Based on the above results, a plausible mechanism is proposed (Scheme 4E). Initially, B(C6F5)3 activates BCB ketone 1a to form complex I. The nucleophilic addition of formaldimine via an SN2like pathway produces intermediate V. Subsequent reaction with another molecule of formaldimine affords intermediate VI. Finally, intramolecular cyclization furnishes product 3a with the release of B(C6F5)3 catalyst. For intermediate V, the preferred intermolecular addition of another formaldimine than the intramolecular attack of the enolate on the iminium group (VI vs 6a) is probably due to the higher nucleophilicity of the Bn-N atom of formaldimine compared to the enolate. Thus, when N-PMP formaldimine with lower nucleophilicity is employed, the intramolecular cyclization of intermediate V' becomes competitive with the intermolecular addition of another N-PMP formaldimine to V', leading to products 6c and 3z simultaneously (path a vs path b, Scheme 4E). In contrast, when N-PMP triazinane 1b and BCB ester 2A were used under condition B, no biscyclobutenyl amine 5w was formed. Instead, two Leitch's products were obtained, the (3+2) aza-BCH 6d (40%) and the cyclobutenyl methanamine 7d (50%) (Scheme 5A). This result suggests that Leitch's carbocation species is involved in this reaction. Moreover, a competition reaction of 1a with 2A and 2G was performed under condition B, leading to three seperable products including the cross-over product 5x confirmed by NMR and HRMS analysis (Scheme 5B). This result indicates iterative additions of Lewis acid-induced enolate II to N-imine and N-iminium electrophiles. Furthermore, the reaction of 1a with 2A in the presence of PMB-NH2 generated two seperable products 5a and 5s, providing evidence for the exchange between PMB-NH2 and triazinane 1a (Scheme 5C). In addition, the reaction of [D6]-1a with 2A afforded the fully deuterated product [D4]-5a (Scheme 5D). Treatment of [D6]-1a with 2A in the presence of formaldehyde produced [D]-5a with 30% of hydrogen incorporation (Scheme 5E). These results demonstrate that under condition B, triazinane decomposes to formaldimine that further hydrolyzes to aniline and formaldehyde, which is consistent with the result that 4Å MS significantly suppressed the reaction (Table , entry 29). Based on the above results, we propose a possible mechanism (Scheme 5F). Initially, complexation of the In(OTf)3 catalyst with BCB ester 2A generates zwitterionic enolate intermediate II. Then, nucleophilic addition of enolate to formaldimine is preferred than the SN1-like attack of formaldimine to the cation carbon center of enolate, leading to Leitch's carbocation species VII. The selectivity may originate from the coordination of formaldimine to Lewis acid. This key intermediate VII undergoes an intramolecular E1 elimination to cyclobutenyl methanamine 7b rather than a nucleophilic ring-closing to 6b, which is mainly attributed to the more basicity of Bn-N atom. N-Bn triazinane 1a decomposes to formaldimine that further hydrolyzes to benzylamine and formaldehyde in the presence of water. A Lewis acid-promoted condensation of 7b with formaldehyde generates an iminium species VIII and a hydroxide ion. A second nucleophilic addition of enolate II to iminium affords carboncation species IX. Finally, deprotonation of IX by the hydroxide ion offers the target product 5a, regenerating Lewis acid and water. When N-PMP formaldimine with lower basicity was employed, the intramolecular nucleophilic ring-closing of intermediate VII' became competitive with the E1 elimination, leading to products 6d and 7d simultaneously. No biscyclobutenyl amine 5w was observed for this reaction, because N-PMP triazinane 1b could not hydrolyze to generate formaldehyde under condition B. This was demonstrated by the results of 1 H NMR analysis, showing that N-Bn triazinane 1a under condition B decomposed to benzylamine and formaldehyde, while no formaldehyde was observed for N-PMP triazinane 1b (Scheme 5G).