MECHANISM OF THE UNUSUAL ETHER CLEAVAGE OF (2-METHOXY-1,3-XYLYLENE)-15-CROWN-4 BY ORGANOMAGNESIUM REAGENTS

The surprisingly easy ether cleavage reaction of (2-methoxy-1,3-xylylene)-15-crown-4 (7) and organomagnesium reagents in diethyl ether solution was investigated. Facile alkyl ether cleavage was found to take place only with 7. (2-Ethoxy-1,3-xylylene)-15-crown-4 (17) was cleaved at a considerably lower rate, while the (2-isopropoxy-1,3-xylylene)-15-crown-4 (18) did not show ether cleavage at all. The sulfur analogues [2-(methylthio)-1,3-xylylene]- (21) and [2-(ethylthio)-1,3-xylyene]-15-crown-4 (22) are unreactive at ambient temperature. The reactivity of the cleaving reagent decreases in the order diphenylmagnesium > phenylmagnesium bromide > magnesium dibromide. The reaction is proposed to proceed via an initially formed 1:1 ''side-on'' complex (13), in which the organomagnesium compound is coordinated to the two central crown ether oxygens. When the solvent is coordinating more strongly to the organomagnesium compound than the crown ether, such as THF, the reaction does not take place because the side-on complex (13) is not formed. As in previously reported metalation reactions, it was found that the 1,3-xylylene-15-crown-4 derivatives react faster than the larger 1,3-xylylene-18-crown-5 derivatives.