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

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.