In this extension to a classic physical chemistry experiment, students record the proton nuclear magnetic resonance spectra of the β-diketones 2,4-pentanedione, 3-methyl-2,4-pentanedione, and 3-chloro-2,4-pentanedione to investigate the effect of substituents on keto–enol tautomerization equilibria. From the integrated intensities of keto and enol methyl proton peaks, students calculate the equilibrium constant for each 2,4-pentanedione. The students then use the effects of electron-donating and electron-withdrawing substituents to elucidate the nature of keto–enol equilibria.
We report detailed studies on the characterization of an intramolecular NH-F hydrogen bond formed within a fluorinated "proton sponge" derivative. An ammonium ion, generated from 8-fluoro-N,N-dimethylnaphthalen-1-amine, serves as a charged hydrogen bond donor to a covalently bound fluorine appropriately positioned on the naphthalene skeleton. Potentiometric titrations of various N,N-dimethylnaphthalen-1-amines demonstrate a significant increase in basicity when hydrogen bonding is possible. X-ray crystallography reveals that NH-F hydrogen bonding in protonated 8-fluoro-N,N-dimethylnaphthalen-1-amine is heavily influenced by ion pairing in the solid state; bifurcated and trifurcated hydrogen bonds are formed depending on the counterion utilized. Compelling evidence of hydrogen bonding in the 8-fluoro-N,N-dimethylnaphthyl-1-ammonium cation is provided by gas-phase cryogenic vibrational photodissociation spectroscopy. Solution-phase infrared spectroscopy provides complementary results, and the frequencies of the N-H stretching mode in both phases are in excellent agreement with the computed vibrational spectra. NMR analysis of protonated 8-fluoro-N,N-dimethylnaphthalen-1-amine demonstrates significant H-F coupling between the N-H hydrogen and fluorine that cannot be attributed to long-range, through-bond interactions; the couplings correlate favorably with calculated values. The results obtained from these experiments are congruent with the formation of an NH-F hydrogen bond upon protonation of 8-fluoro-N,N-dimethylnaphthalen-1-amine.
Trimethylsilyl halide activation of Fmoc-protected N,O-acetals yields reactive intermediates capable of efficiently adding to a variety of enamines. NMR studies have provided evidence that a stable halomethyl carbamate intermediate forms in solution. Good yields are obtained over a broad range of enamine nucleophiles encompassing both cyclic and acyclic ketone-derived and aldehyde-derived enamines. Preliminary studies suggest that the enamine additions occur through a concerted, S(N)2-type mechanism.
Treatment of the Fmoc‐protected amino alcohol (II), a “so‐called” N,O‐acetal, with TmsCl results in formation of a chloromethylcarbamate intermediate, which reacts with enamines to give β‐amino ketenes and aldehydes.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.