Synthesis of P,N-chelate phosphaalkene–oxazoline ligands and their applications in asymmetric catalysis

• Main Author: Dugal-Tessier, Julien
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/31408

This thesis outlines the design, synthesis and utilization of phosphaalkene-based ligands for asymmetric catalysis. Transition metal catalysis studies that utilize achiral phosphaalkene-based ligands are reviewed in Chapter 1. In addition, the synthesis and reactivity of phosphaalkenes are briefly introduced in this chapter. The reactivity of a palladium(II) phosphaalkene complex [MesP=CPh(2-py)⋅PdCl₂] bearing the smaller P-Mes substituent compared to the traditional Mes* is described in Chapter 2. This complex was found to be a competent catalyst for the Overman–Claisen rearrangement with yields ranging from 33% to 91%. In Chapter 3, a modular route to a set of chiral phosphaalkene–oxazoline [PhAk–Ox, R′P=CR′′(C(i-Pr-Ox)R₂)] proligands is described. The synthetic route starts from a chiral pool material (L-valine) and generates the P=C bond by a phospha-Peterson reaction. The electronic and steric properties of the proligands (R′, R′′ and R) were modified using this synthetic route. MesP=CPh(C(i-Pr-Ox)Me₂) was thermally polymerized to generate poly(methylenephosphine). The investigation of the coordination chemistry of PhAk–Ox proligands is described in Chapter 4. Rhodium(I) and iridium(I) PhAk–Ox complexes were characterized by X-ray crystallography and NMR spectroscopy. Rhodium(I) PhAk–Ox complexes were found to be active in the asymmetric allylic alkylation of ethyl (1-phenylallyl) carbonate with dimethyl malonate as a nucleophile. The optimal conditions generated products in 37% yield and 66% ee. The investigations of PhAk–Ox ligands in palladium(0) catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate using malonate type nucleophiles are reported in Chapter 5. The structural modification of the ligand through the incorporation of a gem-dimethyl group [MesP=CPh(C(4-i-Pr-5-Me₂-Ox)Me₂)] was needed to optimize yields (73–95%) and enantioselectivities (79–92%). Ring-closing metathesis processes were used to generate enantioenriched carbocycles. To conclude, the results presented in this dissertation represent the highest reported enantioselectivities for a reaction utilizing a phosphaalkene-based ligand. These results also serve as a proof of concept that phosphaalkene ligands can be used in asymmetric catalysis.