A carbon‐13 NMR study of some alkyl‐substituted indenes. An experimental study of the baker‐nathan order of hyperconjugation in the neutral ground state

Carbon‐13 NMR spectra of several alkyl‐substituted indenes have been assigned and analysed. For structures having an alkyl substituent in the C‐2 position an increased shielding was observed for the remote conjugated positions C‐4 and C‐6, an alternating trend similar to that found for the 2‐cycloaminoindenes. The hyperconjugative behaviour was slightly more pronounced for the methyl group compared with an isopropyl or tert‐butyl substituent, a fact which supports a model where CH σ–π interactions are more efficient than those involving CC bonds.