Enantioselective Organocatalytic Synthesis of γ-Valerolactone by Hydrosilylation of Methyl Levulinate
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Enantiopure γ-valerolactone (GVL) has many applications and can be produced by stereoselectively reducing the inexpensive biomass compound levulinic acid (LA) or derivative alkyl levulinates (ALs). A green and highly selective method of performing this synthesis is by asymmetric organocatalyzed hydrosilylation. Coupling a bulky chiral cation and a fluoride anion catalyst both activates the hydride-donating silane and induces stereoselectivity in the reaction. In this thesis the principle above has been applied in the N-benzylquininium fluoride (BQF) catalyzed hydrosilylation of methyl levulinate (ML) to GVL by tris(trimethylsiloxy)silane (TTSH), leading to an ee of roughly 25%. Likewise the catalyst N-benzylquinidinium fluoride (BQDF), which is the pseudo-enantiomer of BQF, has been utilized resulting in the same selectivity, but of the opposite chirality. To explore the effect of the chiral cation and fluoride catalyst separately, N-benzylquininium chloride (BQC) (or pseudo-enantiomer N-benzylquinidinium chloride (BQDC)) combined with tetra-n-butylammonium fluoride (TBAF) have been used to catalyze the hydrosilylation reaction. Small amounts of BQC or BQDC lead to a stable ee of around 12%, but a diminished yield.