(Z)- and (E)-3-Alkyldine-1,3-dihydroindol-2-ones : influence of configuration on the transmission of the inductive effect to the carbonyl group

A series of (Z-) and (E-)-3-alkylidene-1,3-dihydroindol-2-ones has been prepared and their i.r. spectra in the carbonyl region recorded. Good correlations were obtained between the carbonyl stretching frequency of the E-isomers and the polar parameter σ*, indicating that the inductive effect of the alkyl group is transmitted to the carbonyl group and that the variation in frequency in the series is a function of this effect only. The Z-isomers do not give a linear correlation and it is suggested that this is due to the steric interactions of the alkyl group with the oxygen of the carbonyl group which twists the CC–CO system to a certain extent with consequent variation in conjugation.