Allyltributyltin polymer-supported

Allyltributyltin can be used in combinatorial radical allylation. Moreover, a polymer-supported reagent bearing an allyltin group was also developed and it can be used for the radical allylation of alkyl halides. 

Enholm [12] has also prepared an enantiomerically pure soluble polymer support 82 by couphng xylose-derived chiral auxiliary 81 with 77 (Scheme 18). The chiral support was then treated with bromopropionic acid 83 to give substrate 84. Eree radical allyl transfer from allyltributyltin imder thermal conditions provided 85 in 93% yield, and basic cleavage from the resin gave (R)-(-)-2-methylpent-4-enoic acid 86 in 80% yield and 97% ee, with a 92% yield of recovered 82. Previous studies of the same process in solution had found the addition of Lewis acids to be crucial for high selectivities to be obtained. Interestingly, the addition of Lewis acids to the reaction on polymer support led to cleavage of the carbohydrate from the polymer backbone. En-... 

These polymers underwent radical allylation [13] using allyltributyltin to give adducts 94 and 95, and reduction with tributyltin hydride to give the products 96 and 97 (Scheme 20). All products were obtained in high yield as white crystalline materials which could be easily separated from tin byproducts. The products were released from the polymer support by hydrolysis with lithium hydroxide. 

Similar polymers were formed from norbornene-2,3-dicarboxylic anhydride 101 [13], which allowed for two reactive sites to be incorporated into each monomer unit, effectively providing a loading capacity of 200% (Scheme 21). Treatment of polymer 102 with allyltributyltin or tributyltin hydride resulted in reaction of both bromides in every monomer unit giving products 103 or 104 in 76% and 77% yield respectively. Hydrolysis with LiOH released the expected products 100 and 105 from the polymer support. In principle, it should be possible to recover and reuse the polymers prepared in Enholm s studies. 

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