Synthesis of TSPBA

N,N,N′,N′-tetramethyl-1,3-propanediamine (0.1 g, 0.75 mmol) and 4-(bromomethyl)phenylboronic acid (0.5 g, 2.3 mmol) were dissolved in dimethylformamide (10 ml) and mixed together. After stirring at 60°C overnight, the mixture was poured into tetrahydrofuran (THF) (100 ml), filtered, and washed with THF (3 × 20 ml). After drying under vacuum overnight, pure TSPBA (0.3 g, yield 70%) was obtained. ¹H nuclear magnetic resonance (300 MHz, d-DMSO, δ): 8.132 (s, 4H), 7.85 (d, 4H), 7.49 (d, 4H), 4.58 (s, 4H), 3.26 (s, 4H), 2.97 (s, 12H), 2.38 (m, 2H) (fig. S2).