Y-Conjugated Dianions. A ¹³C Nuclear Magnetic Resonance Study

Tris(diphenylmethylene)methane dianion (1^2-), tetraphenylacetone dianion (2^2-), and diphenylacetate dianion (3^2-) were generated in dimethyl sulfoxide or tetrahydrofuran solutions by deprotonation of the appropriate carbon acid. Chemical shifts, carbon-carbon spin coupling constants, and other spectroscopic evidence indicated an sp²-hybridized central atom for all dianions, as well as D₃ symmetry for the all-carbon dianion. The one-bond CC spin coupling constant for 1^2- (62 Hz) increased upon consecutive replacements of diphenylmethylene by the groups, i.e., to 69 Hz for 2^2- and 76.7 Hz for 3^2-, 2K⁺ (86.0 Hz for the Li⁺ salt) reflecting charge localization at oxygen. An empirical model for the residual charge on phenyl-substituted resonance-stabilized carbanions that allows comparison of dissimilarly substituted carbanions was developed.