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Tetraphenyl-p-benzoquinone, according to its single crystal structure, shows some steric congestion: its quinone ring is distorted by 7° to a chair conformation, and its phenyl substituents are twisted around their CC axes between 46° and 72°. The half-wave reduction potentials of -0.57 and -1.25 V in acetonitrile confirm negligible π interaction of the phenyl substituents. Addition of alkalimetal tetraphenylborate salts lowers the second reduction potential due to contact ion formation, which can be confirmed by UV/VIS spectra recorded under aprotic conditions. Extensive ESR/ENDOR investigations prove the formation of the following species in THF solution: Tetraphenyl-p-benzosemiquinone radical anion contact ion pairs [M·⊖ Me⊕solv]' (Me⊕: Li⊕, Na⊕, Rb⊕, Cs⊕) and contact triple ion radical cations both with identical cations [M·⊖ (Me⊕solv)2]·⊕ (Me⊕: Li⊕, Na⊕, Cs⊕) and different cations [M·⊖ (Li⊕solv)(Me⊕solv)]·⊕ (Me⊕: Na⊕, Cs⊕). Addition of crown ethers can lead to external solvation of the Me⊕ counter cations, whereas cryptands form internal solvation complexes. The radical anion of 2,6-diphenyl-p-benzosemiquinone adds cations at its phenyl-free molecular half. The radical anion salt [tetraphenyl-p-benzosemiquinone·⊖ (Na⊕(tetrahydropyrane) 2)] could be crystallized and its structure determined at 200 K. In agreement with the Hirota sign rules for contact radicals in solution, the Na⊕ ion is found 62 pm above the π plane and 29° outside the axis of the CO bound, which is elongated due to one-electron reduction by 5 pm to 127 pm.
Dichlorido(3-phenylindenylidene)bis(triphenylphosphane)ruthenium(II) tetrahydrofuran disolvate
(2011)
The RuII atom in the title compound, [RuCl2(C15H10)(C18H15P)2]·2C4H8O, has a distorted square-pyramidal conformation. The P and Cl atoms are at the base of the pyramid and the Ru-Cindenylidene bond is in the axial position. The two Cl ligands and the two phosphane ligands are in trans positions. The Cl-Ru-Cl and P-Ru-P angles are 157.71 (2) and 166.83 (2)°, respectively. The two independent tetrahydrofuran (THF) solvent molecules are disordered. One THF molecule was refined using a split-atom model. The second THF molecule was accounted for by using program PLATON/SQUEEZE [Spek (2009). Acta Cryst. D65, 148-155]. The molecular conformation shows three intramolecular C-H...Cl contacts and two C-H...[pi] interactions while the crystal packing features an intermolecular C-H...Cl contact and two very weak intermolecular C-H...[pi] contacts.
The crystal structure of C12H11N2SiCl3 (monoclinic, P21/m, Z = 2, with a: 9.284(4), b: 7.226(2), c: 10.832(5) Å, β = 115.14(3)°) was refined to R(F) =0.035 from 1228 independent reflections. A trigonal bipyramidal, pentacoordinate silicon is observed. The chelated complex shows two different Si−N bonds, a coordinative bond (1.984(2) Å) between Si and N on the axial position and a Si−N single bond (1.737(3) A, equatorial plane), introduced by chemical reaction. The coordinative bond is 14.2% longer than the Si−N single bond. The lengthening of the coordinative bond in the present case is compared with distances in other extracoordinated silicon compounds.
C2-symmetric bisamidines : chiral Brønsted bases catalysing the Diels-Alder reaction of anthrones
(2008)
C2-symmetric bisamidines 8 have been tested as chiral Brønsted bases in the Diels- Alder reaction of anthrones and N-substituted maleimides. High yields of cycloadducts and significant asymmetric inductions up to 76% ee are accessible. The proposed mechanism involves proton transfer between anthrone and bisamidine, association of the resulting ions and finally a cycloaddition step stereoselectively controlled by the chiral ion pair.
In the title compound, C30H34N2O6, the complete molecule is generated by a crystallographic 2/m symmetry operation. The 1-oxyl-3-pyrroline-3-carboxylate group lies on a mirror plane. The dihedral angle between the ring planes of the biphenyl fragment is constrained by symmetry to be zero, resulting in rather short intramolecular H...H contact distances of 2.02 Å. In the crystal, molecules are connected along the a-axis direction by very weak intermolecular methyl-phenyl C-H...[pi] interactions. The C-H bond is not directed to the center of the benzene ring, but mainly to one C atom [C-H...C(x - 1, y, z): H...C = 2.91 Å and C-H...C = 143°]. Key indicators: single-crystal X-ray study; T = 169 K; mean σC–C) = 0.002 Å ; R factor = 0.049; wR factor = 0.126; data-to-parameter ratio = 19.8.
A versatile synthetic procedure is described to prepare the benzimidazole-fused 1,2,4-thiadiazoles 2a–c via a methanesulfonyl chloride initiated multistep cyclization involving the intramolecular reaction of an in-situ generated carbodiimide with a thiourea unit. The structure of the intricate heterocycle 2a was confirmed by single-crystal X-ray analysis and its mechanism of formation supported by DFT computations.
A chiral analog of the bicyclic guanidine TBD : synthesis, structure and Brønsted base catalysis
(2016)
Starting from (S)-β-phenylalanine, easily accessible by lipase-catalyzed kinetic resolution, a chiral triamine was assembled by a reductive amination and finally cyclized to form the title compound 10. In the crystals of the guanidinium benzoate salt the six membered rings of 10 adopt conformations close to an envelope with the phenyl substituents in pseudo-axial positions. The unprotonated guanidine 10 catalyzes Diels–Alder reactions of anthrones and maleimides (25–30% ee). It also promotes as a strong Brønsted base the retro-aldol reaction of some cycloadducts with kinetic resolution of the enantiomers. In three cases, the retro-aldol products (48–83% ee) could be recrystallized to high enantiopurity (≥95% ee). The absolute configuration of several compounds is supported by anomalous X-ray diffraction and by chemical correlation.
The title compound, C12H20N4O, undergoes a phase transition on cooling. The room-temperature structure is tetragonal (P43212, Z′ = 1), with the methoxybornyl group being extremely disordered. Below 213 K the structure is orthorhombic (P212121, Z′ = 2), with ordered molecules. The two independent molecules (A and B) have very similar conformations; significant differences only occur for the torsion angles about the Cbornyl—Ctetrazole bonds. The independent molecules are approximately related by the pseudo-symmetry relation: xB = −1/4 + yA, yB = 3/4 - xA and zB = 1/4 + zA. In the crystal, molecules are connected by N—H⋯N hydrogen bonds between the tetrazole groups, forming a pseudo-43 helix parallel to the c-axis direction. The crystal studied was a merohedral twin with a refined twin fraction value of 0.231 (2).
The six-membered ring of the title compound, C11H16NO, has a distorted envelope conformation. The piperidine N atom deviates by 0.128 (1) Å from the plane through its three neighbouring atoms. In the crystal structure, molecules are connected by intermolecular Cethynyl-H...O contacts to form chains extending in the [10\overline{1}] direction. Key indicators: single-crystal X-ray study; T = 167 K; mean σ(C–C) = 0.001 Å ; R factor = 0.040; wR factor = 0.112; data-to-parameter ratio = 27.3.