Cobalt–Catalyzed Oxidative α-Cyanation of Tertiary Aromatic Amines with Trimethylsilyl Cyanide and tert-Butyl Hydroperoxide

Abstract

In this Letter is described that a cobalt(II)–tert-butyl hydroperoxide oxidizing system was used to catalyze the α-cyanation of aromatic tertiary amines in the presence of trimethylsilyl cyanide to produce the corresponding α-aminonitriles in good yields.

Key words

cobalt - oxidation - cyanation - aminonitriles - tertiary amines

• References and Notes

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• 11 During this study, Doyle and co-workers reported that a Co(OAc)₂-TBHP oxidizing system undertook an oxidative Mannich-type reaction through a single electron transfer, see: Ratnikov MO, Doyle MP. J. Am. Chem. Soc. 2013; 135: 1549
• 12 General Procedure To a 5 mL screw vial containing freshly distilled MeOH (0.6 mL) were successively added 1.0 M CoCl₂ in a MeOH solution (6.0 μL, 0.0060 mmol), aromatic tertiary amine (0.60 mmol), TMSCN (188 μL, 2.50 mmol), and 5.5 M TBHP in decane solution (164 μL, 0.900 mmol) under an ambient atmosphere. The resultant mixture was stirred at 60 °C (bath temperature), and consumption of the starting amine was monitored by GC analysis. After the reaction the resultant mixture was quenched with a aqueous solution of Na₂CO₃. The aqueous layer was extracted with CH₂Cl₂, then the organic phases were dried over anhyd Na₂SO₄, filtered, and evaporated under a reduced pressure. The crude product was purified by silica chromatography (hexane–EtOAc, 4:1) to give the corresponding α-aminonitrile. Selected Data for the Prepared Aminonitriles N-Methyl-N-(4-methylphenyl)aminoacetonitrile (1) ¹H NMR (500 MHz, CDCl₃): δ = 2.28 (s, 3 H), 2.95 (s, 3 H), 4.12 (s, 2 H), 6.78 (d, 2 H, J = 9.0 Hz), 7.11 (d, 2 H, J = 9.0 Hz). ¹³C NMR (125 MHz, CDCl₃): δ = 20.3, 39.4, 42.8, 115.3, 115.4, 129.8, 129.9, 145.6. MS (EI): m/z (%) = 160 (100) [M⁺]. N-Methyl-N-(4-bromophenyl)aminoacetonitrile (7) ¹H NMR (500 MHz, CDCl₃): δ = 2.99 (s, 3 H), 4.15 (s, 2 H), 6.72 (d, 2 H, J = 9.0 Hz), 7.38 (d, 2 H, J = 9.0 Hz). ¹³C NMR (125 MHz, CDCl₃): δ = 39.3, 42.1, 112.5, 115.1, 116.4, 132.2, 146.7. MS (EI): m/z (%) = 224 (100) [M⁺]. The characterization of other α-aminonitriles was confirmed by ¹H NMR and ¹³C NMR spectroscopic analysis.
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• 14 One reviewer pointed out that a reaction of CoCl₂ with TBHP initially generated Co(III)Cl₂(OH) and a tert-butyloxy radical (t-BuO^•, Equation 1, a). Those species are acceptable. However, both species would be promptly oxidized by an excess amount of TBHP to finally produce a tert-butylperoxy radical (t-BuOO^•) through the following plausible oxidation steps (Equation 1, b and c). Also, several groups reported that ruthenium-, rhodium-, or copper-catalyzed oxidative substitution of amines with TBHP, producing not the N,O-aminal derivative, but the amino peroxide derivative.^4f,13b,15 Moreover, Doyle et al. showed that Co(OAc)₂ converted TBHP into the tert-butylperoxy radical.¹¹ Therefore, we proposed the initial formation of the tert-butylperoxy radical by CoCl₂ as the plausible reaction path.
• 15 Catino AJ, Nichols JM, Nettles BJ, Doyle MP. J. Am. Chem. Soc. 2006; 128: 5648