Redox‐Active Frustrated Lewis Pair‐Mediated B─H Bond Activation: From Proton Transfer to THF Ring Opening

The CAAC‐stabilized dithiolene (L 0 ) zwitterion (1), an unusual redox‐active intramolecular frustrated Lewis pair (FLP), activates the B─H bond of boranes via hydride‐coupled reverse electron transfer processes. The reactions of 1 with catecholborane in THF give a zwitterionic bis(dithiolene)‐based spiroborate ( 2 ), in which one sulphur atom (at the C2 carbon) bonds to the (CH 2 ) 4 OB(O) 2 C 6 H 4 chain due to the catecholborane (CatBH)/S thiourea Lewis pair‐mediated THF ring opening. In addition to 2 , [CAAC(H)] + [(Cat) 2 B] − , ( 3 ), CAAC(H) 2 , ( 4 ), and [CAAC(H) 3 ] + [(Cat) 2 B] − , ( 5 ), are also isolated from these reactions. In addition to synthetic, structural, and spectroscopic data, a plausible reaction mechanism is proposed. This finding provides compelling experimental evidence of FLP‐mediated B─H activation via net proton transfer.