Adamantanes, Nortricyclenes, and Dendrimers with Extended Silicon Backbones

By reaction of the hexabromoheptasilane MeSi(SiMe 2 SiMeBr 2 ) 3 ( 1 a ) with H 2 O, H 2 S, NH 3 , and H 2 NMe the heptasilaadamantanes MeSi(SiMe 2 SiMeO) 3 ( 4 ), MeSi(SiMe 2 SiMeS) 3 ( 5 ), MeSi(SiMe 2 SiMeNH) 3 ( 6 a ), and MeSi(SiMe 2 SiMeNMe) 3 ( 6 b ), respectively, were prepared in good to moderate yields. Molecular structures of 4 , 5 , 6 a , and 6 b were determined by X‐ray crystallography. The symmetry of the cages is approximately C 3 v , and the geometry around the nitrogen atoms is essentially planar. Ab initio SCF/HF calculations with the 6‐31G* basis set confirm these results. Reduction of MeSi(SiMe 2 Si t BuBr 2 ) 3 ( 1 b ) with lithium naphthalenide afforded the heptasilanortricyclene MeSi(SiMe 2 Si t Bu) 3 ( 7 ). The 29 Si NMR spectrum of 7 consists of three signals with chemical shifts that agree closely with values predicted by ab initio calculations. 29 Si INADEQUATE spectra also strongly support the nortricyclene structure. Ab initio SCF/HF calculations were performed for the parent molecule Si 7 H 10 , and the ring strain of the cage was estimated as 168.8 kJ mol −1 by using the homodesmic reaction Si 7 H 10 + 3 Si 2 H 6 →Si 13 H 28 . Compound 1 a also served as the starting material for the preparation of first‐generation dendrimer 2 a by reaction with six equivalents of Ph 2 MeSiLi. Subsequent protodearylation with HBr and reaction with (Me 2 PhSi) 2 SiMeK afforded second‐generation dendrimer 3 . All dendrimers were characterized by multinuclear NMR spectroscopy.