α-Cleavage of 2,2-Diazido-2,3-dihydroinden-1-one in Solution and Cryogenic Matrices

Abstract

The Norrish type I (α-cleavage) reaction is an excellent photochemical method for radical-pair formation in solution. However, in cryogenic matrices, the starting material typically re-forms before the radical pair diffuses apart. This study focused on N 2 extrusion from an azido alkyl radical to prevent radical-pair recombination. Irradiation of 2,2-diazido-2,3-dihydroinden-1-one (1) in methanol mainly yielded methyl 2cyanomethylbenzoate (2) and 2-cyanomethylbenzoic acid (3) via α-cleavage. Laser flash photolysis of 1 in argon-saturated acetonitrile resulted in α-cleavage to form triplet biradical 3 1Br1 (λ max ∼ 410 nm, τ ∼ 400 ns). In contrast, upon irradiation in glassy 2-methyltetrahydrofuran matrices, triplet alkylnitrene 3 1N was directly detected using electron spin resonance (D/hc = 1.5646 cm −1 , E/hc = 0.00161 cm −1 ) and absorption spectroscopy (λ max = 276 and 341 nm). Irradiation of 1 in argon matrices generated 3 1N, benzoyl azide 4, singlet benzoylnitrene 1 4N, and isocyanide 5, as revealed by IR spectroscopy. The experimental results supported by density functional theory calculations [B3PW91/6-311+ +G(d,p)] suggest that irradiation of 1 in matrices results in α-cleavage to form biradical 3 1Br1, which extrudes N 2 to yield 3 1Br2. Rearrangement of 3 1Br2 into 3 1N competes with cleavage of a N 3 radical to form radical 1Ra3. The N 3 /1Ra3 radical pair combines to form 4, which upon irradiation yields 1 4N and 5.