Conversion of Azobenzenes into N,N′-Diarylhydrazines by Sodium Dithionite

Abstract

A number of chloro-, methyl- and methoxy-substituted azobenzenes have been reduced to the corresponding hydrazines by using an aqueous solution of Na₂S₂O₄. The yield is generally excellent, but two compounds, viz. 4,4-dimethoxyazobenzene and 2,2,4,4,6,6-hexamethylazobenzene, gave no hydrazine at all.

Key words

azobenzenes - sodium dithionite - reduction - hydrazines

References and Notes

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9

General Procedure for the Preparation of Azobenzenes 1a-k from Anilines
Oxygen was bubbled through a mixture of an aniline derivative (20 mmol), (CuCl)₂ (0.20 g, 2.0 mmol), and dry pyridine (20 mL), which was stirred at r.t. for 1-3 h. The reaction was quenched by addition of H₂O (20 mL), which in some cases gave a precipitate, in other cases not. When no precipitate was formed, the hydrolyzate was extracted with Et₂O (3 × 10 mL) and the combined extracts were dried (MgSO₄), filtered, and concentrated on a rotary evaporator. Purification by flash chromatography (silica, hexane-EtOAc in a ratio of 90:10) gave a solid, which was recrystallized from EtOH and dried under vacuum. When the azobenzene precipitated during the hydrolysis, the precipitate was filtered, washed with H₂O on the filter, and dried under vacuum. The products were characterized by IR, ¹H NMR, and ¹³C NMR spectroscopy, mass spectrometry, and melting points (when appropriate), which were compared with literature data.

10

All the products were known from the literature: 1a [29418-34-6]: ref. 8k, 8m; 1b [29418-25-5]: ref. 8f, 8g, 8j; 1c [29418-31-3]: ref. 8f, 8g, 8i, 8k, 8m; 1d [77611-71-3]: ref. 8c, 8r, 8s; 1e [5692-66-0]: ref. 6l, 8d, 8e, 8g, 8h; 1f [7334-33-0]: ref. 8k, 8m, 8p, 8t, 8u, 8v; 1g [15426-14-9]: ref. 8k, 8m, 8n, 8p, 8q, 8t; 1h [1602-00-2]: ref. 8f, 8g, 8j-m, 8p, 8q, 8s, 8t; 1i [613-55-8]: ref. 8k, 8m, 8n, 8p, 8t, 8u; 1j [6319-23-9]: ref. 8t, 8u, 8v; 1k [501-58-6]: ref. 8j, 8k-m, 8o, 8p, 8u.

11

General Procedure for the Reduction of Azobenzenes to N , N ′-Diarylhydrazines
Azobenzene 1 (0.4-2.1 mmol) was added to a mixture of H₂O (165 mL/mmol of 1), MeOH (165 mL/mmol of 1), and CH₂Cl₂ (25 mL/mmol of 1). The resulting mixture was stirred and heated to reflux, and an excess of sodium dithionite (see Table [1] ) was added. After stirring at reflux (see Table [1] ), the product mixture was poured into ice and extracted with Et₂O (3 × 100 mL). The combined extracts were dried (MgSO₄) and filtered, and then the solvent was removed under reduced pressure to give a residue, from which the product 2 was isolated by flash chromatography (silica, hexane-EtOAc in a ratio of 90:10). The results are compiled in Table [1] .

12

All the N,N′-diarylhydrazines except 2,2′,4,4′-tetramethyl-(N,N′-diphenylhydrazine) (2b) are mentioned several times in the literature: 2a [107418-14-4]: ref. 8c, 8m, 13f, 13j; 2c [63615-06-5]: ref. 8m, 13f; 2d [142068-90-4]: ref. 8c; 2e [5692-66-0]: ref. 8m, 13a; 2f [782-74-1]: ref. 8m, 13g, 13h; 2g [953-01-5]: ref. 8m, 8n, 8q, 13e, 13g; 2h [953-14-0]: ref. 8m, 8p, 8q, 13b-d, 13g, 13h; 2i [787-77-9]: ref. 8m, 8n, 8p, 13e, 13g, 13h, 13j; 2j [1027-32-3]: ref. 13g, 13h, 13j; 2k [1027-40-3]: ref. 8m, 13b-d. The synthesis and isolation of 2b have been reported by Nölting and Stricker,^8c but no data except the melting point were given.
Data for 2b: mp 119-121 °C (lit.^8c mp 120-122 °C). IR (film): 3364, 3228, 3008, 1627, 1510, 1463, 1444, 1276, 1240, 1153, 1012, 875, 814 cm^-1. ¹H NMR (200 MHz, CCl₄): d = 2.16 (s, 6 H), 2.19 (s, 6 H), 5.19 (s, 2 H), 6.59-6.76 (m, 6 H). ¹³C NMR (50 MHz, CCl₄): d = 16.9, 20.3, 111.1, 120.1, 127.4, 127.6, 130.7, 143.7. MS (EI): m/z (%) = 240 (0.5) [M⁺], 194 (5), 182 (2), 172 (2), 163 (3), 147 (2), 131 (5), 110 (2), 107 (6), 106 (100), 105 (93), 98 (4), 88 (7), 87 (8), 85 (5), 78 (17), 77 (98), 76 (5), 74 (12). HRMS (EI): m/z calcd for C₁₆H₂₀N₂ [M⁺]: 240.1626; found: 240.1622.

15

Preparation of 2,4,6-Trimethylaniline
2,2′,4,4′,6,6′-Hexamethylazobenzene (1e, 0.50 g, 1.88 mmol) was added to a mixture of H₂O (200 mL), MeOH (200 mL), and CH₂Cl₂ (30 mL). The resulting mixture was stirred and heated to reflux, and Na₂S₂O₄ (2.00 g, 11.5 mmol) was added. After stirring at reflux for 3 h, the product mixture was poured into ice and extracted with Et₂O (3 × 100 mL). The combined extracts were dried (MgSO₄) and filtered, and the solvent was subsequently removed under reduced pressure to give a residue, from which 0.42 g (82%) of 2,4,6-trimethylaniline was isolated by flash chromatography (silica, hexane-EtOAc in a 90:10 ratio). The spectroscopic and physical properties of the product were identical to those of the aniline derivative used to prepare azobenzene 1e.