Elucidating factors leading to acidolytic degradation of sterically strained oligoether dendrons

Abstract

While steric hindrance can be particularly large in dendritic molecules, it is usually implicated with difficulties in the synthesis of higher generation structures and restricted access of reagents, including bond-cleaving agents, to the dendritic interior. A different situation, where the steric hindrance is translated into a steric strain within the dendritic molecule and, consequently, causes enhanced decomposition of the dendron-containing structure, has only occasionally been reported and exclusively for dendronized polymers. In this work we describe post-synthetic cleavage of sterically congested third-generation oligoether dendrons from solid supports, followed by their disassembly into monomeric building blocks under acidic conditions. This disassembly was monitored by H NMR, revealing the intermediate fragment structures and the exact order of bond cleavage within the dendritic molecule. These conclusions were supported by MS analysis of the cleavage mixtures. Though distinguishing between steric and electronic reasons of molecular disassembly can be a challenging task, we were able to analyze these factors separately by monitoring the dendron disassembly and comparing the rates of “decay” of parent dendrons and their fragments. This comparison reveals that while the electron donation of the steric congestion-inducing alkyl substituents is a prerequisite for the disassembly of the structures, this disassembly is very strongly accelerated in the sterically crowded dendrons and intermediates, where both steric and electronic factors contribute in a synergistic way to the disassembly phenomenon.