Titanium(IV) bromide and boron(III) tribromide promoted reactions of arylaldehydes with 3-butyn-2-one, methyl propiolate and propynenitrile

The reactions of arylaldehydes with 3-butyn-2-one, methyl propiolate or propynenitrile in the presence of Lewis acids such as titanium(IV) bromide (TiBr4) or boron(III) tribromide (BBr3) (1.4 equiv.) can be drastically affected by the reaction temperature. When the reaction of arylaldehydes with 3-butyn-2-one was carried out at <-20degreesC, brominated compounds were obtained as the major product. However, when the reaction was carried out at room temperature (20degreesC), both the brominated compounds and alpha,beta-dibrominated compounds were formed as the major products. Moreover, at 70degreesC in 1,2-dichloroethane, the alpha,beta-dibrominated compounds were obtained as the sole products. On the other hand, the reactions of arylaldehydes with methyl propiolate or propynenitrile in the presence of TiBr4 are very slow at room temperature (20degreesC). The corresponding alpha-brominated compounds, derived from the reactions of arylaldehydes with methyl propiolate, were obtained in low yields. While, at 70degreesC in 1,2-dichloroethane, beta,beta-dibrominated compounds were obtained from the reactions of arylaldehydes with methyl propiolate or propynenitrile in the presence of TiBr4 in moderate yields. The substituent on the phenyl ring can affect the reaction rate and the E/Z ratio. In addition, with two substrate, palladium catalyzed allylic substitution and Suzuki-type coupling reaction have been examined. (C) 2002 Elsevier Science Ltd. All rights reserved.