Protruding interfacial OH groups and 'on-water' heterogeneous catalysis

Abstract

The key aspect of the remarkable organic catalysis that is observed to occur at the organic/water phase boundary, the so-called 'on-water' catalysis (Narayan et al 2005 Angew. Chem. 44 3275), was recently proposed to be the protruding OH groups of water molecules at the interface that interact with the transition state (TS) via hydrogen bonding and lower activation barriers (Jung and Marcus 2007 J. Am. Chem. Soc. 129 5492). In particular, the cycloaddition reaction of quadricyclane (Q) with dimethyl azodicarboxylate (DMAD) on-water was calculated to be more than 100 000 times more efficient in terms of rate constant than the neat reaction. In this paper, we review and consider a related reaction of Q with dimethyl acetylenedicarboxylate, where nitrogen, a good H-bond acceptor, in DMAD is replaced by carbon, a poor H-bond acceptor. A very low rate acceleration of acetylenedicarboxylate on-water relative to the neat reaction is obtained theoretically, as compared to DMAD on-water, due to the relatively low H-bonding ability of acetylenedicarboxylate with water at the TS relative to the reactants. We suggest that there may also be an 'intrinsic steric effect' or orientational advantage in the on-water catalysis in general, and both electronic and steric effects may be in operation for the smaller on-water catalysis for the cycloaddition reaction of quadricyclane and acetylenedicarboxylate. A preliminary quantum mechanical/molecular mechanical (QM/MM) simulation including 1264 water molecules for the on-water reaction of DMAD + Q also suggests that there are indeed approximately two–four more H-bonds between the TS and the dangling OH groups than between the reactants and the surface.