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Interproton coupling constant variations in 3‐membered ring heterocycles. Separation of lone pair and inductive effects
Author(s) -
Crist Delanson R.,
Borsetti Arnold P.,
Jordan Guy J.,
Hammer Charles F.
Publication year - 1980
Publication title -
organic magnetic resonance
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0030-4921
DOI - 10.1002/mrc.1270130110
Subject(s) - lone pair , coupling constant , chemistry , geminal , inductive effect , ring (chemistry) , vicinal , methylene , stereochemistry , crystallography , nuclear magnetic resonance , medicinal chemistry , physics , quantum mechanics , molecule , organic chemistry
In order to separate inductive and lone pair effects on geminal and vicinal coupling constants in a stereochemically well‐defined system, the 1 H NMR spectra of phenylcyclopropane (1), N ‐methyl‐2‐phenylaziridine (2), styrene oxide (3) and 1,1‐dimethyl‐2‐phenylaziridinium fluorosulfonate (4) were compared. In D 2 O 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‐ d 1 , 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 CH 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.