Synthesis of polyaryl rigid-core carbosilane dendrimers for supported organic synthesis

Abstract

Carbosilane dendrimers can be used as soluble supports for organic synthesis, since their structure allows separation of excess reagents from the supported products, eventually yielding products of high purity and in high yield, as in solid-phase organic synthesis (SPOS). In previous studies often loss of dendritic material during filtration was observed, due to the rather flexible structure of the conventional dendrimers. In order to improve the diafiltration retention of the carbosilane dendrimers, the synthesis of carbosilane dendrimers based on more rigid-core molecules was investigated. Both 1,3,5-tris(4-bromophenyl)benzene 1 and tetrakis(4-bromophenyl)silane 2 were selected on the basis of their rigid structure and suitable functional groups for further functionalization using organolithium chemistry. An optimized halogen lithium exchange (HLE) protocol was developed for the synthesis of 2 via 1-bromo-4-lithiobenzene. This protocol involves reaction of an aryl bromide with n-BuLi at room temperature, followed by partial removal of the solvent by evaporation (70%, v/v), addition of pentane to promote precipitation of the aryl lithium compound, and centrifugation and removal of the solvent to obtain, after repeating the last two steps once, the wet aryl lithium compound in pure form. This HLE protocol was proven to be effective for mono- and dilithiation, as well as for polylithiation reactions of aryl bromides. Furthermore, the rigid tris(4-bromophenyl)chlorosilane wedge 3 was synthesized to add a rigid generation to the prepared core molecules, and bromotriallylsilane 4 was synthesized for the introduction of triallylsilyl moieties on the periphery of the core molecules. With these four building blocks several rigid-core carbosilane dendrimers were synthesized, which can be applied as better retainable soluble supports for organic synthesis in a diafiltration setup