How H-Bonding Helps

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Science  01 Jun 2007:
Vol. 316, Issue 5829, pp. 1257
DOI: 10.1126/science.316.5829.1257c

Although hydrogen bonding has long been known to be prevalent in enzymatic substrate-binding motifs, its use in nonmetallic small-molecule catalysis has been adopted comparatively recently. Jensen and Sigman have quantified the impact of catalyst acidity on the rate and enantioselectivity of the hetero-Diels Alder reaction in one such system. The catalyst is a chiral oxazoline derivative that binds aldehydes through the N-H group of a pendant amide; the authors systematically varied the acidity of this group by adding differing numbers of Cl or F atoms to the amide -carbon. They observed linear free energy relationships correlating both rate and enantioselectivity with increased acidity, which they tentatively attribute to a tighter transition state JSY

Angew. Chem. Int. Ed. 46, 10.1002/anie.200700298 (2007).

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