Selective reduction in ring size of silsesquioxane oligomers mediated by potassium metal

The trisilanol (c-C5H9)(7)Si7O9(OH)(3), generated from the hydrolytic conde nsation of cyclopentyltrichlorosilane, is converted in excellent yield to a new siloxane 3-endo-tert-butyldimethylsiloxy-1,3,5,7,9,11,14-heptacyclopen tyltetra- cyclo[7.3.3.1(3,14).1(5,11)]heptasiloxane [(c-C5H9)(7)Si7O9 (O)(O Si-Me-2 Bu-t) or C41H78O11Si8] in two high-yield steps. It is highly unusua l for a polyhedral oligosilsesquioxane to give such well diffracting crysta ls, and even more so for it to crystallize virtually without disorder. Mole cules of this siloxane occur in the crystal as isolated units, free from th e silanol hydrogen-bonding interactions that dominate the structural charac teristics and associated reaction chemistry of the parent polysilanols.