Jacobsen polymer-supported

The synthesis of the first polymer-supported chiral Mn-salen derivatives was reported independently by Sivaram171 and Minutolo.171-173 Different monomeric Jacobsen-type units, containing two polymerizable vinyl groups, were copolymerized with styrene and divinylbenzene to yield the corresponding cross-linked polymers as a monolithic compact block.174-176 The less mobile system (Figure 19) with no spacer between the aromatic ring and the polymer backbone is less enantioselective. 

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

Week et al. [65] further reported the Co salen complex supported on norbomene polymers (23, 24) with stable phenylene-acetylene linker (Figure 8). The polymer-supported salen catalysts were investigated for HKR of the racemic terminal epoxides that showed outstanding catalytic activities and comparable selectivities to the original catalysts reported by Jacobsen. However, the polymeric catalyst was recycled only once after its precipitation with diethylether as the catalyst became less soluble and less reactive in subsequent catalytic... 

ARO reaction with phenols and alcohols as nucleophiles is a logical extension of HKR of epoxides to synthesize libraries of stereochemically defined ring-opened products in high optical purity. To this effect Annis and Jacobsen [69] used their polymer-supported Co(salen) complex 36 as catalyst for kinetic resolution of epoxides with phenols to give l-aiyloxy-2-alcohols in high yield, purity and ee (Scheme 17). Conducting the same reaction in the presence of tris(trifluoromethyl)methanol, a volatile, nonnucleophilic protic acid additive accelerates KR reaction with no compromise with enantioselectivity and yield. Presumably the additive helped in maintaining the Co(III) oxidation state of the catalyst. 

The Jacobsen group also achieved process improvement with respect to catalyst immobilizaion and recycling [10]. In this recycling study a polymer-supported... 

Another recent example by Peukert and Jacobsen (199) took advantage of the first polymer supported Jacobsen s catalyst 8.53 (Fig. 8.31) comparable with the soluble catalyst in asymmetric epoxidation and its full characterization (200, 201). The supported catalyst, prepared from the activated carbonate of hydroxymethyl PS and from a soluble phenolic catalyst (201), was used to catalyze the opening of racemic alkyl epoxides (Mi, Fig. 8.31) with substituted phenols and yielded the 50-member aryloxy alcohol library L15 with good enantiomeric purity (average >90%, never below 80% e.e.). 8.53 was also used to produce the chiral intermediate monomer set M3 (Fig. 8.31) which was used to make two 50-member chiral libraries L16 (1,4-diary-loxy 2-propanols) and L17 (3-aryloxy-2-hydroxy propanamines) with excellent enantiomeric excess following the straightforward synthetic schemes reported in Fig. 8.31. 

Annis DA and Jacobsen EN. Polymer-supported chiral Co(salen) complexes Synthetic apphcations and mechanistic investigations in the hydrolytic kinetic resolution of terminal epoxides. J Am Chem Soc 1999 121(17) 4147-4154. 

Polymer-supported salen catalysts were also developed by employing poly (norbornene)-immobihzed salen complexes 139 of manganese and cobalt (Scheme 3.40) [77]. The poly(norbornene) complexes are highly active and selective catalysts for the epoxidation of olefins. The asymmetric epoxidation of cis-P-methylstyrene 132 occurred smoothly at -20 °C to give the chiral epoxide 133 in 100% conversion with 92% ee. Under the same reaction conditions, Jacobsen s catalyst (an unsupported salen complex) afforded the same product with 93% ee. 

The first example of a polymer-supported Jacobsen catalyst was reported by Dhal [187]. It involved die radical copolymerization of the eorresponding dis ryl monomer widi EGDMA in a ratio of 10/90 to give die macroporous polymer 303 (Scheme 124). The epoxidation of unfimcdonalized olefins led to moderate yields (55-72%) and low ee (up to 30%). Moreover, this catalytic system could be used at least five times without any signifieant loss of its catalytic activity. 

Besides biomimetic complexes, Jacobsen described particularly efficient bis (chromium-salen) catalyst 9 for the asymmetric ring-opening reaction of epoxides with azide (Scheme 9) [42]. The efficiency of this class of catalysts is attributed to a cooperative mechanism, both substrates being activated toward each other by their respective chromium atom. Of note, a less pronounced cooperative effect was initially demonstrated in an intermolecular manner using monomeric Cr(N3)-salen catalyst [43]. Jacobsen also showed that an analogous cooperative mechanism takes place using polymer-supported chiral Co(salen) complexes for the hydrolytic kinetic resolution of terminal epoxides [44, 45]. 

Also in 2000, attachment of the Jacobsen catalyst to polymeric supports such as poly(ethylene glycol) and different polystyrene-based resins through a glutarate spacer was described [28]. Soluble as well as insoluble polymer-bound complexes were employed as catalysts in the epoxidation of styrene, cfs-2-methylstyrene, and dihydronaphthalene with wx-CPBA/NMO. Results were similar to those achieved with the nonsupported catalyst. Catalyst recycling was shown to be possible either by filtration or by precipitation and one catalyst system could be used for three cycles without significant loss of activity and enantioselectivity. 

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