We envisioned another promotion of catalyst-free AAC by secondary covalent interactions. Thus, two functional groups (X, Y) of azide and alkyne rapidly interfere without any catalyst under biorthogonal 15 conditions to form an intermediate that has alkynyl and azide groups in proximal positions (Scheme 1a). The tentative intramolecular and proximal positionings were expected to accelerate the reaction rate and to improve regioselectivity of AAC. We chose formations of aminalimine or boronate ester as the secondary interactions since they proceed rapidly and they are known to exhibit biorthogonality. Thus, we thought that the reaction rate and regioselectivity of SPAAC would be improved in the combination of cyclooctynylamine 1a with azido aldehyde 2 or the combination of cyclooctynol 4a and azido boronic acid 5 to provide triazole 3a or 6a (Scheme 1b). Hosoya's group studied AAC utilizing the proximity effects of borate formation between 2-axidophenylboronic acid and non-strained N-propargyl diethanolamine. Hall reported synergic click chemistry utilizing the formation of boronate and thiosemicarbazone. In this paper, we report the first example of SPAAC by secondary covalent interactions of imines or boronate esters. Scheme 1. Concept for secondary interaction-assisted AAC (SI-AAC, eq a) and the secondary interaction-assisted SPAAC (SI-SPAAC, eq. b and c) Cyclooct-2-yn-1-amine (1a) and cyclooct-2-yn-1-ol (4a) were synthesized from cyclooctene (7) as shown in Scheme 2. Osmium tetraoxide-catalyzed dihydroxylation Cyclooctynes (1a and 4a) and aryl azides (2 and 5) were in hand, and we next studied their AAC. The AAC of alkynyl amine 1a with azido aldehyde 2 in methanol was completed within 2 h at room temperature, and compound 3a was isolated in 87% yield (Scheme 4A). The regioisomer 3a' that could be formed without the secondary interaction of the amino group of 1a and the formyl group of 2 was not isolated by our hands. We could not deny the formation of the regioisomer 3a', but the mass balance suggested that highly regioselective AAC between 1a and 2 took place. On the other hand, the reaction of the N-Boc amine 1b with the azido aldehyde 2 under the same reaction conditions gave 3b' in 55% yield as the major regioisomer along with an inseparable mixture of 3b and 3b-aminal in 37% yield (Scheme 4B). Thus, the regioselectivity was not controlled in AAC with N-Boc amine 1b. A competitive reaction between an equal amount of the amine 1a, N-Boc amine 1b, and 2 was investigated (Scheme 4C). The azide aldehyde 2 was completely consumed at room temperature for 2 h, and the cycloadduct 3a was isolated as a major product in 44% yield along with [3+2] adducts from 1b in 24% combined yields. Therefore, the reaction of azide aldehyde 2 with 1a took place more rapidly than that with 1b. 1 H NMR monitoring of AAC of 2 with 1a or 1b in CD3OD also showed that AAC with 1a proceeded more smoothly (Figure ). The regioselective formation of 3a from 1a and more rapid formation of 3a suggested that AAC of 1a and 2 would take place via an aminal 13 or a (Z)-imine 14 that were formed by the fast secondary covalent interaction between the amino group of 1a and the formyl group of 2 (Scheme 5). Next, the effect of boronic acid ester formation was investigated in AAC of 2-azido boronic acid 5 with hydroxy alkyne 4a and O-TBS alkyne 4b (Scheme 6). It was found that AAC of 4a and 5 proceeded rapidly at room temperature within 0.5 h to give a cycloadduct 6a in 96% yield (Scheme 6A). On the other hand, O-TBS alkyne 4b reacted with 5 very slowly even at 50 °C to afford 6b' as a major product, and boronic acid 5 (6%) was recovered after 23 h (Scheme 6B). These results showed that the formation of boronic acid ester 15 was crucial for the acceleration of the reaction rate and the control of regioselectivity (Scheme 7). We thought that the formation of the boronic acid ester 15 offered not only the proximal effects but also the electron-withdrawing nature that was favorable for AAC. Finally, the secondary interactions of aminal-imine and boronic acid ester for AAC were compared under identical conditions by measuring their reaction progress with 1 H NMR (Figure ). It was clearly shown that the boronic acid ester-assisted AAC to 6a proceeded more readily than the aminal-imine-assisted AAC to 3a. Therefore, the formation of boronate ester was the more effective secondary interaction for AAC.