Ketoisophorone (4-oxoisophorone) is an intermediate product useful as a starting material of medicines, perfumes, condiments, and a component for polymers.
As a process for producing ketoisophorone from isophorone, Japanese Patent Publication No. 30696/1980 (JP-B-55-30696) discloses a method for producing 4-oxoisohporone by oxidizing .alpha.-ishophorone with oxygen in the presence of a phosphomolybdic acid or a silicomolybdic acid. Japanese Patent Application Laid-Open No.191645/1986 (JP-A-61-191645) discloses a process for producing 4-oxoisophorone by oxidizing .alpha.-isophorone with oxygen in the coexistence of a phosphomolybdic acid or a silicomolybdic acid and an alkaline metal compound or an aromatic amine. Japanese Patent Application Laid-Open No. 93947/1975 (JP-A-50-93947) discloses a method for producing 4-oxoisophorone by oxidizing .alpha.-isophorone in the gas phase in the presence of a vanadium catalyst. Japanese Patent Application Laid-Open No. 81347/1974 (JP-A-49-81347) discloses a method for producing 4-oxoisophorone by oxidizing .alpha.-isophorone with an alkaline metal chromic acid salt or a dichromate or a chromium trioxide. In Chem. Lett. (1983), (7), 1081, there is disclosed a method for producing 4-oxoisophorone by oxidizing .alpha.-isophorone using t-butylhydroperoxide in the presence of a palladium catalyst. However, in these methods, the selectivity of ketoisophorone is reduced, therefore separation of the formed by-product(s) or a metal catalyst and purification of the object compound are complicated. Moreover, these methods involve using a heavy metal compound requiring special treatment, such as chromium, or a peroxide needed to be handled with care, which results in a decrease in working efficiency.
Moreover, as a method for producing ketoisophorone from .beta.-isophorone, Japanese Patent Application Laid-Open No. 125316 (JP-A-51-125316) discloses a method for producing an ethylenically unsaturated dicarboxylic acid by oxidizing .beta.-ethylenically unsaturated ketone with molecular oxygen or a molecular oxygen-containing gas in the presence of an inorganic base or an organic base and a cobalt or manganese chelate. In this method, however, the yield of ketoisophorone is low due to the use of a straight-chain secondary or tertiary amine such as triethylamine as the organic base.
Further, Japanese Patent Application Laid-Open No. 53553/1998 (JP-A-10-53553) discloses a method for producing ketoisophorone by oxidizing .beta.-isophorone with molecular oxygen in the presence of a manganese complex salt, an organic base, a specific substance having a catalytic action, and water. In this method, there is disclosed an addition of, e.g., an organic acid having a pKa value of 2 to 7 or a corresponding aldehyde, a C.sub.1-4 aliphatic alcohol or phenol, a compound formable in enolic form, or lithium sulfate as the specific substance having a catalytic action. According to this method, however, the specific substances decrease, depending on the species of the bases, the conversion or selectivity of the substrate considerably or cause the isomerization of .beta.-isophorone to .alpha.-isophorone. Particularly, a higher concentration of .beta.-isophorone in the reaction system causes a considerable decrease in the yield of ketoisophorone. For example, when the concentration of .beta.-isophorone is 20% by weight or more, the conversion and/or the selectivity is decreased to a large extent. Therefore, relatively large amounts of the manganese complex salt and the organic base are required for an improved conversion. Further, a lower concentration of oxygen remarkably decreases the reaction rate and consequently the conversion.