EVIDENCE FOR KETENE INTERMEDIATES IN THE DECARBONYLATION OF 2,4-DIOXO ACIDS AND ESTERS AND 2-OXOBUTANEDIOIC ACID-ESTERS

The mechanism by which alpha, gamma-dioxo carboxylic acid esters 1 and 2-oxobutanedioic acid diesters 2 lose CO was explored. The compounds, 5,5-dimethyl-2,4-dioxohexanoic acid ethyl ester, 1a, alpha,2-dioxocyclohexaneacetic acid ethyl ester, 1b, and alpha,1-dioxotetrahydro-2-naphthaleneacetic acid ethyl ester, 1c, lose CO at 170-190-degrees-C to yield the corresponding beta-keto esters 3a-c. When compounds 1 or the parent acids 4 were heated to 170-190-degrees-C with water in a sealed reactor, they yielded ketones resulting from replacement by H of C(O)CO2R from 1 or C(O)CO2H from 4. beta-Keto esters suffered replacement by H of the carbethoxy group to yield the corresponding ketones when heated with water at about 105-degrees-C. Acylketenes, such as 4,4-dimethyl-1-pentene-1,3-dione, 6a, 2-oxocyclohexylidenemethanone, 6b, 1-oxotetrahydro-2-naphthylidenemethanone, 6c, 3-methyl-1-butene-1,3-dione, 6d, and 1-butene-1,3-dione, 6e, are implicated as the common intermediates that react with water to form beta-keto acids that subsequently decarboxylate to yield the ketones 5. Intense IR frequencies in the region of 2120-2140 cm-1, characteristic of ketenes, are observed when 1,2, or 3 is subjected to GC-FTIR analysis with the injector and light pipe at 280-degrees-C. Loss of carbon monoxide and alcohol at high temperature is required to form 6 from 1, while only the loss of alcohol at lower temperature is needed to form 6 from 3.