Sp2 carbanions

The greater acidity of the a-protons of pyrroline compared to the pyrrolidine is demonstrated by the fact that whereas n-butyllithium is strong enough to form the unsaturated (sp2) carbanion, f-butyllithium is necessary for the saturated (jp3) system. 

Rewcastle, G. W., Katritzky, A. R., Generation and Reactions of sp2-Carbanionic Centers in the Vicinity of Heterocyclic Nitrogen Atoms, 56, 155. 

Generation and reactions of sp2 carbanionic centers vicinal to heterocyclic N atoms 93AHC(56)155. 

The closer that electrons are to the nucleus, the more stable. An s orbital is closer to a nucleus than a p orbital is, as p orbitals are elongated away from the nucleus. An sp2 carbanion is more stable than an sp3 carbanion because the sp2 carbanion has 33% s character and and the electron pair is closer to the positive nucleus than in an sp3 carbanion which is only 25% s character. The sp2 carbanion is easier to form because of its relative stability. 

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