Oxidative rearrangement of chalcones without a hydroxy group by thallium(III) nitrate in methanol

Abstract

Oxidative rearrangement of chalcones with no free hydroxy group with thallium(III) nitrate (TTN) in methanol was studied in detail and the following results were found. The reactions were greatly accelerated when there were methoxy groups at the 2-, 4-, and/or 6-position on the B ring and formed the corresponding 1,2-diaryl-3,3-dimethoxypropan-1-one (acetal) only. The methoxy groups on the A ring also had a tendency to accelerate the reaction, but the 2′,6′-dimethoxy groups greatly retarded the reaction. The 2′-methoxy group in the chalcones with no substituent at the 6′-position apparently accelerated the reaction, although other 2′-substituents retarded it. In the reaction of the 2′,4′-dimethoxychalcones (3a and 3g) with no electron-donating group on the B ring, a mixture of the acetal, methyl 2,3-diaryl-3-methoxypropanoate, and thallium compounds were obtained. The three thallium compounds obtained from compound 3h were assigned as cyclic complexes of thallium() chloride with two molecules of the acetal and/or propanoate. Based on these results, the mechanism of the oxidative rearrangement was proposed as shown in Schemes 3 and 4.