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Synlett 2007(11): 1695-1698  
DOI: 10.1055/s-2007-982565
LETTER
© Georg Thieme Verlag Stuttgart · New York

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

Leiv K. Sydnes*a, Shire Elmia, Per Heggena, Bjarte Holmelida, Didrik Malthe-Sørensenb
a Department of Chemistry, University of Bergen, Allégt. 41, 5007 Bergen, Norway
Fax: +4755589490; e-Mail: leiv.sydnes@kj.uib.no;
b GE Healthcare, Lindesnes Fabrikker, 4510 Spangereid, Norway
Further Information

Publication History

Received 7 March 2007
Publication Date:
25 June 2007 (online)

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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

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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.