Polymer support monolith

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. 

Although the history of rigid monolithic polymers is relatively short, a number of applications have already been explored. These applications cover a rather broad range of fields from heterogeneous catalysis and solid-phase extraction, to polymer-supported chemistry and a variety of separation processes. 

Hoveyda and co-workers immobilized an olefin metathesis catalyst on monolithic sol-gel and claimed that the catalytic material is easily recyclable. Barrett and co-workersprepared a recyclable boomerang polymer supported catalyst for olefin methathesis by grafting the preformed catalyst to a polystyrene... 

Buchmeiser, M.R., Lubbad, S., Mayr, M. and Wurst, K., Access to silica- and monolithic polymer supported C-C-coupling catalysts via ROMP applications in high-throughput screening, reactor technology and biphasic catalysis, Inorg. Chim. Acta, 2003, 345, 145. 

Monolith Column—Porous silica column prepared in situ to completely fill the column tube with a fully porous silica foam skelton. After the organic polymer support is heated off, the silica surface is silylated in place to product bonded-phase surface. Column is high resolution and can be used at high flow rates with relatively low back-pressure (see Chapter 16). 

Composite membrane catalysts can also be assembled with polymeric supports or intermediate layers [117-119]. These membranes were tested as membrane catalysts for selective hydrogenation of some dienic hydrocarbons and proved to be as selective as monolithic palladium alloy membranes [117]. The use of polyarilyde has been proposed in order to widen the temperature range of polymer-supported membrane application... 

The most common polymer supports used for chiral catalyst immobilization are polystyrene-based crosslinked polymers, although poly(ethylene glycol) (PEG) represents an alternative choice of support. In fact, soluble PEG-supported catalysts show relatively high reactivities (in certain asymmetric reactions) [le] which can on occasion be used in aqueous media [le]. Methacrylates, polyethylene fibers, polymeric monoliths and polynorbornenes have been also utilized as efficient polymer supports for the heterogenization of a variety of homogeneous asymmetric catalysts. 

Luis prepared polymeric monoliths 17 containing TADDOL subunits [13] these were synthesized with a thermally induced radical soluhon polymerization of a mixture containing TADDOL monomer, styrene and DVB, using toluene/1-dodecanol as the precipitating porogenic mixture and azoisobutyronitrile (AIBN) as the radical inilialor. The polymer-supported Ti-TADDOLates generated from 17 and Ti(OiPr)4 were then used for the asymmetric alkylation of benzaldehyde to give 1-phenylethanol in 60% yield and 99% ee [13]. 

The use of monolithic columns in LC has advanced rapidly since their first introduction in the 1990s [18-21]. In contrast to capillary columns packed with particulate stationary phases, monolithic columns consist of a single continuous support. Monolithic stationary phases can be subdivided in two classes, i.e., polymer-and silica-based materials. 

Reaction performed with monolithic polymer-supported catalysts, lead generally to good to excellent behaviour in terms of catalytic activity and enantioselectivity and as the reaction is performed without stirring, the catalytic system is generally very stable. 

To increase the accessibility of the inner reactive groups and also the number of functionalities, the monolith surface was grafted as showed in Figure 16. The chloromethyl polystyrene discs (Fig. 16, A) reacted with azobis(4-cyanovaleric acid) (ACVA), a symmetrical azo initiator, to give the polymer-supported acid (Fig. 16, B and C). The pores were then filled with an appropriate monomer and polymerized to afford the final products (Fig. 16, D). The initiator could also react with two chloromethyl groups (Fig. 16, C) and this was indeed preferred to obtain... 

The enantioselective Diels-Alder reaction is another main motif in chiral Lewis acid catalysis. In 1996, Itsuno and coworkers reported an asymmetric Diels-Alder reaction using polymer-supported catalysts under flow conditions. Immobilized chiral oxazoboloridune (34) was prepared from a copolymer of N-sulfonylvabne and borane having styrene moiety, affording the Diels-Alder adduct in an enantioselective manner (up to 71% yield) [126], The authors used a gravity-fed-type column for the flow reaction. Ti-TADDOL-functionalized monolithic resins (35) were developed by Altava and Luis for the asymmetric Diels-Alder reaction (Scheme 7.30). 

---