Phase transfer catalysis: catalyst screening, reaction development, and mechanism analysis

The asymmetric alkylation of a glycine Schiff base in addition to the development of a [2,3]-Wittig rearrangement under phase transfer catalysis (PTC) reaction conditions was investigated. Supplementing these studies, a deeper understanding of the PTC mechanism was sought for the alkylation of phenol. In regards to the asymmetric PTC (APTC) alkylation, a small library of catalysts derived from Cinchona alkaloids was examined for selectivity and activity. An array of substituent modifications were tested, examining the role of steric and electronics on catalyst activity. Catalysts that contained large substituents attached to the quinuclidinium core were found to be the most selective and those in which the hydroxyl group was protected generally afforded faster catalysts. The removal or augmentation of the dipole of the hydroxyl group position significantly impacted catalyst activity. For the development of an APTC [2,3]-Wittig rearrangement, substituted α-allyloxy-phenylacetonitrile substrates were employed. Under liquid-liquid PTC reaction conditions, these substrates were found to rearrange and afford phenyl ketone products. Initial rearrangement products that contained enolizable hydrogens were found to isomerize under the reaction conditions. The use of terminally disubstituted allkyloxy substituents led to the formation of ketones containing adjacent carbon-centered quaternary stereocenters as stable rearrangement products. When a [5,5,5]-pyrolizidinium type of chiral, non-racemic catalyst was employed with a prochiral substrate, an enantioenriched rearrangement product was obtained, albeit in low selectivity (er 55:45). The mechanistic analysis of the base-mediated PTC alkylation of phenol has revealed startling insights. A series of symmetrical, tetraalkylammonium salts were examined for their catalytic and intrinsic reactivity. Tetraalkylammonium phenolate salts were prepared from the corresponding ammonium bromides and their activity was evaluated for the n-butylation of phenol. Small differences in intrinsic reactivity were observed with alkyl lengths greater than C2. In contrast, under phase transfer catalysis conditions large differences in alkylation rate were observed among C2, C3 and C4 tetraalkylammonium catalysts. Consequently, the intrinsic reaction rate plays only a minor role in the reactivity of tetraalkylammonium catalysts using a phase transfer process. Benzyl bromide titration of the organic under PTC reaction conditions revealed the presence of potassium phenoxide complexed with the tetraalkylammonium phenoxide. The relative amounts of complexed potassium phenoxide was found to have a negligible impact on the catalytic activity