Electrophilic groups phase-transfer-catalyzed

Three major topics of research which are based on phase transfer catalyzed reactions will be presented with examples. These refer to the synthesis of functional polymers containing functional groups (i.e., cyclic imino ethers) sensitive both to electrophilic and nucleophilic reagents a novel method for the preparation of regular, segmented, ABA triblock and (A-B)n alternating block copolymers, and the development of a novel class of main chain thermotropic liquid-crystalline polymers, i.e., polyethers. 

The phase-transfer-catalyzed asymmetric alkylation of 1 has usually been performed with achiral alkyl halides, and hence the stereochemistry of the reaction with chiral electrophiles has scarcely been addressed. Nevertheless, several groups have tackled this problem. Zhu and coworkers examined the alkylation of 1 with stereo-chemically defined (5S)-N-benzyloxycarbonyl-5-iodomethyl oxazolidine using 4d to prepare (2S,4R)-4-hydroxyornithine for the total synthesis of Biphenomycin. Unexpectedly, however, product 7 with a 2 R absolute configuration was formed as a major isomer, and the diastereomeric ratio was not affected by switching the catalyst to pseudoenantiomeric 2d and even to achiral tetrabutylammonium bromide (TBAB), indicating that the asymmetric induction was dictated by the substrate (Scheme 2.3) [21]. 

The phase transfer catalyzed reaction of these a,w-di(electrophilic) polyether sulfones with 2-(p-hydroxyphenyl)-2-oxozoline gave a,o)-di(2-oxazo-line) polymers (Scheme 3). The etherification was performed in a two phase liquid-liquid (chlorobenzene, 50% aqueous NaOH) reaction mixture with equimolar amounts of phase transfer catalyst versus phenol groups at room temperature. 

We have also recently described the use of a polystyrene resin with pendant sulflnate groups (Ref. 64) in phase transfer catalyzed reactions with various electrophiles. The reactions afford sulfones in excellent yields, often nearly quantitative, even though sulfinates are not reputed to be very good nucleophiles. In contrast, the same reactions without any added phase transfer catalyst give only very low conversions. The sulfinate resins have also been used extensively in a number of Michael additions and have proved to be excellent for use as regenerable separation media in the removal of allergenic substances from some plant extracts used in the perfume and cosmetics Industry (Ref. 61, 62). 

Having optimized the catalytic enantioselective phase-transfer alkylation system, the group explored the scope and limitations. A variety of electrophiles were reacted with the benzophenone imine glycine tert-butyl ester 1 catalyzed by 5 mol% of the selected chiral dimeric PTCs, benzene-linked-l,3-dimeric PTC 37, 2 -F-benzene-linked-1,3-dimeric PTC 41, and naphthalene-linked-2,7-dimeric PTC 39, at reaction temperatures of 0°C or — 20 °C (Scheme 4.8). 

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