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506 Organic Chemistry Solutions Manual Problem 7 We introduced the idea of a spacer between a benzene ring (in a polystyrene resin) and a functional group in the chapter. If a polymer is being designed to do Wittig reactions, why would it be better to have a Ph₂P group joined directly to the benzene ring than to have a CH₂ spacer between them? polymer polymer PPh₂ PPh₂ useful for Wittig reactions useless for Wittig reactions If you need a hint, draw out the reagents you would add to the polymer to do a Wittig reaction and work out what you would get in each case. Purpose of the problem Introduction to polymer-supported reagents and your chance to avoid a trap. Suggested solution The first polymer, with the Ph₂P group joined directly to the polymer, does a straightforward Wittig reaction without mishap. After the reaction, the P=O groups in the polymer must be reduced back to Ph₂P groups before the next application. polymer polymer polymer polymer R¹ Br base R²CHO + PPh₂ R¹ PPh₂ PPh₂ PPh₂ + The other polymer starts in the same way, but treatment with base removes the wrong proton as the CH₂ group between the polymer and the PPh₂ group is benzylic and has more acidic protons than those on the other CH₂ group unless R¹ is a very anion-stabilizing group like CO₂Et. Worse, the product is left attached to the polymer while the phosphorus atom is lost and the polymer cannot be used again. polymer polymer polymer polymer R¹ Br base R²CHO PPh₂ R¹ PPh₂ PPh₂ PPh₂ CHR² R¹ R¹