Studies into the reactions of some simple cyclic acetals with boron trihalides/lithium aluminium hydride and boron hydrochlorides

Abstract

1. The use of 2-n-propyl-1,3-dioxane as a model substrate has enabled the following conclusions to be drawn with respect to the reaction between cyclic acetals and boron trichloride (or boron tribromide)/lithium aluminium hydride which gave hydroxyether products. (a) The stoichiometry of the reaction is 3:1 with respect to the acetal/ boron trihalide ratio. (b) An a-haloether-containing complex is the substrate which is reduced by the lithium aluminium hydride to an ether function. 2. The use of a number of substituted dioxolanes, dioxanes, dioxepan.es and dioxocanes has enabled the following conclusions to be drawn about the substituent effects upon the cleavage reaction. (a) Substituents at C effect the facility of cleavage, thus electron withdrawing groups at retard cleavage and vice versa. (b) Substituents at C. (or (c) effect the direction of ring cleavage, thus electron withdrawing groups at C (or C ) upon the dioxolane favour cleavage of the - 0 bond farther from the substituent.(c) For a given substituent in a variety of ring sizes the facility of cleavage has been shown to be as follows dioxolane-dioxane dioxene dioxocane 3. The following reactions have been carried out using boron trichloride/lithium aluminium hydride as the active reagent. Rationales for each of these reactions are discussed. 4. The reactions of boron tribromide/lithium aluminium hydride with 1,3-benzodioxoles were considered, thus 1,3-benzodioxole itself was cleaved to 2-methoxy phenol while 2-phenyl-1,3-benzodioxole gave 2-benzyloxy phenol. The mechanism of the reaction is discussed. 5. The reaction of boron monohydrochloride (BHCl

) with 1,3-dioxanes (which yield respective hydroxyethers) is consideredand a possible mechanism discussed. 6. The E.I. mass spectra of some of the acetal cleavage products are discussed along with the C.I. mass spectra of the substrate acetals themselves.