The photodecarbonylation of o-benzoquinone to cyclopentadienone at low temperature was monitored by infrared and ultraviolet spectra which showed that the reaction is highly sensitive to substituents. 3, 5-Di-t-butyl-o-benzoquinone undergoes clean decarbonylation to give cyclopentadienone under irradiation at− 170° C. Similar irradiation of 4-t-butyl-, 4- methyl-, 3-methyl-, and unsubstituted o-benzoquinones also leads to decarbonylation to ...