Understanding the role of adsorption in the reaction of cyclopentadiene with an immobilized dienophile

This paper presents a kinetic characterization for the Diels-Alder reaction of cyclopentadiene with 2-mercaptobenzoquinone chemisorbed to a gold substrate. Cyclic voltammetry was used to investigate the rate of the reaction because the quinone undergoes a reversible two-electron reduction but the product of reaction is not electroactive. Reactions were performed in 1:1 THF:H2O (2 mM phosphate and 70 mM NaCl, pH = 7.4) and monitored by voltammetric scanning between -120 and -20 mV. The rate of loss in peak current correlated with the rate of Diels-Alder reaction and was described well by an exponential decay in agreement with the pseudo-first-order kinetics expected with the high concentration of cyclopentadiene relative to quinone. The first-order rate constants, however, did not increase linearly with the concentration of diene as would be expected for a second-order bimolecular reaction. Instead, the first-order rate constants reached a limiting value with higher concentrations of cyclopentadiene. The kinetic behavior was better explained by a pathway wherein the cyclopentadiene first adsorbs to the monolayer and then reacts with the immobilized quinone. The data were fit well by a rate law based on a Langmuir isotherm for adsorption of diene to the monolayer and a first-order rate constant for the Diels-Alder reaction and gave a first-order rate constant of 0.011 s(-1) and an equilibrium constant for association of cyclopentadiene with the substrate of 65 M-1. The equilibrium constant for adsorption depended on the composition of solvent and was larger in an electrolyte containing 1:2 THF:H2O (163 M-1) and smaller in 2:1 THF:H2O (28 M-1). But in all cases, the first-order rate constant for cycloaddition was unchanged. The combination of self-assembled monolayers as structurally well-defined substrates and cyclic voltammetry to measure the rates of reactions provides a methodology that is well suited for studying many mechanistic features of interfacial reactions.