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 A_X and A_Y. A_X and A_Y 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 ¹J_CH 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 q_c at the carbanionic carbon calculated by using equation (1) for carbanions diactivated with identical groups ^–CHX₂ provides access to the fraction of negative charge q_X withdrawn by each group X: these values allow a precise prediction of q_c in ^–CHXY, and thus, of the ¹³C shift of their carbanionic carbon also. The charge demand values q_X constitute a rank of electron-withdrawing capacities of primary functions: COPh,COMe,CONMe₂,CO₂Me,SO₂R,CN, SOPh,PO(OEt)₂. The values of q_x as obtained from the ^–CHX₂ anions are different but are linearly related to the q_x values derived from the previously studied PhCH^–X systems: they are, furthermore, related to the resonance parameters σ_R^– of functionalities X, in turn obtained from the PhNHX systems.