Catalysts bifunctional polymer

A catalyst such as XXXXIV is referred to as a bifunctional polymer catalyst—two different catalyst moieties function simultaneously in the same reaction. 

Ref. 14S), and the greatest dauylation rate at pH 8 (0.009—0.013 sec" ) was found with bifunctional polymer catalysts (Elm -HA, PVP -HA and HlA-VIm-AAm) (Section S—3). The acylation rate can be further increased by catalysis at higher pH s and by using substrates with longer alkyl chains. 

Kunitake and his coworkers have investigated bifunctional polymer catalysts(29) in micelles(30) and polysoaps.(3L) The bifunctional interaction between a hydroxamate nucleophile and a neighboring imidazole group at the catalytic site is similar to the charge relay system of serine proteases. This interaction leads to remarkably accelerated acylation and deacylation processes. In the hydrophobic environment of cationic micelles, where the reactivity of anionic nucleophiles are remarkably enhanced, the overall catalytic efficiency exceeded even that of a-ch3rmotrypsin at pH 8.(30) Micellar monofunctional catalysts, nonmicellar bifunctional catalysts, and polysoap-bound bifunctional catalysts were less effective. 

An important consideration in catalyst, reagent or substrate recovery is measuring and verifying how effective such recovery actually is. While we have modeled such recovery using dye-modified polymers, analyses of catalysts typically requires additional analytical work. For example, ICP analysis for residual metal can be used as a quantitative and sensitive assay. Such assays are however more problematic with non-metallic catalysts. In this paper, we show that bifunctional polymers where both a catalyst and a colorimetric label are included in the same polyacrylamide polymer provide a simple way to monitor separability and catalyst recovery for non-metallic polymer-bound catalysts. 

The largest turnover rate is realized by the proper balance of acylation and deacylation, and found for imidazole-containing miceUes (SA -Im-CTAB), bifunctional miceUes (LImHA-CTAB), a polyethyleneimine derivative(D(10%)-PEI-Im(15%) (Section 6—2), and bifunctional polymers(EIm -HA and PVP -HA) (Section 5—3). It is remarkable that the turnover rate of synthetic catalysts can amount to 10 to 20% of that of a-chjonotrypsin, although it is to be noted that PNPA is never a good substrate for OE-chymotrypsin. Comparable rates are observed for acylation and deacylation, when they are compared separately. 

In 2012, the first polymer supported bifunctional primaiy amine-ureas were developed by Portnoy and coworkers. This heterogeneous catalytic system was tested in the Michael addition of acetone, cyclic ketones and aldehydes to aromatic nitro-olefins leading to activities and selectivities unprecedented for immobilised catalysts. Catalyst 41 based on (ll ,2f )-diphenylethylene-1,2-diamine and a L-valine spacer provided the Michael products in yields ranging from 23 to 99% and in high enantioselectivity (up to 99% enantiomeric excess) (Scheme 19.43). Unfortunately, recovery of the polymer-catalyst and reuse was only tested for 3 cycles, maintaining the high levels of enantioselectivity, but with a significant loss in the yield. 

If neutral-neutral imidazole interaction or bifunctional catalysis is involved at neutral or near neutral pH, then three mechanisms can be proposed to describe the interactions between the polymer catalyst and the substrate. 

The bifunctional thiourea catalyst was anchored to the developed polymer and subjected to the Michael and aza Henry reactions [93]. 

Fig. 26. Tan 3 of a crosslinking PBD (Mw = 18 000) as a function of reaction time [31]. Parameter is the frequency co. The polymer is vulcanized at the pendant vinyl units with a bifunctional silane crosslinker using a platinum compound as catalyst. The curves intersect at the gel point resulting in...

A modified poly(ethylenimine) also acts as an efficient catalyst for decarboxylation (Suh et al., 1976 Spetnagel and Klotz, 1976). In particular, the partially quaternized polymer [SS] catalyzed the decarboxylation of oxalacetic acid in a bifunctional manner (Spetnagel and Klotz, 1976), as shown in (18). The decarboxylation is thought to occur via pre-equilibrium... 

Shibasaki et al. developed a polymer-supported bifunctional catalyst (33) in which aluminum was complexed to a chiral binaphtyl derivative containing also two Lewis basic phosphine oxide-functionahties. The binaphtyl unit was attached via a non-coordinating alkenyl Hnker to the Janda Jel-polymer, a polystyrene resin containing flexible tetrahydrofuran-derived cross-Hnkers and showing better swelling properties than Merifield resins (Scheme 4.19) [105]. Catalyst (33) was employed in the enantioselective Strecker-type synthesis of imines with TMSCN. 

Suzuki cross-coupling has found applications in the preparation of specialty polymers, too. Rigid rod polymers may have very useful properties (the well-known Kevlar, poly(p-phenyleneterephtalamide) belongs to this family, too) but they are typically difficult to synthetize, characterize and process. Such materials with good solubility in organic solvents [38] or in water [39] were obtained in the reactions of bifunctional starting compounds under conventional Suzuki conditions with [Pd(PPh3)4] and [Pd(TPPMS)3] catalysts, respectively (Scheme 6.15). 

The applied catalytic system consisted of a Ru-Noyori-type racemization catalyst 1 (Fig. 12b) and Novozym 435. This catalyst combination tolerates a wide range of acyl donors, and it was expected that it would allow the use of bifunctional acyl donors for the formation of polycondensates. Before the start of the reaction, the monomer mixture showed the expected diastereomer ratio of (S,S) R,R) R,S) of 1 1 2 of the 1,4-diol employed. After 30 h of reaction the (5,5)-enantiomer almost completely disappeared, whereas the ratio of [R,R)- to (/ , 5)-monomer was ca. 3 1 (R S ca. 7 1). At a hydroxyl group conversion of 92% after 70h, no further conversion was observed and a final ratio of R,R) to R,S) of 16 1 (R S ca. 33 1) was obtained. Unfortunately, the molecular weights of the polymer were moderate at best (Mw = 3.4kDa) and Novozym 435 had to be added every few hours to compensate for the activity loss of the lipase. This suggests that Ru-catalyst 1 and Novozym 435 are not fully compatible. 

The Tsuji-Trost-type reaction is applicable to bifunctional vinyl epoxide 144 and 1,3-diketone using a palladium catalyst as demonstrated by Koizumi, who obtained polymer 145 (Equation (67)). The reaction proceeds at 0 °C to a reflux temperature of THE. The resulting polymer 145 is isolated in a quantitative yield. The molecular weight of 145 is ca. 3000 (PDI = 2.0-2.7) when 5 mol% of Pd(PPh3)4 is employed as a catalyst. Use of Pd2(dba)3 with several bidentate phosphines such as dppe, dppp, dppb, and dppf is also effective for the polymerization reaction. Propargyl carbonate 146 also reacts with bisphenols in the presence of a palladium catalyst to afford polyethers 147 via carbon-oxygen bond formation at s - and r/) -carbon atoms (Equation (68)). 

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