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20th November 2011 @ 06:19
Vilsmeier-Haack Synthesis of 2,5-dimethyl-1H-(p-tolyl)-pyrrole-3-carboxaldehyde



DMF (1.5 mL, 19.4 mmol, 6.5 equiv.) was stirred under a nitrogen atmosphere in an ice-bath. Phosphoryl chloride (0.308 mL, 3.3 mmol, 1.1 equiv.) was added and the reaction stirred for 30 minutes. Reaction mixture was a very pale yellow. A solution of 2,5-dimethyl-1H-(p-tolyl)-pyrrole (556 mg, 3 mmols, 1 equiv.) in DMF (2 mL) was added dropwise over 1 minute. After 5 minutes, flask removed from ice and reaction stirred for a further 55 mins. TLC at 1 hr showed reaction at completion. Reaction mixture was then poured onto ice (50 mL) and 1M NaOH added (pH 11), until adjusted to pH 6. Total volume approximately 50 mL after ice has melted and solution was pH 3 the next day. 20% NaOH was added to achieve pH 11 and flask bathed in a brine ice-bath for 20 minutes. Product filtered and washed with water to produce a wet brown paste. Product was dissolved in MeCN and concentrated under a nitrogen atmosphere. Brown solution with precipitation of fine crystals was observed overnight. Solution was cooled in an ice bath and filtered. Filter cake washed with water (3 x 10 mL). Filtered product was then dried under vacuum to produce a lumpy grey-brown powder (423 mg, 71%).

Collected data: 2,5-dimethyl-1H-(p-tolyl)-pyrrole-3-carboxaldehyde

mpt: 109-111oC
1H-NMR (300 MHz, CDCl3): δ 9.86 (s, 1H), 7.30, 7.33 (d, 2H), 7.06, 7.09 (d, 2H), 6.37 (s, 1H), 2.44 (s, 3H), 2.27 (s, 3H), 1.98 (s, 3H)
13C-NMR (75 MHz, CDCl3): δ 185.18, 138.98, 138.94, 134.30, 131.08, 130.18, 127.65, 121.82, 105.63, 21.19, 12.64, 11.19
m/z (ESI+/-): 214 [M+H]+, 100%
IR: 810.51, 1421.53, 1514.85, 1651.24, ~3934.95

For data files, refer to Vilsmeier Haack Synthesis of 2,5-dimethyl-1-(4-methylphenyl)-1H-pyrrole-3-carbaldehyde (LMW 5-1)

References:
doi: 10.1002/cmdc.200600026
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