I like this one: Tetrahedron Letters 39 (1998), 8013-8016, "The Ultrasound Promoted Knoevenagel Condensation of Aromatic Aldehydes"
In this paper several aromatic aldehydes are reacted with nitromethane in GAA at 22°C for 3h using ammonium acetate catalyst. The reaction is done in an ultrasound cleaning bath. Yields of nitrostyrenes for the following aldehydes are: 2,3-diMeO-benzaldehyde 99%, piperonal 99%, 2,4,6-triMeO-benzaldehyde 85%, 4-AcNH-benzaldehyde 95%, 4-MeS-benzaldehyde 96%, indole-3-carboxaldehyde 93%, vanillin 89%. The reaction provided nitroalcohols instead of nitroalkenes in the case of: 4-Cl-benzahydehyde 70%, 4-nitrobenzaldehyde 61%, 3-nitrobenzaldehyde 51%. For these an additional diisopropylethylamine catalyst was needed. I bet the nitroalcohols could be dehydrated to the nitroalkenes.
The researchers also tried reacting several aldehydes with malonic acid to give the corresponding cinnamic acids in yields ranging from 61% to 99% (3-nitro being the worst, 4-Cl and plain benzaldehyde being the best ones). Pyridine was the solvent and piperidine the catalyst.
"General procedures. Nitromethane and pyridine were freshly distilled. Ultrasound reactions were peroformed using a branson 5510 ultrasound bath or a Crest Tru-Sweep ultrasonic cleaner with little difference. Method A: A mixture of aldehyde (20.0 mmol), nitromethane (13.0 ml) and ammonium acetate (3.324 g), sonicated at 22°C for 3h. After removal of nitromethane, partition between dichloromethane and water then brine gave crude product which was recrystallized from aq. ethanol (except 4-AcNH-benzaldehyde, AcOH). Method B: A solution of aldehyde (1.00 mmol), nitromethane (1.0 ml), ammonium acetate (2.5 mmol) and diisopropylethylamine (0.1 mmol) was sonicated at 22°C for 3-6h, according to TLC. After removal of solvents , work-up as above gave crude product which was purified on silica gel. Method C: A solution of aldehyde (1.00 mmol), malonic acid (2.18 mmol) in pyridine (5.7 mmol) and piperidine (0.20 mmol) was sonicated at 22°C for 3h. Solution added to 5% aq. HCl, chilled and filtered by suction. The precipitate was recrystallized from aq. ethanol."
They also mention that "...the 2,4,6-trimethoxy derivative 1c gave nitroalkene 5c (84%) demonstrating steric factors to be of little detriment." which is nice
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So, how about them 2,5-dimethoxybenzaldehyde or even 4-fluorobenzaldehyde?
understanding is everything