ABSTRACT
A combined experimental-computational analysis of the one- and two-photon absorption (OPA, TPA) and the electronic circular dichroism (ECD) of complex chiral shape-persistent (2,6)-pyrido[142 ]allenoacetylenic cyclophane, (P,P,P,P)-MC[142 ] enantiomers, sheds light on the origin of their peculiar spectral signatures. The study on MC[142 ], which is represented in solution by three possible conformers, indicates that two of them (chair and twist) are the prevalent conformers at room temperature; the population ratio depending on the solvent. The complex shape of the red-edge of the UV and ECD spectra is qualitatively reproduced by using vibronic calculations and assigned to progressions on the ethynyl stretchings observable only in the chair and twist conformers. The picture arising from the analysis of the linear spectra is confirmed by two-photon spectroscopies, showing that they constitute valuable tools for the exploration of high-energy electronic transitions in pyridoallenoacetylenic cyclophanes. Discrepancies among computed and experimental spectra point out that an accurate description of such complex systems with a large delocalized π framework and a dense manifold of electronic excited state remains a challenge for theoretical models.
ABSTRACT
Chiroptically active allenes are employed for the construction of surface-confined nanostructures. Morphological complementarity between the homochiral units leads to self-assembly of two highly-ordered, upstanding, diastereomeric architectures. The novel, intertwined self-assembled layer structures feature reactive terminal alkynes for further functionalization and carry potential for widespread applications exploiting chiroptical amplification.
ABSTRACT
New 2,6-disubstituted pyrido-allenophanes with locked rotation of aromatic spacers were designed and synthesized. The synthesis was accomplished by Pd-catalyzed C(sp(2))-C(sp) Sonogashira cross-coupling reaction between 1,3-diethynylallene (DEA) and 2,6-dibromopyridine followed by an intermolecular ring closure. Because racemic DEA was employed, pyrido-allenophanes were obtained as mixtures of stereoisomers that were resolved by preparative HPLC. The conformational space of all these diastereoisomers was explored at the CAM-B3LYP/6-31+G*//AM1 level of theory. The isomers were characterized through their symmetry properties revealed in NMR, circular dichroism, and chiral stationary-phase HPLC experiments. X-ray diffraction was used to assign and to corroborate the configuration of several diastereoisomers. The unexpected encapsulation of two molecules of CHCl(3) in the crystal structures shows the potential of these conformationally hampered allenophanes as encapsulating hosts.
Subject(s)
Alkadienes/chemistry , Catalysis , Circular Dichroism , Crystallography, X-Ray , Cyclization , Macrocyclic Compounds/chemistry , Molecular Conformation , Palladium/chemistry , StereoisomerismABSTRACT
Chiral pyridoallenophanes! The synthesis and characterization of chiral acetylenic pyridoallenophanes along with 2,4- and 2,5-pyridyl[Re(CO)(4)LBr] complexes are reported. The relative configurations of the four stereoisomers have been unambiguously assigned. Chromatographic resolution of the racemate of the C(4) stereoisomer was accomplished and mirror-image CD spectra have been obtained (see figure).Chiral macrocycles bearing four 2,5-substituted pyridine units have been prepared regioselectively. The four possible diastereoisomers (twist, chair, crown, and boat) were obtained in the reaction and separated by preparative HPLC. The isomers were fully characterized through their chirality and also their symmetry properties, taking into account the number of nonequivalent cumulenic carbon atoms in the (13)C NMR spectra. The reaction of the isolated twist cyclophane with [Re(CO)(5)Br] led to an uncommon tetracarbonyl rhenium complex [Re(CO)(4)L]. The arrangement of the ligands around the metal core was studied through the 2,4- and 2,5-bis(ethynylallenylethynylpyridyl) model compounds by analysis of their NMR, IR, and UV/Vis spectra.
ABSTRACT
[chemical reaction: see text]. A stereoselective total synthesis of the visual pigment A2E has been achieved with use of palladium-catalyzed cross-coupling reactions in all key steps: a regioselective Suzuki or Negishi coupling of 2,4-dibromopyridine, a Sonogashira reaction, and a double Stille cross-coupling to complete the bispolyenyl skeleton.
Subject(s)
Coloring Agents/chemical synthesis , Pyridinium Compounds/chemical synthesis , Retinoids/chemical synthesis , Catalysis , Coloring Agents/chemistry , Molecular Structure , Palladium/chemistry , Pyridinium Compounds/chemistry , Retinoids/chemistry , StereoisomerismABSTRACT
Buta-2,3-dienoate reacts with vinyl ketenimine to give the corresponding substituted aniline through a Diels-Alder cycloaddition. Besides the expected Diels-Alder adduct 3a, the aniline 3b was also obtained in a ratio of 91:9. The observed (13)C exchange is explained on the basis of a reversible [2 + 2] cycloaddition competing with the [4 + 2] process. This is supported by B3LYP DFT computations, as a stepwise pathway lies very close in energy to the [4 + 2] concerted one. [reaction: see text]
Subject(s)
Acetylene/chemical synthesis , Alkadienes/chemical synthesis , Macrocyclic Compounds/chemical synthesis , Acetylene/chemistry , Alkadienes/chemistry , Chromatography, High Pressure Liquid , Macrocyclic Compounds/chemistry , Magnetic Resonance Spectroscopy , Models, Molecular , Molecular Structure , StereoisomerismABSTRACT
The alkaloid lennoxamine (1) was synthesized by transannular cyclization of a 10-membered lactam obtained by intramolecular addition of an aryl radical to a (trimethylsilyl)acetylene. The isoindolo[1,2-b][3]benzazepine skeleton present in lennoxamine was also obtained by means of regioselective 7-endo-trig radical cyclization of methylenephthalimidines.
