Competition between α-Cleavage and Energy Transfer in α-Azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a
(π,π*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited
state (T2K) is 10 kcal/mol higher in energy and has a (n,π*) configuration. In comparison, T1K and T2K
of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,π*) and (π,π*)
configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient
absorption (λmax = 450 nm) due to formation of T1K, which decays with a rate of 2.1 × 105 s-1 to form
triplet alkylnitrene 2b (λmax = 320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast,
laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of
nitrene 2a (λmax = 320 nm) and benzoyl radical 3a (λmax = 370 nm). The decay of 3a is 2 × 105 s-1 in
methanol, whereas nitrene 2a decays with a rate of ∼91 s-1. Thus, T1K (π,π*) in azide 1b leads to
energy transfer to form nitrene 2b; however, α-cleavage is not observed since the energy of T2K (n,π*)
is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both α-cleavage
and energy transfer are observed from T1K (n,π*) and T2K (π,π*), respectively, since these triplet states
are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination
of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the
major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl
radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More
specifically, nitrene 2 does undergo α-photocleavage to form benzoyl radicals and iminyl radicals. The
secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.