Chelation Assistance in the Activation of C-S and C-O Bonds

A chelation approach to activate C-S and C-O bonds is described. In the area of organosulfur chemistry, the theme is based on the formation of a chelation complex which results in the enhancement of the reactivity of the aliphatic carbon-sulfur bonds in the nickel-catalyzed cross coupling reactions with the Grignard reagent. Various neighboring heteroatom substituents (OR, OH, NR₂, SR as well as another dithioacetal group) can facilitate the olefination of a dithioacetal group. Depending on the relative position of the newly formed double bond and the remaining dithioacetal moiety, tandem olefinations of bisdithioacetals occur to yield the corresponding dienes under these conditions. Polythioethers afford the corresponding degradation products via β-sulfur elimination process. 1,3-Dimercapto- and 1,3-dithiolatopropanes undergo desulfurative cyclopropanation under these reaction conditions. The reactions of dihydrothiapyrans with MeMgI in the presence of the nickel catalyst proceeds smoothly to give vinyl cyclopropanes.

Ketals containing a neighboring heteroatom react with the Grignard reagent in aromatic solvents regio- and stereoselectively. The reactions of acetonide derivatives of monosaccharides under these conditions afford the corresponding products which have only one or two free hydroxy groups depending on kind of the Grignard reagents employed. Tunable chiral diols are obtained from the reactions of 2S,3S-threitol bisketals with Grignard reagents or with LiAlH₄/AlCl₃. The reactions of benzylic acetals, prepared from these chiral diols and aromatic aldehydes, with aryl or secondary or sterically hindered Grignard reagents give the corresponding retentive ring-opening products in high diastereoselectivity.

nickel catalysts - Grignard reagents - olefination - acetals - dithioacetals