Studies on new hydride reducing agents

Abstract

Part I. The present study confirms the versatility of zinc borohydride, 8-Oxyquinoline dihydroboronite, and 4-N,N-dimethylaminopyridine-borane complex as reducing agents. Zinc borohydride in anhydrous diethylether reduced tertiary and benzylic halides to corresponding hydrocarbons without attacking primary, sencondary alkyl, and aryl halides. Zinc borohydride in N,N- dimethylformamide showed similar reducing properties as sodium borohydride in polar solvents. Deoxygenation of $\alpha$-methylbenzyl alcohol with zinc cyanoborohydride did not occur, but bis($\alpha$-methyl benzyl) ether was formed. It was found that zinc chloride alone in dicloroethane dehydrated benzylic and allylic alcohols to afford the corresponding ethers. 8-Oxyquinoline dihydroboronite prepared from 8-hydroxyquinoline and boranedimethylsulfide complex in equimolar ratio in tetrahydrofuran was confirmed to have a relatively rigid five-membered ring system. It was found that the reagent reacted with carbonyl compounds very slowly as observed in other amine-borane complexes. However, in the presence of 0.1 equiv of boron trifluoride, this reagent was very effective for selective reduction of aldehydes and other carbonyl compounds. By addition of equimolar amounts of boron trifluoride, the reducing power of this reagent could be enhanced and several carvbonyl compounds were reduced. 4-N,N-Dimethylaminopyridine-borane complex was easily prepared by the reaction with equimolar amounts of 4-N,N-dimethylaminopyridine and borane-dimethyl sulfide in diethyl ether. In its reducing abilities toward carbonyl compound, the reagent was shown to be more reactive than common amine-borane complexes, capable of achieving chemoselective reduction of aldehydes in the presence of ketones. Several carbonyl compounds were also reduced with this reagent in the presence of equimolar amounts of boron trifluoride. Part II. N,N``-Sufinyl bis-1,2,4-triazole was conveniently prepared by the reaction of thionyl chloride wi...