Transmission of electronic effects through cyclopropane. II.—Comparative modulation of 1H chemical shifts by aryloxy and aryl substituents in 2‐(donor)‐1‐(acceptor)cyclopropanes

Abstract

AbstractThe electronic interaction between donor and withdrawing substituents on vicinal carbons of cyclopropane, leading to the possible participation of the cyclopropane ring in the mesomeric transmission, was examined in the terms of the variable electron density release of the donor group consequential to the variable electron density demand by the withdrawer. Based on the large anisotropic effect caused by the donor on the 1e" and 4e′ orbitals of cyclopropane that surround the corresponding geminal cyclopropyl proton, the 1H NMR spectra of 26 2‐(donor)‐1‐(acceptor) three‐membered carbocyclic derivatives were analysed for differential effects of C‐1 substituents, with variable electron‐withdrawing capacity, on their cis protons located on C‐2 and C‐3. It was found that 2‐aryloxy groups exert a strong modulation effect on δH‐2, making this proton notably less sensitive than H‐3 to changes in anisotropy modifications caused by the C‐1 group. 2‐Aryl groups, in contrast, affect proton resonances only to a limited extent. This was interpreted as an increased electron release on the 1e" and 4e′ LUMOs of cyclopropane by the C‐2 donor as electron demand increases in the C‐1 environment, and not as a consequence of conjugation through the ring.