Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Ranaweera A. A. Upul Ranaweera; Yu Zhao; Sivaramakrishnan Muthukrishnan; Christopher Keller; Anna D. Gudmundsdottir

doi:10.1071/CH10331

RESEARCH FRONT

Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Ranaweera A. A. Upul Ranaweera ^A , Yu Zhao ^A , Sivaramakrishnan Muthukrishnan ^A , Christopher Keller ^A and Anna D. Gudmundsdottir ^A ^B

+ Author Affiliations

- Author Affiliations

^A Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172, USA.

^B Corresponding author. Email: anna.gudmundsdottir@uc.edu

Australian Journal of Chemistry 63(12) 1645-1655 https://doi.org/10.1071/CH10331
Submitted: 9 September 2010 Accepted: 10 November 2010 Published: 6 December 2010

Abstract

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 (T_1K and T_2K) 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 T_1K, indicating that azido cleavage is feasible. The calculations place the energy of the triplet azido group (T_A) in 1a, 1b ∼25 kcal mol^–1 below T_1K; thus, this process is also easily accessible via energy transfer. Further, the transition state barrier for T_A to expel N₂ and form triplet nitrenes is less than 1 kcal mol^–1 above T_A 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.

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Competition Between Azido Cleavage and Triplet Nitrene Formation in Azidomethylacetophenones

Abstract

References

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