Selective Propargylation of Diaryl Azo Compounds Using Metallic Barium

Abstract

The Barbier-type propargylation of azo compounds with α,γ-disubstituted propargylic tosylates was achieved by using metallic barium as the promoter. Various propargylated hydrazines (α-adducts) were exclusively synthesized from the corresponding propargylic tosylates and azobenzenes (diaryldiazenes). The thus-obtained propargylic hydrazines were further efficiently converted into propargylic amines by reductive N–N bond cleavage. Benzidine rearrangement of the propargylic hydrazines was also attempted.

Key words

azo compounds - barium - hydrazines - propargylation - propargylic tosylates

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• 15 Typical Experimental Procedure for the Barbier-Type Selective Propargylation of Diaryl Azo Compounds: Synthesis of 1-(6,6-Dimethylhept-4-yn-3-yl)-1,2-diphenylhydrazine (3ea, Table [2], Entry 2) Freshly cut barium (small pieces, 103.0 mg, 0.75 mmol), propargylic tosylate 1h (220.8 mg, 0.75 mmol), and azobenzene (45.6 mg, 0.25 mmol) were placed in a Schlenk tube (25 mL) under an argon atmosphere and covered with dry THF (1 mL). The mixture was stirred for 14 h at room temperature. The mixture was treated with sat. aq NH₄Cl (10 mL), and the aqueous layer was extracted three times with Et₂O (10 mL each). The combined organic extracts were washed with brine, dried over Na₂SO₄, and concentrated in vacuo after filtration. The residual crude product was purified by column chromatography on silica gel (hexane–MeOH, 50:1) to afford propargylic hydrazine 3ea (65.5 mg, 85% yield). ¹H NMR (400 MHz, CDCl₃): δ = 7.25–7.15 (m, 4 H, Ar–H), 7.05–7.03 (d, 2 H, J = 8.2 Hz, Ar–H), 6.91–6.85 (m, 3 H, Ar–H), 6.77–6.74 (t, 1 H, J = 7.3 Hz, Ar–H), 5.67 (br, 1 H, NH), 4.43–4.39 (t, 1 H, J = 7.5 Hz, CH), 1.89–1.66 (m, 2 H, CH₂), 1.16 (s, 9 H, 3 CH₃), 1.04–1.00 (t, 3 H, J = 7.4 Hz, CH₃). ¹³C NMR (99.5 MHz, CDCl₃): δ = 150.8, 149.0, 129.1, 128.9, 120.7, 118.9, 116.2, 112.1, 94.9, 75.1, 58.1, 31.4, 31.1, 27.4, 11.4. IR (neat): 3311, 2965, 2359, 1600, 1496, 1362, 1308, 1239, 1170, 1092, 1025, 992, 857, 819, 745, 691, 628 cm^–1. MS (ESI): m/z calcd for [C₂₁H₂₇N₂]⁺ [M + H]⁺: 307.2169; found: 307.2170; mp 57–60 °C.