In order to perform a systematic study of the effect of substituting a -CH2 group by a -NH or an -O on the ring strain of a three-membered ring and on its intrinsic basicity[1], we have studied at G2 level the following neutral compounds: ciclopropane, oxirane, dioxirane, trioxirane, oxaziridine, dioxaziridine, oxadiaziridine, aziridine, diaziridine and triaziridine. as well as all the possible lithiated and protonated derivatives.
Ring strain energies obtained from the corresponding homodesmotic reactions are quite constant along the series. However the analysis of the laplacian of the charge density reveals that the delocalization of charge inside the ring decreases as the degree of substitution increases.
Regarding the basicities of three membered rings we have found that a systematic substitution of the -CH2 groups of cyclopropane by -O implies a large decrease of the protonation energies. More importantly this decrease is not additive. In contrast when the - CH2 are substituted by -NH groups, the decrease observed in the intrinsic basicity of the system is smaller and practically additive. When the reference acid is Li+ the basicities do not follow regular trends because substitution of -CH2 groups by -NH or -O opens the possibility of yielding cyclic or pyramidal structures which present enhanced stabilities.
This work has been partially supported by the D.G.I.C.Y.T Project No. PB93-0289-C02-01.