The preparation of functionalized 2-substituted allyl bromides and chlorides from carboxylic esters is reported. The carboxylic esters were transformed first to 1-substituted cyclopropanols by treating with ethylmagnesium bromide in the presence of titanium alkoxide. Mesylation of the cyclopropanols followed by halide displacement of the sulfonate group to halogen, accompanied by cyclopropyl-allyl rearrangement, affords the required allyl bromides and chlorides.
The composition of the gaseous products from the reaction of Ti(IV) isopropoxide with ethylmagnesium bromide in diethyl ether evidences the formation of subvalent titanium isopropoxide species in various oxidation states depending on relative amounts of the reactants. Reaction of titanium(IV) isopropoxide with one equivalent of the Grignard reagent gives presumably titanium(III) isopropoxide. The latter is generated as a result of disproportionation of starting Ti(IV) species and titanium(II) isopropoxideethene complex which is formed as a result of ethane extrusion from diethyltitanium(IV) isopropoxide. Titanium(III) isopropoxide prepared by this way transforms the aldehydes and the aromatic ketones into the corresponding pinacols in good yields.
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