Photochemistry of 4,4-Dimethyl-1-mesityl-2-pentyn-1-one.

The photochemistry of 4,4-dimethyl-1-mesityl-2-pentyn-1-one (7) in solution has been examined by using a combination of product studies, quantum yield measurements, and laser flash photolysis techniques. The main products of reaction are the indanones 10 and 11, believed to be formed via the intermediacy of carbene 9. Transient phenomena are dominated by the photoenolization of the parent ketone 7, which occurs following hydrogen abstraction by the triplet leading to biradical (or triplet enol) 8. The biradical has a lifetime of 38 +/- 4 ns in methanol and 45 +/- 7 ns in heptane (for 7D) at room temperature. Biradical decay leads to a mixture of enols 18Z and 18E. The syn enol 18Z usually accounts for >90% of the mixture and decays with first-order kinetics with k/sub d/ = 3.7 x 10/sup 6/ s/sup -1/ in hexane in at 293 K. For comparison the deuterio analogue 18Z-D decayed with k/sub d/ = 3.7 x 10/sup 6/ s/sup -1/ under the same experimental conditions. The anti enol was extremely long-lived, and under carefully controlled conditions its lifetime was 22 s in hexane at room temperature.