Polymer-bound species

A disulfide-linked bis(aminoethanol) 82 prepared from L-cystine also catalyzes the borane reduction of ketones. Other oxazaborolidine derivatives are obtained from 83, 84, " and 85, " and polymer-bound species. Those derived from the ephedra bases find use in the asymmetric reduction of imines. bicyclic oxazaborolidine generated... 

Using soluble polymers under conditions where the products and polymer supports both remain in solution during the separation stage is a scheme that is uniquely applicable to soluble polymers. It is, for the most part, the scheme used in nature where enzyme catalysts are used and separation is based on size. To date, hquid/hquid separations remain a less common way of recovering soluble polymer-bound species. However, the earhest examples where soluble polymers were used in catalysis separated the solutions of soluble polymer-bound catalyst from the products with membranes [ 106,107]. While solid/Uq-uid separations (vide infra) stiU predominate, that situation may change as new separation strategies are invented and perfected or as new more durable and improved membranes are developed. Indeed, as can be seen from the discussion below, a variety of new and improved approaches have recently been developed where soluble polymer-bound catalysts are isolated as solutions. 

The polymer-bound species, a catalyst, a product, or a by-product, can be separated from the reaction mixture. Macroscopic solids and gels can usually be separated from liquids by filtration. Soluble polymers and colloids can be separated from low molecular weight compounds by ultrafiltration. Colloids can be coagulated and filtered. The functionalized polyethylenes described by Bergbreiter in chapter 2 are soluble hot and insoluble cold so that they can be filter. ... 

Site Isolation Using Soluble Polymer-Bound Species... 

On a more fundamental level, changes in reactivity can occur from polymer-binding of a reagent. In most cases the polymer is insoluble in the reaction medium the reaction becomes heterogeneous. Normally such reactions are retarded and the selectivity of the reagent Increases. There are instances, however, where reactivity increases. Normally this event is Interpreted in terms of polymer-bound species which are "site-separated and in an unassociated state are more reactive. 

Polymer-bound a,B-unsaturated nitriles, esters, and ketones could be prepared by modified Wittig reactions of known polymer-bound species such as the B-keto-phosphonate 9 (56). 

The success of both peptide cyclizations and ester enolate tnqiping is due to the lesser mobility of polymer-bound species, which reduces the rates of die bimolecular reactions that lead to higher oligomeric peptides or to ester self-condensation. 

Kinetic investigations of the lifetimes of polymer-bound benzyne (7.81 and of an N-deprotected amino acid active ester (2) put the concept of site isolation into voper po qi ve. Polymra -bound reactive intermediates have substantially longer lifetimes than the analogous micromolecular species, but they arc not completely isolated, and in time react with othra- polymer-bound species. Several reviews summarize the field as of 1978-82 (10-171. 

The courses of bimolecular reactions in a polymer gel depend upon the nK>bilities of the polymer-supported functional group aiid the micromolecular reactants. Functional group mobility is necessa for reaction between two polymer-bound species. In principle, only solute mobility is necessary for reactions b ween a polymer-bound species and a soluble reagent. Motions of both microrttolecular solutes and macro-molecular chains have been studied by spectroscopic techniques that are sensitive to rates of translational and rotational diffusion. 

A similar analysis of the surface area available and the area occupied by the polymer bound species was used to design the synthesis of bis(di-n-butylchlorotin)-tetracarbonylosmium shown in Scheme 7 (S2) The analogous reaction sequence in solution gives bisOi-dibutyltin-tetracailxrttylosniium), a cyclic dimer. A calculation based on a Poisson distribution of sites on a 100 m /g, 20% cross-linked macro-porous polystyrene led to the choice of a substitution level of 0.01-0.02 mmol/g, practical oiily for small scale syntiieses. 

The rates of reactions between polymer-bound species decrease with increased cross-linking, decreased DF, decreased solvent swelling, and decreased temperature. Seldom have all df these variables beoi erqrlored for a single reaction. 

Polymer-bound species can be added to reactions that are programmed to recognize only the target product. This, of course, is not a serendipitous process but requires specific chanical design. Products can then be captured and washed, then rereleased to give clean compounds, a procedure otherwise known as the catch-and-ielease technique for compound purification. 

In the course of developing the diene cyclooligomerization and hydrogenation catalysts described above, we recognized that soluble polymers could also be used in such multi-step processes. However, unlike the case with two different polystyrene-bound species, the use of one soluble and one insoluble polymer bound species would enable the facile isolation of the product and each catalyst or reagent. Flotation techniques can be used to separate two insoluble polymers from one another. However, separation of a soluble polymer from an insoluble one would be simpler since it would only involve a filtration step. Kinetic isolation of each polymer-bound species would be possible even with one soluble polymer-bound species because of the diffusional constraints associated with soluble macromolecules. 

The main featnres to note are that the polymer recovery mass balance can be >97% and only one polymer-bound species should be detected by NMR analysis for each step of the synthesis. 

In solution aliphatic esters are acylated or alkylated at the a position by treatment with a hindered organolithium base at dry ice temperature to generate an enolate, followed by addition of a carboxylic acid chloride or an alkyl bromide. At higher temperatures the ester self-condenses rapidly during the time of enolate generation. When the ester is bound to a slightly swellable 10-20% cross-linked PS, acylations and alkylations proceed at room temperature with 73-90% yields and little or no competing self-condensation of the ester as shown in Scheme 26. Self-condensation is retarded because it requires reaction of two polymer-bound species with each other. 

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