Transforming Triplet Vinylnitrene into Triplet Alkylnitrene at Cryogenic Temperatures

Abstract

Photolysis of 2,3-diazidonaphthalene-1,4-dione (1) in methyltetrahydrofuran matrices forms 2-(λ 1 -azaneyl)-3-azidonaphthalene-1,4-dione (vinylnitrene 3 2), as confirmed by electron paramagnetic resonance spectroscopy. The zero-field splitting (zfs) parameters for 3 2 (D/hc = 0.5338 cm −1 , and E/hc = 0.0038 cm −1 ) reveal significant 1,3biradical character. Irradiating 3 2 yields 2-(λ 1 -azaneyl)-1,3-dioxo-2,3dihydro-1H-indene-2-carbonitrile (alkylnitrene 3 3), which has zfs parameters typical of a cycloalkylnitrene (D/hc = 1.57 cm −1 , and E/hc = 0.00071 cm −1 ). Photolysis of 1 in argon matrices verifies that 3 2 forms 3 3.

Photoinduced α-Cleavage of 2-Azido-2-phenyl-1,3-indandione at Cryogenic Temperatures

“…The calculated zero-field splitting parameters D / hc and E / hc were 1.5837 and 0.0039 cm –1 , respectively . The D / hc value of 3 2 is similar to those of triplet 2-(λ 1 -nitreno)-1,3-dioxo-2,3-dihydro-1 H -indene-2-carbonitrile (1.57 cm –1 ) and triplet methylnitrene (1.595 cm –1 ) and its derivatives …”

Preparation and Photochemistry of Parent Triplet Vinylarsinidene

“…An example is the use of vinylazides as versatile reagents in the synthesis of heterocyclic compounds under the action of light. − However, the widespread use of photoreactive vinyl azides depends on a deeper understanding of complex reaction mechanisms. These mechanisms depend on various factors: the structure of the vinyl azides, the exposure wavelength, and whether light is absorbed directly or passes through sensitizers. − The potential energy surface surrounding parent vinylnitrene ( 1 ) reveals intriguing isomers that push the boundaries of our understanding of structure and bonding. − Nitrene 1 has thus far escaped direct spectroscopic identification because of its significant 1,3-diradical character, which renders it susceptible to structural rearrangement or fragmentation. − The attempted photochemical or thermal preparation of 1 from vinyl azide gives the singlet state that undergoes rapid isomerization (Scheme A), although the triplet is the electronic ground state (Δ E ST = ∼14 kcal mol –1 ). ,− As the rate for intersystem crossing is considered to be lower than the rate for intramolecular isomerization, it is highly unlikely to access the triplet state through conventional nitrene preparation methods. − However, the structural rigidity of vinylnitrenes attached to cyclic structural moieties (e.g., naphthalene-1,4-dione and 1 H -inden-1-one) can make them accessible at cryogenic temperatures, favoring the formation of their triplet states. − …”