Room temperature cupric halides mediated olefin alkoxylation of BODIPYs with methanol: mechanisms and scope

Abstract

We disclose an efficient methodology for olefin alkoxylation of fluorescent BODIPYs (boron-dipyrromethene) at the 3,5-styryl group with methanol by cupric halide (chloride or bromide) at room temperature. Mechanistic studies provide evidence for the alkoxylation reaction firstly initiated by a radical cation, that is, halide promotes the oxidizing ability of the Cu(II) center to an extent that the single electron transfer (SET) from BODIPYs to the cupric ion and results in the production of a BODIPYs radical cation and Cu(I), then the BODIPYs radical cation subsequently reacts with methanol to afford the alkoxylated product. As the dialkoxylated product complexes with cuprous halide and further decreases its reducing ability, which is supported by DFT calculations, only strongly oxidative cupric bromide can mediate tetraalkoxylation and give rise to the tetraalkoxylated product. In addition, the expanded scope studies suggest that this method is also well suited for the alkoxylation of electron-rich conjugated olefins. The active benzyl bromide derivative may be another intermediate in the presence of cupric bromide. Therefore, the reaction is highly dependent on the anions of cupric salts; Cu(OAc)₂, CuSO₄ and Cu(NO₃)₂ containing weakly nucleophilic anions show no activity in alkoxylation.