When I used the search engine to find prep of this compnd, on excellent Rhodium's
page I found Vilsmeyer formylation with only 40% yield. I decided to try more recent
modification of this reaction and here is the result...
The Vilsmeyer complex was prepared by 30 min addition of 92 ml phosphoryl chloride
(freshly opened ampoule) to 150 ml anhydrous DMF with stirring and cooling in ice.
Light orange solution was then added dropwise during 2 hrs to a stirred solution
of 116 g 1,4-dimethoxybenzene (prepared from hydroquinone and dimethyl sulphate
in water, not purified, well dried) in 150 ml dry DMF at 100 - 110 deg C (bath),
a calcium chloride guard tube was used. Heating was continued for 3 hrs. Resulting
black mixture was poured onto 1 liter of ice-water, neutralized with 20% NaOH
(pH = 7-8) and extracted with CH2Cl2 (3 x 150 ml). Combined extracts were washed
with 100 ml water, dried over Na2SO4 and distilled at atmospheric pressure (bath
temp. lower than 80 deg C). Crude black product was crystallized from a mixture of
120 ml ethanol and 80 ml water. Crystallization was finished in freezer. Yield 106 g
pure, light brown 2,5-dimethoxybenzaldehyde (76%, m.p. 52-53 deg C). Isolation
of product by distillation under diminished pressure (b.p. 146 deg C / 10 mm.) should
provide somewhat higher yield, of course.
J.C.S. Perkin 1, 1974, 1353-1354
Note by Rhodium: This procedure does not work as advertised, see Post 299121
(karel: "Revision of post 70528", Chemistry Discourse) for details