Lapworth mechanism, benzoin condensation

Although the catalysis of the dimerization of aldehydes to acyloins by thiazolium ion has been known for some tlrae, the development of procedures using anhydrous solvents which give satisfactory yields of acyloins on a preparative scale was first realized in the submitters laboratories. The mechanism proposed by Breslow - for the thiazolium ion-catalyzed reactions is similar to the Lapworth mechanism for the benzoin condensation with a thiazolium ylide replacing the cyanide ion. Similar mechanisms are involved... 

The mechanism of the cyanide- and thioazolium ion-catalyzed conjugate addition reactions is considered to be analogous to the Lapworth mechanism for the cyanide-catalyzed benzoin condensation. Thus the cyano-stabilized carbanion resulting from deprotonation of the cyanohydrin of the aldehyde is presumed to be the actual Michael donor. After conjugate addition to the activated olefin, cyanide is eliminated to form the product and regenerate the catalyst. 

Lapworth, A. J. J. Chem. Soc. 1903, 83, 995. Arthur Lapworth (1872—1941) was bom in Scotland. He was one of the great figures in the development of the modem view of the mechanism of organic reactions. Lapworth investigated the Benzoin condensation at the Chemical Department, The Goldsmiths Institute, New Cross, UK. 

The benzoin condensation catalyzed by N-heterocyclic carbenes has been investigated intensively. First investigations date back to 1832 when Wohler and Liebig discovered the cyanide-catalyzed coupling of benzaldehyde to benzoin (Wohler and Liebig 1832). In 1903 Lapworth postulated a mechanism for this reaction in which an intermediate car-banion is formed by hydrogen cyanide addition to benzaldehyde fol-... 

In the benzoin condensation, both aromatic and heterocyclic aldehydes are transformed into a-hydroxy ketones of the general formula ArCHOHCOAr, often called benzoins. This class of compounds is frequently encountered in natural products, hence the benzoin and related reactions have received much at-tention. The reaction employs a cyanide ion as the catalyst and the mechanism, proposed by Lapworth, involves formation of carbanions stabilized by the nitrile group (Scheme 1). 

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