Carbon-13 substituent chemical shifts in the side-chain carbons of aromatic systems: the importance of π-polarization in determining chemical shifts

Abstract

¹³ C Substituent chemical shifts of the carbonyl sites in the side-chains of meta- and para-substituted benzenes of the type XC₆H₄COZ have been measured. Analysis of this data using the dual substituent parameter method shows that inductive effects are predominant. The reverse inductive contribution observed is explained in terms of a π-polarization mechanism. Critical support for this mechanism is obtained from additional series where the carbonyl is complexed with Lewis acids. The concepts of ‘extened’ and ‘localized’π-polarization are discussed.