Synthesis of Aryl Thiocyanates via Copper-Catalyzed Aerobic Oxidative Cross-Coupling between Arylboronic Acids and KSCN

Abstract

A new protocol for the preparation of aryl thiocyanates by the cross-coupling reaction of arylboronic acids with KSCN salt is described. The coupling reaction was catalyzed by 20 mol% of copper acetate in the presence of 2.0 equivalents 4-methylpyridine serving both as ligand and base under 0.2 MPa of molecular oxygen. A variety of arylboronic acids, including those with substituents at ortho position, were suitable under the reaction conditions.

Key words

aryl thiocyanates - arylboronic acids - potassium thio­cyanate - copper acetate - oxidative cross-coupling reaction

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• 18 General Procedure for the Preparation of Aryl Thiocyanates To a Teflon-lined stainless steel autoclave (250 mL) was added aryl boronic acid (10 mmol), KSCN (1.07 g, 11 mmol), Cu(OAc)₂ (0.36 g, 2 mmol), 3 Å MS (2.5 g), and MeCN (25 mL). The autoclave was closed and charged with oxygen to 0.2 MPa. The autoclave was then set into the preheated (80 °C) oil bath. After the reaction proceeded within 12 h, the autoclave was cooled to r.t. and carefully depressurized. The mixture in the autoclave was filtered to remove the catalyst and 3 Å MS and washed with MeCN. The combined filtrate was concentrated under reduced pressure. The residual was purified by flash chromatography through a silica column using PE–EtOAc as the eluent under the protection of N₂ to afford the desired products. The products were characterized by IR, ¹H NMR, ¹³C NMR, and HRMS. Phenyl Thiocyanate (2a) Yellowish oil. IR (neat): ν = 2173 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.43–7.56 (m, 5 H). ¹³C NMR (125 MHz, CDCl₃): δ = 110.5, 124.4, 129.5, 130.06, 130.23. HRMS (EI): m/z calcd for C₇H₅NS [M]⁺: 135.0143; found: 135.0140. CAS Reg. No. 5285-87-0. 2-Methoxyphenyl Thiocyanate (2c) Yellow oil. IR (neat): ν = 2153 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 3.94 (s, 3 H), 6.94–7.59 (m, 4 H). ¹³C NMR (125 MHz, CDCl₃): δ = 56.2, 110.5, 111.4, 113.1, 122.1, 129.9, 130.5, 156.5. HRMS (EI): m/z calcd for C₈H₇NOS [M]⁺: 165.0248; found: 165.0252. CAS Reg. No. 14372-66-8. 4-Fluorophenyl Thiocyanate (2i) Colorless oil. IR (neat): ν = 2153 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.15–7.59 (m, 4 H). ¹³C NMR (125 MHz, CDCl₃): δ = 110.6, 117.6 (d, J = 22.7 Hz), 119.2 (d, J = 3.3 Hz), 133.2 (d, J = 8.9 Hz), 163.6 (d, J = 250.1 Hz). HRMS (EI): m/z calcd for C₇H₄NSF [M]⁺: 153.0048; found: 153.0048. CAS Reg. No. 2924-02-9. 2-Bromophenyl Thiocyanate (2k) Pink oil. IR (neat): ν = 2161 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 7.25–7.74 (m, 4 H). ¹³C NMR (125 MHz, CDCl₃): δ = 109.6, 121.8, 127.1, 129.07, 129.44, 130.1, 133.6. HRMS (EI): m/z calcd for C₇H₄NSBr [M]⁺: 212.9248; found: 212.9238. CAS Reg. No. 55757-32-9. 4-(Methoxycarbonyl)phenyl Thiocyanate (2n) White soild; mp 62.7 °C. IR (KBr): ν = 2153, 1708 cm^–1. ¹H NMR (500 MHz, CDCl₃): δ = 3.96 (s, 3 H), 7.57–8.12 (m, 4 H). ¹³C NMR (125 MHz, CDCl₃): δ = 52.6, 109.1, 128.5, 130.47, 130.93, 131.21, 165.8. HRMS (EI): m/z calcd for C₉H₇NO₂S [M]⁺: 193.0198; found: 193.0203. CAS Reg. No. 1879-22-7.