H-Bonding Additives Act Like Lewis Acid Catalysts

Abstract
A combination of NMR, IR, and ab initio techniques reveals the striking structural similarities of an exemplary H-bonded complex of an N-acyloxazolidinone with an N,N-disubstituted electron-poor thiourea and the corresponding Lewis acid complex. Although the H-bond association constant is lower than for the Lewis acid adduct, Diels−Alder reactions are accelerated and stereochemically altered in a fashion similar to weak Lewis acids.