Close Amide NH···F Hydrogen Bonding Interactions in 1,8-Disubstituted Naphthalenes

Abstract

In this note, we present a series of N-(8-fluoronaphthalen-1-yl)benzamide derivatives designed to maximize amide-NH•••F hydrogen bond interactions therein. A combination of IR and NMR spectroscopy indicates a linear correlation between the high energy shift in NH stretching frequency and the electron withdrawing nature of the substituent, consistent with the trend predicted by DFT calculations. Additionally, a limiting case of hydrogen bonding is observed when the benzamide derivatives are replaced with trifluoroacetamide, causing an additional red shift of 44 cm −1 in the NH stretching frequency. Most importantly, 1 H− 19 F coupling constants in this series are among the largest measured for amide-NH•••F interactions. X-ray crystallography reveals face-to-face alignment of naphthalene rings in these derivatives resulting in part from the NH•••F hydrogen bonds. This motif also dictates the formation of sheets composed of stacked naphthalene rings in the crystal structure as opposed to unfluorinated analogues wherein NH•••OC hydrogen-bonding interactions force benzamide and naphthalene rings to engage in T-shaped π−π interactions instead. Additionally, the NH proton in the trifluoroacetamide derivative engages in extended H-bond interactions in its crystal structure. Note pubs.acs.org/joc

Spectroscopic and Crystallographic Characterization of the R3N+−C−H⋅⋅⋅X Interaction

“…All solvents were dried and distilled by standard methods. All 1 H spectra were acquired on a 400 MHz NMR spectrometer in CD 3 CN or DMSO-d 6 , 19 F spectra were acquired on a 300 MHz NMR spectrometer in CD 3 CN, and 13 C NMR spectra were acquired on a 400 MHz NMR spectrometer in CD 3 CN or DMSO-d 6 . The 1 H, 13 C, and F NMR chemical shifts are given in parts per million (δ) with respect to an internal tetramethylsilane (TMS, δ 0.00 ppm) standard or 3-chlorobenzotrifluoride (δ À 64.2 ppm relative to CFCl 3 ).…”

“…All 1 H spectra were acquired on a 400 MHz NMR spectrometer in CD 3 CN or DMSO-d 6 , 19 F spectra were acquired on a 300 MHz NMR spectrometer in CD 3 CN, and 13 C NMR spectra were acquired on a 400 MHz NMR spectrometer in CD 3 CN or DMSO-d 6 . The 1 H, 13 C, and F NMR chemical shifts are given in parts per million (δ) with respect to an internal tetramethylsilane (TMS, δ 0.00 ppm) standard or 3-chlorobenzotrifluoride (δ À 64.2 ppm relative to CFCl 3 ). NMR data are reported in the following format: chemical shift (integration, multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, dd = doublet of doublets, dm = doublet of multiplets, m = multiplet), coupling constants (Hz)).…”

“…This trend is consistent with previously reported IR data involving NÀ H•••F hydrogen bonds. [13] Next, we investigated computationally the relative (and very qualitative) hydrogen bond strength of an R 3 N + À CÀ H•••O interaction in contrast to the corresponding CÀ H•••O interaction. [14] Employing an intramolecular pyridinium model similar to compounds 4-8, we found that the calculated energy of the R 3 N + À CÀ H•••O arrangement was 12.6 kcal lower than that of the analogous CÀ H•••O conformation (Figure 10).…”

“…White solid (302 mg, 58 %); 1 H NMR (400 MHz, DMSO-d 6 ): δ 4.92 (1H, br. s), 4.75 (1H, dd, J = 10.8 Hz, J = 2.6 Hz, 4.22-4.10 (3H, m), 3.73-3.54 (9H, m), 3.40-3.28 (1H, m), 2.38-2.29 (1H, m), 2.14 (1H, q, J = 11.7 Hz), 1.77-1.48 (6H, m), 1.47-0.90 (12H, m), 0.89 (3H, s), 0.78 (3H, s), 0.76-0.68 (1H, m) ; 13 Compound 2: DABCO (250 mg, 2.23 mmol, 1.0 equiv.) was dissolved in MeCN and then NaOH (196 mg, 4.91 mmol, 2.2 equiv.)…”

“…White solid (0.95 g, 68 %); 1 H NMR (400 MHz, CD 3 CN): δ 8.67 (1H, m), 8.58 (1H, dm, J = 5.8 Hz), 8.33 (1H, dm, J = 9.0 Hz), 8.05 (1H, dd, J = 8.2 Hz, J = 2.3 Hz), 4.33 (3H, s), 2.37 (3H, s); 13 Compound 6: Prepared according to the General Procedure for the Synthesis of Pyridinium Salts, employing 2-aminopyridine as the nucleophile and 4-nitrobenzoyl chloride as the electrophile.…”