Photoinduced Decarboxylative Borylation of N-Hydroxyphthalimide Esters with Hypoboric Acid
Bálint Nagy, Zsombor Gonda, Tamás Földesi, Péter Pál Fehér, András Stirling*, Gergely L. Tolnai*, Zoltán Novák*
*HUN-REN, H-1117 Budapest; Eötvös Loránd University, Institute of Chemistry, H-1117 Budapest, Hungary,
Email: stirling.andras
ttk.mta.hu, tolnaichem.elte.hu,
novakzttk.elte.hu
B. Nagy, Z. Gonda, T. Földesi, P. P. Fehér, A. Stirlig, G. L. Tolnai, Z. Novák, Org. Lett., 2024, 26, 2292-2296.
DOI: 10.1021/acs.orglett.4c00511
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Abstract
Visible-light mediates a photochemical conversion of redox-active esters of primary, secondary, and tertiary alkylcarboxylic acids with hypoboric acid into the corresponding boronic esters. Quantum chemical calculations and mechanistic considerations provide deeper insights into the mechanism of photochemical borylation reactions.
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proposed mechanism
Details
The document discusses a study on the photoinduced decarboxylative borylation of N-hydroxyphthalimide (NHPI) esters using hypoboric acid. This visible-light-driven photochemical transformation converts primary, secondary, and tertiary alkylcarboxylic acids into boronic esters without the need for catechol or added photocatalysts. The process utilizes hypoboric acid and redox-active esters to achieve a simple and economical synthesis. Quantum chemical calculations and mechanistic studies provide insights into the reaction mechanism, revealing that the formation of boron radicals is thermodynamically favorable. The optimal conditions involve using 1.5 equivalents of hypoboric acid and pinacol in DMF under 440-445 nm light at 21°C, achieving up to 81% yield. The study also explores the synthetic utility of this method across various carboxylic acids, producing boronic esters in moderate to excellent yields. The reaction mechanism suggests that boron radicals interact with the redox-active ester to form radical species, which then undergo visible light-induced decomposition to yield the desired boronic esters. This method offers a cost-effective and efficient alternative to traditional borylation techniques, providing valuable mechanistic insights and broad applicability in organic synthesis.
General procedure for borylation
The appropriate redoxactive ester (0.50-3 mmol, 1.0 equiv) and tetrahydroxydiboron (0.75-4.5 mmol, 1.5 equiv) were measured to a 20 mL screw capped vial with rubber septa and a magnetic stir bar. The vial was sealed, evacuated and refilled with Ar three times and 2-12 mL of abs. DMF (0.25 M) was added. The reaction vial was put into a thermostated photoreactor and stirred at 21°C for 3 hours. The reaction was monitored with GC and after the reaction was completed, the mixture was removed from the reactor. Then pinacol (0.75-4.5 mmol, in triethylamine (2-12 mL, 0.325M) was added and stirred at room temperature for 1 h. The reaction mixture was diluted with ethyl acetate (60 mL) or diethyl ether (60 mL, if the product was volatile). The mixture was extracted with water (2 × 30 mL) and the combined organic layer was washed with saturated LiCl (30 mL). The organic phase was filtered over anhydrous Na2SO4 and concentrated. The crude product was purified with column chromatography using silica gel, with a gradient of hexanes - EtOAc or pentane - Et2O if the product was volatile.
Key Words
alkylboronates, photochemistry
ID: J54-Y2024
