Reactive Barium-Promoted Benzylation of Diaryl Azo Compounds

Abstract

The Barbier-type benzylation of azo compounds with benzylic chlorides was achieved by using reactive barium as the promoter. Various benzylic hydrazines were synthesized from the corresponding benzylic chlorides. The thus-obtained benzylic hydrazines were further efficiently converted into benzylic amines by reductive N–N bond cleavage.

Key words

azo compounds - barium - benzylation - benzylic amines - hydrazines

• References and Notes

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• 7 The major byproducts were dimers of the benzylic chloride and 1,2-diphenylhydrazine.
• 8 Typical Experimental Procedure for the Benzylation: Synthesis of 1,2-Diphenyl-1-(1-phenylethyl)hydrazine (Entry 5 in Table 1, Entry 2 in Table 2, and Entry 1 in Table 3) To a solution of biphenyl (463 mg, 3.0 mmol) in dry THF (7.5 mL) was added freshly cut lithium (21.0 mg, 3.0 mmol). Stirring the mixture at r.t. for 2 h gave the dark-blue lithium biphenylide solution. Anhydrous BaI₂ (587 mg, 1.5 mmol) was placed in a separate flask and covered with dry THF (7.5 mL), and the mixture was stirred for 20 min at r.t. To the solution of BaI₂ in THF was added at r.t. a solution of lithium biphenylide in THF under an argon stream. The reaction mixture was stirred for 30 min at r.t. and for 10 min at –78 °C. Then, a solution of 1-phenylethyl chloride (0.199 mL, 1.5 mmol) and azobenzene (91 mg, 0.5 mmol) in THF (4 mL) was added to the resulting dark-brown suspension of reactive barium (1.5 mmol) in THF (15 mL) at –78 °C with a syringe pump (6 mL/h). After being stirred for 2 h at this temperature, the mixture was treated with sat. NH₄Cl aq (10 mL) at –78 °C, and the aqueous layer was extracted three times with Et₂O (10 mL each). The combined organic extracts were washed with Na₂S₂O₃ (10%, w/v) and brine, dried over Na₂SO₄, and concentrated in vacuo after filtration. The residual crude product was purified by column chromatography on silica gel (hexane–EtOAc, 80:1) to afford the benzylic hydrazine. The chemical yield (79%) was determined by ¹H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as an internal standard. Spectral Data of the Product ¹H NMR (400 MHz, CDCl₃): δ = 7.30–7.15 (m, 9 H, ArH), 6.94–6.92 (m, 2 H, ArH), 6.81–6.72 (m, 4 H, ArH), 5.30 (br s, 1 H, NH), 5.28–5.27 (q, 1 H, J = 7.1 Hz, CH), 1.64 (d, 3 H, J = 7.1 Hz, CH). ¹³C NMR (99.5 MHz, CDCl₃): δ = 149.7, 148.8, 140.7, 129.2, 128.4, 127.3, 119.3, 119.2, 114.5, 112.0, 59.9, 18.2. ESI-MS: m/z calcd for [C₂₀H₂₁N₂]⁺: 289.1699[M + H]⁺; found: 289.1697.
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