Calix[4]thiacrowns as ditopic hosts for homo- and heterobinuclear accommodation: first report of a chopsticks-type pi-coordination

[reaction: see text] Calix[4]thiacrowns with hetero-L(1) and a homodonor set (L(2)) were synthesized and characterized by X-ray analysis. From the reaction of L(2) with AgPF(6), disilver complex [Ag(2)(L(2))](PF(6))(2) (1) was isolated. By comparing 1,3-alternate conformations of L(2) and 1, we found a considerable decrease of dihedral angles of two opposite aromatic rings upon complexation. This finding can be explained in terms of the "chopsticks process", which illustrates how Ag(+)-arene pi-coordination occurs. Heterobinuclear complexation of L(1) in solution was also observed.     

The conformations of bicyclo[3.1.0]hexane derivatives by 1H and 13C NMR

The ¹H and ¹³C spectra of bicyclo[3.1.0]hexanes and thujanes have been recorded and assigned. Application of the Karplus equation has yielded dihedral angles, and a computer calculation of the angle of ring buckle as a function of the main dihedral angles has been carried out. The calculated angles of ring buckle agree well with known values in the bicyclo[3.1.0]hexanes, but for 1-methylbicyclo[3.1.0]hexanes and thujanes the results are not self consistent. It is suggested that the bridgehead substituent causes the boat to twist, although the twist can be reduced by an axial methyl substituent on C-4.     

Development of Axially Chiral Cyclo‐Biaryldiol Ligands with Adjustable Dihedral Angles

A new type of axially chiral cyclo‐[1,1′‐biphenyl]‐2,2′‐diol (CYCNOL) ligands with adjustable dihedral angles have been developed by varying the bridge chain length. Eight‐, nine‐ and ten‐membered cyclo‐ligands were prepared and evaluated by using two representative examples: enantioselective additions of diethylzinc to aldehydes and organometallic reagents to enones. The results revealed that the fine regulation of dihedral angles through variation of the bridge chain length was effective in the asymmetric synthesis.     

Preferred Conformations of Calix[4]- and Calix[6]arenes,Calculated ab initio

Ab initio STO-2G calculations showed that the preferred conformation of a series of calix[6]arenes substituted at the upper rim is a compressed cone stabilized by a pseudo-ring of six hydrogen bonds with close characteristics. The endocyclic dihedral angles between the benzene ring planes, characterizing the molecular shape of the above macrocyclic compounds in this conformation, were calculated. The calculation results are consistent with the conformational data obtained by other theoretical methods and describe the characteristics of hydrogen bonds more adequately, in agreement with the experimental data and with the existing concepts of the hydrogen bonding efficiency.     

Understanding rubredoxin redox sites by density functional theory studies of analogues

Determining the redox energetics of redox site analogues of metalloproteins is essential in unraveling the various contributions to electron transfer properties of these proteins. Since studies of the [4Fe-4S] analogues show that the energies are dependent on the ligand dihedral angles, broken symmetry density functional theory (BS-DFT) with the B3LYP functional and double-ζ basis sets calculations of optimized geometries and electron detachment energies of [1Fe] rubredoxin analogues are compared to crystal structures and gas-phase photoelectron spectroscopy data, respectively, for [Fe(SCH(3))(4)](0/1-/2-), [Fe(S(2)-o-xyl)(2)](0/1-/2-), and Na(+)[Fe(S(2)-o-xyl)(2)](1-/2-) in different conformations. In particular, the study of Na(+)[Fe(S(2)-o-xyl)(2)](1-/2-) is the only direct comparison of calculated and experimental gas phase detachment energies for the 1-/2- couple found in the rubredoxins. These results show that variations in the inner sphere energetics by up to ～0.4 eV can be caused by differences in the ligand dihedral angles in either or both redox states. Moreover, these results indicate that the protein stabilizes the conformation that favors reduction. In addition, the free energies and reorganization energies of oxidation and reduction as well as electrostatic potential charges are calculated, which can be used as estimates in continuum electrostatic calculations of electron transfer properties of [1Fe] proteins.     

Conformationally locked bridged bicyclic diselenides: synthesis, structure, Se...O interaction, and theoretical studies

A general synthetic methodology has been developed for the synthesis of a conformationally locked, bridged diselena-bicyclo[3.2.1]octane skeleton by regio- and stereospecific tandem nucleophilic ring opening of cis-1,4-aziridino-epoxides with tetraethylammonium tetraselenotungstate [Et4N]2WSe4, 1, in a one-pot synthesis. Some correlations have been made on the physicochemical characteristics of the diselenides with a change in the dihedral angles.     

银桥联六苯基苯配合物的结构与导电性

在一定研究基础上合成出分子式为{Ag~2(hpb)(ClO~4)~2[(CH~3)~2CO]~2}1(hpb=C~6Ph~6)的二核银配合物,并制备出自由配体hpb的单晶2,其结构由X射线单晶衍射表征。将这两种化合物与先前研究结果比较发现,配合物的分子空间结构不仅依赖于阴离子大小、溶剂性质,还取决于溶剂类型和自由配体的分子空间结构与排列,并且这种结构变化导致了导电性的不同。     

Highly enantioselective hydrogenation of enol acetates catalyzed by Ru-TunaPhos complexes

[reaction: see text] The chiral disphosphines with tunable dihedral angles (TunaPhos) have been used for asymmetric hydrogenation of enol acetates and dihedral-angle-dependent enantioselectivities were observed. C2-TunaPhos has been proved to be effective for Ru-catalyzed asymmetric hydrogenation of electron-deficient and other enol acetates.     

Conformational Analysis of p-Substituted Calix[6]arenes by Molecular Mechanics and Quantum-Chemical Methods

The molecular mechanics (AMBER 3.0) and quantum-chemical (AM1) calculations indicated that in the series of calix[6]arenes substituted on the upper rim, the preferable conformation is a pinched cone stabilized by the nonequivalent hydrogen bonds on the lower rim of the macrocycle molecule. For the basic conformers of the macrocycles under study, the endocyclic dihedral angles between the planes of the benzene rings characterizing the shape of the macrocycles were calculated. The results of AM1 and AMBER 3.0 calculations agree with the results of the conformational analysis of calix[6]arene molecules by experimental and theoretical methods.     

A new atropisomeric P,N ligand for rhodium-catalyzed asymmetric hydroboration

A new optically active and large dihedral angle atropisomeric P,N ligand, pyphos, which contains a tertiary phosphine and pyridine moiety, was prepared and resolved through diastereomeric complexation with chiral palladium amine complexes. The hexafluorophosphate salt of the diastereomers were found to be separable by fractional recyrstallization, while the corresponding chloride salt did not. [Rh(COD)pyphos]PF(6) complex was synthesized by metalation of pyphos with [Rh(COD)Cl](2) followed by anion exchange with NH(4)PF(6) in excellent yield, and the target rhodium complex was characterized by single-crystal X-ray crystallography. The chiral cationic rhodium complex was utilized in the enantioselective hydroboration of vinylarenes. Excellent regioselectivity and good enantioselectivity were observed, and the ee values were found to be dependent on the electronic properties of para-substituted styrenes.     

Synthesis of unsymmetrical substituted 1,4-dihydropyridines through thermal and microwave assisted [4+2] cycloadditions of 1-azadienes and allenic esters

Thermal and microwave assisted [4+2] cycloadditions of 1,4-diaryl-1-aza-1,3-butadienes with allenic esters lead to cycloadducts, which after a 1,3-H shift afford variedly substituted unsymmetrical 2-alkyl-1,4-diaryl-3-ethoxycarbonyl-1,4-dihydropyridines in high yields. Reactions carried out under microwave irradiation are cleaner and give higher yields with much shortened reaction times. Density functional theory (DFT) at the B3LYP/6-31G* level has been used to calculate geometric features of the reactants, barrier for s-trans to s-cis and reverse isomerization of azadienes (5a-d, 10a-e), dihedral angles between N(1), C(2), C(3), and C(4) atoms of azadienes along with various indices such as chemical hardness (eta), chemical potential (micro), global electrophilicity (omega), and the difference in global electrophilicity (Deltaomega) between the reacting pairs and Fukui functions (f (+) and f(-)). The results revealed that s-trans is the predominant conformation of azadienes at ambient temperature and the barrier for conversion of the s-trans rotamer of 1-azadienes to s-cis may be the major factor influencing the chemoselectivity, i.e., [4+2] verses [2+2] cycloaddition. The regiochemistry of the observed cycloadditions is collated with the obtained local electrophilicity indices (Fukui functions). Transition states for the formation of both [4+2] and [2+2] cycloadducts as located at the PM3 level indicate that the transition state for the formation of [4+2] cycloadducts has lower energy, again supporting the earlier conclusion that preferred formation of [4+2] cycloaaducts at higher temperature may be a consequence of barrier for s-trans to s-cis transformation of 1-azadienes.     

香豆素-咪唑类荧光染料的设计与研究

合成了不同给电子取代基(羟基、丁氧基、二乙基氨基等)的菲并[9,10-d]咪唑(CA1~CA6)或4,5-二苯基咪唑(CB1~CB6)修饰的香豆素衍生物,初步考察了它们的溶液发光和固体发光现象.研究表明,当香豆素取代基为氨基时,化合物在二氯甲烷中的荧光较强,而羟基取代、丁氧基取代或者无取代的衍生物在二氯甲烷中的荧光都很弱,而菲并[9,10-d]咪唑修饰的衍生物CA1~CA5的溶液荧光要比4,5-二苯基取代咪唑修饰的衍生物CB1~CB5的溶液强.另外,染料分子的分子内氢键强度及咪唑基-香豆素环间二面角大小都会对染料分子的发光性能产生影响.     

Synthesis, Crystal Structure, Fluorescence Properties and Geometrical Calculation of 4,8,10-Trimethyl-2-(3,5-dimethylpyrazol)pyrido-[2',3':3,4]-pyrazol[1,5-a]-pyrimidin

A novel compound 4,8,10-trimethyl-2-(3,5-dimethylpyrazol)-pyrido-[2',3':3,4]-pyrazol-[1,5-5-a]-pyrimidine (C17H18N6) has been synthesized from 2,6-dichloro-4-methylnico-tinonitrile, hydrazine and acetylacetone by substitution and cyelolization. The structure was characterized by elemental analysis, 1H NMR and IR, and single-crystal structure was determined by X-ray diffraction analysis. The compound crystallizes in the monoclinic system, space group P21/c with a = 8.0338(9), b = 28.618(3), c = 7.2688(8) (A),β = 111.051(2)°, V= 1559.6(3) (A)3, Z = 4, Dc=1.305 g/cm3,μ = 0.083 mm-1, Mr = 306.37, F(000) = 648, S = 0.971, the final R = 0.0795 and wR =0.1746 for 1457 observed reflections (Ⅰ > 2σ(Ⅰ)). The results demonstrate that the pyridine, pyrazole and pyrimidine rings are nearly eoplanar, which is evident from the dihedral angles among the four rings in the range of 0.13～4.15°. The face-to-face π-π stacking interactions among pyridine,pyrazol and pyrimidine rings result in a supramolecular crystal, in which they seem to be effective in stabilizing the structure. Meanwhile, the fluorescence properties of the title compound were discussed, which showed very strong fluorescence. The calculated results (selected bond lengths,bond angles and torsion angles) are all typical and agree well with the experimental results, which demonstrates that B3LYP/6-31+G* is suitable for the title compound.     

3‐[2,6‐Bis­(diethyl­carbamoyl)­pyridin‐4‐yl]‐N‐(tert‐butoxy­carbonyl)­alanine methyl ester: a chiral tridentate ligand that causes a diastereomeric excess of its lanthanide complexes in solution

L⁴, or 3‐[2,6‐bis­(diethyl­carbamoyl)­pyridin‐4‐yl]‐N‐(tert‐but­oxy­car­bonyl)­alanine methyl ester, C₂₄H₃₈N₄O₆, crystallizes in neat [010] laths stabilized by abundant intra‐ and intermolecular hydrogen bonds. The strongest of these form [010] chains of mol­ecules, thus rationalizing the fastest growth direction, while the slowest direction coincides with the normal to the (110) layers, which are linked by very weak hydrogen bonds. There exist two independent mol­ecules, the distances and bond angles of which differ in a random manner only. The torsion and dihedral angles, however, differ so as to achieve optimal packing. The influence of the chiral group in the 4‐position of the pyridine ring on the helical wrapping and on the ensuing diastereomeric induction is briefly discussed.     

DFT conformational studies of alpha-maltotriose

Recent DFT optimization studies on alpha-maltose improved our understanding of the preferred conformations of alpha-maltose. The present study extends these studies to alpha-maltotriose with three alpha-D-glucopyranose residues linked by two alpha-[1-->4] bridges, denoted herein as DP-3's. Combinations of gg, gt, and tg hydroxymethyl groups are included for both "c" and "r" hydroxyl rotamers. When the hydroxymethyl groups are for example, gg-gg-gg, and the hydroxyl groups are rotated from all clockwise, "c", to all counterclockwise, "r", the minimum energy positions of the bridging dihedral angles (phi(H) and psi(H)) move from the region of conformational space of (-, -), relative to (0 degrees , 0 degrees), to a new position defined by (+, +). Further, it was found previously that the relative energies of alpha-maltose gg-gg-c and "r" conformations were very close to one another; however, the DP-3's relative energies between hydroxyl "c" or "r" rotamers differ by more than one kcal/mol, in favor of the "c" form, even though the lowest energy DP-3 conformations have glycosidic dihedral angles similar to those found in the alpha-maltose study. Preliminary solvation studies using COSMO, a dielectric solvation method, point to important solvent contributions that reverse the energy profiles, showing an energy preference for the "r" forms. Only structures in which the rings are in the chair conformation are presented here.     

Conformational investigations of 6-EXO and 6-ENDO- methylbicyclo[3.1.0] hexan-3-one

R band (26.5–40 GHz) microwave spectra have been recorded for the common isotopic species of 6-endo-methylbicyclo[3.1.0]hexan-3-one and 6-exo-methylbicyclo[3.1.0]hexan-3-one. These spectra are shown to be consistent with a ground vibrational state boat conformation having five membered ring dihedral angles of 7.2 ± 0.3° and 20.5 ± 1.4° respectively. This information is correlated with product quantum yields obtained on photolysis of these molecules.     

The optical activity of β,γ-enones in ground and excited states using circular dichroism and circularly polarized luminescence

The circularly polarized luminescence (CPL) and electronic circular dichroism (CD) spectroscopic parameters corresponding to the n←π* and n→π* transitions, respectively, have been calculated for selected β,γ-enones using density functional theory. For the smallest β,γ-enone, (1R,4R)-bicyclo[2.2.1]hept-5-en-2-one (norbornenone), coupled-cluster calculations have also been carried out. The excited-state potential energy surface for three of the five enones studied reveals two minima with different C[double bond, length as m-dash]OC[double bond, length as m-dash]C dihedral angles, and with rotatory strengths of opposite sign. The relative energies of the minima determine the sign of the CPL intensity, which may be the same or opposite as in the CD spectrum, in agreement with experimental data. The results obtained in this first computational study of CPL demonstrate its usefulness as an indicator of excited-state structures of chiral species.     

Ethylenedithio?Tetrathiafulvalene?Helicenes: Electroactive Helical Precursors with Switchable Chiroptical Properties

Electroactive fused ethylenedithio? tetrathiafulvalene? [4]helicene and ‐[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3‐dibromo‐helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single‐crystal X‐ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid‐state architectures show interplay between S???S and π???π intermolecular interactions. The chiroptical properties of the enantiopure EDT? TTF? [6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical‐cation species has been achieved.     

Spectroscopic behavior on the formation complex of three double-armed calix[4]arene derivatives with lanthanoid nitrates in acetonitrile

The complexation spectroscopic behavior of three p-tert-butylcalix[4]arene Schiff bases i.e. 5,11,17,23-tetra-tert-butyl-25,27-bis[2-[N-(3-nitrobenzylidene)amino]ethoxy]-26,28-dihydroxycalix[4]arene (1), 5,11,17,23-tetra-tert-butyl-25,27-bis[2-[N-(2-hydroxybenzylidene)amino]ethoxy]-26,28-dihydroxycalix[4]arene (2), and 5,11,17,23-tetra-tert-butyl-25,27-bis[2-[N-(2-hydroxy-3-methoxybenzylidene)amino]ethoxy]-26,28-dihydroxycalix[4]arene (3) with lanthanoid nitrates (Tb3+ and Eu3+) has been investigated in anhydrous acetonitrile at 25 degrees C by using UV-vis and FT-IR as well as fluorescence spectra. The results obtained indicated that the spectroscopic behavior of compounds 1-2 upon complexation with lanthanoid ions did not show any significant larger difference in comparison with free compounds 1 and 2, which may be contributed to the poor binding ability. Contrary to compounds 1 and 2, the lower rim functional groups in compound 3 can form two large pi electron conjugate system with lanthanide ion and encapsulate lanthanide ions tightly, displaying the novel spectroscopic behavior upon complexation with lanthanide ions. As compared with compound 3, the formation complexes of compound 3 with Tb3+ and Eu3+ showed new broad intense absorption at 398 nm, respectively, and IR spectra showed that O-H stretching vibration at 3413.40 cm(-1) displayed a large drop. It is interestingly noted that the narrow emission line spectra were observed only for 3 complex with Tb3+, but did not for 3-Eu3+ complex. In the 3-Eu3+ complex, the broad-band emission at lambda(max) = 534 nm was obtained at the excitation of 398 nm. The spectroscopic behavior of three calix[4]arene derivatives upon complexation with lanthanoids was discussed from the relationship between the host structure and the properties of guest lanthanide ions.