Mechanistic aspects of ketene formation deduced from femtosecond photolysis of diazocyclohexadienone, o-phenylene thioxocarbonate, and 2-chlorophenol.

The photochemistry of diazocyclohexadienone (1), o -phenylene thioxocarbonate (2), and 2-chlorophenol (3) in solution was studied using time-resolved UV-vis and IR transient absorption spectroscopies. In these three cases, the same product cyclopentadienyl ketene (5) is formed, and two different mechanistic pathways leading to this product are discussed: (a) rearrangement in the excited state (RIES) and (b) a stepwise route involving the intermediacy of vibrationally excited or relaxed carbene. Femtosecond UV-vis detection allows observation of an absorption band assigned to singlet 2-oxocyclohexa-3,5-dienylidene (4), and this absorption feature decays with an 30 ps time constant in hexane and acetonitrile. The excess vibrational energy present in nascent carbenes results in the ultrafast Wolff rearrangement of the hot species. IR detection shows that photoexcited o -phenylene thioxocarbonate (2) and 2-chlorophenol (3) efficiently form the carbene species while diazocyclohexadienone (1) photochemistry proceeds mainly by a concerted process.