Reducing agents polymer-supported

In cases where Noyori s reagent (see p. 102f.) and other enantioselective reducing agents are not successful, (+)- or (—)-chlorodiisopinocampheylborane (Ipc BCl) may help. This reagent reduces prochiral aryl and tert-alkyl ketones with exceptionally high enantiomeric excesses (J. Chandrasekharan, 1985 H.C. Brown, 1986). The initially formed boron moiety is usually removed hy precipitation with diethanolamine. Ipc2BCl has, for example, been applied to synthesize polymer-supported chiral epoxides with 90% e.e. from Merrifield resins (T. Antonsson, 1989). 

Devaky and Rajasree have reported the production of a polymer-bound ethylenediamine-borane reagent (63) (Fig. 41) for use as a reducing agent for the reduction of aldehydes.87 The polymeric reagent was derived from a Merrifield resin and a 1,6-hexanediol diacrylate-cross-linked polystyrene resin (HDODA-PS). The borane reagent was incorporated in the polymer support by complexation with sodium borohydride. When this reducing agent was used in the competitive reduction of a 1 1 molar mixture of benzaldehyde and acetophenone, benzaldehyde was found to be selectively reduced to benzyl alcohol. 

To overcome problems associated with the removal of iodobenzene and its derivatives formed upon fluorination of arylalkenes and arylalkynes with (difluoroiodo)arenes, polymer-supported (difluoroiodo)arenes were proposed.139 With these agents, the separation procedures are reduced to filtration of the iodinated polymer. For this purpose popcorn polystyrene is io-dinated and then transformed into the difluoroiodide by treatment with xenon difluoride in the presence of hydrogen fluoride in dichloromelhane at 25 C. The amount of active fluorine bonded to iodine atoms on the polymer support is estimated by iodometric titration. The reactions with phenyl-substituted alkenes result in rearranged gew-difluorides. The procedure provides the same fluorination products as with (difluoroiodo)benzenc (see Section 4.13.) but in much higher yields, e.g. PhCF2CH2Ph (96%), PhCF2CH(Me)Ph (95%). PhCH2CF2H (86%), and l,l-difluoro-2-phenylcyclopentanc (91 %). 

Blasi et al. (2008) developed an experimental method for removing carbonyl compounds from wine. It used selective liquid-solid extraction, with phenylsulfonylhydrazine as a scavenging agent, bonded to a porous polymer support. The method was efficient for reducing the SO2 binding power of botrytized wines, without impairing their sensory qualities. 

Polymer-supported germanium hydrides have been investigated by Mochida and coworkers who report the preparation of poly(4-diethylhydrogermane)styrene and related polymers and examine their reactivity as free-radical reducing agents (equation 8)25. 

Dumartin and associates described the preparation of in situ polymer-supported organ-otin hydrides for use as clean reducing agents (equation 15)47, while Deleuze and coworkers reported the preparation of a novel, macroporous polymer-supported organotin hydride (37), for use in catalytic free-radical reductions (equation 16)48,49. 

The polymer-supported organotin dihydride 70 was shown to be an efficient reducing agent for aldehydes and ketones, but substantial loss of activity was observed after regeneration. More recently, various polymer-supported butyltin reagents (71, 72) were studied as reagents for the acetylation of sucrose158. 

---