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 M1. 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.