Selective Formation of Triplet Alkyl Nitrenes from Photolysis of β-Azido-Propiophenone and Their Reactivity

Abstract

Photolysis of beta-azido propiophenone derivatives, 1, with built-in sensitizer units, leads to selective formation of triplet alkyl nitrenes 2 that were detected directly with laser flash photolysis (lambdamax = 325 nm, tau = 27 ms) and ESR spectroscopy (|D/hc| = 1.64 cm-1, |E/hc| = 0.004 cm-1). Nitrenes 2 were further characterized with argon matrix isolation, isotope labeling, and molecular modeling. The triplet alkyl nitrenes are persistent intermediates that do not abstract H-atoms from the solvent but do decay by dimerizing with another triplet nitrene to form azo products, rather than reacting with an azide precursor. The azo dimer tautomerizes and rearranges to form heterocyclic compound 3. Nitrene 2a, with an n,pi* configuration as the lowest triplet excited state of the its ketone sensitizer moiety, undergoes intramolecular 1,4-H-atom abstraction to form biradical 6, which was identified by argon matrix isolation, isotope labeling, and molecular modeling. beta-Azido-p-methoxy-propiophenone, with a pi,pi* lowest excited state of its triplet sensitizer moiety, does not undergo any secondary photoreactions but selectively yields only triplet alkyl nitrene intermediates that dimerize to form 3b.

Photolysis of 3,5‐diphenylisoxazole in argon matrices

“…For example, triplet carboalkoxynitrenes are short‐lived, as they abstract H atoms from the solvent . In comparison, triplet alkylnitrenes and arylnitrenes are less reactive and longer‐lived, as they do not react with the solvent, but mainly decay by dimerization (Scheme ) . Because triplet alkylnitrenes and arylnitrenes have high spin properties and are long‐lived, they have a potential as building blocks for high spin assemblies.…”

Photolysis of acetophenone derivatives with α‐cyclopropyl substituents

“…Although intramolecular sensitization with a built‐in triplet acetophenone sensitizer has been shown to be a valuable tool for forming triplet biradicals such as triplet alkylnitrenes and vinylnitrenes, intramolecular sensitization using acetophenone derivatives with α‐cyclopropyl substituents is complicated by slow intersystem crossing rates.…”

A Laser Flash Photolysis Study of Azo-Compound Formation from Aryl Nitrenes at Room Temperature

“…8 We note that Gudmundsdottir et al have demonstrated that triplet alkyl nitrenes (formed by internal triplet sensitization) do not react with other alkyl azides. 22 In this contribution, we describe the photochemistry of 4-azidoquinoline 1-oxide in solution, as determined via a combined spectroscopic, laser flash photolysis (LFP), and computational study. The kinetics of the reaction measured by LFP allows us to test various possible mechanisms for azide formation.…”