Sandmeyer-Type Reaction to Pinacol Arylboronates in Water Phase: A Green Borylation Process

Abstract

Copper(I)-catalyzed cross-coupling reactions of aryl diazonium salts with bis(pinacolato)diboron can proceed smoothly in the water phase at room temperature to give the corresponding arylboronate esters in good to high yields. The Sandmeyer-type borylation not only provides direct access to arylboronates bearing halo and acidic substituents, but also achieves a green borylation process.

Key words

aryldiazonium salts - pinacol arylboronate - borylation - Sandmeyer reaction - 4-carboxylphenylboronate ester

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• 14 General Procedure for the Synthesis of 2a–n Aryldiazonium tetrafluoroborate salts (2 mmol) and B₂pin₂ (2 mmol) were resolved in MeCN (6 mL) and H₂O (3 mL). After the solution was clear, CuBr (0.1 mmol) was added. The resulting reaction mixture was allowed to stir for 5–6 h at r.t. The solution was then concentrated under reduced pressure. The residue was diluted with brine. The H₂O layer was extracted with EtOAc several times. The combined organic layer was dried over anhyd Na₂SO₄ and concen-trated under reduced pressure. The crude product was purified on silica gel by flash column chromatography with EtOAc and hexane. Compound 2a: ¹H NMR (400 MHz, CDCl₃): δ = 1.39 (s, 12 H, CH₃), 7.93 (d, J = 8.1 Hz, 2 H, HAr), 8.12 (d, J = 8.1 Hz, 2 H, HAr). MS (EI): m/z (%) = 248 (2.8) [M⁺], 233 (57.3), 162 (57.9), 149 (100), 148 (24.9), 85 (10.9), 43 (10.1)