Protruding interfacial OH groups and ‘on-water’ heterogeneous catalysis

Abstract

The key aspect of the remarkable organic catalysis that is observed to occur at theorganic/water phase boundary, the so-called ‘on-water’ catalysis (Narayan et al 2005Angew. Chem. 44 3275), was recently proposed to be the protruding OH groups of watermolecules at the interface that interact with the transition state (TS) via hydrogenbonding and lower activation barriers (Jung and Marcus 2007 J. Am. Chem. Soc. 1295492). In particular, the cycloaddition reaction of quadricyclane (Q) with dimethylazodicarboxylate (DMAD) on-water was calculated to be more than 100 000 timesmore efficient in terms of rate constant than the neat reaction. In this paper, wereview and consider a related reaction of Q with dimethyl acetylenedicarboxylate,where nitrogen, a good H-bond acceptor, in DMAD is replaced by carbon, a poorH-bond acceptor. A very low rate acceleration of acetylenedicarboxylate on-waterrelative to the neat reaction is obtained theoretically, as compared to DMADon-water, due to the relatively low H-bonding ability of acetylenedicarboxylate withwater at the TS relative to the reactants. We suggest that there may also bean ‘intrinsic steric effect’ or orientational advantage in the on-water catalysisin general, and both electronic and steric effects may be in operation for thesmaller on-water catalysis for the cycloaddition reaction of quadricyclane andacetylenedicarboxylate. A preliminary quantum mechanical/molecular mechanical(QM/MM) simulation including 1264 water molecules for the on-water reaction ofDMAD + Q also suggests that there are indeed approximately two–four more H-bonds between theTS and the dangling OH groups than between the reactants and the surface.