The aminodihydropentalene derivative la reacts with the Lewis acidic RB(C6F5)(2) boranes (2a-c) by C-C bond cleavage to yield the formal borylene insertion products 3. In contrast, 1a,b react with HB(C6F5)(2) at 55 degrees C by elimination of dihydrogen to yield the iminium-stabilized zwitterionic heterofulvenes 10a,b. The reaction pathways were studied by preparation of the kinetically controlled intermediates 7a,b and the thermodynamically controlled products 9a,b, monitored by variable-temperature NMR experiments, and supported by DFT calculations. The trapping reactions of 9a with HCl and PhCHO, respectively, led to the addition products 13 and 14. Compounds 3c, 7a,b, 10a,b, 11, 13, and 14 were characterized by X-ray diffraction.