Polymer-bound catalysts Michael addition

Evidently, many simple chiral organic compounds that act as catalysts can be covalently bound to polymer backbones. Polymer-anchored quinine catalyzes the asymmetric Michael addition of a jS-keto ester to methyl vinyl ketone, which proceeds in 22-42% optical yield (Scheme 13) (29). 

Michael additions of C-nudeophiles such as the indanone 1 have been the subject of numerous further studies For example, the reaction between the indanone 1 and methyl vinyl ketone was effected by a solid-phase-bound quinine derivative in 85% yield and with remarkable 87% ee by d Angelo, Cave et al. [5], Co-polymers of cinchona alkaloids with acrylonitrile effected the same transformation Kobaya-shi and Iwai [6a] achieved 92% yield and 42% ee and Oda et al. [6b] achieved almost quantitative yield and up to 65% ee. Similarly, partially resolved 2-(hydroxy-methyl)quinudidine was found to catalyze the reaction between 1 and acrolein and a-isopropyl acrolein with induction of asymmetry, but no enantiomeric excesses were determined [7]. As shown in Scheme 4.4, the indanone 7 could be added to MVK with up to 80% ee under phase-transfer conditions, by use of the Cinchona-derived PT-catalysts 9a and 9b, affording the Michael-product 8 or enf-8, respectively [8]. The adducts 8 or ent-8 were intermediates in the stereoselective Robinson anellation of a cydohexenone ring to the indanone 7 [8],... 

Computational studies concerning theoretical approaches to the intrinsic basicity of neutral nitrogen bases have been reported, including those of phos-phoranimines. The non-ionic phosphazene bases BEMP (112), BTPP (113) and (114, R = Ph) appear to be excellent catalysts for the Michael addition reactions. Thus the yield of the coupling reaction of ethyl isocyanoacetate with l,2-bis(4-bromomethylphenyl)ethane is increased by the addition of the phosphazene base BEMP. Polymer-supported BEMP (P-BEMP) has been applied for the allylation of 2H-benzo[d]l,3-dioxolan-5-ol by allyl bromide. " Cyclodehydration of 1,2 diacylhydrazines by tosyl chloride in the presence of P-BEMP leads to excellent yields of 1,3,4,-oxadiazoles. Addition of P-BEMP also improves the yield of the Hofmann elimination step in the synthesis of tertiary mines using REM resin (polymer-bound acrylate ester). ... 

Immobilization of chiral ligands to effect asymmetric induction in alkylation of aromatic aldehydes by diorganozinc reagents promoted by PEG-im-mobilized ligands 54-57 can also be promoted by soluble polystyrene-bound species. A recent example of this is work where a polystyrene-bound BINOL was prepared [ 105]. This polymer 69 was used to form titanium-BINOLate and AlLibis(binaphthoxide) catalysts for Et2Zn reaction with benzaldehyde and for asymmetric Michael additions of stabilized carbanions to cyclohexenone. While good stereoselectivities were obtained with these catalysts, the synthetic yields were modest. 

Recently, Hodge et prepared polymer-bound chiral quaternary ammonium chlorides by the reaction of chlomethylated crosslinked polystyrene with cinchonidine, cinchonine or (— )-A -methylepledrine and carried out several Michael reactions. Catalysts prepared from cinchonidine(I) achieved optical yields of < 27% of the S-adduct in the addition of methyl-l-oxoindan-2-carboxylate(II) to methyl vinyl ketone. 

The polymeric catalysts used can be divided into two main classes polymer bound alkaloids " and polymeric amines ", even if it is fair to mention that the first known asymmetric Michael addition was carried out in the presence of alcoholates supported on optically active natural quartz. ... 

Polymer-bound Cinchona alkaloids (53) are catalysts for asymmetric Michael additions of carbanions and thiols to a,j -unsaturated ketones, esters and nitro compounds as shown in Scheme 22. Attachment of the alkaloids to the polymer through the remote vinyl group by copolymerization with acrylonitrile or by addition of a polymeric thiol to the vinyl group was required for catalytic activity and stereoselectivity. " " The enantiomeric excesses attained with the polymeric catalysts (28-51 %) are lower than with the soluble alkaloids (36-63%), but the ease of recovery and reuse of the polymers is usually an advantage. 

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