Another route to 2C-[BDE]

by Yellium

2,5-Dimethoxytoluene [1]

A solution of 100 mmol (13.8 gr) 1,4 dimethoxybenzene in dry THF (80 ml) was added dropwise to a solution of 1.6 M BuLi (69 mL, 110 mmol) in dry THF (30ml) at rt under argon and stirred for 1h. Reaction was brought to approx. -80'C using a bath of ethylacetate, cooled with liquid nitrogen. MeI (25 ml, 400 mmol) was added via syringe at -78'C to the above solution and after 1 h at rt the mixture was quenched with saturated aqeous NH4Cl solution. Separation of the organic layer, and extraction of the water layer with CH2Cl2, drying of the combined organic fractions over Na2SO4, and removal of the solvent under vacuum afforded 18,4 gr 2,5-dimethoxytoluene as an oil, which was used in the next step without further purification.

Reaction of this compound with BuLi at 0'C for 10 minutes, followed by addition of 1 equivalent DMF, 10 minutes stirring, and workup with dilute HCl yielded only a small amount of impure aldehyde. This was most likely due to a too short reaction time, and wet DMF. Destillation of the reaction mixture yielded 13.4 gr 2,5-dimethoxytoluene, followed by 3 gr of an unknown yellow/greenish liquid.

2,5-dimethoxytoluene -> 4-bromo-2,5-dimethoxytoluene (after [2])

13.4 gr 2,5-dimethoxytoluene was dissolved in 200 ml DCM, to which 15.5 gr nbs was added. After 39h stirring at room temperature, followed by the classical workup (washing with sodium sulfite, drying over Na2SO4, and evaporation of the solvent), 18.9 gr of slightly brown-colored 4-bromo-2,5-dimethoxytoluene was obtained. Impurities could be removed by washing with a little ether, yielding almost colorless crystals.

Attempts to prepare the grignard reagent from the above bromide were futile.

4-bromo-2,5-dimethoxytoluene -> 2,5-dimethoxy-4-methylbenzaldehyde

17.1 gr of the above bromide was dissolved in 200 mL dry ether, cooled to -80 using ethylacetate/LN2, was added 55 ml of 1.6M BuLi, letting the mixture return to room temperature in the course of one hour, followed by the addition of 5.9 gr dry DMF in 20 ml ether over a 2 minute period. After stirring at room temperature for another hour, a few hundred mL of aqeous HCl was added, the organic layer was separated, and the aqeous phase was extracted with 3x100ml DCM. The organic extracts were combined, and the solvent was removed under vacuum yielding 8.2 gr brown-colored aldehyde (61%), which crystalized upon standing.

2,5-dimethoxy-4-methylbenzaldehyde -> 2,5-dimethoxy-4-methyl-beta-nitrostyrene [3]

8.2 gr impure aldehyde was dissolved in 40 ml IPA, followed by addition of 3.4 gr nitromethane and 0.55 gr ethylenediammonium diacetate. After 24h stirring at room temperature the solid chunk of orange stuff was filtered and washed with a little cold IPA. After drying, 9.0 gr nitrostyrene was obtained. Reduction of this nitrostyrene using 8.2 gr LAH gave about 6 gr 2-CD hydrochloride. [4]

This reaction was also succesfully employed using ethyl iodide in the first step, giving 2C-E. Yields are comparable, at some points slightly higher. (FYI:12.8 gr 2.5-dimethoxyethylbenzene was obtained from 10.3 gr 1,4-dimethoxytoluene, 51.5 ml 1.6 M BuLi and 24 ml EtI; which gave 18.1 gr 4-bromo-2,5-dimethoxyethylbenzene from the reaction with NBS. 18.1 gr 2.5dmebr, 53 mL 1.6M BuLI and 5.7 ml dry DMF in 20 ml dry Et2O gave 9.0 gr aldehyde, which finally resulted in 10.2 gr nitrostyrene.)

Care must be taken that the DCM layer in the bromination step is thoroughly washed with sodium sulfite, removing all color. If not, a nasty brown contamination will follow you through the formylation, hindering crystallization. It does not affect nitrostyrene formation, however. Another point: 4-bromo-2,5-dimethoxyethylbenzene is a lot more soluble in Et2O than its methyl counterpart.

And regarding BuLi: 1.6M solutions in hexane can be handled with syringes & septums. As long as you flush your environment with N2 or Ar, nothing spectacularly explosive happens. Drying solvents ghetto style was done by dumping a lot of NaH in Et2O/THF, letting it stand for a few hours and then taking the required amount of solvent by syringe. Sure, not as good as refluxing with benzoquinone & distillling, but a hell of a lot easier :-)

Another bedtime story:

1,4-dimethoxybenzene-> 1-bromo-2,5-dimethoxybenzene. [2]

14.1 gr 1,4-dimethoxybenzene was dissolved in 190 ml ch2cl2, and 18.2 gr nbs was added. After 36h stirring at room temperature, the mixture was washed with sodium sulfite, dried over Na2SO4, and the solvent evaporated. In contradiction with the results of the original authors, there was still an appreciable amount of 1,4-dimethoxybenzene present, which was removed by filtering the liquid 1-bromo-2,5-dimethoxybenzene through a pasteur pipette with a small amount of glass wool in it. Final yield: 14.7 gr 1-br-2.5dmb.

Reaction of this bromide in a similar way as described above with BuLi and DMF gave about 8 gr 2,5-dimethoxybenzaldehyde.

(For those who don't want to use BuLi, it could be useful to try to form the grignard reagent with Mg, and reacting that with DMF as in [5]. Experiences in this lab has shown that formation of the grignard reagent of 1-bromo-2,5-dimethoxybenzene is quite difficult to get started.)

Excerpt from [5]:

A solution of 20 mmol N,N, dimethylformamide in Et2O or THF (3oml) is slowly added to the corresponding freshly prepared grignard reagent (21mmol) in Et2O or THF (50 ml) at 0'C under nitrogen over a period of 5 minutes. An instant exothermic reaction takes place with the formation of a sticky white precipitate. The mixture is then brought to room temperature and stirred for one hour. Subsequently, the reaction mixture is carefully quenched with 3 N HCl until the solution becomes acidic. The product is extracted with ether (3x50ml), the ether layer washed with water (150ml), aqeous sodium hydrogen carbonate solution (100ml) and saturated sodium chloride solution (150 ml). After drying of the ether layer with anhydrous magnesium sulfate, the solvent is evaporated to provide the carbonyl product. The carbonyl product is further purified by recristallization or distillation.

Yield for benzaldehyde, from bromobenzene: 88%.


[1] Tetrahedron Asymmetry, 1997, 913
[2] Syn. Comm. 28, 1998, 2087-2095
[4] PIHKAL, #23
[5] Synthesis 1984, 228