Metal-free catalytic hydrogenation

Abstract

Addition of hydrogen to unsaturated organic compounds has produced a plethora of chemical products in food, agricultural, pharmaceutical, and oil industry. The current hydrogenation methods are usually accompanied by transition-metal-based catalysts. However, transition-metal-free catalytic hydrogenation is yet to be explored. In this study, Stephan and his group demonstrated the first hydrogenation process that does not need metal catalysts. Their strategy involved the so-called "frustrated Lewis pairs" such as phosphonium borates (R2PH)(C6F4)BH(C 6H5)2 (R = 2,4,6-Me3C 6H2 1 or tBU 2) to catalyze the hydrogenation of imines (Table 1, entries 1-7), nitriles (entries 8-10), and aziridines (entry 11), affording primary and secondary amines in high yields. It was observed that electron-rich imines (entries 1 and 2) have high reduction rates and low pressure (1 atm) requirements and need only moderate temperatures. These findings are in contrast to those found for the electron-poor imines (entries 3 and 4). The results suggest that basicity of the N center determines the rate of the reaction, indicating that the reduction was due to proton transfer from P to N. The absence of reactivity between phosphonium borate Cy 3P(C6F5)3BH(C6F 5)2 and tBuN-CPh(H) even after heating at 120°C for 24 h supported this contention. It is also noteworthy that less bulky imine and nitriles must first be bound to B(C6H5)3 to successfully effect their catalytic reduction (entries 7-10). In these cases, the reduction involved hydride transfer from the catalyst to the carbon of borane-bound imine. Subsequent protonation of the amido borate anion by the phosphonium center produced the desired amine (Scheme 1). While the hydrogenation of imines, nitriles, and aziridines occurred in catalytic fashion, the reduction of benzaldehyde with 1 or 2 was limited only to the formation of the zwitterion in which the benzyloxy is still bound to the boron center of the catalyst (Scheme 2). The final proton transfer from P center to O did not occur because the basicity of O is lower than that of N. (Chemical Equation Presented). © 2007 Data Trace Publishing Company.