The addition of Grignard reagents or lithium alkyls to aliphatic a-chlorocarbonyl compounds is shown to be a stereoselective process. The chlorohydrins produced are converted into olefins by three stereospecific steps, the intermediates being epoxides and iodohydrins. A new procedure for making iodohydrins from epoxides, and a new reagent for reduction of iodohydrins to olefins, are described, as well as a direct but non-stereospecific reduction of epoxides to olefins. The geometry of the two 3-methylpent-2-enes is established by unequivocal synthesis of each isomer.[ 19591 A General Stereoselective Synthesis of Olefins.113 chlorohydrins, and we find also that lithium alkyls react in the same sense. The general reaction is:We habitually mix the reagents a t -70" and add acetic acid soon afterwards to decompose the complex. This technique, adopted to minimise rearrangement of the complex, is so rapid and convenient that there has been no incentive to compare it with operation at room temperature; with the one (Grignard) reaction where such a comparison was made, the yield at the higher temperature was a little lower. Comparisons of lithium alkyls with Grignard reagents were made more extensively. In general, the Grignard reagent gave the superior yield of chlorohydrin from substances containing the group -CO*CH,Cl, probably because the lithium alkyl induced self-condensation of the ketone. The lithium alkyl had the advantage with more hindered carbonyl compounds; these are less prone to enolisation but are liable to reduction by the Grignard reagent. Comparative yields are assembled in the Table. Chloro-carbon yl Yield (%) with, Chloro-carbon yl Yield (%) with compound BunLi BunMgBr compound BunLi BPMgBr
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