H-bonding additives act like Lewis acid catalysts.

Peter R Schreiner, Alexander Wittkopp

Index: Org. Lett. 4(2) , 217-20, (2002)

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Abstract

[reaction: see text] 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.