The use of chlorodimethylsilane-mediated reductive etherification reaction in macromolecular engineering

Abstract

The reaction of a carbonyl compund (aldehydes or ketones) with a silane-based reducing agent along with a Brønsted or a Lewis acid yields symmetric or unsymmetrical ethers, and this reaction is called a reductive etherification reaction (RER) in organic chemistry. This strategy has been applied at the macromolecular level to some extent and has been out of scenery for about 20 years. Chlorodimethylsilane (CDMS) is a silane compound that act as both a Lewis acid and a reducing agent, and has been empolyed in RER in molecular level and showed promising results to obtain unsymmetrical ethers in the presence of an alcohol. Since it does not require additional addivites, CDMS-mediated RER can be considered as a new and straightforward method to synthesize ethers. In this thesis, we present three different examples of the CDMS-mediated RER in macromolecular levels to revive RER in order to explore the scope of this reaction. In the first study, we describe a straightforward post-polymerization modification (PPM) methodology from polyketone via CDMS-mediated RER. The polyketone platform was prepared via acyclic diene metathesis (ADMET) polymerization and reacted with a variety of alcohols in the presence of CDMS to create a wide range of polymers possessing pendant functional alkoxy structures under mild conditions. The effect of parameters such as solvent and the amount of reactants on RER were studied. We have also explored the effect of other silane compounds on RER, none of which provided as high efficiency as CDMS-mediated one. It has been found that the primary alcohols yielded corresponding alkoxy structures in high efficiencies, high isolated yields, and a wide range of functional group tolerance. It was also found that the formation of polyalcohol was inevitable, to some extent, during the RERs in addition to the alkoxy-functional polymers. All the polymers obtained were characterized in detail by various spectroscopic measurements and a mechanistic aspect was also presented to evaluate the product distributions. Since operationally simple chemical methods, with high yield and functional group tolerance, that can be easily adapted to macromolecular level are always desired in synthetic polymer chemistry, this study promises remarkable opportunities for synthesizing functional polymers and may pave a way for the development of a novel metal-free PPM method. In the second study, CDMS-mediated reductive etherification reaction is introduced as a versatile strategy for polyether synthesis. Accordingly, terephthalaldehyde (TPA) and 1,4-butanediol were first reacted at room temperature in the presence of CDMS using nitromethane as the polymerization solvent to reveal the optimum conditions for the proposed system. Subsequently, a variety of diols ranging from linear to sterically congested diols were reacted with TPA (and its isomers) under the optimized conditions to create a polyether library. Meanwhile, in addition to polyether having the expected alternating units, the formation of polyether stem from the self-condensation of TPA was found to be inevitable in all cases. From the proposed strategy, polyethers with a molecular weight up to 110.4 kDa and a high alternating unit up to 93% were obtained. The versatile and robust character of the presented strategy was supported by a model end-group study and the polymerization behavior was examined mechanistically. It is anticipated that the presented method might be a strong candidate for polyether synthesis with different backbones, given the unlimited sources of diols. In the third study, we have focused on synthesising polythioacetals (PTAs) using CDMS as a catalyst. In one of our previous works, when CDMS was used to perform RER on a pendant aldehyde polymer platform along with thiols, thioacetal formation was observed. This result was not surprising since CDMS has both Lewis acid and reducing agent character, yet, strikingly unexpected. Inspired by this study, PTA synthesis using CDMS as a catalyst was proposed. Optimization of polymerization conditions was first carried out by comparing traditional acid sources with CDMS. The progress of polymerizations were kinetically monitored and CDMS was found to perform best. Various parameters such as equivalent of reactants, solvent, and time on polymerization were then examined. Finally, a library of PTA was created using a variety of aldehydes and dithiols from moderate to high molecular weights. A mechanistic approach for the polymerization was proposed and finally a model degradation study on a representative PTA was performed in the presence of hydrogen peroxide.