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

Abstract

AbstractIn 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.