1,3-Cyclobutanedione is an organic compound with the formula . It is an isomer of 1,2-cyclobutanedione. The compound would be of little interest except that its tautomer is a subunit in some commercial dyes.
In solution, 1,3-cyclobutanedione exists in equilibrium with a less stable tautomer, called squaraine, 3-hydroxycyclobut-2-enone.[1] Squaraine dyes are, formally at least, derivatives of squaraine. In such dyes, the ene-one tautomer predominates.The mixture of tautomers can be prepared by hydrolysis of 1-ethoxycyclobutene-3-one, which is prepared from the cycloaddition of ethoxyacetylene to ketene.[2]
A variety of substituted 1,3-Cyclobutanediones form upon spontaneous dimerization of disubstituted ketenes.[3] 2,2,4,4-Tetramethylcyclobutanedione is thus formed by dehydrochlorination of isobutyryl chloride:
Ketene itself dimerizes mainly to give the lactone called diketene as well as a small amount of 1,3-cyclobutanedione.