HETEROCYCLES, Vol. 86, No. 1, 2012
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appearance of a doublet at (! 38, d, J = 112 Hz) with the corresponding bromomagnesium
-
aminoborohydride signal (MgBr+ BH3-NiPr2, ! -28, q, J = 88 Hz). The reaction was then cooled to
25 °C and acidified with 3M HCl (3mL) (CAUTION: hydrogen evolution). After 10 min of stirring the
reaction mixture was warmed to 65 °C and stirred for an additional 15 min. The reaction mixture was
then transferred to a separatory funnel and extracted with Et2O (3 x 15 mL). The combined organic
layers were dried over anhydrous MgSO4, filtered, and concentrated in vacuo (25 °C, 1 Torr) to afford
napthylboronic acid as a white solid. The results for the other boronic acids prepared by this method are
summarized in Table 1. For copies of the 1H, 13C and 11B NMR spectrum see Supporting Information.
Because of their facile dehydration, boronic acids tend to provide inconsistent melting points. Therefore,
the melting points for boronic acids were not taken. (J. Org. Chem., 2011, 76, 3571.)
Phenylboronic acid (Table 1, entries 1, 11, 12);16 White powder; (0.234 g, 95%). 1H NMR (500 MHz,
CDCl3): ! 7.52 (t, J = 6 Hz, 2H), 7.61 (t, J = 7 Hz, 1H), 8.26 (d, J = 5.5 Hz, 2H); 13C NMR (125.7 MHz,
CDCl3): d 128.0, 132.7, 135.7, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): d
+30.7.
o-Tolylboronic acid (Table 1, entry 2);16 White powder; (0.226 g, 88%). 1H NMR (500 MHz,
13
CDCl3): ! 2.82, 7.27 (m, 2H), 7.459 (dt, J = 1.5, 7 Hz, 1H), 8.22 (dd, J = 7 Hz, 1H); C NMR (125.7
MHz, CDCl3): ! 23.1, 125.3, 130.7, 132.3, 137.4, 146.4; 11B NMR (160.4 MHz, CDCl3): ! +31.9.
p-Tolylboronic acid (Table 1, entry 3);16 White powder; (0.345 g, 95%). 1H NMR (500 MHz, CDCl3):
! 2.44 (s, 3H), 7.32 (d, J = 7.5 Hz, 1H), 8.13 (d, J = 7.5 Hz, 1H); 13C NMR (125.7 MHz, CDCl3): ! 21.9,
11
128.9, 133.7, 135.9, 143.1, carbon attached to boron not observed; B NMR (160.4 MHz, CDCl3): !
+30.4.
t-Butylboronic acid (Table 1, entry 4);25 White powder; (0.481g, 94%). 13C NMR (125.7 MHz,
CDCl3): ! 27.8, carbon attached to boron not observed; 11B NMR (160.4 MHz, CDCl3): ! +32.7.
Cyclohexylboronic acid (Table1, entry 5);26 White powder; (0.497g, 97%). 13C NMR (125.7 MHz,
11
CDCl3): ! 27.3, 27.5, 28.3, carbon attached to boron not observed; B NMR (160.4 MHz, CDCl3): !
+33.5.
n-Hexylboronic acid (Table 1, entry 6);27 White powder; (0.404g, 95%). 13C NMR (125.7 MHz,
CDCl3): ! 14.7, 23.4, 24.2, 25.0, 32.5, 32.8; 11B NMR (160.4 MHz, CDCl3): ! +34.4.
n-Decylboronic acid (Table 1, entry 7);28 White powder; (0.888g, 95%). 13C NMR (125.7 MHz,
CDCl3): ! 14.1, 22.7, 23.4, 24.4, 29.4, 29.5, 29.7, 31.9, 32.4, carbon attached to boron not observed; 11B
NMR (160.4 MHz, CDCl3): ! +33.9.
1-Napthylboronic acid (Table 1, entry 8);16 White powder; (0.253g, 79%). 1H NMR (500 MHz,
DMSO-d6): ! 3.44 (brs, 1H), 7.50 (m, 3H), 7.78 (d, J = 5 Hz, 1H), 7.91 (t, J = 9.5 Hz, 2H), 8.36 (brs, OH),