Investigation of the Formation of Methyl 2-Cyano-2--4-oxopentanoate from Methyl 2,4-Dioxopentanoate. A Clarification of the Pathway for the Reaction of Trimethylsilyl Cyanide with Enolized β-Diketones

Article abstract of DOI:10.1021/jo00225a044

The possible reaction pathways for the reaction of methyl 2,4-dioxopentanoate (1) with trimethylsilyl cyanide, with and without catalysts present, have been investigated by monitoring these reactions by 1H NMR.While all reactions of 1 with Me3SiCN, whether catalyzed or uncatalyzed, involve the initial formation of the silyl enol ether 5, it is not an intermediate in the formation of the cyanohydrin 3 from the reaction of 1 with Me3SiCN alone or in the presence of ^-CN.Evidence is presented which indicates that the silyl enol ether 5 and HCN are in equilibrium with 1 and Me3SiCN and that is the keto tautomer 1a, containing a very reactive α-keto ester group, which reacts with Me3SiCN to produce 3 (Scheme III).The existence of the equilibrium 5 + HCN <*> 1 + Me3SiCN is demonstrated by isolation of the cyclic silyl enol ether cyanohydrin 9 when the silyl enol ether 5 is allowed to react with Me2Si(CN)2.Evidence that trialkylsilyl cyanides react with the keto tautomer of 1, and the exclusion of other possible reaction pathways, is obtained on reaction of 1 with t-BuMe2SiCN to give the tert-butyldimethylsilyl enol ether 10 and the unprotected cyanohydrin 11.The reaction of 2,4-pentanedione (4) with Me3SiCN, with and without the presence of catalysts, was reinvestigated and the mechanism is shown, in contrast with the earlier proposals, to be consistent with the mechanistic ideas presented here for the reaction of 1 with Me3SiCN.