cyclo-Boratrisiloxane and cyclo-diboratetrasiloxane derivatives and their reactions with amines: crystal and molecular structure of (p-BrC6H4BO)2(Ph2SiO)2

The new cyclo-diboratetrasiloxanes (RBO)2(R′2SiO)2 (R=p-BrC6H4, o-MeC6H4, m-NH2C6H4, m-NO2C6H4, Bun; R′2=Ph2, R=Ph, p-BrC6H4, m-NO2C6H4; R′2=octn/Me) and cyclo-boratrisiloxanes (RBO)(R′2SiO)2 (R=p-BrC6H4, m-NH2C6H4; R′2=Me2, Ph/Me) were prepared in high yield by condensation reactions. cyclo-Diboratetrasiloxane·amine adducts (1:1) were obtained for (PhBO)2(Ph2SiO)2 with cyclohexylamine, triethylamine, piperidine, and isobutylamine and for (o-MeC6H4BO)2(Ph2SiO)2 with morpholine; in contrast the cyclo-boratrisiloxanes were unreactive towards amines. The Lewis acidity of these borasiloxane rings was determined by the Gutmann’s acceptor number (AN) method with well-defined ranges observed for the cyclo-diboratetrasiloxanes (46–62) and the cyclo-boratrisiloxanes (22–28). The crystal and molecular structure of (p-BrC6H4BO)2(Ph2SiO)2 is reported. New compounds were characterised by elemental analysis, m.p., 1H- and 11B-NMR, and IR spectroscopy; solids were characterised by solid-state 11B MAS-NMR spectroscopy.