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

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.