Selective Prins reaction of styrenes and formaldehyde catalyzed by 2,6-di-tert-butylphenoxy(difluoro)borane

The sterically congested Lewis acid 1 was used as a catalyst in the Prins reaction of various styrenes and formaldehyde. 4-Aryl-1,3-dioxanes 5 were selectively formed as the exclusive products of the reaction with styrenes 4a-i and vinylthiophenes 4j-k. The reaction proceeded in most cases with good to excellent yields (55-99%). Styrenes which carried a strongly electron-withdrawing group (CN, CO2Me) did not react. The reaction with beta-alkylstyrenes 6 was successful for the methyl substituted substrate 6a and yielded (88%) trans-5-methyl-4-phenyl-1, 3-dioxane (7a) preferentially (dr = 75:25). For steric reasons, other beta-alkylstyrenes 6b-d did not react. The remarkable stereodiscrimination attained by catalyst 1 was employed in the regioselective transformation of 4-propenyl styrene (10) to dioxane 11 (83% yield).