Scandium triflate-catalyzed intramolecular Friedel–Crafts acylation with Meldrumʹs acids: insight into the mechanism

The intramolecular Friedel–Crafts acylation of arenes with Meldrumʹs acid derivatives catalyzed by Sc(OTf)3 was reported as a mild and general entry into functionalized 1-indanones. Mechanistic investigations were undertaken to determine the rate-determining step in the acylation sequence using Meldrumʹs acid, as well as to examine the role of the Lewis acid catalyst. Enolizable Meldrumʹs acid derivatives react via an acyl ketene intermediate under thermal conditions, while quaternized Meldrumʹs acid derivatives are thermally stable and only act as effective Friedel–Crafts acylating agents in the presence of a Lewis acid catalyst. The acylation was postulated to proceed through direct acylation of a Lewis acid-activated carbonyl. In the catalytic Friedel–Crafts acylation of Meldrumʹs acids, triflic acid appeared to be the active catalytic species, with Sc(OTf)3 serving as a very mild and convenient reagent for its delivery.