1H, 13C and 77Se NMR study of tetraselenafulvalene derivatives supported by novel H(Se)C and H(C)Se triple-resonance experiments at natural abundance

Pd/Cu-Catalyzed Cross-Coupling of Bis(2-bromovinyl) Selenides with Terminal Acetylenes: Unusual Involvement of Selanyl Function in the Sonogashira Reaction

The Pd/Cu-catalyzed Sonogashira reaction of (E,E)-bis(2-bromovinyl) selenide and (E,E)-bis(1-bromo-1-hexen-2-yl) selenide with terminal alkynes was found to proceed at room temperature involving both bromine atoms and the selanyl function. As a result, new bis-(1,3-enynyl) selenides and enediyne hydrocarbons are formed with a complete retention of the stereoconfiguration of the initial selenides. Due to steric hindrances in the cross-coupling at the selenyl function in the case of (E,E)-bis(1-bromo-1-hexen-2-yl) selenide, the second process is realized to a lesser extent than with unsubstituted (E,E)-bis(2-bromovinyl) selenide.

Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth-Rhodium Paddlewheel Complexes

A H(C)Rh triple resonance NMR experiment makes the rapid detection of ¹⁰³Rh chemical shifts possible, which were previously beyond reach. It served to analyze a series of dirhodium and bismuth–rhodium paddlewheel complexes of the utmost importance for metal–carbene chemistry. The excellent match between the experimental and computed ¹⁰³Rh shifts in combination with a detailed analysis of the pertinent shielding tensors forms a sound basis for a qualitative and quantitative interpretation of these otherwise (basically) inaccessible data. The observed trends clearly reflect the influence exerted by the equatorial ligands (carboxylate versus carboxamidate), the axial ligands (solvents), and the internal “metalloligand” (Rh versus Bi) on the electronic estate of the reactive Rh(II) center.

J(77 Se,1 H) and J(77 Se,13 C) couplings of seleno-carbohydrates obtained by 77 Se satellite 1D 13 C spectroscopy and 77 Se selective HR-HMBC spectroscopy

Seleno-carbohydrates are those in which the oxygen of the glycosidic bond or the hydroxyl group is artificially replaced with selenium. This substitution changes ¹ H and ¹³ C chemical shifts and produces spin coupling constants involving ⁷⁷ Se. Coupling constants, such as ^2-3 J(⁷⁷ Se, ¹ H), are likely to be useful for conformational analyses of glycans because such couplings are never observed in natural glycans. Several papers have discussed the relationship between ^2-3 J(⁷⁷ Se, ¹ H) and conformation; however, only few reports describe ^1-3 J(⁷⁷ Se, ¹³ C), which could also be useful. Here, we obtain ⁷⁷ Se coupling constants of seleno-carbohydrates from ⁷⁷ Se-selective HR-HMBC and ⁷⁷ Se satellites in 1D ¹³ C spectra and examine their conformations using the Newman projection scheme.

H(C)Ag: a triple resonance NMR experiment for (109) Ag detection in labile silver-carbene complexes

In silver complexes, indirect detection of (109) Ag resonances via (1) H,(109) Ag-HMQC frequently suffers from small or absent JHAg couplings or rapid ligand dissociation. In these cases, it would be favourable to employ H(X)Ag triple resonance spectroscopy that uses the large one-bond JXAg coupling (where the donor atom of the ligand X is the relay nucleus). We have applied an HMQC-based version of the H(C)Ag experiment to a labile silver-NHC complex (NHC=N-heterocyclic carbene) at natural (13) C isotopic abundance and variable temperature. In agreement with simulations, H(C)Ag detection became superior to (1) H,(109) Ag-HMQC detection above -20 °C.

Experimental and computational studies of nJ(77Se, 1H) selenium-proton couplings in selenoglycosides

Selenoglycosides are important starting materials in synthetic carbohydrate chemistry and play a role in biological interactions as well. Both aspects are influenced by the conformation around the glycosidic bond. Here, we present a combined experimental and computational approach to measure and evaluate (n)J((77)Se, (1)H) coupling constants for their use in conformational analysis. The measurements were carried out using a modified CPMG-HSQMBC pulse scheme which yields pure absorption antiphase multiplets to allow accurate determination of the (n)J(XH) values regardless of the size of the proton-proton couplings. Theoretical calculations were performed at the Second-Order Polarization Propagator Approach (SOPPA) level. Population-averaged values calculated for geminal and vicinal couplings are in a good agreement with experiment indicating an adequate theoretical level of the calculations. Experimental observations and computations alike have indicated that two-bond (77)Se-(1)H couplings, (2)J((77)Se, (1)H), in a H1-C1-Se-X moiety are very sensitive to the torsion angle around the C1-Se-bond and will, therefore, be useful for conformational studies.