Experimental charge maps in di-activated carbanions: access to charge demands of primary electron-withdrawing functionalities

Abstract

It is found that the 13C shifts of C-1 in numerous, 1,1-disubstituted ethylene derivatives, R–CHCX2 and RCHCXY, are predictable, relative to the unsubstituted system RCHCH2 on the basis of additivity of the shielding contributions AX and AY. AX and AY are obtained by subtracting the shift of parent ethylene from that of C-1 in monosubstituted ethylenes CH2CHX or CH2CHY. This result allows an assessment of the scope of our previously proposed charge–13C relationship, equation (1) which is now applied to numerous carbanions di-activated with both identical and with different ‘primary’ electron-withdrawing functionalities. It is found that the 13C shift of the carbanionic carbon is almost insensitive to the dipolar-aprotic or protic nature of the solvent, Me2SO or MeOH. The 1JCH coupling constants of the carbanionic carbon provide unequivocal evidence for the trigonal hybridization of the charged carbon in all the carbanions examined, including the sulphonyl-, sulphinyl-, and cyano-stabilized systems. The π-electron density qc at the carbanionic carbon calculated by using equation (1) for carbanions diactivated with identical groups –CHX2 provides access to the fraction of negative charge qX withdrawn by each group X: these values allow a precise prediction of qc in –CHXY, and thus, of the 13C shift of their carbanionic carbon also. The charge demand values qX constitute a rank of electron-withdrawing capacities of primary functions: COPh,COMe,CONMe2,CO2Me,SO2R,CN, SOPh,PO(OEt)2. The values of qx as obtained from the –CHX2 anions are different but are linearly related to the qx values derived from the previously studied PhCH–X systems: they are, furthermore, related to the resonance parameters σR– of functionalities X, in turn obtained from the PhNHX systems.