Measurement of Heterogeneous Reaction Rates: Three Strategies for Controlling Mass Transport and Their Application to Indium-Mediated Allylations

Solvent Effects on Heterogeneous Rate Constants for Indium Mediated Allylations

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.

A Modern Twist to a Classic Synthetic Route: Ph3Bi-Based Redox Transmetalation Protolysis (RTP) for the Preparation of Barium Metalorganic Species

This paper reports advances in redox transmetalation/protolysis (RTP) utilizing the readily available Ph₃Bi for the synthesis of a series of barium metal-organic species. On the basis of easily available starting materials, an easy one-pot procedure, and workup, we have obtained BaL₂ compounds (L = bis(trimethylsilyl)amide, phenyl(trimethylsilyl)amide, pentamethylcyclopentadienide, fluorenide, 2,6-di-isopropylphenolate, and 3,5-diphenylpyrazolate) quantitatively by sonication of an excess of barium metal with triphenylbismuth and HL in perdeuterotetrahydrofuran, as established by NMR measurements. Rates of conversion are affected by both pK_a and bulk of HL. Competition occurs from direct reaction of Ba with HL, thereby enhancing the overall conversion, the effect being pronounced for the less bulky and more acidic ligands. Overall, the method significantly adds to the synthetic armory for barium metal-organic/organometallic compounds.