Polymer-Bonded Lewis Acids in Organic Synthesis: Conversion of Carboxylic Acids into Acyl Chlorides and of Alcohols into Alkyl Chlorides or Bromides by Polymer-Bonded Phosphorus Reagents (original) (raw)

Relatively few examples of polymer-supported reducing agents have been reported '). In this context, carbonyl groups, except a,gunsaturated ketones, have been efficiently reduced by a poly(2-~inylpyridine)-borane complex exclusively in benzene in the presence of equimolar amounts of BF30Et,. In the course of our investigation on polymer-supported reagents3), we used weak anion-exchangers as suitable supports for bonding Lewis acids such as phosphorus(II1, V) halides 4). Now we refer that also aluminium hydride, a Lewis acid which is known to form adducts with tertiary amines5), can be attached to Amberlite IRA 93 (Rohm and Haas), a macroporous styrene-divinylbenzene copolymer functionalized with tertiary amino groups. General part Aluminium hydride, by which a number of organic functional groups can be reduced6), is conveniently prepared6) by addition of 0,5 mol of 100% sulfuric acid to 1 mol of lithium aluminium hydride in THF solution. Hydrogen is evolved, and lithium sulfate precipitates quantitatively following the equation: 2LiAIH, + H,SO, Li2S0, 1 + 2A1H3 + 2H2 Amberlite IRA 93 is treated with the obtained THF solution of AIH, under argon. This resin reduces aldehydes, ketones and esters in good yields and, stored in a desiccator, is stable for almost a month. The capacity of the polymeric reagent has been evaluated by treating a weighed amount of the resin with an excess of acetic acid and backtitrating with aqueous 0,l M sodium hydroxide: a mean value of 5 3 mmol of aluminium hydride per gram of dry resin is obtained. Reactions are performed in batch, at room temperature under an inert atmosphere, by stirring the substrate, dissolved in a dry solvent, usually diethyl ether (hexane, THF and methylene chloride are also suitable), with a twofold molar excess of resin: