2C-B synthesis via 4-Bromo-2,5-Dimethoxyphenylacetonitrile

2C-B synthesis via 4-Bromo-2,5-Dimethoxyphenylacetonitrile by Karel & Rhodium

2-Bromo-1,4-dimethoxybenzene

Method A – bromine and hydrogen peroxide in acetic acid

2 ArH + Br2 + H2O2 —> 2 ArBr + 2 H2O

To ice-cooled and stirred mixture of 138 g 1,4-dimethoxybenzene (DMB) and 600 ml acetic acid was carefully added a solution of 26 ml bromine in 50 ml acetic acid during 20 min. Then 52 ml of 30% hydrogen peroxide were dropwise added during 30 min. Ice bath was then removed and stirring was continued for 30 min. Then the mixture was poured into 3 l water and dark red liquid product was extracted with CH2Cl2. Work-up and yield – see Method B.

Method B – hydrobromic acid and hydrogen peroxide in dichloromethane/water

ArH + HBr + H2O2 —> ArBr + 2 H2O

To water-cooled and stirred mixture of 138 g 1,4-DMB, 200 ml CH2Cl2, 150 ml water and 115 ml 48% hydrobromic acid were dropwise added 110 ml of 30% hydrogen peroxide during 1 h. The reaction mixture was stirred for 1 day (TLC control) and then filtered through Celite to allow separation of layers. Water layer was extracted with 50 ml CH2Cl2, organic extracts were dried over Na2SO4 and evaporated in vacuo to give 193 g of dark red liquid (1). Crude product was distilled at 0.2 kPa with aid of 15 cm Vigreux column – 30 g of forerun were collected at 55-90°C (2) and then 144 g (66% yield) of pure product at 90-100°C (3).

Notes:

1. Method A and B gives similar crude products consisting of 82% 2-Br-1,4-DMB, 12% unreacted 1,4-DMB and 6% 2,5-diBr-1,4-DMB (molar % estimated from NMR).
2. It is mixture containing cca 15 g of 1,4-DMB and 15 g 2-Br-1,4-DMB, from which part of 1,4-DMB crystallizes on standing; and can be reused in another bromination of 1,4-DMB.
3. From brown distillation residue, which often spontaneously crystallizes on standing, 2,5-diBr-1,4-DMB can be isolated by dissolving it in warm CH2Cl2 and slow evaporation of majority CH2Cl2 on air to afford 10 g of yellowish crystals of 2,5-diBr-1,4-DMB.

4-Bromo-2,5-dimethoxybenzylchloride

ArH + CH2O + HCl —> ArCH2Cl + H2O

To vigorously stirred mixture of 130g 2-Bromo-1,4-Dimethoxybenzene, 58 ml 35% HCl, 24 g 1,3,5-trioxane (1) and 130 mg cetyltrimethylammonium bromide (2) in 500 ml RBF was introduced gaseous HCl (about 17 g was necessary) at 80°C (oil bath) during 2 h (3). The hot mixture was then poured into 1000 mL ice/water while stirring with a glass rod to avoid the crude product forming lumps. Filtration, several washings with cold water and drying over KOH pellets in vacuum dessicator provided 156 g of light yellow crude product (4). One crystallization from minimum amount of dry acetone gave 81 g (51% yield) of white crystals (5). The pure material melts at 110°C [6]

Notes:

1. This is a modification of the method of Quelet[6], who used 325g 2-Bromo-1,4-Dimethoxybenzene and 175g 35% CH2O and bubbled gaseous HCl during 2 h. My «38% CH2O» gave only 40% yield of chloromethylated product in that reaction (probably due to partial decomposition and polymerization of old formalin solution), so I chose stable and water soluble 1,3,5-trioxane as formaldehyde source.
2. Tetradecyltrimethylammonium bromide as PTC has been used in bromomethylation reaction of arenes with 1,3,5-trioxane and 48% HBr/acetic acid [1].
3. The crude product often crystallized from the reaction mixture. If not, continue heating for another hour.
4. This is a mixture of 68% 4-Br-2,5-diMeO-BnCl, 30% 6-Br-2,5-diMeO-BnCl and trace probably 3-Br-2,5-diMeO-BnCl as indicated NMR spectrum.

   4-Br-2,5-diMeO-BnCl:
   1H NMR (CDCl3,400MHz) 7.10 (s,1H), 6.95 (s,1H), 4.61 (s,2H), 3.87 (s,3H), 3.84 (s,3H).
   13C NMR (CDCl3,100MHz) 151.4 (s), 149.9 (s), 125.6 (s), 116.4 (d), 114.1 (d), 112.0 (s), 56.9 (q), 56.4 (q), 41.1 (t).

   6-Br-2,5-diMeO-BnCl:
   1H NMR (CDCl3,400MHz) 6.86 (s,1H), 6.84 (s,1H), 5.47 (s,2H), 3.86 (s,6H).

5. From NMR. Purification of crude product at benzylchloride stage is much easier than at next benzylcyanide stage. Even chromatographical separation of 4-Br-2,5-diMeO-BnCN and 6-Br-2,5-diMeO-BnCN was fruitless.

2C-B analytical profile [4,5]

2CB freebase
1H NMR (CDCl3,400MHz) 7.02 (s,1H), 6.74 (s,1H), 3.84 (s,3H), 3.77 (s,3H), 2.90 (t,J=6.9Hz,2H), 2.72 (t,J=6.9Hz,2H), 1.35 (broad s,2H)
13C NMR (CDCl3,100MHz) 151.9 (s), 149.6 (s), 128.3 (s), 115.7 (d), 114.8(d), 108.8 (s), 56.9 (q), 56.0 (q), 42.1 (t), 34.9 (t)
2CB hydrochloride
1H NMR (D2O,400MHz) 7.14 (s,1H), 6.88 (s,1H), 3.78 (s,3H), 3.74 (s,3H), 3.17 (t,J=6.9Hz,2H), 2.88 (t,J=6.9Hz,2H)

2C-B freebase FTIR: 1039, 1210, 1381 and 1491 cm-1[2].

Mass Spectrum of 2C-B [3]

References

[1] Synlett 55 (1989)
[2] J. Chrom. Sci. 33, 583-590 (1995)
[3] J. Health Sci. 48(1), 14–21 (2002)
[4] (GC) J. Chrom. Sci. 36, 23-28 (1998)
[5] (MS/NMR/FTIR/HPLC) J. Anal. Toxicol. 22(5), 345–354 (1998)
[6] Quelet, Bull. Soc. Chim. Fr. 1953, C46
[7]
[8]
[9]
[10]