Pure and Applied Chemistry

Abstract: 3-halopropenyl esters, readily prepared by the addition of acyl halides to acrolein, react with zinc, indium, and chromium(II), thus opening a route to a new class of oxygen-substituted allylic organometallic compounds. Indium and zinc reagents smoothly add to carbonyl compounds, affording alk-1-en-3,4-diol derivatives in a variety of synthetic procedures which include typical Grignard stepwise conditions as well as Barbier one-pot protocols. Using zinc and indium in water or aprotic solvents, simple diastereoselectivity was found to depend on the nature of the carbonyl compound; conjugated aldehydes favor formation of syn-adducts while unconjugated aldehydes favor anti-adducts. Moving to chromium, a reversal of regioselectivity was observed in favor of (Z)-4-hydroxy-enolacetates, flexible protected forms of homoaldols. Chromium complexes are generated in a catalytic cycle based on the combined use of the redox Mn(0)/Cr(III) couple and of TMSCl. When the Cr-catalyzed reaction is carried out in the presence of Jacobsen's Salen ligand, the regiochemical outcome of the reaction is again reversed, and syn-alk-1-en-3,4-diols are formed in high ee's.