Photoinduced C-N bond cleavage in 2-azido-1,3-diphenyl-propan-1-one derivatives: Photorelease of hydrazoic acid

Article abstract of DOI:10.1021/jo070558q

(Chemical Equation Presented) Photolysis of 3-azido-1,3-diphenyl-propan-1- one (1a) in toluene yields 1,3-diphenyl-propen-1-one (2), whereas irradiation of 3-azido-2,2-dimethyl-1,3-diphenyl-propan-1-one (1b) results in the formation of mainly 2,2-dimethyl-1,3-diphenyl-propan-1-one. Laser flash photolysis (308 nm) of 1a,b in acetonitrile reveals a transient absorption (λ_max = ～310 nm) due to the formation of radicals 4a and 4b, respectively, which have lifetimes of ～14 μs at ambient temperature. TD-DFT calculations (B3LYP/6-31+G(d)) reveal that the first and second excited states of the triplet ketone (T_1K (n,π*) and T_2K (π,π*)) in azide 1a are almost degenerate, at ～74 and 76 kcal/mol above the ground state (S₀), respectively. Similarly, azide 1b has T_1K and T₂K 75 and 82 kcal/mol above S₀, respectively. The calculated transition state for cleaving the C-N bond is located 71 and 74 kcal/mol above S₀ in azides 1a and 1b, respectively. The calculated bond dissociation energies for breaking the C-N bond are 55 and 58 kcal/mol for azides 1a and 1b, respectively, making C-N bond breakage accessible from T _1K in azides 1 at ambient temperature. In comparison, the irradiation of azides 1 in argon matrices at 14 K lead to the formation of the corresponding triplet alkyl nitrenes (1-n), via intramolecular energy transfer from T_2K- The characterization of 1-n was supported by isotope labeling, IR spectroscopy, and molecular modeling.