The Synthesis of Immobilised Reagents Based on a Polymerised Triphenylphosphine Scaffold for Use in Flow Chemistry Applications

Abstract

Throughout the years, there have been challenges associated with the purification of some critical reactions which have proved to be time consuming and tedious. However, these difficulties have been grandly simplified by the use of immobilized reagents which can be readily removed by filtration and thus decreasing the purification time. An added advantage, is the incorporation of the solid-supported reagents onto flow reactors to facilitate in-line purification thus minimizing exposure to hazardous chemicals, waste and reaction time for some protocols. In this thesis the application of polymer-bound triphenylphosphine was compared using commercial unsupported, commercial polystyrene-supported and in-house modified polymer-supported triphenylphosphine reagents in the ozonolysis, Wittig and Suzuki-Miyaura coupling reactions. Validation of the supports was compared under both batch and flow conditions in each case. Our in-house polymer-supported triphenylphosphine reagent, formed by precipitation polymerisation of diphenyl (4-vinylphenyl)phosphine, showed a phosphorus loading of 1.85 mmol/g and was benchmarked against commercial polystyrene-supported triphenylphosphine which had a phosphorus loading of 3.00 mmol/g. Firstly, we demonstrated the application of polymer-bound triphenylphosphine as a reductant in ozonolysis reactions under batch-flow hybrid conditions. The transformations were conducted using our newly developed ozonolysis flow setup which allowed reactions to be performed at ambient temperatures. Aldehydes were formed in moderate to high yields of 52-70% at room temperature, with cooled temperatures favouring the formation of the analogous acids. Of key importance was that the in-house polymer-supported triphenylphosphine reagent showed an improvement in yields after 48 hours relative to the commercial polystyrene reagent, with a four-fold reduction in the amount of reductant used. Secondly, we demonstrated the conversion of commercial polystyrene-supported triphenylphosphine and in-house polymer-supported triphenylphosphine to Wittig benzyl phosphonium salts which gave a phosphorous elemental loading of 0.818 mmol/g and 1.614 mmol/g respectively. The formation of alkene products was observed at 30 °C when using in-house polymer-supported phosphonium salt but product formation was only observed at 60 °C for the analogous commercial polystyrene support. The Wittig reactions under batch and flow conditions gave high yields for the unsupported phosphonium salt. In contrast, the commercial and in-house polymer-supported phosphonium salts gave varying yields, which were substrate dependant. Lastly, we transformed in-house polymer-supported triphenylphosphine reagent into polymer-supported tetrakis(triphenylphosphine)palladium catalysts under batch and flow conditions affording palladium loadings of 0.460 mmol/g and 0.689 mmol/g respectively. The prepared catalysts were benchmarked against commercial polystyrene-supported tetrakis(triphenylphosphine)palladium catalyst which has a reported palladium loading of 0.500 mmol/g. Demonstrating their use in Suzuki-Miyaura coupling reactions under batch and flow conditions showed comparative results. We observed an increase in yield for both the commercial and in-house catalysts under flow conditions compared to batch use with varying yields achieved with a single reaction. We were also able to readily recycle the substrates across the catalyst house in a packed-bed reactor affording close to quantitative conversion. Overall, our in-house polymer-supported reagent and catalyst were an improvement to the commercially available polymer-supported counterparts. For the ozonolysis reactions, fourfold less reductant was used, reaction activity was observed at lower temperatures for the Wittig reaction and our catalyst offered an economical alternative to expensive reagent in Suzuki-Miyaura reactions. Although the unsupported reagents gave higher yields, the approach suffered from complicated purification protocols to remove unreacted triphenylphosphine and spent triphenylphosphine oxide. When employing flow conditions using solid-supported reagents/catalysts this was greatly simplified as these reagents remained in the packed-bed reactor.