RESUMO
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.
RESUMO
We describe three new strategies for determining heterogeneous reaction rates using photomicroscopy to measure the rate of retreat of metal surfaces: (i) spheres in a stirred solution, (ii) microscopic powder in an unstirred solution, and (iii) spheres on a rotating shaft. The strategies are applied to indium-mediated allylation (IMA), which is a powerful tool for synthetic chemists because of its stereoselectivity, broad applicability, and high yields. The rate-limiting step of IMA, reaction of allyl halides at indium metal surfaces, is shown to be fast, with a minimum value of the heterogeneous rate constant of 1 × 10(-2) cm/s, an order of magnitude faster than the previously determined minimum value. The strategies described here can be applied to any reaction in which the surface is retreating or advancing, thereby broadening the applicability of photomicroscopy to measuring heterogeneous reaction kinetics.