Influence of the correlation,aggregation, and solvation on ab initio computed Li-C,Li-N,and Li-Li NMR coupling constants

The (1)J and (3)J(C-Li), (1)J(N-Li), and (2)J(Li-Li) NMR coupling constants have been calculated for various homogeneous and heterogeneous aggregates of methyllithium and lithium dimethylamide at the HF and MP2 levels of calculation. Ethereal solvation has also been taken into account either through a continuum model or through the explicit introduction of Me(2)O molecules. The results obtained are in good general agreement with the experimental data available for methyllithium itself or model alkyllithiums and supports the empirical rule proposed by Bauer, Winchester, and Schleyer to evaluate (1)J(C-Li) provided that calculations include solvent and/or aggregation effects.     

Efficient synthesis of novel dipyridoimidazoles and pyrido[1′,2′;1,2]imidazo[4,5-d]pyridazine derivatives

Novel dipyrido[1,2-a;3′,4′-d]imidazoles 7a-d, dipyrido[1,2-a;4′,3′-d]imidazoles 8a,c and pyrido[1′,2′;1,2]imidazo[4,5-d]pyridazine derivatives 9a-d were synthesized by two pathways: thermal electrocyclic reaction of 3-alkenylimidazopyridine-2-oximes 10 and direct condensation of ethyl glycinate (or hydrazine) with 2,3-dicarbonylimidazo[1,2-a]pyridines 11.     

β-Cyano meso-unsubstituted porphyrins for intramolecular charge transfer

β-Cyano meso-unsubstituted porphyrin 10, synthesized via a ‘3+1’ condensation using a bromotripyrrane, shows an effective charge transfer interaction across the porphyrin between electron-acceptor and -donor groups in β-positions.     

A new accurate calorimetric method for the enthalpy of fusion measurements up to 1000 °C

A new metrological approach is proposed by the Laboratoire National de Metrologie et d’Essais to lower the uncertainty of measurements of enthalpy of fusion from 23 to 1000 °C. It consists, as in our previous works, in putting the studied sample inside a calibration crucible and in performing the heat calibration of the calorimeter by electrical substitution at two temperatures before and after the temperature of fusion of the sample during the same low-rate heating run. The novelty lies in the “electrical compensation” of the endothermic reaction of fusion. The quantification of the electrical energy needed to melt the sample, which can be measured with high accuracy, and the integrated remaining heat flow rate divided by the calibration factor (sensitivity) of the calorimeter at the temperature of fusion, leads to the determination of the enthalpy of fusion of the sample with low uncertainty. This methodology reduces the influence of the uncertainty component associated with the area of the peak of fusion by comparison with the former procedure when no electrical compensation was applied. The relative expanded uncertainty (k = 2) associated with the enthalpy of fusion measurements, performed using this facility and this new procedure, has been assessed to be < 0.2% for tin and indium and to be about 0.3% in the case of silver.

     

1,2‐Oxazine N‐Oxides as Catalyst Resting States in Michael Additions of Aldehydes to Nitro Olefins Organocatalyzed by α,α‐Diphenylprolinol Trimethylsilyl Ether

By combining enamines, derived from aldehydes and diphenylprolinol trimethylsilyl ether (the Hayashi catalyst), with nitroethenes ((D₆)benzene, 4‐Å molecular sieves, room temperature) intermediates of the corresponding catalytic Michael‐addition cycles were formed and characterized (IR, NMR, X‐ray analysis; Schemes 3–6 and Fig. 1–3). Besides cyclobutanes 2 , 1,2‐oxazine N‐oxide derivatives 3 – 6 and 8 have been identified for the first time, some of which are very stable compounds. It may not be a lack of reactivity (between the intermediate enamines and nitro olefins) that leads to failure of the catalytic reactions (Schemes 3–5) but the high stability of catalyst resting states. The central role zwitterions play in these processes is discussed (Schemes 1 and 2).     

Bifunctional Rhenium Complexes for the Catalytic Transfer‐Hydrogenation Reactions of Ketones and Imines

The silyloxycyclopentadienyl hydride complexes [Re(H)(NO)(PR₃)(C₅H₄OSiMe₂tBu)] (R=iPr ( 3 a ), Cy ( 3 b )) were obtained by the reaction of [Re(H)(Br)(NO)(PR₃)₂] (R=iPr, Cy) with Li[C₅H₄OSiMe₂tBu]. The ligand–metal bifunctional rhenium catalysts [Re(H)(NO)(PR₃)(C₅H₄OH)] (R=iPr ( 5 a ), Cy ( 5 b )) were prepared from compounds 3 a and 3 b by silyl deprotection with TBAF and subsequent acidification of the intermediate salts [Re(H)(NO)(PR₃)(C₅H₄O)][NBu₄] (R=iPr ( 4 a ), Cy ( 4 b )) with NH₄Br. In nonpolar solvents, compounds 5 a and 5 b formed an equilibrium with the isomerized trans‐dihydride cyclopentadienone species [Re(H)₂(NO)(PR₃)(C₅H₄O)] ( 6 a,b ). Deuterium‐labeling studies of compounds 5 a and 5 b with D₂ and D₂O showed H/D exchange at the H_Re and H_O positions. Compounds 5 a and 5 b were active catalysts in the transfer hydrogenation reactions of ketones and imines with 2‐propanol as both the solvent and H₂ source. The mechanism of the transfer hydrogenation and isomerization reactions was supported by DFT calculations, which suggested a secondary‐coordination‐sphere mechanism for the transfer hydrogenation of ketones.     

19F Magic Angle Spinning Dynamic Nuclear Polarization Enhanced NMR Spectroscopy

The introduction of high‐frequency, high‐power microwave sources, tailored biradicals, and low‐temperature magic angle spinning (MAS) probes has led to a rapid development of hyperpolarization strategies for solids and frozen solutions, leading to large gains in NMR sensitivity. Here, we introduce a protocol for efficient hyperpolarization of ¹⁹F nuclei in MAS DNP enhanced NMR spectroscopy. We identified trifluoroethanol‐d₃ as a versatile glassy matrix and show that 12 mm AMUPol (with microcrystalline KBr) provides direct ¹⁹F DNP enhancements of over 100 at 9.4 T. We applied this protocol to obtain DNP‐enhanced ¹⁹F and ¹⁹F–¹³C cross‐polarization (CP) spectra for an active pharmaceutical ingredient and a fluorinated mesostructured hybrid material, using incipient wetness impregnation, with enhancements of approximately 25 and 10 in the bulk solid, respectively. This strategy is a general and straightforward method for obtaining enhanced ¹⁹F MAS spectra from fluorinated materials.