A Dendralenic C–H Acid
Received: 16 January 2019
Accepted after revision: 01 February 2019
14 February 2019 (online)
Published as part of the 30 Years SYNLETT – Pearl Anniversary Issue
The design and synthesis of a strong, dendralenic C–H acid is described. Crystal structure analyses confirm the proposed structure. Despite the moderate stability of our motif, an application to Brønsted acid catalysis has been explored.
Key wordsdendralenic C–H acids - cross-conjugated acids - triflyl groups - strong acids - non-coordinating anions - Brønsted acids
References and Notes
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- 7 Triformylmethane was prepared in two steps from commercially available bromoacetic acid following our recently reported procedure, see Ref. 2.
- 8 HTMP·TBTA Schlenk flask was charged with bis(triflyl)methane (1.7 g, 6.0 mmol, 6.0 equiv) and dry CH2Cl2 (1 mL) was added under argon. The colorless, clear solution so obtained was cooled to –78 °C in an acetone/dry ice bath. Triformylmethane (0.10 g, 1.0 mmol) was added and a slurry was obtained. Trimethyl orthoacetate (0.47 g, 3.9 mmol, 0.50 mL, 3.9 equiv) and acetic anhydride (1.6 g, 16 mmol, 1.5 mL, 16 equiv) were added and the reaction mixture was allowed to reach RT overnight. A dark red solution was obtained. All volatiles were removed under reduced pressure and the solid mixture so obtained was dissolved in CH2Cl2 (5 mL). 2,2,6,6-Tetramethylpiperidine (0.85 g, 6.0 mmol, 1.0 mL, 6.0 equiv) was added and the reaction mixture was subsequently concentrated under reduced pressure. Almost complete removal of all volatiles was achieved by dissolving in CHCl3 and evaporation to dryness. The solid mixture was transferred to a separation funnel with CHCl3 (20 mL) and washed with sat. aq NaHCO3 (20 mL). Both phases were separated and the aq phase was washed with CHCl3 (20 mL), acidified with aq HCl (conc.) to a pH of 1, and extracted with CH2Cl2 (20 mL). The pooled CH2Cl2 phases were concentrated under reduced pressure to give the TMP salt of the title compound as an orange solid (0.35 g). As this compound still contained significant amounts of bis(triflyl)methane, the solid mixture was dissolved in CHCl3, washed with aq HCl (conc.), dried over MgSO4, filtered, and concentrated under reduced pressure. The red solid so obtained was dissolved in CHCl3 and all volatiles were removed under reduced pressure. This procedure was repeated once more with CHCl3 and then with 1,2-dichloroethane. A red solid was obtained, which was triturated with CHCl3 to give the TMP salt of the title compound as a yellow solid (60 mg, 0.058 mmol, 5.8% yield). 1H NMR (500 MHz, CDCl3): δ = 8.31 (s, 3 H), 5.61 (s, 2 H), 1.86–1.81 (m, 2 H), 1.75–1.72 (m, 4 H), 1.49 (s, 12 H). (14N couplings can be observed in the 1H NMR spectrum. However, the signals of NH2 group was not sharp enough for an accurate determination of 1 J 1H14N.) 13C NMR (126 MHz, CDCl3): δ = 165.51, 119.82 (q, 1 J CF = 328 Hz), 114.79, 108.53, 59.94, 35.23, 27.87, 15.95; 19F NMR (471 MHz, CDCl3): δ = –73.96; HRMS (ESIneg): m/z [M – H+] calcd for C13H3O12F18S6 –: 884.7667; found: 884.7667. Single crystals suitable for structural analysis were obtained after dissolving the initially obtained orange solid [containing bis(triflyl)methane impurities] in CHCl3 or 1,2-dichloroethane and slowly evaporating the solvent.
- 9 Synthesis of HTBT as the Free Acid HTMP·TBT (17 mg, 0.017 mmol) was dissolved in CH2Cl2 (10 mL) and conc. H2SO4 (10 mL) was added. The mixture was stirred at RT for 30 min and the sulfuric acid phase was removed using a Pasteur pipet. BaCl2 (dry) was added and after stirring for 30 min, the solution was filtered and all volatiles were removed under reduced pressure. A yellowish solid was obtained. NMR spectra (1H and 19F) in CDCl3 were acquired. After 3 d, 1H and 19F NMR spectra were acquired once again, but after this time period, all product signals had vanished due to deprotonation and decomposition. 1H NMR (501 MHz, CDCl3): δ = 8.80 (t, J = 1.7 Hz, 1 H), 8.59 (t, J = 1.7 Hz, 1 H), 6.52 (d, J = 10.9 Hz, 1 H), 5.36 (d, J = 10.9 Hz, 1 H); 13C NMR (126 MHz, CD2Cl2): δ = 162.66, 158.32, 136.73, 123.98, 119.52 (q, J = 331 Hz), 76.06. (Due to the fast decomposition of the desired product and the observed 13C to 19F coupling, not all signals in the 13C NMR spectrum were obtained.) 19F NMR (471 MHz, CDCl3): δ = –71.48, –72.25, –72.52, –72.87, –73.00. The HTMP·TBT salt (0.017 g, 0.017 mmol) was dissolved in CH2Cl2 (10 mL) and conc. H2SO4 (10 mL) was added. The mixture was stirred at RT for 30 min and the sulfuric acid phase was removed. Treatment with BaCl2 was omitted. After three months, single crystals of the title compound were obtained which were suitable for structure analysis.
- 10 Höfler D. Ph.D. thesis. Universität zu Köln: Germany 2018
- 11 Conversion of HTBT into the Etherate SaltHTMP·TBT (43 mg, 0.042 mmol) was dissolved in CH2Cl2 (10 mL) and conc. H2SO4 (10 mL) was added. The mixture was stirred at RT for 30 min and the sulfuric acid phase was removed. All volatiles were removed under reduced pressure and a colorless solid was obtained. Et2O was added, which afforded a clear, yellow solution. All volatiles were removed under reduced pressure and a yellow solid was obtained. CH2Cl2 (10 mL) was added and the formation of a biphasic mixture was noticed. Slow evaporation over 14 d led to the formation of single crystals of the etherate salt, which were suitable for structure analysis.
- 12 Jutzi P, Müller C, Stammler A, Stammler H.-G. Organometallics 2000; 19: 1442