Selective lithiation and crystal structures of G1-carbosilane dendrimers with dimethoxybenzene functionalities

Synthetic routes for the following dendrimers are described: Si[CH2CH2Si(Me)2CH2-3,5-(MeO)2-C6H3]4 (2) and Si[CH2CH2Si(Me)2-2,4-(MeO)2-C6H3]4 (4). The crystal structures of these dendrimers have been determined by single crystal X-ray diffraction. One of the dendrimers (2) shows crystallographic S4-symmetry and intramolecular C–H⋯O bonding, whereas the other (4) shows crystallographic C2-symmetry and is imbedded in a zipper-like network of intermolecular C–H⋯O bonds. Direct lithiation of these dendrimers with n-BuLi in Et2O yields complete and selective lithiation in the aryl ring between the methoxy substituents. However, dendrimer 2 is also partially lithiated in the benzylic group. For the solid state structure of such lithiated dendrimers, a diamond-type assembly is predicted.