Visible Light Activated Benzimidazolequinone Alkoxyamines of 1,1,3,3‐Tetramethylisoindolin‐2‐yloxyl (TMIO)

Abstract

1,1,3,3‐Tetramethylisoindolin‐2‐yloxyl (TMIO) is a well‐known stable 2,3‐dihydroisoindoline aminoxyl radical, which is thermally generated (at >100 °C) from literature alkoxyamine derivatives. Herein is the synthesis of visible light activated benzimidazolequinone alkoxyamines of TMIO using oxidative methods and the first room temperature nitroxide‐exchange experiments with 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) analogues. For the alkoxyamine TMIO‐Vis dissociation rate constants are provided using TEMPO and oxygen as radical traps.

CF3-Bis-TEMPO-Vis: New Visible Light Active Bis-Benzimidazolequinone Alkoxyamine

“…The kd of 1 increases 2-fold when the homolysis solution was diluted to 0.25 mM, which is expected due to the relative increase in photon to molecule ratio using the same LED setup. The rate of visible light induced decay of alkoxyamines and bis-alkoxyamines of TEMPO is influenced by the concentration of released nitroxide (i.e., free TEMPO) [4]. The addition of 1.3 equiv.…”

“…The addition of 1.3 equiv. of free TEMPO to the [Bis-TEMPO-Vis]0 = 5 mM experiment slowed The rate of visible light induced decay of alkoxyamines and bis-alkoxyamines of TEMPO is influenced by the concentration of released nitroxide (i.e., free TEMPO) [4]. The addition of 1.3 equiv.…”

“…of free TEMPO to the [Bis-TEMPO-Vis] 0 = 5 mM experiment slowed the rate of homolysis (Figure 3A). TEMPO significantly absorbs in the visible region (Figure 4) [4], and quenches excited states by electron transfer [9]. However, the addition of free TEMPO will also slow alkoxyamine decay through increased recombination.…”

“…Alkoxyamines usually dissociate thermally at >100 • C to bench-stable five or six membered cyclic α-tetraalkyl-substituted aminoxyl (nitroxide) and reactive carbon-centered radical [1,2]. Recently, we reported homolysis of benzimidazolequinone alkoxyamines of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) [3], and 1,1,3,3-tetramethylisoindolin-2yloxyl (TMIO) [4], activated by visible light (Scheme 1). Room temperature homolysis is favored through extensive delocalization of the quinone methide (QM) radical, including onto the quinone 7-O atom.…”

Investigation of the Structure of the Complex Oligomeric Mixture Formed During Grignard Tetramethylation of N-benzylphthalimide

“…TMIO or 1,1,3,3-tetramethylisoindolin-2-yloxyl (5, Scheme 1) is one of the most versatile isoindoline nitroxides due to its applications [1,2] and a variety of important advantages such as thermal stability, low reactivity towards olefins [3], symmetrical nature [4], UV detectability [5] and relative inertness to free radical attack [4] [4]. The most common and effective pathway for synthesizing 5 remains the Grignard-based approach in which N-benzylphthalimide 1 is treated with methylmagnesium halide in refluxing toluene, followed by subsequent debenzylation and oxidation of the resulting secondary amine (Scheme 1) [6].…”