Diol-functionalised polystyrene resins : synthesis and applications
Chapter 1 introduces the motivation behind and the methods for the use of insoluble polymers for the heterogenisation of well-defined homogenous catalysts. The use of diol ligands and polymer-supported catalysts in the Diels-Alder reaction and dialkylzinc addition to aldehydes is also reviewed. Finally, the aims of this work are outlined. Chapter 2 evaluates various routes for the synthesis of diol-functionalised resins. The grafting of triol compounds to preformed chloromethylated resins was undertaken through formation of an ether linkage, leaving a pendant diol. Emphasis was placed on incorporating an alkyl spacer between the diol moiety and the resin in order to distance the supported Lewis acid from the polymer matrix in subsequent catalysis applications. A relationship between the size of alcohol substrates and ease of reaction with chloromethylated resins is reported; due to mass transport effects, the grafting of larger residues is considerably more difficult. Chapter 3 discusses the synthesis of polystyrene resins possessing a chiral diol-functionality introduced via asymmetric dihydroxylation of alkenes. The asymmetric dihydroxylation process is reviewed. Two contrasting strategies for diol-functionalised resin preparation are described: (a) solution-phase alkene dihydroxylation followed by grafting and (b) dihydroxylation of an alkene moiety already bound to the solid-phase. Chapter 4 investigates supported-diol-titanium species as Lewis acids and probes the effect of alkyl spacer length on their activity in the Diels-Alder reaction of methyl acrylate and cyclopentadiene. The incorporation of a nine-carbon methylene spacer led to an improvement in catalyst activity. The application of polymer-bound Ti-based Lewis acids in the asymmetric Diels-Alder reaction of methacrolein and cyclopentadiene and diethylzinc addition to benzaldehyde is also reported. In both cases low ee's were observed.