Reactions That Float

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Science  06 May 2005:
Vol. 308, Issue 5723, pp. 759
DOI: 10.1126/science.308.5723.759d

Solvents are generally thought to accelerate bimolecular reactions by increasing the mixing of the substrates and by stabilizing key structural changes along the pathway. Both factors would seem to rely on intimate contact between the solvent and the reactants. For over a half-century, water has been known to accelerate some organic coupling reactions, such as Diels-Alder cyclization, but the effect has remained largely unexploited because of the poor aqueous solubility of most reagents.

Narayan et al. have achieved rate enhancements for a wide range of cycloadditions and ring-opening reactions simply by stirring the insoluble reaction partners in an aqueous suspension. Remarkably, several reactions involving azodicarboxylates are accelerated beyond the rate achieved by solvent-free mixing of miscible liquid reagents: Coupling of neat quadricyclane and dimethyl azodicarboxylate takes 2 days as compared to only 10 min “on water.” Hydrogen bonding appears to increase the reaction rate, yet heterogeneity is a surprisingly important factor. When a suspension was homogenized by adding methanol, the reaction slowed down. A molecular explanation for the phenomenon is elusive, but the authors have encouraged those who make related observations to share their thoughts. — JSY

Angew. Chem. Int. Ed. 10.1002/anie.200462883 (2005).

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