Krapcho decarboxylation

The Krapcho decarboxylation, sometimes referred to as Krapcho reaction or Krapcho Decarbalkoxylation is a name of reaction in organic chemistry. It was named after its discoverer, the response Andrew Paul Krapcho, who published them in 1967. With the help of this reaction can be α, α -disubstituted esters and α -monosubstituted ester decarboxylation.

Survey

The reaction takes place (e.g., DMSO, DMF, DMA or HMPT ) in an aprotic - polar solvent. (Shown as X) in the reaction of esters, in the α -position of an electron withdrawing group having, with cyanide ion ( for example, KCN), the ester is decarboxylated and carbon dioxide is released as a gas.

Instead of potassium cyanide, the reaction can also be performed with other salts: NaCN, LiCl, NaCl, NaBr, NaI, LiI · H2O, Na2CO3 · H2O, Na3PO4 · 12 H2O.

Mechanism

The variation of the mechanism depends on the radicals R, which are bonded to the α - carbon atom.

α, α -disubstituted esters

Are both radicals R are different from hydrogen, the ester is thus α, α -disubstituted, the salt takes (in this case represented by the cyanide anion ), the alkyl group (in this case a methyl group) 1 in the course of a SN2 reaction in and goes out as methyl cyanide. Leaving a carboxylate 2 from the now carbon dioxide is cleaved by rearrangement of electrons and a carbanion 3 formed as a transition state. This is protonated by water to the decarboxylated product 4.

α - mono-substituted ester

Applies for a radical R = H, the starting material is an α - monosubstituted esters 5 In this case, the cyanide ion attacks the carbonyl group of 5 and it creates a tetrahedral transition state 6 under formation of a C = O double bond is formed on the one hand the carbanion 3, that is formed also on α, α -disubstituted esters and other ethyl cyanoformate (7). The carbanion 3 is again protonated by water to the product 4. The ethyl cyanoformate (7 ) reacts with water liberating hydrogen cyanide and carbon dioxide byproduct 8 in the form of an alcohol.