A heterogeneous Pd complex catalyst for allylation with allylic alcohols enhanced by an aluminum-doped mesoporous silica support

Abstract

Allylation using allylic alcohols is environmentally friendly because water is the only by-product. Allylic alcohols are one of the most unreactive allylating agents for homogeneous Pd-catalyzed allylation; however, they can undergo electrophilic activation with acidic active species. Heterogeneous catalysts are known for their unique catalytic performance due to the concerted effect of the immobilized metal center and support material surface. Our group has developed a mesoporous-silica-supported Pd complex catalyst that promotes the Tsuji–Trost allylation of dicarbonyl compounds with allylic alcohols through the concerted effect of Pd and surficial silanol groups. In this work, to enhance the catalytic activity of the supported Pd complex for allylation with allylic alcohols, the acidity of the support was increased by doping the silicate backbone with aluminum. Several spectroscopic techniques, such as Pd K-edge X-ray absorption fine structure (XAFS), solid-state NMR, and pyridine adsorption FT-IR measurements, were applied to confirm the structure of the catalyst. The Pd complex catalyst immobilized on an Al-doped support showed enhanced catalytic activity in allylation owing to the activation of the allylic alcohol by both the Al Lewis acid sites and silanol groups. The site-selective immobilization of the Pd complex through a silane-coupling reaction on Brønsted acid sites (Si–O(H⁺)–Al) enhances the concerted effect between the Pd complex and Lewis acidic Al sites on the MS surface.