Anomeric effects in fluoro and trifluoromethyl piperidines: a computational study of conformational preferences and hydration

An Examination of Factors Influencing Small Proton Chemical Shift Differences in Nitrogen-Substituted Monodeuterated Methyl Groups

Monodeuterated methyl groups have previously been demonstrated to provide access to long-lived nuclear spin states. This is possible when the CH2D rotamers have sufficiently different populations and the local environment is chiral, which foments a non-negligible isotropic chemical shift difference between the two CH2D protons. In this article, the focus is on the N-CH2D group of N-CH2D-2-methylpiperidine and other suitable CH2D-piperidine derivatives. We used a combined experimental and computational approach to investigate how rotameric symmetry breaking leads to a 1H CH2D chemical shift difference that can subsequently be tuned by a variety of factors such as temperature, acidity and 2-substituted molecular groups.

Fluorinated Rings: Conformation and Application

The introduction of fluorine atoms into molecules and materials across many fields of academic and industrial research is now commonplace, owing to their unique properties. A particularly interesting feature is the impact of fluorine substitution on the relative orientation of a C-F bond when incorporated into organic molecules. In this Review, we will be discussing the conformational behavior of fluorinated aliphatic carbo- and heterocyclic systems. The conformational preference of each system is associated with various interactions introduced by fluorine substitution such as charge-dipole, dipole-dipole, and hyperconjugative interactions. The contribution of each interaction on the stabilization of the fluorinated alicyclic system, which manifests itself in low conformations, will be discussed in detail. The novelty of this feature will be demonstrated by presenting the most recent applications.

Origins of Small Proton Chemical Shift Differences in Monodeuterated Methyl Groups

We have recently shown that the small proton chemical shift difference in 2-methyl-1-(methyl-d)piperidine supports a long-lived nuclear spin state. To identify additional candidate molecules with CH₂D groups exhibiting accessible long-lived states, and to investigate the factors governing the magnitude of the shift differences, we report a computational and experimental investigation of methyl rotational equilibria and proton chemical shifts in a variety of 2-substituted 1-(methyl-d)piperidines. The polarity and size of the 2-substituent affect the 1,2-stereoisomeric relationship, and consequently, the strength of the rotational asymmetry within the CH₂D group. Nonpolar and large 2-substituents prefer the equatorial position, and relatively large shift differences (i.e., > 13 ppb) are observed. Polar and small substituents, however, increasingly prefer the axial position, and medium to small shift differences (i.e., 0 to 9 ppb) are observed. In addition, the diastereotopic CH₂D proton chemical shift difference for tricarbonyl(1-chloro-2-deuteriomethylbenzene) chromium(0) was computed, showing that reasonable predictions of these small shift differences can be extended to more complex, organometallic species.

Stereoselective Rearrangement of (Trifluoromethyl)prolinols to Enantioenriched 3-Substituted 2-(Trifluoromethyl)piperidines

3-Substituted 2-(trifluoromethyl)piperidines B were synthesized by ring expansion of (trifluoromethyl)prolinols A, which were obtained from L-proline via an aziridinium intermediate C. The ring opening of the (trifluoromethyl)aziridinium intermediate by different nucleophiles is regio- and diastereoselective.