Thieme Chemistry Journal Awardees - Where Are They Now? Pentafluorophenyl End-Capped Polyynes as Supramolecular Building Blocks

Abstract

The synthesis of pentafluorophenyl end-capped polyynes up to an octayne has been developed based on the use of a Fritsch-Buttenberg-Wiechell rearrangement as the key step. UV/Vis spectroscopic analysis shows little change in the electronic nature of these structures as phenyl is formally replaced by pentafluorophenyl. The ability of pentafluorophenyl polyynes to function as supramolecular building blocks in host-guest systems is demonstrated through the formation of an inclusion complex between triyne 5 and decafluorotolan.

Key words

alkynes - aromatic interactions - carbenes - carbenoids - crystal engineering - polyynes - supramolecular chemistry

References and Notes

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10

Data for 1 (C₈HBr₂F₅), F _w = 351.91; monoclinic crystal system; space group P2₁/n (an alternate setting of P2₁/c [No. 14]), a = 4.2574(6), b = 37.836(5), c = 5.8166(8) Å; β = 96.478(2)˚; V = 931.0(2) Å^³; Z = 4; ρ _calcd = 2.511 g
cm^-³; 2θ _max = 52.68˚; µ = 8.744 mm^-¹; T = -80 ˚C; total data collected = 7332; R ₁ = 0.0688 [1617 observed reflections with F _o ^² ÷ 2σ(F _o ^²)]; wR ₂ = 0.1884 for 136 variables and 1900 unique reflections with F _o ^² ÷ -3σ(F _o ^²); residual electron density = 1.576 and -1.452 e Å^-³. CCDC 725485.

25

Data for 11 (C₁₆H₁₅F₅Si), F _w = 330.37; monoclinic crystal system; space group P2₁/c (No. 14), a = 8.3640(5), b = 18.3726(11), c = 10.9822(7) Å; β = 104.5167(9)˚; V = 1633.74(17) Å^³; Z = 4; ρ _calcd = 1.343 g cm^-³; 2θ _max = 52.76˚; µ = 0.186 mm^-¹; T = -80 ˚C; total data collected = 12874; R ₁ = 0.0381 [2844 observed reflections with F _o ^² ÷ 2σ(F _o ^²)]; wR ₂ = 0.1125 for 201 variables and 3346 unique reflections with F _o ^² ÷ -3σ(F _o ^²); residual electron density = 0.210 and -0.241 e Å^-³. CCDC 725483.

26

Data for 27b (C₂₃H₂₁F₅Si), F _w = 420.49; triclinic crystal system; space group P1 (No. 2), a = 7.5471(8), b = 16.2227(18), c = 18.215(2) Å; α = 83.2514(15), β = 81.0827(16), γ = 84.9478(16)˚; V = 2182.3(4) Å^³; Z = 4; ρ _calcd = 1.280 g cm^-³; 2θ _max = 52.00˚; µ = 0.155 mm^-¹; T =
-80 ˚C; total data collected = 14651; R ₁ = 0.0460 [6050 observed reflections with F _o ^² ÷ 2σ(F _o ^²)]; wR ₂ = 0.1342 for 523 variables and 8490 unique reflections with F _o ^² ÷
-3σ(F _o ^²); residual electron density = 0.329 and -0.165 e Å^-³. CCDC 725482.

27

Intermolecular distances are based on the separation of planes generated from all non-hydrogen atoms of neighboring molecules using Mercury 1.3.

28

Data for 5×decafluorotolan (4:1, C₈₆H₂₀F₃₀), F _w = 1623.02; triclinic crystal system; space group P1 (No. 2), a = 7.2628(8), b = 13.1573(14), c = 18.0306(19) Å; α = 80.3101(17), β = 82.1018(17), γ = 81.9049(18)˚; V = 1670.0(3) Å^³; Z = 1; ρ _calcd = 1.614 g cm^-³; 2θ _max = 52.80˚; µ = 0.152 mm^-¹; T = -80 ˚C; total data collected = 13414; R ₁ = 0.0460 [4226 observed reflections with F _o ^² ÷ 2σ(F _o ^²)]; wR ₂ = 0.1494 for 523 variables and 6819 unique reflections with F _o ^² ÷ -3σ(F _o ^²); residual electron density = 0.397 and -0.189 e Å^-³. CCDC 725484.

29

The synthesis of 5, 10, 12, 17, and 24 has been communicated, see ref. 7.

30

TLC analysis often indicated that reactions were complete soon after addition of base (ca. 30 min), although warming of the reaction solution in the TLC capillary could influence this analysis.

32

General procedure for FBW rearrangements29Unless otherwise noted in the individual procedures, in a flame-dried flask, a solution of the dibromoolefin (2.00 mmol) in anhydrous hexanes (30 mL, freshly distilled from CaH2) was cooled to -78 ˚C under a positive pressure of N2. n-BuLi (2.5 M in hexanes, 1.05 equiv) was slowly added dropwise over a period of ca. 5 min. The reaction was allowed to warm to r.t. over ˜1 h, stirred for 3 h,30 and then quenched through the addition of aqueous NH4Cl (50 mL). Et2O (50 mL) was added, and the organic layer was separated, washed with distilled H2O (3 25 mL), dried over MgSO4, filtered, and the solvent removed in vacuo. Column chromatography (silica gel) gave the desired product.
3-Pentafluorophenyl-2-propynoic acid (2)
Using the general procedure, 1 (0.500 g, 1.42 mmol) in hexanes (30 mL) was reacted with n-BuLi (1.15 mL, 2.90 mmol, 2.5 M in hexanes) for 1 h and then warmed to r.t. MgBr2 (0.29 g, 1.6 mmol) was added in one portion, and the inhomogeneous mixture was stirred for 15 min. Dry CO2 gas was bubbled through the solution as it was allowed to warm to r.t. and stirred overnight. Et2O (50 mL) and aqueous 10% HCl (50 mL) were added and the organic phase was separated and washed with distilled H2O, followed by 10% aqueous NaOH (3  30 mL). The aqueous phases were collected and acidified with aqueous 10% HCl (50 mL). Et2O (3  30 mL) was added and the organic phases were separated, dried over MgSO4, treated with activated charcoal, and filtered. Solvent removal gave 2 (0.182 g, 28%) as a colorless solid. Data were consistent with those reported.11 Mp 96-98 ˚C (Lit.11 105-106 ˚C). IR (neat): 3055 (s), 2987 (s), 2241 (s), 1694 (s), 1522 (s) cm^-1; 1H NMR (400 MHz, CDCl3): d = 10.19 (br); 19F (376 MHz, CDCl3): d = -132.9 to -133.0 (m, 2 F), -146.6 (tt, 3J = 21 Hz, 5J = 4 Hz, 1 F), -160.2 to -160.4 (m, 2 F); 13C{1H} NMR (125 MHz, CDCl3): d = 157.0, 148.4 (d mult, 1J = 258 Hz), 143.8 (d mult, 1J = 260 Hz), 137.8 (d mult, 1J = 260 Hz), 96.8 (app td, J = 17.4, 3.9 Hz), 90.1-90.0 (m), 72.4-72.2 (m); EIMS: m/z (%) = 192.0 (100) [M - CO2]+; HRMS: m/z [M - CO2]+ calcd for C8HF5: 191.9998; found: 191.9974. Anal. Calcd for C9HF5O2: C, 45.76; H, 0.42. Found: C, 45.98; H, 0.67.
1,4-[Bis(pentafluorophenyl)]-1,3-butadiyne (4)
Using the general procedure, 1 (0.700 g, 1.99 mmol) in hexanes (30 mL) was reacted with n-BuLi (1.7 mL, 4.1 mmol, 2.4 M in hexanes). Aqueous work up gave a faint-yellow oil that was immediately dissolved in CH2Cl2 (ca. 5 mL) and added dropwise over 30 min to a solution of Hay catalyst [CuCl (0.200 g, 2.04 mmol) and TMEDA (1.21 g, 10.4 mmol, 1.6 mL)] in CH2Cl2 (50 mL). The reaction was stirred overnight, Et2O (50 mL) and aqueous 10% HCl (50 mL) were added, and the organic phase was separated, washed with distilled H2O (3  50 mL), dried over MgSO4, filtered, and the solvent removed in vacuo. Column chromatography (pentane) gave 4 (0.146 g, 37%) as a colorless solid. Spectral data for this compound were consistent with those reported.12 1-Pentafluorophenyl-5-(tert-butyldimethylsilyl)-2,4-pentadiyn-1-one (19)
To anhydrous CH2Cl2 (100 mL, freshly distilled from CaH2) was added pentafluorophenyl benzoyl chloride (1.61 g, 11.4 mmol) and 18 (1.50 g, 6.35 mmol). The temperature of the solution was lowered to 0 ˚C under a positive pressure of N2, and AlCl3 (1.01 g, 7.63 mmol) was added portion-wise over 5 min. The reaction was stirred at 0 ˚C, monitored by TLC until starting material was no longer observed (typically ca. 3 h), and then carefully poured into aqueous 10% HCl (50 mL) in ice (50 g); CAUTION: exothermic. Pentane (50 mL) was added and the organic layer was separated, washed with distilled H2O (3  50 mL), dried over MgSO4, treated with activated charcoal, filtered, and the solvent removed in vacuo. Column chromatography (CH2Cl2-pentane, 1:4) gave 19 (0.754 g, 2.11 mmol, 33%), as a clear pale-yellow oil. Rf = 0.35 (CH2Cl2-pentane, 1:4). IR (neat): 2955 (m), 2193 (s), 2099 (s), 1659 (s), 1522 (m) cm^-1; 1H NMR (400 MHz, CDCl3): d = 0.97 (s, 9 H), 0.19 (s, 6 H); 19F (376 MHz, CDCl3): d = -140.3 to -140.4 (m, 2 F), -146.8 (tt, 3J = 21 Hz, 5J = 6 Hz, 1 F), -160.3 to -160.5 (m, 2 F); 13C{19F} NMR (100 MHz, CDCl3): d = 166.9, 145.6, 144.0, 137.8, 113.9, 100.5-100.4 (m), 86.1, 79.3, 72.6, 25.5 (q sept, J = 126, 6 Hz), 16.9-16.8 (m), -5.2 (q mult, 1J = 126 Hz); HRMS (EI): m/z [M+] calcd. for C17H15F5OSi: 358.0812; found: 358.0809. Anal. Calcd for C17H15F5OSi: C, 56.98; H, 4.19. Found: C, 57.35; H, 4.20. 5-tert-Butyldimethylsilyl-2,4-butynoic acid 1-(tert-Butyldimethylsilyl)butadiyne31 (1.30 g, 7.93 mmol) was added to anhydrous THF (75 mL) and the mixture was cooled to -78 ˚C under a positive pressure of N2. CH3Li (6.0 mL, 9.0 mmol, 1.5 M in Et2O) was added and the mixture was stirred for 1 h. Dry CO2 gas was bubbled through the solution as it was allowed to warm to r.t. and stirred overnight. Et2O (50 mL) and aqueous 10% HCl were added and the organic phase was separated, washed with distilled water (3  50 mL), dried over MgSO4, treated with activated charcoal, and filtered. Solvent removal in vacuo gave 5-tert-butyldimethylsilyl-2,4-butynoic acid (1.35 g, 82%) as a pale-yellow solid. Mp 52-53 ˚C. IR (neat): 3400-2700 (br), 2955 (s), 2248 (m), 2206 (s), 2108 (s), 1691 (s) cm^-1; 1H NMR (400 MHz, CDCl3): d = 10.28 (s), 0.96 (s, 9 H), -0.17 (s, 6 H); 13C NMR (100 MHz, CDCl3): d = 156.8, 95.5, 86.0, 73.7, 65.4, 26.0, 16.8, -5.2; MS (EI): m/z (%) = 208.1 (8) [M+], 151.0 (64) [M - t-Bu]+, 107.0 (100) [M - t-Bu - CO2]+; HRMS: m/z [M+] calcd. for C11H16O2Si: 208.0920; found: 208.0922. Anal. Calcd for C11H16O2Si: C, 63.46; H, 7.69. Found: C, 63.28; H, 7.96. 1-tert-Butyldimethylsilyl-5-dibromomethylene-7-pentafluorophenyl-1,3,6-heptatriyne (26a) SOCl2 (1.49 g, 0.9 mL, 12.5 mmol) was added to 5-tert-butyldimethylsilyl-2,4-butynoic acid (0.520 g, 2.50 mmol) in a flask protected from moisture with a drying tube containing CaCl2. The mixture was stirred at r.t. overnight. The excess SOCl2 was removed in vacuo to provide 25a, which was subjected to acylation with acetylene 8 (0.580 g, 2.20 mmol) and AlCl3 (0.351 g, 2.64 mmol) in CH2Cl2 (75 mL) as described for compound 19. The ketone product was immediately dissolved in anhydrous CH2Cl2 (50 mL, freshly distilled from CaH2) and carried on to the next step.CBr4 (0.476 g, 1.44 mmol) and Ph3P (0.754 g, 2.88 mmol) were added to CH2Cl2 (50 mL) and allowed to stir for 5 min at r.t. until the mixture turned bright orange. This solution was cooled to 0 ˚C, the reserved ketone was slowly added over a period of 20 min, and the reaction was stirred overnight. The solvent was reduced to ca. 10 mL, pentane (100 mL) was added and the inhomogeneous mixture was filtered through a plug of Celite and the solvent removed in vacuo. Column chromatography (pentane) gave 26a (0.219 g, 0.407 mmol, 19%) as a pale-yellow solid. Mp 104-105 ˚C. Rf = 0.66 (pentane). IR (neat): 2954 (m), 2197 (w), 2098 (m), 1502 (s) cm^-1; 1H NMR (400 MHz, CDCl3): d = 0.97 (s, 9 H), 0.18 (s, 6 H); 19F (376 MHz, CDCl3): d =
-134.8 to -134.9 (m, 2 F), -150.7 (app t, J = 21 Hz, 1 F),
-161.6 to -161.7 (m, 2 F); 13C {1H} NMR (100 MHz, CDCl3): d = 147.0 (d mult, 1J = 256 Hz), 142.6 (dtt, 1J = 259 Hz, J = 14 Hz, J = 5 Hz), 137.6 (d mult, 1J = 252 Hz), 114.4 (br), 112.4, 99.2 (app td, J = 18 Hz, J = 4 Hz), 96.3 (app q, J = 4 Hz), 94.8, 87.5, 81.4, 79.9 (app q, J = 4 Hz), 70.7, 26.0, 16.8, -5.0; HRMS (EI): m/z [M+] calcd. for C20H1579Br81BrF5Si: 537.9210; found: 537.9213. Anal. Calcd for C20H15Br2F5Si: C, 44.61; H, 2.79. Found: C, 45.01; H, 3.06.