Solvent‐Induced Deuterium Isotope Effects in ¹³C‐ and ¹⁵N‐NMR. Spectra of Enaminones

Abstract

SummaiyThe effect of deuterium on the 13C and 15N chemical shifts of enaminones has been investigated. D/H isotope shifts are reported for neutral and protonated species, i.e., when the isotope is exchanged on the C(2)-, N-, or 0-atoms. In cases of slow exchange the isotope shifts were obtained from solutions containing both isotopomers, whereas for fast exchange (acidic solutions) either separate NMR. sample tubes ( 15N-NMR.) or coaxial tubes ( 13C-NMR.) were used.In neutral molecules the isotope effects 6,-(D,H) are intrinsic in nature. In acidic solutions, the enaminocarbonyl cations formed exhibit 6,-(D, H)and d N (D, H)values which are discussed in terms of the proton transfer. The mesomeric character of the cations is reflected by characteristic features in the d,-(D,H)-and d N (D,H)values, which can be ascribed to isotopic perturbation of resonance.0-Protonation shifts in the '%-resonance, observed for the first time, are large and positive (+60 to +76 ppm), in contrast to amides, where the effects are of the same sign but an order of magnitude smaller. Both protonation shifts and solventinduced isotope effects are discussed in connection with the nucleophilic character of the reactive centers in the enaminone synthon.

A Spectroscopic Overview of Intramolecular Hydrogen Bonds of NH…O,S,N Type

“…2.4.1. RAHB Cases (a "Double Bond" Connecting Cα and Cβ) n ΔC(ND) were early on studied in enaminones [74]. Later this study was extended 12,18,32,75,76].…”

Proton-Coupled15N NMR spectra of neutral adn protonated ethenoadenosine and ethenocytidine

“…N NMR 3NEPT spectra of ethenoadenosine /0. 4.5f (4.5)g (N3-H5) 3.3f Table 2. Values in parentheses refer to protonated ethenoadenosine (2a).…”

Deuterium isotope effects on 14N and 15N nuclear shielding in simple nitrogen‐containing compounds

“…Surprisingly little is known about deuterium isotope effects on nitrogen shielding. 5. An attempt at the separation of direct and equilibrium contributions to A15N(D) in o-hydroxyazo dyes has been carried out recently.…”