Elucidating multicomponent mechanisms in the catalytic hydrogenation of 2-methylquinoline under crude-H2 conditions: a key H2-cleavage process by a boron–olefin Lewis pair

Abstract The mechanisms of the triarylborane-catalyzed hydrogenation of 2-methylquinoline (MeQin) in the presence of CO2 were investigated using the artificial force induced reaction method. When B(C6F5)3 (B1) is used as the catalyst, the hydrogenation proceeds via intermolecular proton and hydride transfer from [MeQin–H][H−B1] to a 1,4-dihydroquinoline intermediate (p-H2-MeQin). However, we confirmed that B1 quickly decomposes via a CO2-capture reaction with 2-methyl-1,2,3,4-tetrahydroquinoline (H4-MeQin) followed by thermally induced proto-deboronation. When B(2,6-Cl2C6H3)(3,5-Br2-2,6-F2C6H)2 (B2) is employed, the dissociation of CO2 from the corresponding CO2-capture product can occur prior to the irreversible proto-deboronation step, and the hydrogenation of MeQin thus continues efficiently. Moreover, the artificial force induced reaction analysis suggested that a frustrated Lewis pair comprising the boron atom of B2 and the olefinic carbon in p-H2-MeQin would mediate the heterolytic cleavage of H2. Based on these mechanistic details, we identified the modified catalyst B(2-Cl-6-FC6H3)3 (B7) that demonstrates a remarkable catalyst turnover number (TON = 4,000) in the hydrogenation of MeQin under H2/CO2 conditions.