Synthesis of 2-Alkynyl-, 4-Alkynyl-, and 2,7-Dialkynyl-1,8-bis(dimethyl­amino)naphthalenes and the Unexpected Influence of ortho-Alkynyl Groups on Their Basicity

Abstract

Novel 2-alkynyl-, 4-alkynyl-, and 2,7-dialkynyl derivatives of 1,8-bis(dimethylamino)naphthalene (‘proton sponge’) have been synthesized and their basicity values have been measured by competitive NMR studies in DMSO. These indicate that, while the para-alkynyl groups decrease the basicity of the parent proton sponge approximately by one order of magnitude in accordance with their electron-accepting nature, the influence of ortho-alkynyl functionalities is unexpectedly base-enhancing. The latter phenomenon has been ascribed to the appearance of a buttressing effect, but some other factors can be also at work.

Key words

amines - proton sponge - cross-coupling - alkynes - ­buttressing effect - basicity

• References and Notes

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• 13 1,8-Bis(dimethylamino)-2-(phenylethynyl)naphthalene (6a) Yellow solid; mp 98–100 °C (from EtOH). ¹H NMR (250 MHz, CDCl₃): δ = 2.78 (6 H, s, NMe₂), 3.16 (6 H, s, NMe₂), 6.95 (1 H, dd, J = 6.0, 2.8 Hz, H_arom), 7.28–7.40 (7 H, m, H_arom), 7.54 (2 H, dd, J = 7.7, 1.8 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 45.0, 45.2, 91.5, 94.5, 114.2, 114.7, 122.0, 123.0, 124.8, 126.8, 128.0, 128.8, 131.0, 131.3, 138.2, 152.0, 152.5. MS: m/z (%) = 314 (90) [M]⁺, 299 (24), 282 (72), 268 (77), 254 (29), 226 (35), 207 (37), 196 (27), 167 (26), 157 (25), 149 (29), 141 (29), 133 (25), 127 (53), 113 (38), 103 (26), 91 (70), 77 (68), 58 (72), 51 (35). 44 (100). Anal. Calcd for C₂₂H₂₂N₂: C, 84.04; H, 7.05; N, 8.91. Found: C, 83.87; H, 7.23; N, 9.03. 1,8-Bis(dimethylamino)-2-(p-tolylethynyl)naphthalene (6b) Yellow solid; mp 89–92 °C (from EtOH). ¹H NMR (250 MHz, CDCl₃): δ = 2.37 (3 H, s, CMe), 2.79 (6 H, s, NMe₂), 3.18 (6 H, s, NMe₂), 6.95 (1 H, dd, J = 6.3, 2.4 Hz, H_arom), 7.16 (2 H, d, J = 8.1 Hz, p-Tol), 7.24–7.40 (4 H, m, H_arom), 7.43 (2 H, d, J = 8.1 Hz, 4-Tol). ¹³C NMR (62.9 MHz, CDCl₃): δ = 22.0, 45.0, 45.2, 91.0, 94.8, 114.0, 115.0, 119.2, 121.8, 122.1, 123.0, 126.8, 129.8, 131.0, 131.2, 138.2, 138.3, 152.0, 152.1. MS: m/z (%) = 328 (100) [M]⁺, 313 (25), 296 (74), 282 (66), 268 (17), 206 (18), 196 (20), 164 (21), 149 (22), 141 (17), 134 (18), 127 (20). Anal. Calcd for C₂₃H₂₄N₂: C, 84.11; H, 7.37; N, 8.53. Found: C, 84.24; H, 7.26; N, 8.37. 1,8-Bis(dimethylamino)-2-(trimethylsilylethynyl)-naphthalene (6c) Dark yellow oil. ¹H NMR (250 MHz, CDCl₃): δ = 0.25 (9 H, s, SiMe₃), 2.78 (6 H, s, NMe₂), 3.09 (6 H, s, NMe₂), 6.91 (1 H, m, H_arom), 7.24–7.35 (4 H, m, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 0.5, 45.0, 45.1, 82.4, 99.1, 107.3, 114.0, 122.0, 122.7, 126.8, 131.3, 138.4, 138.5, 152.1, 153.1. IR: 2140 (C≡C), 1553 (ring) cm^–1. MS: m/z (%) = 310 (51) [M]⁺, 295 (29), 279 (24), 264 (22), 238 (30), 221 (17), 206 (51), 192 (30), 73 (100). Anal. Calcd for C₁₉H₂₆N₂Si: C, 73.49; H, 8.44; N, 9.02. Found: C, 73.58; H, 8.29; N, 9.23. 2-Ethynyl-1,8-bis(dimethylamino)naphthalene (6d) Dark yellow oil. ¹H NMR (250 MHz, CDCl₃): δ = 2.86 (6 H, s, NMe₂), 3.20 (6 H, s, NMe₂), 3.45 (1 H, s, ≡CH), 7.04 (1 H, m, H_arom), 7.31–7.45 (4 H, m, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 45.0, 45.2, 82.4, 85.6, 113.6, 114.2, 122.1, 122.9, 123.0, 126.9, 131.4, 138.5, 152.1, 153.3. IR: 3303 (≡CH), 2093 (C≡C), 1600, 1553 (ring) cm^–1. MS: m/z (%) = 238 (95) [M]⁺, 223 (18), 206 (70), 192 (83), 178 (21), 165 (18), 152 (31), 139 (15), 111 (21), 103 (23), 89 (25), 76 (33), 63 (34), 58 (69), 32 (100). Anal. Calcd for C₁₆H₁₈N₂: C, 80.63; H, 7.61; N, 11.75. Found: C, 80.50; H, 7.79; N, 11.81. 1,8-Bis(dimethylamino)-2,7-bis(phenylethynyl)-naphthalene (7a) Yellow solid, mp 156–157 °C (from EtOH or n-octane). ¹H NMR (250 MHz, CDCl₃): δ = 3.20 (12 H, s, 2 NMe₂), 7.33–7.47 (10 H, m, H_arom), 7.55–7.62 (4 H, m, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 45.0, 91.0, 94.5, 117.0, 123.5, 124.4, 126.3, 128.0, 128.5, 131.0, 131.5, 138.0, 153.0. IR: 2211 (C≡C) cm^–1. MS: m/z (%) = 414 (94) [M]⁺, 399 (39), 382 (100), 368 (31), 307 (25), 91 (29), 58 (23). Anal. Calcd for C₃₀H₂₆N₂: C, 86.92; H, 6.32; N, 6.76. Found: C, 87.09; H, 6.17; N, 6.84. 1,8-Bis(dimethylamino)-2,7-bis(p-tolylethynyl)-naphthalene (7b) Yellow-orange solid, mp 155–156 °C (from i-PrOH). ¹H NMR (250 MHz, CDCl₃): δ = 2.36 (6 H, s, 2 CMe), 3.15 (12 H, s, 2 NMe₂), 7.17 (4 H, d, J = 7.9 Hz, 4-Tol), 7.32 (2 H, d, J = 8.5 Hz, H_arom), 7.40 (2 H, d, J = 8.5 Hz, H_arom), 7.43 (4 H, d, J = 7.9 Hz, 4-Tol). ¹³C NMR (62.9 MHz, CDCl₃): δ = 22.0, 45.0, 90.5, 95.0, 117.5, 121.5, 124.0, 126.8, 129.5, 131.5, 131.8, 138.0, 138.5, 153.0. IR: 2210 (C≡C) cm^–1. MS: m/z (%) = 442 (100) [M]⁺, 427 (40), 410 (89), 396 (33), 320 (22), 221 (20), 205 (21), 191 (17), 175 (26). Anal. Calcd for C₃₂H₃₀N₂: C, 86.84; H, 6.83; N, 6.33. Found: C, 87.00; H, 6.95; N, 6.23. 1,8-Bis(dimethylamino)-2,7-bis(trimethylsilylethynyl)-naphthalene (7c) Dark yellow oil. ¹H NMR (250 MHz, CDCl₃): δ = 0.24 (18 H, s, 2 SiMe₃), 3.06 (12 H, s, 2 NMe₂), 7.20 (2 H, d, J = 8.5 Hz, H_arom), 7.27 (2 H, d, J = 8.5 Hz, H_arom). IR: 2133 (C≡C), 1525 (ring) cm^–1. Anal. Calcd for C₂₄H₃₄N₂Si₂: C, 70.88; H, 8.43; N, 6.89. Found: C, 71.04; H, 8.54; N, 6.71. 2,7-Diethynyl-1,8-bis(dimethylamino)naphthalene (7d) Yellow oil. ¹H NMR (250 MHz, CDCl₃): δ = 3.09 (12 H, s, 2 NMe₂), 3.37 (1 H, s, ≡CH), 7.27 (2 H, d, J = 8.9 Hz, H_arom), 7.33 (2 H, d, J = 8.95 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 45.2, 83.0, 85.0, 115.8, 123.6, 126.1, 132.5, 138.4, 154.0. IR: 3300 (≡CH), 2090 (C≡C), 1540 (ring) cm^–1. MS: m/z (%) = 262 (100) [M]⁺, 230 (59), 216 (41). Anal. Calcd for C₁₈H₁₈N₂: C, 82.41; H, 6.92; N, 10.68. Found: C, 82.62; H, 6.71; N, 10.78. 1,8-Bis(dimethylamino)-4-(phenylethynyl)naphthalene (8a) Yellow solid; mp 147–149 °C (from EtOH). ¹H NMR (250 MHz, CDCl₃): δ = 2.80 (6 H, s, NMe₂), 2.85 (6 H, s, NMe₂), 6.87 (1 H, d, J = 8.0 Hz, H_arom), 6.97 (1 H, d, J = 7.7 Hz, H_arom), 7.31–7.41 (4 H, m, H_arom), 7.58–7.67 (3 H, m, H_arom), 8.04 (1 H, d, J = 8.3 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 44.3, 44.5, 89.6, 93.1, 111.9, 112.9, 113.3, 119.9, 120.6, 124.7, 126.8, 128.1, 128.7, 131.1, 131.8, 138.1, 151.5, 151.9. IR: 2201 (C≡C) cm^–1. MS: m/z (%) = 314 (100) [M]⁺, 299 (19), 283 (44), 268 (76), 226 (21), 134 (13). Anal. Calcd for C₂₂H₂₂N₂: C, 84.04; H, 7.05; N, 8.91. Found: C, 83.92; H, 7.21; N, 9.10. 1,8-Bis(dimethylamino)-4-(p-tolylethynyl)naphthalene (8b) Yellow solid; mp 94–96 °C (from EtOH). ¹H NMR (250 MHz, CDCl₃): δ = 2.36 (3 H, s, CMe), 2.78 (6 H, s, NMe₂), 2.82 (6 H, s, NMe₂), 6.84 (1 H, d, J = 8.0 Hz, H_arom), 6.95 (1 H, dd, J = 7.7, 1.1 Hz, H_arom), 7.16 (2 H, d, J = 8.1 Hz, H_arom), 7.39 (1 H, dd, J = 7.7, 8.1 Hz, H_arom), 7.49 (2 H, dm, J = 8.1 Hz, H_arom), 7.57 (1 H, d, J = 8.0 Hz, H_arom), 8.00 (1 H, dd, J = 8.1, 1.1 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 21.9, 44.4, 44.5, 88.8, 93.2, 112.0, 113.3, 119.8, 119.9, 120.0, 121.6, 126.7, 129.5, 130.9, 131.7, 138.0, 138.1, 151.4, 151.7. IR: 2140 (C≡C) cm^–1. MS: m/z (%) = 328 (100) [M]⁺, 313 (19), 297 (44), 284 (38), 268 (14), 239 (18), 149 (21), 140 (30), 134 (14), 127 (13). Anal. Calcd for C₂₃H₂₄N₂: C, 84.11; H, 7.37; N, 8.53. Found: C, 84.28; H, 7.15; N, 8.46. 1,8-Bis(dimethylamino)-4-(trimethylsilylethynyl)-naphthalene (8c) Yellow solid; mp 108–109 °C (from MeOH). ¹H NMR (250 MHz, CDCl₃): δ = 0.27 (9 H, s, SiMe₃), 2.74 (6 H, s, NMe₂), 2.79 (6 H, s, NMe₂), 6.77 (1 H, d, J = 8.0 Hz, H_arom), 6.91 (1 H, dd, J = 7.6, 1.1 Hz, H_arom), 7.36 (1 H, dd, J = 7.6, 8.2 Hz, H_arom), 7.51 (1 H, d, J = 8.0 Hz, H_arom), 7.88 (1 H, dd, J = 8.2, 1.1 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 0.7, 44.3 44.4, 97.5, 105.3, 111.6, 112.7, 113.2, 119.6, 119.8, 126.8, 131.5, 138.2, 151.4, 152.0. IR: 2144 (C≡C) cm^–1. Anal. Calcd for C₁₉H₂₆N₂Si: C, 73.49; H, 8.44; N, 9.02. Found: C, 73.41; H, 8.27; N, 9.19. 4-Ethynyl-1,8-bis(dimethylamino)naphthalene (8d) Dark yellow oil. ¹H NMR (250 MHz, CDCl₃): δ = 2.76 (6 H, s, NMe₂), 2.81 (6 H, s, NMe₂), 3.36 (1 H, s, ≡CH), 6.79 (1 H, d, J = 7.9 Hz, H_arom), 6.92 (1 H, dm, J = 7.6 Hz, H_arom), 7.37 (1 H, dd, J = 7.6, 8.2 Hz, H_arom), 7.54 (1 H, d, J = 8.0 Hz, H_arom), 7.90 (1 H, dd, J = 8.2, 1.0 Hz, H_arom). ¹³C NMR (62.9 MHz, CDCl₃): δ = 44.3, 44.4, 80.5, 83.8, 105.3, 111.6, 113.3, 119.6, 121.9, 126.9, 131.9, 138.3, 151.5, 152.2. IR: 2085 (C≡C), 3290 (≡CH) cm^–1. Anal. Calcd for C₁₆H₁₈N₂: C, 80.63; H, 7.61; N, 11.75. Found: C, 80.74; H, 7.47; N, 11.59. For general procedures and other experimental and analytical details see the Supporting Information.
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