α-Selective Allylation of Azo Compounds Using Allylic Barium Reagents

Abstract

The addition of allylic barium reagents to azo compounds was achieved with high α-regioselectivity. The double-bond geometry of allylic barium reagents was retained throughout the reaction at –78 °C and E- or Z-enriched allylic hydrazines were selectively obtained from the corresponding allylic barium reagents. An allylic hydrazine was efficiently converted into an allylic amine by reductive N–N bond cleavage.

Key words

allylation - allylic amines - azo compounds - barium - hydrazine

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• 11 α-Selective Allylation: Synthesis of (E)-1-(3,7-Dimethylocta-2,6-dien-1-yl)-1,2-diphenylhydrazine (Scheme 2 and Table 2, Entry 2); Typical Procedure: To a solution of biphenyl (370 mg, 2.4 mmol) in anhydrous THF (5 mL) was added freshly cut lithium (16.7 mg, 2.4 mmol). Stirring the mixture at r.t. for 2 h gave a dark-blue lithium biphenylide solution. Anhydrous BaI₂ (470 mg, 1.2 mmol) was placed in a separate flask, and this was covered with anhydrous THF (5 mL) and stirred at r.t. for 20 min. 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 at r.t. for 20 min, then to the resulting dark-brown suspension of reactive barium (1.2 mmol) in THF (10 mL) was added a solution of geranyl chloride (0.185 mL, 1.0 mmol) in THF (2 mL) at –78 °C by syringe pump (6 mL/h). After stirring for 20 min, the mixture was treated with a solution of azobenzene (91.1 mg, 0.5 mmol) in THF (2 mL) at –78 °C (slow addition at 6 mL/h) and stirred for 10 min at this temperature. To the mixture was added sat. aq NH₄Cl (5 mL), and the aqueous layer was extracted with Et₂O (3 × 10 mL). The combined organic extracts were washed with sodium thiosulfate (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, 20:1 then 80:1) to afford the allylic hydrazine. The chemical yield was determined to be 77% by ¹H NMR spectroscopic analysis using 1,4-bis(trimethylsilyl)benzene as an internal standard, and the α/γ and E/Z ratios were determined to be >99:1 and 99:1, respectively, by ¹H NMR and HPLC analyses. ¹H NMR (500 MHz, CDCl₃): δ = 7.26–7.20 (m, 2 H, ArH), 7.01 (dd, J = 7.7, 1.0 Hz, 2 H, ArH), 6.84–6.79 (m, 4 H, ArH), 5.57 (br s, 1 H, NH), 5.28 (dt, J = 1.5, 6.8 Hz, 1 H, CH), 5.03 (tt, J = 8.0, 1.4 Hz, 1 H, CH), 4.10 (br s, 2 H, CH), 2.11–2.00 (m, 4 H, CH), 1.68 (d, J = 0.8 Hz, 3 H, CH), 1.59 (s, 3 H, CH), 1.58 (d, J = 0.85 Hz, 3 H, CH); ¹³C NMR (125 MHz, CDCl₃): δ = 150.2, 147.8, 141.3, 131.8, 129.3 (2C), 129.1 (2C), 123.9, 119.7, 118.7, 117.8, 113.5 (2C), 112.9 (2C), 48.1, 39.6, 26.3, 25.7, 17.7, 16.2; IR (neat): 3321, 2965, 2922, 1600, 1495, 1596, 1254, 749, 692 cm^–1; HRMS (ESI): m/z [M + H]⁺ calcd for [C₂₂H₂₉N₂]⁺: 321.2325; found: 321.2322.