An efficient method for constructing C3-stannyl-tetrahydroquinoline: cascade hydroboration and hydrostannation of quinoline

Abstract

Given the ability of carbon-tin bonds to undergo multiple transformations and the widespread application of C3-functionalized tetrahydroquinoline in the fields of pharmaceuticals, agrochemicals, and organic synthesis, the construction of C3-stannyl-tetrahydroquinoline holds significant importance. Guided by the hard–soft acid–base theory (HSAB), we used borane as hard acid to selectively reduce the hard base carbon–nitrogen double bonds in quinoline. In contrast, the soft acid hydrostannane is effective for reducing soft base carbon–carbon double bonds in quinoline. By using hydroborane and hydrostannane as co-reducing agents to reduce quinoline in the presence of B(C₆F₅)₃ as a catalyst, a series of novel and potentially important N-borylated-C3-stannyl-tetrahydroquinoline compounds can be obtained with high yields (up to 98%) and selectivity (100%) at room temperature within 2 h. Based on the characterization of key intermediates by a series of in situ NMR reactions and detailed experimental data, we proposed the key to this selective cascade reaction. The hard acid, HBcat, reacts rapidly with the hard base, carbon–nitrogen double bonds in quinoline, to form a Lewis acid–base adduct, which, in the presence of B(C₆F₅)₃, leads to the formation of N-borylated dihydroquinolines. This reaction is less feasible for the soft acid (ⁿBu)₃SnH. The soft acid (ⁿBu)₃SnH, activated by B(C₆F₅)₃, tends to attack soft base carbon–carbon double bonds in N-borylated dihydroquinolines and give the final selective products. Additionally, the ability to obtain allylamine derivatives after the product has been modified further confirms its potential application value.