The molecular structure of allenes and ketenes. Part 16. Phenyl carbon chemical shifts of γ-substituted phenylallenes as a probe for the transmissions of substituent effects across the allenic system

Abstract

¹³ C Chemical shifts of γ-substituted phenylallenes PhHCCCHX have been measured. In order to investigate how substituent effects are transmitted across the allenic system emphasis is on the ¹³C chemical shifts of the phenyl site. Analysis of the data using the dual substituent parameter approach reveals that inductive effects (polarizations) originating from the substituents' dipole moments are predominant. For the phenyl ipso-atom, however, delocalization effects are non-negligible. In order to account for the observations several types of ‘hyperconjugative interactions’ are discussed. Semiempirical CNDO/S molecular orbital calculations are used to inquire further into the electron density distributions of phenylallenes and into the modes of the transmissions of substituent effects across the allenic system. Electron density distributions of phenylallenes turn out to be significantly affected through hyperconjugation.