Alkylation with Carbonyl Compounds The Prins Reaction

Alkylation with Carbonyl Compounds The Prins Reaction. Carbonyl compounds react with alkenes in the presence of Brpnsted acids to form a complex mixture of products known as the Prins reaction. The use of appropriate reaction conditions, solvents, and catalysts allows one to perform selective syntheses. Characteristically formaldehyde is the principal aldehyde used. Mineral acids (sulfuric acid, phosphoric acid), p-toluenesulfonic acid, and ion exchange resins are the most frequent catalysts. Certain Lewis acids (BF3, ZnCl2, SnCl4) are, however, also effective. 

Simple mono- and disubstituted alkenes react to yield 1,3-diols, when the Prins reaction is carried out at elevated temperature. Diols originate from the attack of water on carbocation 18, or through the acidolysis of dioxanes under the reaction conditions. When the reaction is conducted in acetic acid, monoacetates are formed by acetate attack on 18. Dienes resulting from the dehydration of intermediate diols are the products of the transformation of more substituted alkenes. Monoacetates and diols may react further to yield 1,3-diol diacetates. When the Prins reaction 

3-Dioxacyclohexanes can be produced in excellent yields from aliphatic or aryl-substituted alkenes.64 Dilute sulfuric acid at or above room temperature with paraformaldehyde appears to give the best results. Dioxane-water or acetic acid as solvent was found to afford increased yields in the Prins reaction of arylalkenes. 

The stereochemistry of the Prins reaction is complex. In the transformation of cyclohexene and 2-butenes anti stereoselective addition was observed,67-69 whereas syn addition of two formaldehyde units takes place in the formation of 1,3-dioxanes from substituted styrenes.70 Most of the transformations are, however, nonstereo-selective,71 72 accounted for by carbocation 18. 

Few examples of the Prins reaction of homologous aldehydes and ketones are known.73,74 Acetaldehyde was shown to react smoothly with isobutylene under mild conditions, whereas other, less substituted alkenes were found to be very unreactive.73 

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