Martin Silicates as Versatile Radical Precursors in Photoredox/ Nickel Dual Catalysis

Abstract

A cross-coupling methodology is described based on the nickel-catalyzed connection of an aryl moiety to an alkyl radical generated by photoinduced single-electron oxidation of a silicate formed from a Martin spirosilane. Complementary to the other anionic radical precursors in photoredox catalysis, Martin silicates permit access to highly reactive alkyl radicals directly engageable in smooth C(sp²)–C(sp³) bond-formation reactions.

Key words

dual catalysis - photoredox catalysis - nickel catalysis - cross-coupling - Martin silicates - C–C bond formation

• References and Notes

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• 21 Cross-Coupling Reaction; General Procedure A flame-dried, sealed, microwave tube, purged by three argon/vacuum cycles, was charged with the appropriate silicate 1a–h (0.2 mmol, 1 equiv), NiBr₂BPhen (5.5 mg, 0.01 mmol, 5 mol%), and photocatalyst PC1 (5.7 mg, 0.01 mmol, 5 mol%) under argon, along with the appropriate aryl bromide or iodide (0.4 mmol, 2 equiv), if solid. (When the aryl bromide was a liquid, it was introduced from a microsyringe after adding the solvent under argon.) Distilled CH₂Cl₂ was then added (0.2 M) under argon, followed by three freeze–pump–thaw cycles to degas the mixture. The mixture was then irradiated with a blue LED for 24 h at r.t. The reaction was then quenched with sat. aq K₂CO₃ (10 mL) and the mixture was extracted with Et₂O (3 × 20 mL) and H₂O (3 × 20 mL). The combined organic layers were concentrated under vacuum, and the crude product was purified by flash chromatography (silica gel). 1-[4-(Tetrahydro-2H-pyran-4-yl)phenyl]-1H-pyrrole (4) Brown solid; yield: 19.2 mg (51%); mp 119.8–124.1 °C. ¹H NMR: (300 MHz, CDCl₃): δ = 7.40–7.34 (m, 2 H), 7.33–7.27 (m, 2 H), 7.09 (t, J = 2.2 Hz, 2 H), 6.37 (t, J = 2.2 Hz, 2 H), 4.19–4.07 (m, 2 H), 3.58 (dt, J = 11.3, 3.1 Hz, 2 H), 2.81 (tt, J = 10.6, 5.5 Hz, 1 H), 1.96–1.75 (m, 4 H). ¹³C NMR: (75 MHz, CDCl₃): δ = 143.5, 139.3, 127.9, 120.9, 119.5, 110.4, 68.5, 41.2, 34.1. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₅H₁₈NO: 228.1383; found: 228.1384. Methyl 6-(Tetrahydro-2H-pyran-4-yl)-2-naphthoate (11) White solid; yield: 37.9 mg (70%); mp 115.1–119.1 °C. ¹H NMR: (400 MHz, CDCl₃): δ = 8.58 (s, 1 H), 8.05 (dd, J = 8.6, 1.7 Hz, 1 H), 7.91 (d, J = 8.5 Hz, 1 H), 7.83 (d, J = 8.6 Hz, 1 H), 7.68 (d, J = 1.7 Hz, 1 H), 7.44 (dd, J = 8.5, 1.8 Hz, 1 H), 4.13 (ddt, J = 11.5, 4.3, 1.6 Hz, 2 H), 3.98 (s, 3 H), 3.59 (td, J = 11.6, 2.5 Hz, 2 H), 2.95 (tt, J = 11.6, 4.2 Hz, 1 H), 2.06–1.78 (m, 4 H). ¹³C NMR: (101 MHz, CDCl₃): δ = 167.3, 146.0, 135.8, 131.3, 130.8, 129.5, 127.8, 126.9, 126.5, 125.4, 124.6, 68.3, 52.2, 41.8, 33.7. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₇H₁₉O: 271.1329; found: 271.1327. tert-Butyl 4-(Tetrahydro-2H-pyran-4-ylmethylene)piperidine-1-carboxylate (21)Brown solid; yield: 27.0 mg (48%); mp 97–100.5 °C. ¹H NMR: (300 MHz, CDCl₃): δ = 5.06 (d, J = 8.5 Hz, 1 H), 3.99–3.87 (m, 2 H), 3.50–3.32 (m, 6 H), 2.55–2.33 (m, 1 H), 2.16 (dt, J = 31.4, 5.7 Hz, 4 H), 1.42 (s, 13 H). ¹³C NMR: (75 MHz, CDCl₃): δ = 154.9, 134.8, 128.8, 79.6, 67.8, 35.9, 33.5, 33.4, 28.8, 28.6. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₂₈NO₃: 282.2064; found: 282.2068. Methyl 6-sec-Butyl-2-naphthoate (24) Yellow oil; yield: 19.7 mg (41%). ¹H NMR: (400 MHz, CDCl₃): δ = 8.57 (d, J = 1.8 Hz, 1 H), 8.03 (dd, J = 8.6, 1.7 Hz, 1 H), 7.85 (dd, J = 27.2, 8.6 Hz, 2 H), 7.64 (d, J = 1.8 Hz, 1 H), 7.41 (dd, J = 8.5, 1.8 Hz, 1 H), 3.98 (s, 3 H), 2.86–2.73 (m, 1 H), 1.78–1.63 (m, 2 H), 1.33 (d, J = 6.9 Hz, 3 H), 0.86 (t, J = 7.4 Hz, 3 H). ¹³C NMR: (101 MHz, CDCl₃): δ = 167.6, 148.1, 136.0, 129.5, 127.9, 126.9, 125.4, 125.3, 52.3, 42.1, 31.1, 21.8, 12.4. HRMS (ESI): m/z [M + H]⁺ calcd for C₁₆H₁₉O₂: 243.1380; found: 243.1376.
• 22 For the introduction of a methyl group and its ¹³C- and ²H-labeled analogues, see: Abdellaoui M, Deis T, Wiethoff M.-A, Bahri C, Lemière G, Ollivier C, Fensterbank L. Adv. Synth. Catal. 2023; 365: 884