Recent Results in Carbanion Chemistry

The Wittig rearrangement of ethers metalated in the alpha position with alkali metals is a 1,2‐shift to an atom with a lone pair of electrons. The alkyl shift proceeds with racemization (and partial retention) at the migrating carbon atom. The experimental observations suggest a cleavage‐recombination mechanism involving a radical pair, as must also be assumed for the ylide rearrangements. In the allyl migration, the double bond is displaced within the migrating residue (allyl inversion). This is readily explained by a one‐step S ′ mechanism, the transition state of which is allowed on symmetry grounds. In the case of benzyl 2‐butynyl ether metalated on the butynyl residue, the ortho isomerization (corresponding to the Sommelet rearrangement) predominates. – α‐Metalated alkyl isocyanides can react with carbonyl compounds to form olefins and metal cyanate (carbonyl olefination). Isocyanoacetic esters occupy a special place; they react with aldehydes and ketones to form β‐substituted α‐formylaminoacrylates (formylaminoalkoxycarbonylmethylenation), which can enter into a wide range of reactions. β‐Hydroxyalkyl isocyanides or 2‐oxazolines can be isolated as intermediates in the carbonyl olefination. α‐Metalated isocyanides react with acylating agents to form β‐ketoalkyl isocyanides, which cyclize to give oxazoles.