RESUMEN
Indium mediated allylation is a highly selective tool for synthetic chemists to create carbon-carbon bonds, but the first step, heterogeneous reaction of allyl halides at solid indium surfaces, is still poorly understood. For example, the nature of the solvent dramatically affects the rate of reaction, but solvent choice is often based on empirical experiments. Fundamental kinetic studies are the best way to study this effect, but the determination of heterogeneous rate constants is challenging. In an effort to better understand solvent effects, we use optical microscopy to determine heterogeneous rate constants for IMA in aqueous acetonitrile, methanol, ethanol, and 2-propanol. We fit the reaction rate data over a range of mass transport rates using only two adjustable parameters, the heterogeneous rate constant and the mass transport rate. The results emphasize the critical importance of water in determining the rate of reaction. Surprisingly, the polarity of the organic solvent in the mix does not have a major effect on the rate. It is hypothesized that the oxygen atom in water and alcohols is an especially effective Lewis base to stabilize the transition state and the organoindium intermediates, similar to the importance of the oxygen in ethers for the formation of Grignard reagents. This study again demonstrates the power of microscopy for the study of heterogeneous reactions.
RESUMEN
Indium mediated allylation (IMA) offers a powerful tool to synthetic chemists for creating carbon-carbon bonds. However, its rate limiting step, the heterogeneous reaction of allyl halides at solid indium surfaces, is still poorly understood. For example, solvent effects, especially the presence of water, on IMA are dramatic. We report for the first time rate constants for the heterogeneous rate limiting step of IMA. The rate constant for reaction of cinnamyl chloride on indium decreases from 5.5 × 10(-4) cm/s in 80% ethanol/20% water to 1 × 10(-4) cm/s in 99.8% ethanol/0.2% water. In addition, the percent water has a dramatic effect on induction time. This study further establishes photomicroscopy as a powerful tool for the determination of heterogeneous rate constants.
