Solvent Effects on Heterogeneous Rate Constants for Indium Mediated Allylations

Abstract

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.

Silver Iodide‐mediated Insertion of Indium to Synthesize Water‐tolerant Organoindium Reagents and the Application in Cross‐coupling Reactions

“…However, when palladium catalysts were Pd (OAc) 2 and Ruphos, Pd(dppf)Cl 2 , Pd 2 (dba) 3 and PPh 3 , the amount of catalyst was also 5mol%, the product 3 a was obtained with lower yield (Table 3, entries 2,4,5). Among them, the use of PdCl 2 (PPh 3 ) 2 (5 mol %) as the catalyst afforded the product 3 a with an excellent yield of 90% (Table 3, entry 6) [28] . After a series of screening, we finally got an optimal reaction system, the following cross‐coupling reactions were chosen to be carried out at 100 °C for 24 h using PdCl 2 (PPh 3 ) 2 as the catalyst, LiCl as an additive, and DMA as the solvent (Table 4, substrate 3 a ).…”

“…Among them, the use of PdCl 2 (PPh 3 ) 2 (5 mol %) as the catalyst afforded the product 3 a with an excellent yield of 90% (Table 3, entry 6). [28] After a series of screening, we finally got an optimal reaction system, the following cross-coupling reactions were chosen to be carried out at 100 °C for 24 h using PdCl 2 (PPh 3 ) 2 as the catalyst, LiCl as an additive, and DMA as the solvent (Table 4, substrate 3 a).…”