LETTER
Substituent Effects in Carboni–Lindsey Reactions of Alkynylboronates
2887
(5) Gomez-Bengoa, E.; Helm, M. D.; Plant, A.; Harrity, J. P. A.
J. Am. Chem. Soc. 2007, 129, 2691.
(6) For related mechanistic studies of cycloadditions involving
1, see: Rusinov, G. L.; Ishmetova, R. I.; Latosh, N. I.;
Ganebnych, I. N.; Chupakhin, O. N.; Potemkin, V. A. Russ.
Chem. Bull. 2000, 49, 355.
(7) For an excellent overview of [4+2] cycloadditions of
1,2,4,5-tetrazines, see: Sauer, J.; Heldmann, D. K.;
Hetzenegger, J.; Krauthan, J.; Sichert, H.; Schuster, J. Eur.
J. Org. Chem. 1998, 2885.
(11) Characterisation Data for Compounds 3c–e:
3c: isolated as a light yellow solid; mp >184 °C (dec.). 1H
NMR (250 MHz, CDCl3): d = 0.99 (s, 12 H, Me), 1.97 (s, 3
H, Me), 2.01 (s, 3 H, Me), 2.27 (s, 3 H, Me), 2.70 (s, 3 H,
Me), 5.70 (s, 1 H, CH), 6.02 (s, 1 H, CH), 6.85–6.95 (m, 2 H,
CH), 7.12–7.22 (m, 2 H, CH). 13C NMR (62.9 MHz, CDCl3):
d = 11.3, 13.3, 13.9, 14.9, 25.3, 84.3, 106.2, 111.3, 114.6 (d,
J = 22.0 Hz), 129.8, 131.4 (d, J = 8.5 Hz), 141.2, 142.5,
144.7, 149.7, 150.5, 152.4, 157.6, 162.9 (d, J = 249.0 Hz).
FTIR: 2980 (m), 2922 (w), 1604 (w), 1579 (m), 1535 (w),
1505 (s), 1475 (s), 1422 (s), 1402 (s), 1341 (s), 1234 (m),
1162 (w), 1142 (m), 1123 (m) cm–1. HRMS: m/z [M + H+]
calcd for C26H31BN6O2F: 489.2586; found: 489.2581. 3d:
isolated as a light yellow solid; mp 191.9–193.4 °C. 1H NMR
(250 MHz, CDCl3): d = 0.98 (s, 12 H, Me), 1.98 (s, 3 H, Me),
2.01 (s, 3 H, Me), 2.26 (s, 3 H, Me), 2.70 (s, 3 H, Me), 5.71
(8) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett.
1988, 29, 2631.
(9) (a) Carboni, R. A.; Lindsey, R. V. J. Am. Chem. Soc. 1959,
81, 4342. (b) Boger, D. L. Chem. Rev. 1986, 86, 781.
(10) Representative Experimental Procedure for
Alkynylboronate Cycloaddition Reactions; Synthesis of
3,6-Bis(3,5-dimethyl-1H-pyrazol-1-yl)-4-(4-methoxy-
phenyl)-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)pyridazine (3b): 3,6-Bis(3,5-dimethylpyrazol-1-yl)-
1,2,4,5-tetrazine (1; 131 mg, 0.486 mmol) and 2b (125 mg,
0.486 mmol) were dissolved in nitrobenzene (2 mL) and
heated at 140 °C until the red colour of the tetrazine had
faded. The nitrobenzene was then removed by bulb-to-bulb
distillation under reduced pressure and the residue was
purified by chromatography on silica gel to give the title
compound as a light yellow solid (202 mg, yield: 83%); mp
169.3–170.2 °C. 1H NMR (250 MHz, CDCl3): d = 1.03 (s, 12
H, Me), 1.92 (s, 3 H, Me), 2.09 (s, 3 H, Me), 2.29 (s, 3 H,
Me), 2.72 (s, 3 H, Me), 3.74 (s, 3 H, Me), 5.72 (s, 1 H, CH),
6.04 (s, 1 H, CH), 6.71–6.80 (m, 2 H, CH), 7.10–7.20 (m, 2
H, CH). 13C NMR (69.2 MHz, CDCl3): d = 11.2, 13.4, 13.9,
14.8, 25.2, 55.3, 84.3, 106.0, 111.1, 113.0, 126.0, 130.9,
141.0, 142.4, 145.5, 149.5, 150.4, 152.6, 157.7, 160.0. FTIR:
2977 (m), 2930 (m), 2837 (w), 1611 (m), 1576 (m), 1536
(w), 1507 (s), 1498 (s), 1421 (s), 1402 (s), 1370 (s), 1302
(m), 1251 (s), 1179 (m), 1140 (m), 1124 (m) cm–1. HRMS:
m/z [M + H+] calcd for C27H34BN6O3: 501.2785; found:
501.2800.
(s, 1 H, CH), 6.01 (s, 1 H, CH), 7.09–7.22 (m, 4 H, CH). 13
C
NMR (62.9 MHz, CDCl3): d = 11.3, 13.3, 13.9, 14.9, 25.3,
84.4, 106.3, 111.3, 127.7, 130.9, 132.3, 134.8, 141.2, 142.5,
144.5, 149.8, 150.6, 152.1, 157.5. FTIR: 2980 (m), 2929
(m), 1600 (w), 1579 (m), 1560 (m), 1532 (m), 1493 (s), 1475
(s), 1448 (m), 1421 (s), 1356 (s), 1140 (s) cm–1. HRMS: m/z
[M + H+] calcd for C26H31BN6O2Cl: 505.2290; found:
505.2310. 3e: isolated as a light yellow solid; mp 215.3–
217.0 °C. 1H NMR (250 MHz, CDCl3): d = 1.00 (s, 12 H,
Me), 1.97 (s, 3 H, Me), 2.14 (s, 3 H, Me), 2.32 (s, 3 H, Me),
2.76 (s, 3 H, Me), 5.77 (s, 1 H, CH), 6.08 (s, 1 H, CH), 7.30–
7.38 (m, 2 H, CH), 7.52–7.61 (m, 2 H, CH). 13C NMR (62.9
MHz, CDCl3): d = 11.5, 13.2, 13.9, 14.9, 25.3, 84.4, 106.6,
111.6, 112.2, 118.4, 130.3, 131.2, 138.9, 141.4, 142.5,
143.4, 149.9, 150.7, 151.5, 157.4. FTIR: 2992 (m), 2925 (w),
1857 (w), 2232 (m), 1579 (m), 1534 (m), 1503 (m), 1469 (s),
1419 (s), 1369 (m), 1141 (m) cm–1. HRMS: m/z [M + H+]
calcd for C27H31N7O2B: 496.2632; found: 496.2608.
(12) Unfortunately, the relative rates depicted in Table 4 did not
provide a good linear slope when a Hammett plot was drawn
up.
Synlett 2007, No. 18, 2885–2887 © Thieme Stuttgart · New York