Reduction to Methylene Halides

Another variation of this method involves the treatment of an acetonitrile solution of the aryl aldehyde, trimethylsilyl chloride, and either sodium iodide, if iodide products are desired, or lithium bromide, if bromide products are desired, with TMDO. After an appropriate reaction time (5-195 minutes) at a temperature in the range of —70° to 80°, the upper siloxane layer is removed and the benzyl iodide or bromide product is isolated from the remaining lower portion after precipitation of the inorganic salts by addition of dichloromethane. For example, p-anisaldehyde reacts to form /i-rnethoxybenzyl bromide in 84% isolated yield under these conditions (Eq. 200).314,356 

In the preparation of iodides, but not bromides, PMHS may be substituted for the TMDO. Chlorides can be obtained if thionyl chloride and zinc iodide are added to suppress the formation of symmetrical ethers.314 An example of this type of reductive chlorination is shown by the TMDO-mediated conversion of p-tolualdehyde into p-methylbenzyl chloride (Eq. 201).313 To obtain chlorides from aldehydes having electron-withdrawing groups such as nitro or carbomethoxy, the initial reaction is first carried out at —70° and the mixture is then heated to reflux in order to reduce the formation of symmetrical ether by-products. Zinc chloride is substituted for zinc iodide for the synthesis of chlorides of substrates with electron-donating groups such as methoxy and hydroxy.314 

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