Electron Pushing -- Basic and Nucleophilic Catalysis

  A basic atom is one bearing a lone pair of electrons, or a strongly polarized sigma bond (such as C-Li or C-MgBr) or pi bond (such as an enamine).

  Common bases: NaOH, NaOAc, Na2CO3, KOtBu, NaNH2, n-BuLi, tBuLi, NEt3, N(iPr)2Et (Hünig's base)

  

  Bases/nucleophiles interact with molecules in four main ways. All of these reactions are especially facile when the charge ends up on an electronegative heteroatom (O, N, S, Cl, Br, F)

  1. Deprotonation of acidic hydrogens. To form finite amounts of the anionic product X- from X-H at equilibrium, the pKA of X-H must be comparable or lower than the pKA of B-H. To get reasonable rates of formation of the anionic product, the X-H bond being broken must have a pKA that is no more than about 10 pKA units higher than that of B-H; e.g., hydroxide ion (pKA of H2O ca 15) can effectively catalyze the formation ketone enolates (pKA of ketones ca 20) but not those of alkyl sulfones (pKA ca 30).

  

  2. Nucleophilic addition to C=X and activated C=C multiple bonds (e.g., Michael Reaction). These reactions also work best when a reasonably stabilized anion is being formed. The reverse reaction (retro-Michael) occurs readily if the nucleophile was a stabilized anion.

  

  3. Nucleophilic substitutions.

  

It is permissible to violate the octet rule with heavier elements, but always be careful not to lose track of charges and electrons when you do so.

Aromatic nucleophilic substitution - requires strong anion stabilizing groups on the aromatic ring.

  

  4. Nucleophilic additions to sextet atoms:

  

  Carbanion rearrangements are much less common than those of carbocations. Here are a few name reactions:

    Ramberg-Backlund Reaction

    Grovenstein-Zimmerman Rearrangement

    Favorskii Rearrangement

    Stevens Rearrangement

    Wittig ([2,3] sigmatropic) Rearrangement

  Carbanions with unusual properties:

  

Links: Basic Electron Pushing, Acid Catalysis


Web page created by WINPLT