Sivaramakrishnan Muthukrishnan

Intramolecular H-Atom Abstraction in γ-Azido-Butyrophenones: Formation of 1,5 Ketyl Iminyl Radicals

Photolysis of gamma-azidobutyrophenone derivatives yields 1,4 ketyl biradicals via intramolecular H-atom abstraction. The 1,4 ketyl biradicals expel a nitrogen molecule to form 1,5 ketyl iminyl biradicals, which decay by ring closure to form a new carbon-nitrogen bond. The 1,5 ketyl iminyl biradicals were characterized with transient spectroscopy. In argon/nitrogen-saturated solutions, the biradicals have lambda(max) approximately 300 nm and tau = 15 micros. DFT-TD calculations were used to support the proposed mechanism for formation of the 1,5 ketyl iminyl radicals.

Effect of alkyl substituents on photorelease from butyrophenone derivatives.

Photolysis of 1a yields 4a in argon-saturated methanol, whereas 1b is photostable. Laser flash photolysis of 1a in acetonitrile shows formation of biradical 2a (lambda(max) = 340 nm, tau = approximately 60 ns), which undergoes intersystem crossing to form Z-3a (lambda(max) = 380 nm, tau = 270 ns) and E-3a (lambda(max) = 380 nm, tau = 300 ms). Z-3a regenerates the starting material, whereas E-3b undergoes intramolecular lactonization to release the alcohol moiety and form 4a. Similar laser flash photolysis of 1b shows formation of biradical 2b (lambda(max) = 340 nm, tau = 1.9 micros in acetonitrile), which is longer-lived than 2a is. However, 2b only undergoes intersystem crossing to form Z-3b (lambda(max) = 380 nm, tau = 4.3 micros). Calculations demonstrate that intramolecular pseudo hydrogen bonding between the OH moiety and the radical centered on the isopropyl carbon in 2b and the bulkiness of the isopropyl group prevent the necessary rotation to form E-3b. In comparison, 2a does not form an intramolecular pseudo hydrogen bond between the methylene radical center and the OH group, and as a consequence, it undergoes intersystem crossing to form both E- and Z-3a.

Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Photolysis of p- and m-azidomethylacetophenone (1a, 1b) in argon-saturated solutions yields predominantly imine 2a, 2b, whereas irradiation of 1a, 1b in oxygen-saturated solutions results in heterocycles 3a, 3b, aldehydes 4a, 4b and nitriles 5a, 5b. Density functional theory calculations place the energy of the first and second excited state of the triplet ketones (T1K and T2K) in 1a, 1b in close proximity to each other. The triplet transition state for cleaving the C–N bond in 1a, 1b to form azido and benzyl radicals 1aB, 1bB is located only 3 kcal mol–1 (1 kcal = 4.184 kJ) above T1K, indicating that azido cleavage is feasible. The calculations place the energy of the triplet azido group (TA) in 1a, 1b ∼25 kcal mol–1 below T1K; thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for TA to expel N2 and form triplet nitrenes is less than 1 kcal mol–1 above TA in 1a, 1b. Laser flash photolysis of 1a, 1b reveals the formation of the triplet excited ketones of 1a, 1b, which decay to form benzyl radicals 1aB, 1bB and triplet alkylnitrenes. The triplet ketones and the benzyl radicals are quenched with molecular oxygen at rates close to diffusion, whereas the triplet nitrenes react more slowly with oxygen (∼5 × 105 M–1 s–1). We conclude that the triplet alkylnitrenes intercept the benzyl radicals to form 2 in argon-saturated solution, whereas the benzyl radicals are trapped to form 4 in oxygen-saturated solution; thus, the triplet nitrenes react with oxygen to form 3.

Comparison of photoenolization and alcohol release from alkyl-substituted benzoyl benzoic esters

Photolysis of 1B in argon-saturated solutions yields 4B and releases methanol. Laser flash photolysis of 1B shows formation of biradical 2B, which has a lifetime of ~50 ns and a lmax at 330 nm. Biradical 2B undergoes an intersys- tem crossing to form photoenols E-3B and Z-3B with a lmax at 390 nm. Laser flash photolysis shows that the lifetimes of E-3B and Z-3B are affected by the solvent. Density functional theory calculations demonstrate that the transition-state bar- rier for a 1,5-H atom shift from Z-3B to regenerate 1B is affected by the ortho-alkyl substituents, whereas the stereoelec- tronics of the alkyl substituent affect the transition-state barrier of E-3B as it undergoes electrocyclic ring closure to form 4B. The photoreactivity of 1B was compared with its analogous methyl and isopropyl derivatives 1A and 1C, respectively, to better estimate the effect of the alkyl substituent on reactivity.