Dipole-stabilized carbanions in the gas phase

The energetics, structures, and reactivities of several dipole-stabilized carbanions from formamides, aldehydes, and formamidines have been examined by ICR mass spectrometry and by MNDO calculations. Experimentally in the amides, only one of the alkyl groups on nitrogen appears to be deprotonated, but a stereochemical assignment cannot be made. The order of acidity is aldehyde > amide > ester based on the MO calculations, indicating that the external dipole moment of these carbon acids is not a good measure of acidity. The acidities follow the inductive ordering, based on uI constants. The effect of adjacent substituents on the stability, structure, and reactivity of carbanions has been t h e object of considerable investigation. While it is well established that *-acceptor groups can stabilize carbanions appreciably,' the effect of *-donor groups is not so well understood. First-row heteroatom groups, such as OMe and F, which are ?r donors but u acceptors, have variable and small effects on anion stability in both experiment2 and in theory,3 compared to *-acceptor groups, depending on the nature of the other groups attached to the anionic atom. Recently there has been considerable synthetic interest in lithiocarbon reagents apparent ly stabilized not by *-acceptor substituents, but by an adjacent heteroatom as the positive end of a dipole! The observed stereochemistry of products from anions such as 1 is stated to be consistent with internal chelation of the lithium was the negative end of the dipole, as in 1!.5 This is in contrast to molecular orbital