The synthesis of acyclic and cyclic acetals are described in the chemical literature, and in patent literature such as U.S. Pat. Nos. 2,678,950; 2,888,492; 2,915,530; 2,987,524; 3,014,924; and the references cited therein.
Conventional processes for the direct condensation of alpha, beta-unsaturated aldehydes and polyols in the presence of acid catalysts yield large portions of impurities, principally beta-alkoxyacetals and beta-alkoxyaldehydes which are formed by the addition of the polyols across the alpha, beta-double bond of the aldehydes. As a consequence of these undesirable side reactions, attempts have been made to prevent the formation of the aforementioned impurities by reducing the temperature and catalyst concentration; however, these modifications lead to impractically low rates of reaction.
To overcome the disadvantages of conventional processes, U.S. Pat. No. 3,014,924 proposes reacting an alpha, beta-unsaturated aldehyde with an alphatic polyol bearing at least two hydroxyl groups bonded to different atoms in the polyol molecule in the presence of a catalyst comprising a highly-porous solid carrier having a surface area of at least 75 square meters per gram and about 0.025 to 1.0 millimole per unit weight of carrier of a strong mineral acid.
Preferably, polyol and catalyst are added to a suitable reaction vessel together with a water-immiscible solvent which forms an azeotrope with water and the aldehyde. Next, the the alpha, beta-unsaturated aldehyde is added slowly to the reaction mixture. During the reaction, water, unreacted aldehyde and azeotroping agent are continuously distilled, the water is separated, then the agent and the aldehyde are returned to the reaction vessel. Preferably the azeotroping agent is at least partially miscible with both reactants. Such preferred agents are, for example, xylene, toluene, benzene, cyclohexane, chloroform, diisobutylene, and hexane.
U.S. Pat. No. 2,888,492 describes a process for producing cyclic acetals which involves reacting an acrolein type aldehyde with a polyol in the presence of 0.02 to 0.06 mole percent based on the amount of ethylenic aldehyde present of a sulfo acid such as sulfuric acid, p-toluene sulfonic acid, ethanesulfonic acid, and the like. The reaction is carried out by heating a mixture of the chosen alpha, beta-ethylenic aldehyde and polyol, preferably containing about 5 to 50% excess of aldehyde over the stoichiometric requirement for the reaction, dissolved, or suspended in a suitable liquid such as, for instance, benzene, dichloroethylene, and the like. By refluxing at about 50.degree. C to 90.degree. C under a phase-separating head until the theoretical amount of water is removed, the reaction is completed in about 1 to 3 hours, and moderate yields of unsaturated cyclic acetals are obtained.
As it is apparent from consideration of the above described processes, there remains a need for process for direct condensation of alpha, beta-unsaturated aldehydes and polyols which is rapid and efficient and amenable to plant-scale operation.
Accordingly, it is an object of the present invention to provide an improved process for condensation of alpha, beta-unsaturated aldehydes and polyols to form acetals.
It is another object of this invention to provide a continuous process for converting acrolein into 2-vinyl-1,3-dioxane compounds.
It is a further object of the present invention to provide a commercially feasible process for converting acrolein into 3-(1',3'-dioxane)propionaldehyde derivatives.
Other objects and advantages shall become apparent from the accompanying description and examples.