Accumulation, Characterization and Reactivity of Chiral Ammonium-Carboxonium Dications in Superacid.

The accumulation of chiral ammonium-oxocarbenium dications in superacid is evidenced by low-temperature NMR spectroscopy, X-ray diffraction analysis and confirmed by DFT calculations. Its potential for the diastereoselective remote hydrofunctionalization of non-activated alkene is also explored.

Synthesis and Structure of Protonated Sulfur Dioxide.

Salts of protonated sulfur dioxide were synthesized by recrystallization of [FS(OX)2][SbF6] (X=H, D) in aprotic solvents at low temperatures. Hemiprotonated sulfur dioxide [(SO2)2H][Sb2F11] was obtained from the solvent SO2 and the monoprotonated sulfur dioxide [OSOD][Sb2F11], using 1,1,1,2-tetrafluoroethane as solvent. For both compounds, single crystals were obtained and an X-ray structure analysis was conducted. Furthermore, the salts were characterized by Raman spectroscopy and the results were discussed together with quantum chemical calculations on the M06-2X/aug-cc-pVTZ level of theory.

Synthesis and Structure of the Small Superelectrophile [C2 (OH)2 Me2 ]2.

The acid-activation of 1,2-dicarbonyl compounds plays a key role in a variety of electrophilic reactions, some of which are only accessible in superacidic media when a superelectrophilic dication is formed. To obtain structural and electronic information about these elusive species, the vicinal dication [C2 (OH)2 Me2 ]2+ is synthesized and characterized by Raman spectroscopy and X-ray diffraction. Since this superelectrophile could not be stabilized in convenient superacids, the usage of liquid SO2 turned out to be crucial. The experimental data are discussed together with quantum-chemical calculations on the B3LYP/aug-cc-pVTZ level of theory. Natural Bond Orbital (NBO) analyses quantify the superelectrophilic interactions found in the solid state.

Two Roomtemperature-stable Trications - Triprotonated Triamino- and Tricyanobenzene.

Protonation of 1,3,5-tricyano- and 1,3,5-triaminobenzene was achieved in various superacidic media, resulting in the formation of the respective trinitrilium and triammonium species. Furthermore, the respective N-methyl nitrilium species was synthesized by methylation. Characterization was performed by NMR and vibrational spectroscopy, followed by single-crystal X-ray diffraction analyses of selected species. Fourfold protonation of the amine, which would have led to the triammonium arenium species, could not be achieved. Quantum chemical calculations are employed to enable full vibrational assignment as well to quantify charge localization.

Strengthening of the C-F Bond in Fumaryl Fluoride with Superacids.

The reaction of fumaryl fluoride with the superacidic solutions XF/MF5 (X=H, D; M=As, Sb) results in the formation of the monoprotonated and diprotonated species, dependent on the stoichiometric ratio of the Lewis acid to fumaryl fluoride. The salts [C4 H3 F2 O2 ]+ [MF6 ]- (M=As, Sb) and [C4 H2 X2 F2 O2 ]2+ ([MF6 ]- )2 (X=H, D; M=As, Sb) are the first examples with a protonated acyl fluoride moiety. They were characterized by low-temperature vibrational spectroscopy. Low-temperature NMR spectroscopy and single-crystal X-ray structure analyses were carried out for [C4 H3 F2 O2 ]+ [SbF6 ]- as well as for [C4 H4 F2 O2 ]2+ ([MF6 ]- )2 (M=As, Sb). The experimental results are discussed together with quantum chemical calculations of the cations [C4 H4 F2 O2 ⋅ 2 HF]2+ and [C4 H3 F2 O2 ⋅ HF]+ at the B3LYP/aug-cc-pVTZ level of theory. In addition, electrostatic potential (ESP) maps combined with natural population analysis (NPA) charges were calculated in order to investigate the electron distribution and the charge-related properties of the diprotonated species. The C-F bond lengths in the protonated dication are considerably reduced on account of the +R effect.

Ring Opening Reactions of ß-Propiolactam in Superacidic Media.

The reaction of ß-propiolactam in the superacidic systems HF/MF5 (M=Sb, As) led to the formation of monoprotonated 3-aminopropanoyl fluoride in the form of [C(O)F(CH2 )2 NH3 ][SbF6 ] and [C(O)F(CH2 )2 NH3 ][AsF6 ]. In the presence of traces of water, the diprotonated species ß-alanine [C(OH)2 (CH2 )2 NH3 ][AsF6 ]2 was synthesized for the first time. All salts were characterized by low-temperature infrared and Raman spectroscopy. Additionally, single-crystal X-ray analyses were conducted in the case of [C(O)F(CH2 )2 NH3 ][SbF6 ] and [C(OH)2 (CH2 )2 NH3 ][AsF6 ]2 . By using SO2 instead of HF as the solvent, the salt [C(OH)2 (CH2 )2 NHSO][SbF6 ]2 was obtained, and single-crystal X-ray analysis of this salt containing a thionylimide moiety was conducted. For the formation of these open-chain compounds, an acyl cationic species as intermediate is assumed, which is formed from N-protonated ß-propiolactam. Quantum chemical calculations at the B3LYP/aug-cc-pVTZ and MP2/aug-cc-pVTZ levels of theory were carried out to gain a better understanding of the formation and the structural properties of protonated ß-propiolactam.

Protonated Ethylene Carbonate: A Highly Resonance-Stabilized Cation.

Salts containing the monoprotonated ethylene carbonate species of were obtained by reacting it with the superacidic systems XF/MF5 (X=H, D; M=Sb, As). The salts in terms of [C3 H5 O3 ]+ [SbF6 ]- , [C3 H5 O3 ]+ [AsF6 ]- and [C3 H4 DO3 ]+ [AsF6 ]- were characterized by low-temperature infrared and Raman spectroscopy. In order to generate the diprotonated species of ethylene carbonate, an excess of Lewis acid was used. However, this only led to the formation of [C3 H5 O3 ]+ [Sb2 F11 ]- , which was characterized by a single-crystal X-ray structure analysis. Quantum chemical calculations on the B3LYP/aug-cc-PVTZ level of theory were carried out for the [C3 H5 O3 ]+ cation and the results were compared with the experimental data. A Natural Bond Orbital (NBO) analysis revealed sp2 hybridization of each atom belonging to the CO3 moiety, thus containing a remarkably delocalized 6π-electron system. The delocalization is confirmed by a 13 C NMR-spectroscopic study of [C3 H5 O3 ]+ [SbF6 ]- .

Protonated Ethylene Carbonate: A Highly Resonance-Stabilized Cation.

Invited for this month's cover picture is the group of Prof. Andreas J. Kornath at the LMU in Munich (Germany). The cover picture shows the crystal structure of the salt of protonated ethylene carbonate [C3 H5 O3 ][Sb2 F11 ]. Protonated ethylene carbonate was synthesized by reacting the neutral compound in the superacidic system HF/SbF5 at low temperature. Interestingly, the single-crystal X-ray structure analysis revealed a planar CO3 moiety with nearly equal CO bond lengths. Further theoretical investigations of the cation led to the result, that it contains a remarkably delocalized 6π-electron system. On the cover this resonance stabilization is highlighted by p-orbitals on the CO3 moiety. Read the full text of their Research Article at 10.1002/open.202100229.

Protonation of γ-Butyrolactone and γ-Butyrolactam.

Invited for this month's cover is the group of Andreas Kornath at the Ludwig-Maximilian University of Munich (Germany). The cover picture shows the crystal structures of protonated γ-butyrolactone ([(CH2 )3 OCOH][AsF6 ]) and γ-butyrolactam ([(CH2 )3 NHCOH][AsF6 ]). Both salts were synthesized by reacting the neutral compounds in the superacidic system HF/AsF5 at low temperature. Interestingly, the X-ray structure analyses revealed not only structural parameters of the salts, but also the existent of C⋅⋅⋅F contacts in both species. Quantum chemical calculations were performed to investigate the nature of these contacts. Read the full text of their Full Paper at 10.1002/open.202000220.

Protonation of γ-Butyrolactone and γ-Butyrolactam.

γ-Butyrolactone and γ-butyrolactam were reacted in the superacidic systems XF/MF5 (X=H, D; M=As, Sb). Salts of the monoprotonated species of γ-butyrolactone were obtained in terms of [(CH2 )3 OCOH]+ [AsF6 ]- , [(CH2 )3 OCOH]+ [SbF6 ]- and [(CH2 )3 OCOD]+ [AsF6 ]- and the analogous lactam salts in terms of [(CH2 )3 NHCOH]+ [AsF6 ]- , [(CH2 )3 NHCOH]+ [SbF6 ]- and [(CH2 )3 NDCOD]+ [AsF6 ]- . The salts were characterized by low temperature Raman and infrared spectroscopy and for both protonated hexafluoridoarsenates, [(CH2 )3 OCOH]+ [AsF6 ]- and [(CH2 )3 NHCOH]+ [AsF6 ]- , single-crystal X-ray structure analyses were conducted. In addition to the experimental results, quantum chemical calculations were performed on the B3LYP/aug-cc-pVTZ level of theory. As in both crystal structures C⋅⋅⋅F contacts were observed, the nature of these contacts is discussed with Mapped Electrostatic Potential as a rate of strength.

Investigations on Squaric Acid in Superacidic Media.

The syntheses of [OC(COX)3 ][MF6 ] and [(COX)4 ][MF6 ]2 ⋅2 HF were carried out in superacidic media XF/MF5 (M=As, Sb; X=H, D). The degree of protonation is highly dependent on the stoichiometric ratio of the Lewis acid with regard to squaric acid. The salts of diprotonated squaric acid were characterized by Raman spectroscopy and, in the case of [(COH)4 ][MF6 ]2 ⋅2 HF (M=As, Sb), by single-crystal X-ray structure analyses. [(COH)4 ][AsF6 ] crystallizes in the monoclinic space group P21 /n with two formula units per unit cell. Analysis of the vibrational spectra was achieved with the support of quantum chemical calculations of the cation [(COH)4 ]2+ ⋅4 HF on the PBE1PBE/6-311G++(3df,3pd) level of theory. Furthermore, a salt of monoprotonated squaric acid, [OC(COH)3 ][AsF6 ], was characterized by a single-crystal X-ray structure analysis. It crystallizes in the monoclinic space group P21 /n with four formula units per unit cell. The protonation of squaric acid leads to a change of the carbon skeleton, which is discussed for the entire series, starting with the dianion of squaric acid and ending with the tetrahydroxy dication.

One-Pot Synthesis of Substituted Trifluoromethylated 2,3-Dihydro-1H-imidazoles.

An operationally simple one-pot reaction for the preparation of a novel class of racemic trifluoromethylated 2,3-dihydro-1H-imidazoles derived from electron-poor N,O-acetals and aryl Grignard reagents is described. In addition, access to highly functionalized 2-trifluoromethyl-2,3-dihydro-1H-imidazoles was accomplished by reaction of N-aryl hemiaminal ethers and N-aryl trifluoroethylamines in the presence of an excess of n-butyllithium.