Carbon‐13 nuclear magnetic resonance spectroscopy. Substituent‐induced chemical shift effects on cyclopropyl carbons of 4‐substituted cyclopropylbenzenes

Abstract

Abstract Carbon‐13 chemical shifts of the cyclopropyl carbons of eleven 4‐substituted cyclopropylbenzenes have been measured under conditions effectively corresponding to infinite dilution in DCCI 3 . The substituent‐induced chemical shifts (SCS) of both the α and β carbons of the cyclopropane ring were found to be downfield with electron‐attracting groups and upfield for electron‐donating groups. The trends for the β carbons correspond to those observed for the β carbons of 4‐substituted phenylethenes, while the trends of the α carbons are similar to those found for the α carbons of 4‐substituted isopropyl benzenes. The results for the β carbons can be rationalized by postulating a substantial contribution from a hyperconjugative resonance effect involving the σ system of the benzene ring (and its 4‐substituent) and the C‐α—Cβ bonds of the cyclopropane ring. The effects on the α carbons are in accord with a very reasonable smaller inductive polarization of the C‐αC‐β bonds than encountered for the carbons of corresponding ethenyl‐ or ethynylbenzenes.