Cyclobutenediones 1,2-bisketene

One of the reasons for confusion in this area is the fact that cyclobutenediones absorb at much shorter wavelengths than their saturated counterparts, prompting the use of short wavelength light in photolyses this can result in formation of secondary photoproducts. It seems likely that formation of cyclopropenones and of acetylenes is the result of photochemical reaction of bisketenes, the primary products of irradiation. Acetylenes could also be formed from cyclopropenones. 

The photolysis of bis-benzhydrylidenecyclobutanedione 24 to cyclic anhydride 26 in aqueous methanol may involve prior photocyclization to cyclobutenedione 25 followed by photolysis to bisketene and reaction with solvent rather than formation of biradical 27 and subsequent reactions as originally suggested 51 ... 

Bisketenes 104 and 105 were generated by photolysis of 1,2-cyclobutenediones 103 and identified by their characteristic IR bands. 

Estimated upper limit based on the calculated equilibrium constant, the bisketene is more stable than the cyclobutenedione and ring closure has not been observed. 

Rates of thermal ring closure of aryl-substituted bisketenes 107 generated by photolysis of cyclobutenediones 106 (Eqn (4.56)) are correlated... 

Bisketenes 111 with nitrogen substituents were also obtained by photolysis of the corresponding cyclobutenediones (Eqns (4.59) and (4.60)), and the kinetics of their ring closure measured. 

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