Investigations on the 2-fluorobenzoin and 2-fluoro-3',5'-dimethoxybenzoin photochemistry

Photoremovable protecting groups are used for a broad range of applications in organic synthesis, biophysics and biology. The study of the photorelease mechanisms is key for understanding and controlling their activities and to determine release rates. The benzoin group presents a lot of advantages as a caging compound because it releases the protected group rapidly and in high yields with λ > 300 nm forming an inert benzofuran by-product. 2-Fluorobenzoin (1) and 2-fluoro- 3’,5’-dimethoxybenzoin (2) were studied by steady-state irradiation and laser flash photolysis in order to establish product distributions and to observe transient intermediates. Our investigations show that the two benzoin derivatives have a different photocyclisation mechanism. Irradiation ofin MeCN produces the 2-phenylbenzofuran (within 25 ns) through a fast cyclisation-elimination process involving 31, observed in the femtosecond experiments. In water or trifluoroethanol heterolytic dissociation of 31 forms the triplet α-ketocation (570 nm, 400 ns), which reacts with the solvent after intersystem crossing to its singlet state. Photocyclisation ofin MeCN, on the other hand, produces the dimethoxyphenylbenzofuran via the singlet state. . We observe a species absorbing at 365 nm, with a lifetime of aboutns. This transient, observed during the femtosecond LFP experiments, is tentatively assigned to the biradical 8, because its decay corresponds to the growth of the cationat 485 nm and gives the rate of F release (k = (5.05 ± 0.89) × 108 in air saturated MeCN). The ultimate intermediate has been assigned (Corrie, Wan)20 to the dimethoxycyclohexadienyl cation (3) (485 nm, τ = 425 ns).