Photoinduced metal-free α-selenylation of ketones

Herein, we report an efficient photoinduced α-selenylation of ketones without metal, additives or under photosensitizer-free conditions, providing a green protocol using light energy to synthesize a variety of α-selenoketones. This new methodology proved to be a mild, simple and eco-friendly tool for the efficient synthesis of the desired products.

Herein, we report an efficient photoinduced α-selenylation of ketones without metal, additives or under photosensitizer-free conditions, providing a green protocol using light energy to synthesize a variety of α-selenoketones.

Modes of neighbouring group participation by the methyl selenyl substituent in β-methylselenylmethyl-substituted 1-phenylethyl carbenium ions

Selenium substituents which are disposed β to an electron deficient centre, such as a carbocation p-orbital, or the π* orbital of an electron deficient p-system, interact in a stabilising way by a combination of C-Se hyperconjugation (σ_Se-C–π* interaction), and a through-space homoconjugative n_Se–π* interaction. The relative importance of these two modes of interaction is dependant on the electron demand of the cation, with hyperconjugation predominating for low electron demand systems, and the n_Se–π* interaction predominating for high electron demand cations.

Orbital interactions in selenomethyl-substituted pyridinium ions and carbenium ions with higher electron demand

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a-c and 11a-c) provides strong evidence for C-Se hyperconjugation (σ(C-Se)-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (n(Se)-π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (n(Se)-σ*(CC)) involving the selenium p-type lone pair electrons and the polarized CH(2)-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C-Se hyperconjugation (σ(C-Se)-π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (n(Se)-π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (n(Se)-σ*(CC)) is relatively weak in all ions.

WW, Pinto BM. Ab initio DFT calculations of the configurational and conformational preferences of 2-phenylsulfinylcyclohexanones — Evidence for "exo-anomeric" interactions

The conformational analysis of 2-phenylsulfinylcyclohexanone by ab initio density functional calculations is described. Six conformations corresponding to axial/equatorial isomers and rotation about the exocyclic C2—S bond in each of the RR or RS diastereomers were calculated and the results were examined in terms of relative energies, electrostatic interactions, orbital interactions, and geometrical variations. The global minimum conformation was the RS isomer that positioned the phenylsulfinyl moiety in an equatorial orientation and the sulfinyl oxygen in an anti orientation with respect to the carbonyl carbon atom. Of the other three low energy conformations, only one had a gauche arrangement of these atoms, and only in one of the four lower energy conformations was evidence found for a S-O(–)···C(+)-O electrostatic interaction. In contrast, the results were consistent with the operation of nS → [Formula: see text] stabilizing orbital interactions. Further support for this hypothesis was obtained from the increased C=O bond lengths in these four conformations relative to the other conformations, and by the torsional angle distortion away from ideal geometry, presumably to maximize the stabilizing orbital interaction. We propose that this conformational preference is a manifestation of a generalized exo-anomeric effect. The longer C2—S bond in the axial isomers was also interpreted in terms of a stabilizing πC=O → [Formula: see text] interaction, analogous to an endo-anomeric interaction. Comparison of the computational results to available experimental data on the conformational equilibrium of each diastereomer in solution suggests which conformers are present in each of the equilibria. The available data for the solid state indicate that the RR and RS isomers both crystallize in high energy conformations, stabilized by intermolecular interactions.Key words: 2-phenylsulfinylcyclohexanones, configurational isomers, conformational preferences, DFT calculations, exo-anomeric effect.