Summary: | 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.
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