Conjugate addition between syringol and a captodative olefin catalyzed by BF3

The regioselectivity of the conjugate addition between syringol (1) and the captodative olefin 3-(p-nitrobenzoyloxy)-but-3-en-2-one (2) is evaluated under diverse solvent and Lewis acid catalyst conditions, providing a mixture of the para/meta phenol adducts. Experimental results revealed for most of the trials that the para regioisomer is the major one. However, there is not a clear correlation between the regioselectivity of products and the polarity of the tested solvents. In this work, the reactivity descriptors and the reaction mechanism are studied in the framework of the density functional theory (DFT) to give insights about the reactivity and regioselectivity of this reaction. The Parr functions difference is used as the reactivity descriptor in each stage of the reaction explaining quite well the involved transition states. Overall, it is found that the reactivity of 1 is strongly influenced by the presence of the catalyst, while the regioselectivity of the products depends on both the catalyst and the polarity of the solvent, finding that, the meta adduct is highly dependent of the presence of nonpolar solvents.