Substituent,solvent and steric effects on the electrochemical reduction of aziridinium salts

The polarographic behavior of a series of N,N-dimethyl-2-arylaziridinium fluorosulfonates was investigated as another example of ring-opening reactions of this important class of compounds. Half-wave potentials showed that in water reduction was facilitated by electron-donating groups. This unusual result was explained in terms of electron transfer to an aminocarbonium ion-like species. In methylene chloride, however, two one-electron waves were observed, with the first showing a normal substituent effect (? = 0.17 ± 0.03 V). This result indicated direct electron transfer to the aziridinium ion itself to form a radical species which was detected by cyclic voltammetry. Reasons for the change in mechanism are presented, and E_1/2 values for selected aziridinium salts illustrate the importance of steric effects on the reduction process.     

Interproton coupling constant variations in 3-membered ring heterocycles. Separation of lone pair and inductive effects

In order to separate inductive and lone pair effects on geminal and vicinal coupling constants in a stereochemically well-defined system, the ¹H NMR spectra of phenylcyclopropane (1), N-methyl-2-phenylaziridine (2), styrene oxide (3) and 1,1-dimethyl-2-phenylaziridinium fluorosulfonate (4) were compared. In D₂O the heterocyclic ring protons of 4 were split into an ABX pattern which gave J(cis) = 8.5, J(trans) = 7.4 and J(gem) = ?4.8 Hz (signs consistent with INDOR results). From the small solvent effects on J(vic) determined from 4-d₁, it was concluded that J(gem) is ?5.0 ± 1.0 Hz in methylene chloride. The absolute values for the coupling constants for 1 and 4 provide a measure of the inductive effect of the ring hetero group on J. Values of J(gem) for 2 and 3 deviated from those predicted on the basis of the above inductive effect, suggesting lone pair contributions to J(gem) of c. +5.5 Hz per lone pair. With this estimate it was possible to predict accurately the J(gem) values for 2-t-butyloxaziridine and 1-t-butyldiaziridine. The values of J(cis) and J(trans) for 2 and 3 likewise suggested a contribution of ?2.5 Hz to J(cis) and ?2.7 Hz to J(trans) per lone pair. The present results suggest that the major factors causing positive J(gem) values in epoxides and aziridines are increased s character to the C? H bonds and lone pair effects, while the so-called electronegativity effect actually operates in the opposite direction to decrease J(gem). Also, the unusually low J(vic) values of epoxides relative to cyclopropanes are now seen to be due more to negative lone pair contributions than to the electron withdrawing ability of oxygen.