Polymer-bound reagents

Modes of attachment of functional groups to crosslinked polystyrene are discussed ( 1). Attention is drawn to improved stability and activity of polymer-bound reagents and catalysts incorporating dimethylene spacer between polystyrene aryl and functional group heteroatom, and the simplicity and versatility of their synthesis through high-conversion functional group modifications. 

As an example of the use of a polymer-bound reagent 228, the transformation of alkoxyallenes 8 into vinyl halides 229 is depicted in Scheme 8.57 [131]. 

In this study, a nonchemical means of encoding the identity of each compound was used. The original polymer-bound reagent was placed in a porous microreactor equipped with a radio-frequency device that can be used for identification.168 The porous micro-... 

Polymer-bound reagent. Reaction of a quaternary ammonium anion-exchange resin (Amberlites) wth NaBH4 results in an immobilized borohydride reducing agent (1), which is somewhat less reactive than NaBH4.4... 

C. Parallel Solution Strategies Using Polymer-Bound Reagents... 

Vinyl triflates (10, 123).1 The most satisfactory conditions for preparation of vinyl triflates from carbonyl compounds using triflic anhydride and catalyzed by the pyridine 1 or the polymer-bound reagent 2 are discussed by Wright and Pulley. In the case of aldehydes, the reaction is best carried out in CHC13 or (CH2C1)2 at 50-70°, and results in a gem-bistriflate, which decomposes thermally to the vinyl triflate. These conditions, when applied to a ketone, particularly aldol-prone ones, are usually not useful. The preferred solvent is nonpolar (CClt) with a reaction temperature of about 50°. In addition, the Tf20 should be freshly distilled from P205. 

A modification of GSR has been reported by Classon and co-workers.190 The idea remains the same create a covalently bound phosphorus cation and displace with a nucleophile—in this case, a halogen. Both bromine and iodine have been used.190 Three different systems were evaluated (1) chlorodiphenylphophine, iodine-bromine, and imidazole (2) p-(dimethylamino)phenyldiphenylphosphine, iodine-bromine, and imidazole or (3) polymer-bound triphenylphosphine, iodine-bromine, and imidazole. The last two were found to be very similar to just triphenylphosphine itself, and displayed reactivity inferior to the first system. The polymer-bound reagent does allow for easier removal of triphenylphosphine oxide produced in the course of the reaction. As with the original procedure, and consistent with a Sn2 mechanism, inversion of configuration occurred. Again, as with the original method vicinal diols were readily converted into alkenes.191 This... 

Polymer-bound reagents are functional polymers designed to perform synthetic transformations in the same way as their solution counterparts. Excess reagent and by-products remain attached to the resin, while the end product is purified by simple filtration. 

Thefcphosphine oxide can be reduced back to the phosphine (for example, with Cl- SiH) while still bound to the polymer and the polymer-bound reagent can be used again. Separation of Ph3P=0 from alkene products after a Wittig reaction can be quite a nuisance so the ease of work-up alone makes this an attractive procedure. 

Automated peptide synthesis uses polymer-bound reagents... 

What would be the advantage of the polymer-bound reagent over normal PCC ... 

A useful reagent for the oxidation of alcohols is PCC (pyridinium chlorochromate). Design a polymeric (or at least polymer-bound) reagent that should show similar reactivity. 

Scavenger resins and polymer-bound reagents are routinely used to prepare medium-sized (500-1000 member) libraries. Polymer-bound isocyanates (9) and aldehydes (10) are used to remove amines from reaction mixtures, while polymer-bound thiols (11) are used to scavenge halides. 

So far only we have scratched the surface of what might be possible with polymer-bound reagents in multi-step organic synthesis. We believe it is possible to use these reagents in many elaborate one-pot multi-reagent combinations, even with the aim of discovering new chemical reactions. 

In active supports, the substrate attached to the insoluble polymer effects a synthetic or a catalytic transformation on a soluble substrate. Thus, this group includes the polymer-bound reagents 13 15), in which the active site is consumed during the course of the reaction, and the polymer-bound catalysts 9), in which the active site on the heterogeneous matrix effects catalytic transformations on a soluble substrate. 

Jayalekshmy and Mazur illustrated the reduced mobility of polymer-bound reagents by delayed trapping of reactive intermediates generated within a polymer matrix 139). 

Polymer-bound reagent. A polymer-bound form of PdCU (1) has been prepared by anchoring anthranilic acid to chloromethylated polystyrene followed by complexation with PdCU in CH3CN. 

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