Cyclopropanone acetal 2 was found to cleave by two competing mechanisms. An A-1 reaction predominates at high acidity as evidenced by the activation parameters ($\Delta$H$\sp{\ddagger}$ = 25.0 Kcal/mol, $\Delta$S$\sp{\ddagger}$ = 11.5 eu) and the deuterium solvent isotope effect (k$\sb{\rm H\sb3 O}\sp{+}$/k$\sb{\rm D\sb3 O}\sp{+}$ = 0.66). A mechanistic changeover to an A-S$\sb{\rm E}$2 reaction was observed at the low end of the acid range studied. A physical-organic study of the (3+2) cycloadditions of acetals 2 and 3 and their sulfur analogues, 1,1-dithiomethyl-2-phenylcyclopropane (4) and 2-phenyl-4,7-dithiospiro(2.4) heptane (5) with tetracyanoethylene (TCNE) was undertaken with a view to elucidate the reaction mechanism. The kinetic results indicate that the substrates exhibit a wide range in reactivity towards TCNE, spanning a factor of 10$\sp6$. The rates of cycloaddition of 2 and 3 with TCNE were found insensitive to solvent polarity. Intermolecular trapping experiments did not provide evidence for the intermediacy of zwitterions in the cycloadditions of 2, 3 and 4 with TCNE. However, the reaction of 3 with 1-chloro-2,2-di-cyanoethylene resulted in an open-chain adduct, which may be viewed as arising from the efficient intramolecular trapping of a 1,5-zwitterion by cholride ion. Acetal 3 was also found to participate readily in a (3+2) cycloaddition with 4-phenyl-1,2,4-triazoline-3,5-dione. The solvent effect on the rate of this cycloaddition is similar to that observed for the cycloaddition of 3 with TCNE implying a common mechanism. A comparison of the experimentally obtained free energy of activation, $\Delta$G$\sp{\ddagger}$, with the calculated free energy change, ($\Delta$G$\sb{\rm O}$), for the formation of radical-ion pair intermediates from reactants indicate that, for several of these cycloadditions, a single electron-transfer step may well be operative