Visible‐Light‐Driven Hydrophosphorylation of Azobenzenes Enabled by trans‐to‐cis Photoisomerization

Abstract

A visible‐light‐driven hydrophosphorylation of azobenzenes with diarylphosphine oxides was successfully developed. The reaction proceeded smoothly under blue light irradiation at room temperature without any photocatalyst required, providing a mild approach for this transformation with new strategy. Of important note is that the visible light‐driven trans‐to‐cis photoisomerization of azobenzenes was found to be the key for the process of the reaction, highlighting this underestimated reactivity of azobenzenes in visible‐light‐promoted mechanism.

Recent Advances in Radical Reactions of Azo Compounds

“…After that, the same group reported the visible‐light‐promoted hydrophosphorylation of azobenzenes with diarylphosphine oxides 56 (Scheme 25). [45] The reaction worked well at room temperature without photocatalyst and provided the functionalization products 57 in moderate to good yield. In addition, a gram‐scale reaction of azobenzene was performed to address the synthetic utility of this protocol.…”

“…Although the reactivity of N=N double bond in azoarenes is relatively low, there are still a few examples on the radical functionalization of the N=N bonds of azoarenes [40–45] . In 1964, Hess and co‐workers firstly observed a long‐lived radical by spectroscopic irradiation, which was photochemically produced in azobenzene‐cumene solution and then reduced by cumene [40] .…”

Visible‐Light‐Mediated Cycloaddition of Azobenzenes and Nonstabilized Azomethine Ylides to Access 1,2,4‐Triazolidines

“…Taking into account the redox potentials of azobenzene 1 [ E p red =−1.06 V vs SCE; E p ox =+1.08 V vs SCE], [17a] tertiary amines [ E 1/2 ox =+0.78 V vs SCE] [15,19] and excited RB* [ E 1/2 (RB*/RB⋅ − )=+0.81 V vs SCE; E 1/2 (RB⋅ + /RB*)=−0.96 V vs SCE], [20] we propose that a photoinduced electron transfer (PET) occurs between RB* and benzyl tertiary amine 2 rather than azobenzene 1 . Moreover, based on our previous research [17b] and the observation of cis ‐azobenzene during the reaction, we speculate that the excited RB* ( E T =40.9 kcal/mol) [21] may sensitize 1 ( E T =35.4 kcal/mol) [22] through an energy transfer (EnT) pathway, leading to a trans ‐to‐ cis isomerization of azobenzene.…”

“…This groundbreaking finding enables the formation of this highly reactive intermediate under mild conditions, eliminating the need for external trifluoroacetic acid addition, [16] thereby opening up new possibilities for subsequent transformations in a sustainable manner. Based on our previous research on visible light-mediated functionalization of azobenzenes, [13,17] we envisioned that 1,2,4-triazolidines may be synthesized by 1,3-dipolar cycloaddition of azobenzenes with in situ formed nonstabilized azomethine ylides through photoredox catalysis. Here, we report the successful implementation of this strategy (Scheme 1d), which provides a mild, simple and efficient way for specific synthesizing 4-alkyl-1,2diaryl-1,2,4-triazolidines.…”

“…It is worth noting that the azobenzenes with electron-withdrawing substituents reacted faster than those with electrondonating substituents. Besides, the di-substituted symmetrical azobenzene (17) was found to be a suitable However, ortho-substituted symmetrical azobenzenes led to poor outcomes. For instance, orthofluoro-substituted azobenzene the desired product 18 with moderate yield, while ortho-methyl-substituted azobenzene only produced the hydrogenated product 19.…”