Wittig reactions polymer cross-linking

Polymer-supported Wittig reagents were first prepared more than 20 years ago [32]. It has been shown that the success of the reaction depends strongly upon (i) the preparation of the reagent by bromination and phosphination of cross-linked polystyrene rather than by co-polymerization using styryldi-phenyl phosphine, and (ii) the generation of the phosphorane with a base/ solvent system that swells the phosphonium sites in the polymer network (Scheme 6) [33]. Thus, bromination of polystyrene 1 yielded phenyl bromide 32, and this was followed by phosphination with n-butyUithium and chlor-odiphenylphosphine or with Hthium diphenylphosphide to give 33, a compound which is commercially available (Scheme 6). 

Table V compiles the ZIE product isomer ratios of reactions from Table I for which such information has been reported. With 0.5-2% cross-linked polystyrene supptxis, the isomer ratios are similar to those obtained from Wittig reactions in solution. The exceptions appear to be the reactions on more highly cross-linked supports shown in Table IV. There is a clear trend toward greater E selectivity as the degree of cross-linking of the polymer increases. This probably should be explained as an environmental effect, but comparison with solvent effects on stereochemistry of Wittig reactions in the literature reveals no tendency for aromatic solvents, structurally similar to polystyrene, to increase E selectivity.

Reuse of the phosphine resins for Wittig reactions has been reported without details several times. In the only detailed results 92% conversion of phosphine oxide to phosphine with trichlorosilane was attained with a 2% cross-linked polymer, and repeated synthesis of stilbene by quatemization with benzyl bromide and Wittig reaction with benzaldehyde gave 97% gc yields in both the second and the third cycles based on the amount of phosphonium bromide used (H). Identical recycling of a 20% cross-linked macroporous polymer gave 75% gc yield of stilbene compared with 80% from the first use (131. 

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