Octaphenyl POSS: Synthesis, Structure and Ortho Bromination
Octaphenyloctasilsesquioxane, Ph 8 Si 8 O 12 , is the aromatic counterpart of octamethylsilsesquioxane and one of the most consequential building blocks in cage chemistry, because its eight phenyl rings are not inert decoration but eight addressable reaction sites. First prepared by K. Olsson in 1958 in 9% yield, the compound is now accessible in 95% yield by a two-step route. What makes it distinctive is that electrophilic substitution on those rings disobeys the regiochemical rules learned for isolated arenes: bromination proceeds in the ortho position, with 85% selectivity, and requires no Lewis acid catalyst at all. Topics covered in this post: preparation of octaphenyl-POSS from Olsson's hydrolysis to the Kawakami two-step route via a cyclic tetrasiloxanetetraol; the single-crystal structure and its O h symmetry; uncatalyzed ortho bromination, iron-mediated ortho / meta bromination and regioselective para iodination with ICl; type II halogen bonds and the Br 4 synt...