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. 2023 Jun 15;3(6):100603.
doi: 10.1016/j.checat.2023.100603. Epub 2023 Apr 12.

Photochemical iron-catalyzed decarboxylative azidation via the merger of ligand-to-metal charge transfer and radical ligand transfer catalysis

Shih-Chieh Kao  1 , Kang-Jie Bian  1 , Xiao-Wei Chen  1 , Ying Chen  1   2   3 , Angel A Martí  1   2   3 , Julian G West  1   4
Affiliations

Photochemical iron-catalyzed decarboxylative azidation via the merger of ligand-to-metal charge transfer and radical ligand transfer catalysis

Shih-Chieh Kao et al. Chem Catal. .

Abstract

Ligand-to-metal charge transfer (LMCT) using stoichiometric copper salts has recently been shown to permit decarboxylative C-N bond formation via an LMCT/radical polar crossover (RPC) mechanism; however, this method is unable to function catalytically and cannot successfully engage unactivated alkyl carboxylic acids, presenting challenges to the general applicability of this approach. Leveraging the concepts of ligand-to-metal charge transfer (LMCT) and radical-ligand-transfer (RLT), we herein report the first photochemical, iron-catalyzed direct decarboxylative azidation. Simply irradiating an inexpensive iron nitrate catalyst in the presence of azidotrimethylsilane allows for a diverse array of carboxylic acids to be converted to corresponding organic azides directly with broad functional group tolerance and mild conditions. Intriguingly, no additional external oxidant is required for this reaction to proceed, simplifying the reaction protocol. Finally, mechanistic studies are consistent with a radical mechanism and suggest that the nitrate counteranion serves as an internal oxidant for turnover of the iron catalyst.

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Conflict of interest statement

DECLARATION OF INTERESTS This section is required for all research articles, resources, reviews, and perspectives. Please use it to disclose any competing interests in accordance with Cell Press’s Declaration of Interests policy. If there are no interests to declare, please write, “The authors declare no competing interests.” The text in this section should match the text provided in the Declaration of Interests form.

Figures

Figure 1.
Figure 1.. Importance and Approaches to Decarboxylative C–N Bond Formation.
Yoon has recently disclosed an approach to decarboxylative C–N bond formation of activated carboxylic acids using tandem ligand-to-metal charge transfer (LMCT)/radical polar crossover (RPC) using stoichiometric copper, whereas our approach to decarboxylative C–N bond formation via tandem LMCT/radical ligand transfer (RLT) using catalytic iron.
Figure 2.
Figure 2.. Development of Iron-Photocatalyzed Decarboxylative Azidation
a Reactions performed on 0.1 mmol scale with CH2Br2 added as an internal standard (NMR yield). b MeCN/H2O=4/1 co-solvent system was used.
Figure 3
Figure 3. Scope of photocatalytic decarboxylative azidation of carboxylic acidsa
aStandard conditions: Substrate (0.1 mmol), Fe(NO3)3·9H2O (0.02 mmol), TMSN3 (0.4 mmol), Na2CO3 (1.1 mmol), MeCN (0.25 mL), 390 nm LED light, rt, 24 h. bCH2Br2 was added as an internal standard (NMR yield). cFe(NO3)3·9H2O (0.03 mmol) was used. dNa2CO3 (0.04 mmol) was used. eReaction performed on 0.3 mmol scale with (2 X 52W) purple LEDs (390 nm). fReaction was performed under the following conditions: Substrate (0.1 mmol), Fe(NO3)3·9H2O (0.03 mmol), TMSN3 (0.4 mmol), Na2CO3 (0.02 mmol), MeCN/DCM=1/1 (0.25 mL), Mg(NO3)2·6H2O (0.1 mmol), 390 nm LED light, rt, 24 h. NMR yield is in the parentheses (CH2Br2 as an internal standard). gsame conditions as f but MeCN (0.25 mL) was used. NMR yield is in the parentheses (CH2Br2 as an internal standard). hsame conditions as f but Fe(NO3)3·9H2O (0.02 mmol) was used. NMR yield is in the parentheses (CH2Br2 as an internal standard). iReaction performed on 500 mg scale and yield is in the parentheses. j Same conditions as f but Fe(NO3)3·9H2O (0.02 mmol) and MeCN (0.25 mL) were used. NMR yield is in the parentheses (CH2Br2 as an internal standard).
Figure 4.
Figure 4.. Mechanistic studies on the photocatalytic decarboxylative azidation.
a NMR yield with CH2Br2 added as internal standard. b yield without NaNO3 as additive (Figure 2, entry 1). cReaction performed on a 0.26 mmol scale.
Figure 5.
Figure 5.. Proposed Mechanism
See this image and copyright information in PMC

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