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The methods available for the methylenation of aromatic o-dihydroxy compounds to the methylenedioxy compounds are unsatisfactory. The reaction has invariably been carried out by the action of dihalogenomethanes an the alkali salts of catechol and derivatives. The so-called methylene sulphate is here shown to be a satisfactory agent for tlie methylenation of o-dihydric phenols, monohydric phenols, alcohols, and glycols.
Methylene sulphate has been described, and was first prepared by Delepine1 by the action of paraformaldehyde on fuming sulphuric acid. He described it as a colourless crystalline solid, mp about 155°C (decomposes), which was hydrolysed to formaldehyde and sulphuric acid by heating with water or aqueous alkaline solutions and gave acetals of formaldehyde when heated with alcohols. Methylene sulphate figures incidentally in certain patents for the preparation of aromatic hydroxy-aldehydes2, and Delaby3 has prepared the methylene ether of thymol by the interaction of sodium thymoxide and methylene sulphate in acetone solution. This author refers to a thesis by Darrigarde (Paris, 1912) in which certain methylene ethers are prepared by means of methylene sulphate, but the work has apparently not been published.
Methylene sulphate has now been prepared by a modification of the methods described by Delepine and Delaby in a yield of over 60%. It is completely stable, can be kept indefinitely, and can be used in much the same way as dimethyl sulphate. The reactions of this compound may be summarised as follows.
Reaction with o-Dihydric Phenols
Catechol reacts readily with methylene sulphate in 50% acetone solution in presence of sodium hydroxide to give catechol methylene ether (methylenedioxybenzene), which thus becomes a readily accessible substance. Protocatechuic aldehyde in dilute alcoholic solution similarly gives piperonal, but the yield in this case is not so satisfactory. It was not found possible to effect the methylation of protocatechuic acid.
Methylene Sulphate1,3
To stirred molten fuming sulphuric acid containing 50% of sulphur trioxide (500 g), finely powdered paraformaldehyde (100 g) is added, the temperature being kept between 60°C and 70°C (2 hours). After 12 hours the pasty product is drained an a sintered glass filter and, air having been sucked through it for a short time, dropped in portions into a stirred mixture of water and ice. The methylene sulphate is collected, washed with cold water till almost neutral, and dried in the air and finally in a vacuum over calcium chloride; it now consists of small colourless crystals (228 g), mp 155°C (partial decomposition when rapidly heated; much slower when slowly heated). Washing the product with alcohol and ether as recommended by Delepine is only necessary for purposes of analysis. Methylene sulphate in small quantities may be crystallised from acetone, paracetaldehyde, methylal, glycol methylene ether, and dioxan; the hot solutions are not very stable.
Methylenedioxybenzene
The methylenation of catechol in benzene containing potassium carbonate, or of its disodium salt in acetone. To a mixture of catechol (80 g) in acetone (300 ml) and sodium hydroxide (80 g) in water (300 ml), air being excluded by a current of coal gas, methylene sulphate (80 g) was added in portions with vigorous stirring, so that the temperature kept at about 45°C. After 1 hours stirring, water was added, acetone and catechol methylene ether were distilled in steam, and the layer was extracted with ether; it was obtained as a colourless oil (22 g).
Piperonal
Protocatechuic aldehyde (1.4 g) in alcohol (10 ml) was treated with potassium hydroxide (4.9 g) in water (10 ml) and methylene sulphate (4.4 g), and warms till a vigorous reaction set in. After further heating an the water-bath for 5 minutes, cooling, and dilution with water, crude piperonal (0.15 g) was extracted with ether, mp 37°C after crystallisation from ligroin.