Tributyltin polymer-supported

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. 

Caddick [19] has reported the use of a novel polymer-supported tetra-fluorophenol-Unked acrylate as an activated acceptor for intermolecular radical reactions. Treatment of immobiUzed acrylate 132 with a variety of alkyl iodides in the presence of tributyltin hydride and AIBN gave the corresponding esters 133 (Scheme 29). NucleophiUc cleavage using amines gave amides 134 in good overall yield whilst regenerating phenol resin 131. 

Enholm [26] has reported the first examples of asymmetric radical cy-clizations on soluble polymer supports. The stereocontrol element employed consists of a (+)-isosorbide group attached by a 4-carbon chain to each subunit of a soluble succinimide-derived ROMP backbone. Treatment of the radical cychzation substrate 162 with tributyltin hydride in the presence of zinc chloride followed by hydrolysis of the resulting polymer-supported ester 163 gave the desired product 164 in 80% yield and > 90% ee (Scheme 38). The use of alternative Lewis acids, such as magnesium bromide etherate and ytterbiiun (III) triflate, resulted in lower enantioselectivities, 84% and 72% respectively. No such decrease in selectivity was observed in analogous reactions carried out off-support [27], suggesting that the polymer backbone is somehow responsible for this phenomenon. 

Tributyltin hydride is the most popular reagent in preparative free radical chemistry. The majority of the published work deals with the use of stoichiometric quantities, although alternative approaches such as catalytic or polymer-supported procedures have been developed. Occasionally other substituted organotin hydrides have been used, Ph3SnH being the most representative. 

Since Pereyre and coworkers reported the catalytic activity of tributyltin alkoxide for transesterification reaction [87], organotin derivatives have been used for various kinds of organic transformations. Distannoxane compounds were found to play an important role in the reaction [88-90] and act as pure Lewis acid [91, 92]. Although the transesterification reactions proceeded by using low-molecular-weight catalyst in homogeneous medium, a serious drawback for the industrial processes is a removal of the toxic tin derivatives. In such cases the use of polymer-supported version... 

Radical cyclization reactions are not particularly popular, owing in part to the lack of specificity often observed, so it is interesting to note that the cyclization shown in Scheme 22 proceeds in up to 90% yield when a polymer-supported organotin reagent is used. Lower yields result from the use of BusSnH, and this is attributed to the purification procedure that is necessary to remove the tributyltin bromide at the end of the reaction. 

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