Zinc-Catalyzed Markovnikov-Type Hydroisothiocyanation of Alkenes with Ammonium Thiocyanate

Abstract

A ZnI₂-catalyzed addition of ammonium thiocyanate to olefins in the presence of 4-toluenesulfonic acid and tetrabutylammonium iodide has been developed. The reaction proceeds by a Markovnikov-type process and a radical isomerization, and gives the corresponding isothiocyanates selectively and in good yields.

Key words

zinc catalysis - Markovnikov reaction - isothiocyanates - thiocyanates - alkenes - isomerization

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• 12 (2-Isothiocyanatoethyl)benzene (2a); ⁵ Typical Procedure ZnI₂ (9.6 mg, 0.03 mmol) was added to a mixture of styrene (31.2 mg, 0.3 mmol), NH₄SCN (25.1 mg, 0.33 mmol), and Bu₄NI (22.2 mg, 0.06 mmol) in DCE (1.0 mL), and the mixture was stirred at 80 °C for 18 h. The residue was dissolved in Et₂O and the solution was washed with H₂O and sat. aq NaCl, dried (MgSO₄), and purified by TLC (silica gel, hexane) to give a colorless liquid; yield: 39.3 mg (80%). ¹H NMR (400 MHz, CDCl₃): δ = 7.40–7.37 (m, 2 H), 7.34–7.31 (m, 3 H), 4.91 (q, J = 7.0 Hz, 1 H), 1.67 (d, J = 7.0 Hz, 3 H). ¹³C{¹H} NMR (100 MHz, CDCl₃): δ = 140.1, 132.0, 128.9, 128.2, 125.4, 57.0, 25.0.
• 13 2-Phenylethyl Thiocyanate (3a);^3d Typical Procedure ZnI₂ (9.6 mg, 0.03 mmol) was added to a mixture of styrene (1a; 31.2 mg, 0.3 mmol), NH₄SCN (25.1 mg, 0.33 mmol), and Bu₄NI (22.2 mg, 0.06 mmol) in DCE (1.0 mL), and the mixture was stirred at 40 °C for 18 h. The residue was dissolved in Et₂O, the solution was washed with H₂O and sat. aq NaCl, dried (MgSO₄), and purified by TLC (silica gel, 5% Et₂O–hexane) to give a colorless liquid; yield: 26.3 mg (54%). ¹H NMR (500 MHz, CDCl₃): δ = 7.40–7.35 (m, 5 H), 4.61 (q, J = ­7.3 Hz, 1 H), 1.88 (d, J = 7.3 Hz, 3 H). ¹³C{¹H} NMR (125 MHz, CDCl₃): δ = 139.0, 129.1, 129.0, 127.1, 111.7, 48.5, 22.0.