Polystyrene noncross-linked

New approaches to catalyst recovery and reuse have considered the use of membrane systems permeable to reactants and products but not to catalysts (370). In an attempt to overcome the problem of inaccessibility of certain catalytic sites in supported polymers, some soluble rho-dium(I), platinum(II), and palladium(II) complexes with noncross-linked phosphinated polystyrene have been used for olefin hydrogenation. The catalysts were quantitatively recovered by membrane filtration or by precipitation with hexane, but they were no more active than supported... 

A flexible means of access to functionalized supports for solid-phase synthesis is based on metallated, cross-linked polystyrene, which reacts smoothly with a wide range of electrophiles. Cross-linked polystyrene can be lithiated directly by treatment with n-butyllithium and TMEDA in cyclohexane at 60-70 °C [1-3] to yield a product containing mainly meta- and para-Iithiated phenyl groups [4], Metallation of noncross-linked polystyrene with potassium ferf-amylate/3-(lithiomethyl)heptane has also been reported [5], The latter type of base can, unlike butyllithium/TMEDA [6], also lead to benzylic metallation [7]. The C-Iithiation of more acidic arenes or heteroar-enes, such as imidazoles [8], thiophenes [9], and furans [9], has also been performed on insoluble supports (Figure 4.1). These reactions proceed, like those in solution, with high regioselectivity. 

Although a large variety of reactions were successfully transferred to the solid phase there are only a few examples of free radical reactions on solid phase [173,174]. The free allyl radical transfer was achieved when the carbohydrate auxiliary was linked to a noncross-linked polystyrene polymer (NCPS) [175]. The use of NCPS instead of cross-linked supports has several advantages, including complete solubility in many organic solvents [176]. The polymer 269 was obtained in a radical polymerization of two equivalents of styrene and one equivalent /7-chloromethylstyrene. The protected D-xylose 270 was covalently linked to the support by a Williamson synthesis generating compound 271 (Scheme 10.88). 

For the preparation of soluble supported organocatalysts, noncross-linked polystyrene (PS) and poly(ethylene glycol) derivatives (PEG) are the most usual... 

Scheme 10.11 Other soluble noncross-linked polystyrene resins containing DMAP equivalent functions.

Scheme 21.13 Synthesis of prostaglandin using isolation tags (Janda et a .). NCPS noncross-linked polystyrene.

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