Polystyrene‐Anchored Cinchona Ammonium Salts: Easily Recoverable Phase‐Transfer Catalysts for the Asymmetric Synthesis of α‐Amino Acids

Abstract

Cinchonidine, epicinchonidine, cinchonine or quinine have been N-anchored to cross-linked polystyrene (Merrifield resin) or grafted polystyrene (SynPhase TM Lanterns) affording polymer-supported chiral ammonium salts which are employed as phasetransfer catalysts in the asymmetric alkylation of N-(diphenylmethylene)glycine esters. These new polymeric catalysts can be easily recovered after the reaction by simple filtration and reused.

Recent Developments in Enantioselective Phase Transfer Catalysis Using Chiral Ammonium Salts

“…An O-bound Merrifield polymer with an N-(9-anthracenylmethyl)cinchon idinium unit displayed very high selectivity (ee up to 96%) and recyclability (three times with almost no loss of activity) [63]. Further examples showing similar results in terms of selectivity and reactivity were also reported [72] [73].…”

Recyclable Organocatalysts in Asymmetric Reactions

“…Catalyst 12 was used with a set of bromides to give the corresponding alkylated glycine derivatives (Scheme 2.5 ) [18,19] . They employed several supports such as polystyrene [1% divinylbenzene (DVB)] ( 12 -14 ), polymer -bound triphenylchloromethane (1% DVB) ( 19 ), JandaJel (2% cross -linked) ( 20 ), Wang -Br resin (1% DVB) ( 21 ), polyethyleneglycol -polystyrene copolymer (1% DVB) ( 22 , 23 ), and a mercaptomethyl polystyrene ( 24 ) (Scheme 2.4 ).…”

Immobilised chiral inducer on Pt-based mesoporous titanate nanotubes as heterogeneous catalysts for enantioselective hydrogenation

“…Catalyst based on cinchona grafted silica has proven to be very successful i.e., dihydroxylation of olefin [21], Michael addition [22], synthesis of α-Amino Acids [23], enantioselective reduction with organometallic agents [24] and hydrogenation of ethyl pyruvate [25] and PPD [26][27][28]. The covalent anchoring of inducers suffers from the lengthy process involved in functionalization of inducer and effective covalent anchoring onto the support.…”