Photoinduced Metal-Free Arylboration of Unactivated Alkenes: Synthesis of Indoline Boronic Ester

Abstract

The environmentally benign synthesis of indoline boronic esters, especially through a way of arylboration to alkenes, remains a challenge due to the use of transition metals or high-temperature conditions. We described a photoinduced metal-free arylboration of unactivated alkenes for the synthesis of indoline boronic esters and 1,2,3,4-tetrahydroquinoline boronic ester in good yields. This approach showed good compatibility and great efficiency for a range of allylphenylamines as well as alkylamine. Remarkably, this transformation also suggested that the base is not necessary for photosensitizer-free diboron reagent mediated mild generation of aryl radical. Furthermore, compared to previously reported methods, this approach is mild and environmentally benign.

Key words

arylboration - photoinduced - indoline boronic ester - metal-free - unactivated alkenes

Publication History

Received: 18 August 2023

Accepted after revision: 29 September 2023

Accepted Manuscript online:
29 September 2023

Article published online:
03 November 2023

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• 18a Procedure for the Synthesis of Compound 2a To a Schlenk tube were added 1a (0.2 mmol) and bis(catecholato)diboron (61.8 mg, 1.3 equiv). The reaction vessel was evacuated and backfilled with argon for three times. Dimethylformamide (2.0 mL) was added. The reaction mixture was stirred under blue LEDs (30 W) irradiation at room temperature for 36 h. A solution of pinacol (95.0 mg, 4.0 equiv) in triethylamine (0.3 mL, 11.2 equiv) was added to the mixture. After 3 h, water (5 mL) was added, and the aqueous layer was extracted with ethyl acetate (5 × 3 mL). The combined organic layers were dried over magnesium sulfate, filtered, and concentrated. The product was purified by flash column chromatography on silica gel with PE/EtOAc = 10:1 as eluent to give product 2a in 84% yield.
• 18b Analytical Data of 2a ¹H NMR (400 MHz, CDCl₃): δ = 7.68 (d, J = 8.2 Hz, 2 H), 7.60 (d, J = 8.1 Hz, 1 H), 7.22 (d, J = 8.2 Hz, 2 H), 7.17 (t, J = 7.7 Hz, 1 H), 7.08 (d, J = 7.4 Hz, 1 H), 6.97 (t, J = 7.4 Hz, 1 H), 4.13 (t, J = 9.7 Hz, 1 H), 3.48 (dd, J = 10.4, 7.5 Hz, 1 H), 3.34–3.26 (m, 1 H), 2.36 (s, 3 H), 1.22 (s, 6 H), 1.19 (s, 6 H), 1.14 (dd, J = 16.2, 5.2 Hz, 1 H), 0.80 (dd, J = 16.1, 9.3 Hz, 1 H). ¹³C NMR (101 MHz, CDCl₃): δ = 143.9, 141.4, 137.3, 134.1, 129.6, 127.7, 127.4, 124.0, 123.6, 114.7, 83.5, 57.6, 35.9, 24.9, 24.7, 21.6, 17.0 (br)* (*the carbon attached to boron is broadened due to quadrupolar relaxation). HRMS (ESI⁺): m/z calcd for C₂₂H₂₈BNO₄S [M + H]⁺: 414.1905; found: 414.1902.
• 18c Analytical Data of 2g<¹H NMR (400 MHz, CDCl₃): δ = 7.64 (d, J = 8.2 Hz, 2 H), 7.55 (dd, J = 8.8, 4.5 Hz, 1 H), 7.23 (d, J = 8.1 Hz, 2 H), 6.87 (td, J = 8.8, 2.5 Hz, 1 H), 6.80 (dd, J = 8.2, 1.9 Hz, 1 H), 4.14 (dd, J = 10.7, 9.1 Hz, 1 H), 3.49 (dd, J = 10.8, 7.5 Hz, 1 H), 3.33–3.08 (m, 1 H), 2.38 (s, 3 H), 1.23 (s, 6 H), 1.21 (s, 6 H), 1.06 (dd, J = 16.2, 5.6 Hz, 1 H), 0.76 (dd, J = 16.2, 9.0 Hz, 1 H). ¹³C NMR (101 MHz, CDCl₃): δ = 159.83 (d, J = 242.0 Hz), 144.1, 139.70 (d, J = 8.0 Hz), 137.44 (d, J = 2.0 Hz), 133.7, 129.7, 127.4, 116.01 (d, J = 8.5 Hz), 114.16 (d, J = 23.5 Hz), 111.46 (d, J = 24.0 Hz), 83.6, 58.0, 36.0 (d, J = 1.6 Hz), 24.9, 24.7, 21.6, 16.7 (br)* (*the carbon attached to boron is broadened due to quadrupolar relaxation). ¹⁹F NMR (376 MHz, CDCl₃): δ = –119.33. HRMS (ESI⁺): m/z calcd for C₂₂H₂₇BFNO₄S [M + H]⁺: 432.1811; found: 432.1810.
• 18d Analytical Data of 2r ¹H NMR (400 MHz, CDCl₃): δ = 7.64 (dd, J = 8.2, 6.4 Hz, 2 H), 7.53 (dd, J = 8.0, 4.7 Hz, 1 H), 7.16 (d, J = 8.0 Hz, 2 H), 7.12–7.08 (m, 1 H), 7.01 (t, J = 7.4 Hz, 1 H), 6.91– 6.85 (m, 1 H), 3.89 (td, J = 10.1, 2.7 Hz, 1 H), 3.71 (dd, J = 10.2, 7.0 Hz, 0.43 H), 3.56 (dd, J = 10.5, 7.0 Hz, 0.69 H), 3.38–3.33 (m, 0.69 H), 3.25 (dd, J = 15.6, 6.5 Hz, 0.43 H), 1.29 (tt, J = 14.6, 7.4 Hz, 1 H), 1.14 (d, J = 12.2 Hz, 7.5 H), 0.99 (d, J = 18.4 Hz, 4.4 H), 0.80 (d, J = 7.6 Hz, 1.3 H), 0.60 (d, J = 7.6 Hz, 1.9 H). ¹³C NMR (101 MHz, CDCl₃): δ = 143.9, 143.8, 142.3, 142.1, 134.8, 134.7, 134.1, 133.9, 129.6, 127.7, 127.4, 127.4, 125.0, 124.3, 123.4, 123.0, 114.1, 83.4, 83.2, 55.0, 53.3, 42.0, 41.2, 24.9, 24.7, 24.6, 24.5, 21.6, 20.6 (br)* (*the carbon attached to boron is broadened due to quadrupolar relaxation), 12.3, 10.3. HRMS (ESI⁺): m/z calcd for C₂₃H₂₀BNO₄S [M + H]⁺: 428.2061; found: 428.2061.