Polymer-Supported Metal Lewis Acids

Another approach to the preparation of polymer-supported metal Lewis acids is based on polymerization of functional monomers. If synthesis of the functional monomer is not difficult, polymerization should afford structurally pure functional polymers, because the polymer formed requires no further complicated chemical modification. A variety of substituted styrene monomers are now commercially available styrene monomers with an appropriate ligand structure can be prepared from these. Several other interesting functional monomers such as glycidyl methacrylate, 2-hydr-oxyethyl methacrylate, and other acrylics have also been used extensively to prepare functional polymers. 

In this chapter recent developments in the use of polymer-supported metal Lewis acids in organic synthesis will be discussed. Although most consist of crosslinked polystyrene-based supports, other organic polymers and inorganic supports are included as support materials. 

Aluminum chloride and its derivatives are the most familiar Lewis acids and are routinely employed in many Lewis acid-promoted synthetic transformations. The first polymer-supported metal Lewis acids to be studied were polymers attached by weak chemical or physical interactions to a Lewis acid. In the 1970s Neckers and coworkers reported the use of styrene-divinylbenzene copolymer-supported AlCl,- or BF3 as catalyst in condensations, esterifications, and acetalization of alcohols [11,12]. This type of polymer-supported AICI3 (1) is readily prepared by impregnation of a polystyrene resin with AICI3 in a suitable solvent. Subsequent removal of the solvent leaves a tightly bound complex of the resin and AICI3. The hydrophobic nature of polystyrene protects the moisture-sensitive Lewis acid from hydrolysis, and in this form the Lewis acid is considerably less sensitive to deactivation by hydrolysis. This polymer complex could be used as a mild Lewis acid catalyst for condensation of relatively acid-sensitive dicyclopropylcarbinol to an ether (Eq. 1) [13],... 

I 79 Polymer-Supported Metal Lewis Acids AICI3... 

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