I have been searching and searching and trying and trying for years to find a cheap condensation/dehydration catalyst which doesn't require refluxing GAA or nitromethane to get good yields. Finally I have found it. Methylamine.
A general method for nitrostyrenes is as follows:
100 mmol of a aromatic aldehyde
105 mmol nitromethane or nitroethane
15-20 mmol methylamine as a 10-20% aqueous solution
Enough MeOH, EtOH or IPA to make the mixture stirable. Usually about 25 ml
To a 250 ml rb-flask equipped with a magnetic stirbar is added the aromatic aldehyde, nitromethane and the alcohol. Sirring is started and the aqueous methylamine solution is added in one portion. The reaction flask is placed in a water bath and heated to 40-50°C. The reaction progress can be monitored by TLC to see when the aldehyde is consumed. In most cases 45 minutes in the water bath is enough.
When the reaction is over 25 mmol GAA is added to the reaction mixture and the flask is placed in the freezer until the crystallisation is complete [1]. The solids are then broken up with a spatula, transferred to a filter funnel and washed with water to remove most of the methylamine acetate. The nitrostyrene is then recrystallised from MeOH, EtOH or IPA.
1-phenyl-2-nitroethene, 92%
1-phenyl-2-nitropropene, 93%
1-(4-ethoxyphenyl)-2-nitroethene, 88%
1-(4-ethoxyphenyl)-2-nitropropene, 90%
1-(2-methoxyphenyl)-2-nitroethene, 94%
1-(2-methoxyphenyl)-2-nitropropene, 89%
1-(4-methoxyphenyl)-2-nitroethene, 92%
1-(4-methoxyphenyl)-2-nitropropene, 88%
1-(2,4-dimethoxyphenyl)-2-nitroethene, 97%
1-(2,4-dimethoxyphenyl)-2-nitropropene, 74%
1-(2,5-dimethoxyphenyl)-2-nitroethene, 97%
1-(2,5-dimethoxyphenyl)-2-nitropropene, 89%
1-(2,4,5-trimethoxyphenyl)-2-nitroethene, 89%
1-(2,4,5-trimethoxyphenyl)-2-nitropropene, 94%
1-(2,4,6-trimethoxyphenyl)-2-nitroethene, 95%
1-(2,4,6-trimethoxyphenyl)-2-nitropropene, 96%
1-(3,4,5-trimethoxyphenyl)-2-nitroethene, 92%
1-(3,4,5-trimethoxyphenyl)-2-nitropropene, 93%
3-(2-nitrovinyl)-1H-indole, 79%
3-(2-nitropropenyl)-1H-indole, 72%
5-methoxy-3-(2-nitrovinyl)-1H-indole, 83%
5-methoxy-3-(2-nitropropenyl)-1H-indole, 91%
[1] In case of 1-(2,5-dimethoxyphenyl)-2-nitroethene crystallisation occurs during the reaction. After 20 minutes at 50°C the reaction mixture becomes a bright neon-orange solid cake.