A study of electronic structures of some C9H9- carboanions

The electronic structures of five C₉H₉^-, carboanions have been studied by ab initio STO-3G calculations, and some general conclusions on related C₉H₉^- and C₉H₉⁺ structures are presented. Large antibonding interactions in one occupied MO make barbaral-9-yl anion (2) unstable as its cationic counterpart (8). The proposed D_9h-symmetrical cation and D_3h-symmetrical anion (3) do not exist due to Jahn-Teller distortions. A study of the MO correlations confirms that the two tetracyclic anions with C_2v symmetry (5 and 6) are the results of the Jahn-Teller distortions of 3. Anion 5 is identified as the proper intermediate of the Cope rearrangement of anion 2.     

Exploiting Incommensurate Symmetry Numbers: Rational Design and Assembly of M2M3′L6 Supramolecular Clusters with C3h Symmetry

Mixed-metal mesocates [M₂Pd₃Br₆L₆]⁴⁻ (M=Ti^IV, Sn^IV; L=4-diphenylphosphanyl-catecholate) have been synthesized, in which the two incommensurate symmetry elements generated by the different metal ions are linked by a rigid, bifunctional ligand to generate a C_3h-symmetrical cluster (see picture).     

Selective functionalization of a resorcin[6]arene

The Mannich-type condensation of hexaethylresorcin[6]arene with achiral primary amines results in S₆-symmetrical hexadihydro-1,3-oxazine derivatives, which are mesoforms. The reaction with individual enantiomers of α-phenylethylamine leads to C₃-symmetrical enantiomeric hexaoxazines, which crystallize from reaction mixtures in an analytically pure form.     

Tris-ACE-(6,6′-bipyridyl)-α-cyclodextrin: synthesis and chirality of the tripodal metal complexes

This Letter describes the synthesis of a C₃-symmetrical tris-^ACE-(6,6′-bipyridyl)-α-CyD via ‘one-pot’ Staudinger-Aza-Wittig reaction (SAW). Metal complexation behavior was investigated by UV–vis and Circular Dichroism. Coordination of 6,6′-bipyridyl units with Cu^II and Ni^II cations, spontaneously generates a strong exciton coupling-type positive Cotton effect proving the formation of a Λ-helix with a left-handed screw propeller (M).     

Anion dependent molecular recognition of cations

In DMF-d₇ tetrabenzimidazole cavitands 2 exist as monomeric species and vase-like conformers. Several possible arrangements of the four benzimidazole NHs are indicated by ¹H NMR spectroscopy. The cavitands form 1:1 inclusion complexes with tetraethyl ammonium and phosphonium cations only when strong hydrogen bonding anions like chloride or acetate are present. These complexes are stable on the NMR time scale at 295 K feature a C_2V-symmetrical arrangement of benzimidazole functions. The stability of the C_2V-symmetrical tetramethylammonium acetate complex is independent of the temperature. In contrast, tetramethylammonium and phosphonium chloride complexes exist at 233 K as several isomers. This complicated behavior is, in part, attributed to the hydrogen bonding interactions between the anions and the NH groups of benzimidazole functions.     

Synthesis,Separation, and Characterization of Optically Pure C76 Mono-Adducts

Nucleophilic Bingel cyclopropanation of D₂-C₇₆ with bis[(S)-1-phenylbutyl] 2-bromomalonate in toluene in the presence of base yielded three constitutionally isomeric pairs of diastereoisomeric mono-adducts together with one other constitutional isomer. All seven mono-adducts were isolated in optically pure form by prep. HPLC on a (S,S)-Whelk-O1 chiral stationary phase. They represent the first optically pure adducts of an inherently chiral fullerene. Characterization by UV/VIS, CD, ¹³C- and ¹H-NMR spectroscopy allowed identification of pairs of stereoisomers and symmetry assignments: the two pairs of diastereoisomers which were isolated as the major product possess C₁ symmetry, whereas the third pair of diastereoisomers, which is a minor product, is C₂-symmetrical. The circular dichroism spectra of the optically active C₇₆-adducts showed very pronounced Cotton effects resulting from strong chiroptical contributions of the chiral fullerene chromophore with the maximum observed Δε values being twice as high than those previously measured for optically active adducts of achiral fullerenes with a chiral addition pattern. Whereas the regioselectivity of mono-additions to C₇₀ correlates with the degree of local bond curvature and the regioselectivity of multiple Bingel cyclopropanations of C₆₀ with electronic parameters such as coefficients of the lowest unoccupied molecular orbital (LUMO), no such simple predictive correlations exist for the nucleophilic addition to C₇₆. Despite full spectral characterization, an unambiguous structural assignment of the isolated compounds was not possible, except for the two C₂-symmetrical isomers. Based on considerations of local bond curvature and the previous experiences with the chemistry of C₇₀, the structures of the C₂-symmetrical stereoisomers were assigned as (S,S,^fC)- 3 and (S,S,^fA)- 3 , resulting from addition to the polar α-type C(1)? C(6) bond.     

Correlation analysis of characteristic infrared spectral data of hydantoin derivatives: evidence for vibrational coupling

The characteristic vibrations (ν_CO and ν_CC) of a large number of hydantoin derivatives are reported. Especially the very fine correlations ν_CO(sym) versus ν_CO(asym) (r²=0.985) but also successful correlations of the vibration wave numbers to HAMMETT's substituent constants and some other experimental parameters (pK_s, OxPot, RedPot) as well, corroborate reassignments of previously obtained results [Monatsh. Chem. 92 (1961) 361] and prove the doublet obtained in the region of the CO stretching vibrations to be the symmetrical and anti-symmetrical vibrational modes of a mechanically coupled system of two quasi-symmetrical CO bonds.     

A practical, solvent free, one-pot synthesis of C2-symmetrical secondary amines

A novel one-pot reductive amination of ketones using the combination Ti(OⁱPr)₄/H₂/Pd-C is reported. This practical procedure does not require any solvent, and affords C₂-symmetrical secondary amines in high yields and excellent diastereoselectivities.     

Facile and efficient synthesis of C2-symmetrical 1,3,5-triazine polycarboxylate ligands under microwave irradiation

A rapid and efficient method for preparation of C₂-symmetrical 1,3,5-triazine polycarboxylate ligands was developed. The reactions included either selective mono- or di-substitution of 2,4,6-trichloro-1,3,5-triazine with various nucleophiles containing carboxyl group(s), followed by nucleophilic displacement of the remained chloride(s) in aqueous media under microwave irradiation. Novel C₂-symmetrical tripodal ligands were afforded in good yields and purities under short reaction time with simple work-up, which are potentially useful as structural directing units in metal-organic frameworks.     

Iridium-catalyzed consecutive and enantioselective [2+2+2] cycloaddition of tetraynes and hexaynes for the construction of an axially chiral biaryl system

A chiral iridium complex catalyzed a consecutive and enantioselective [2+2+2] cycloaddition of polyynes to give axially chiral compounds. Intermolecular reaction of tetraynes, possessing aryl groups on their termini, with protected but-2-yne-1,4-diols gave C₂-symmetrical quateraryl compounds. Intramolecular reaction of hexaynes, possessing aryl or alkyl groups on their termini gave C₂-symmetrical biaryl compounds. The catalytic synthesis of a pentacene derivative with axial chiralities is also discussed.     

Oligosaccharide Analogues of Polysaccharides. Part 12. Synthesis of ‘acetyleno-saccharide’-derived cyclodextrin analogues

‘Acetyleno-oligosaccharides’ in which two terminal ethynyl substituents enclose an angle (significantly) below 180° are building blocks for the preparation of cyclodextrin analogues. This is illustrated by the preparation of a cyclotrimer and a cyclotetramer; the C₃-symmetrical cyclotrimer 18 (Scheme 1) was synthesized in 13 steps (7.7%) and the C₄-symmetrical cyclotetramer 51 (Scheme 3) in 14 steps (4.3%) from the known dialkyne 21. The solubilities of 18 and 51 in H₂O were determined by gravimetry; a saturated solution is 130 mM in the trimer 18 and 12.8 mM in the tetramer 51 . The dependence of the optical rotation of 18 and 51 in H₂O on the concentration, and the concentration dependence of the ¹H-NMR chemical shift of the signals of the ¹CH groups of 51 (D₂O) suggest that there is no significant self-association of 18 and 51 .     

Reversible reduction drives anion ejection and C60 binding within an FeII4L6 cage

Fe^II₄L₆ tetrahedral cage 1 was prepared from a redox-active dicationic naphthalenediimide (NDI) ligand. The +20 charge of the cage makes it a good host for anionic guests, with no binding observed for neutral aromatic molecules. Following reduction by Cp₂Co, the cage released anionic guests; subsequent oxidation by AgNTf₂ led to re-uptake of anions. In its reduced form, however, 1 was observed to bind neutral C₆₀. The fullerene guest was subsequently ejected following cage re-oxidation. The guest release process was found to be facilitated by anion-mediated transport from organic to aqueous solution. Cage 1 thus employs electron transfer as a stimulus to control the uptake and release of both neutral and charged guests, through distinct pathways.

Fe^II₄L₆ cage 1 binds anionic guests but not neutral guests. In its reduced form, the cage can bind neutral C₆₀. Reduction and oxidation of the cage could thus be used as a stimulus to control the uptake and release of both neutral and charged guests.     

A Study of the Interaction Between Cucurbit[8]uril and Alkyl‐Substituted 4‐Pyrrolidinopyridinium Salts

The interaction between cucuribit[8]uril (Q[8]) and a series of 4‐pyrrolidinopyridinium salts bearing aliphatic substituents at the pyridinium nitrogen, namely 4‐(C₄H₈N)C₅H₅NRBr, where R=Et (g1), n‐butyl (g2), n‐pentyl (g3), n‐hexyl (g4), n‐octyl (g5), n‐dodecyl (g6), has been studied in aqueous solution by ¹H NMR spectroscopy, electronic absorption spectroscopy, isothermal titration calorimetry and mass spectrometry. Single crystal X‐ray diffraction revealed the structure of the host–guest complexes for g1, g2, g3, and g5. In each case, the Q[8] contains two guest molecules in a centrosymmetric dimer. The orientation of the guest molecule changes as the alkyl chain increases in length. Interestingly, in the solid state, the inclusion complexes identified are different from those observed in solution, and furthermore, in the case of g3, Q[8] exhibits two different interactions with the guest. In solution, the length of the alkyl chain plays a significant role in determining the type of host–guest interaction present.     

Regioselective acylation of aminoresorcinarenes

The acid catalyzed hydrolytic cleavage of the oxazine rings in the readily available tetraoxazine derivatives of resorcinarenes results in tetraaminoresorcinarenes. A similar process applied to C₂-symmetrical bisoxazine resorcinarene tetratosylates affords C_2v-symmetrical resorcinarenediamines. The mild acylation of these resorcinareneamines with BOC-anhydride or para-nitrophenyl ester proceeds selectively at the nitrogen atoms without affecting the hydroxyl groups. Most of the resulting resorcinareneamides are thus obtained in preparative yields and can be easily purified by simple crystallizations. In the crystalline state the compounds obtained are found to bind chloride anions through hydrogen bonds and electrostatic interactions and to display a chiral arrangement of hydrogen bonded functional groups at the wide rim of the macrocycle.     

Quantitative tests of mixed crystal excition theory. I. Naphthalene monomer1B2u and 3B1u spectra

A quantitative, computer processed spectroscopic study, using photon counting, on the first excited triplet and singlet states of dilute isotopic mixed crystals of naphthalene at 2 K is presented for C₁₀H₈; 1-DC₁₀H₇; 2-DC₁₀H₇; 1,4-D₂C₁₀H₆; 1,4,5-D₃C₁₀H₅; 1,4,5,8-D₄C₁₀H₄; 1,2,4,5,8-D₅C₁₀H₃; a β-D₄C₁₀H₄ and a β₂-D₆C₁₀H₂ as guests in C₁₀D₈ host crystals (and, for comparison, also for the same guests in a durene host crystal). The guest—host relative polarization Rashba formula has been verified quantitatively, and, as an added bonus, the elusive polarization ratio of the pure naphthalene crystal singlet Davydov components has been found to be 80 ± 20 (b/a), which is in poor agreement with the transition octupole—transition octupole model. The experimental guest energies and their concomitant quasiresonance shifts for bound singlet states (as well as the occurrences of unbound states) are in excellent quantitative agreement (about 1 cm^?1) with those calculated using a Green's function formalism based on the ideal mixed crystal approximation and on a restricted Frenkel type dispersion relation derived from resonance pairs. The same Green's function also accounts quantitatively (within 10%) for the guest singlet state exciton localizations (guest excitation amplitudes). The triplet exciton state reveals an orientational site splitting (about 0.7 cm^?1) for the 0—0 transition of the I-DC₁₀H₇ guest in C₁₀D₈ host. The order of the α and β substituted deuteronaphthalenes in the triplet state is reversed from that of the singlet state. The last two observations are related to the different nature of the lowest Π-Π^* singlet and triplet states of naphthalene.     

A straightforward and modular synthesis of enantiopure C2- and C1-symmetrical 2,2′′-phosphino-1,1′′-biferrocenes

A new practical and highly flexible synthesis of enantiopure C₂- and C₁-symmetrical 2,2′′-phosphino-1,1′′-biferrocenes is presented. The structural properties of two of their PdCl₂ complexes and preliminary Ru-catalysed hydrogenations using the biferrocene ligands, giving enantioselectivities of up to 82%, are described.     

Crystal structure of the 1:3 thiourea-hexachloroethane inclusion compound at 295 K

The crystal structure of the 1:3 thiourea-hexachloroethane inclusion compound at 295 K has been determined. The parameters of the rhombohedral Bravais cell (space group R3ˉc) are a = 16.097(2) Å, c = 12.450(3) Å, V = 2793.6(8) ų, d _calc = 1.659 g/cm³, d _exp = 1.661±0.005 g/cm³, Z = 6 for C₅H₁₂Cl₆N₆S₃. The disordered guest molecules lie in the channels (parallel to the c axis) of the clathrate framework constructed from thiourea molecules linked by N-H...S hydrogen bonds. The mutual arrangement of the carbon and chlorine atoms is such that they define four orientations of the C-C bond relative to the channel of the host framework with nearly eclipsed conformations of hexachloroethane: one orientation along the triple bond axis (channel axis) and three equiprobable orientations at an angle of 74(2)° to the axis; for the coaxial orientation, there are two different configurations of the guest with slightly different geometries. The relative contributions of each of the five orientations were determined from the site occupancies of the guest atoms: 26.18 (coaxial orientations) and 3×19%. The resulting structure model is compared with the literature data about guest disordering in the structure of an adduct of the same composition determined at 233 K and with other similar structures. Original Russian Text Copyright ? 2007 by S. F. Solodovnikov, G. N. Chekhova, G. V. Romanenko, N. V. Podberezskaya, Z. A. Solodovnikova, D. V. Pinakov, and A. R. Semenov __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 48, No. 2, pp. 348–357, March–April, 2007.     

Asymmetric synthesis of α-substituted alkylphosphonates based on symmetrical dialkyl phosphites

Chiral C₂-symmetrical dialkyl phosphites and C₃-symmetrical trialkyl phosphites, derived from (−)-borneol, (−)-menthol, and 1,2∶5,6-di-O-isopropylidene-α-d-glucofuranose, were studied as the starting reagents for the preparation of chiral organophosphorus compounds. The reactions of C₂-symmetrical dialkyl phosphites and C₃-symmetrical trialkyl phosphites with aldehydes and amines or aldehydes are accompanied by asymmetrical induction at the α-carbon atom to yield optically active α-aminoalkylphosphanates or α-hydroxyalkylphosphonates, respectively. The stereoselectivity of the reaction depends on the structure of the starting compounds and the reaction conditions. Translated fromIzvestiya Akademii, Nauk, Seriya Khimicheskaya, No. 8, pp. 1588–1593, August, 1999.     

Ineffective OH Pinning of the Flipping Dynamics of a Spherical Guest within a Tight‐Fitting Tube

A supramolecular/synthetic method has been devised to affix a sterically hindered substituent onto a fullerene guest encapsulated in a tubular host. A two‐wheeled complex of (C₅₉N)‐(C₅₉N) with a tubular host was oxidatively bisected to afford a C₅₉N⁺ cation captured in the tube. The C₅₉N⁺ cation in the tube was then trapped by ethanol or water, which led to an oxy substituent pinned on the guest. The guest motions within the tube were modulated by the pinned substituent, and up‐and‐down flipping motions were halted by an ethoxy substituent. A hydroxy substituent, however, was ineffective in halting the flipping motions, despite the tight‐fitting relationship between the tubular host and the spherical guest. Theoretical calculations of the dynamics revealed that the flipping motions were assisted by OH‐π hydrogen bonds between the guest and the carbon‐rich wall and that sliding motions of the OH group were also facilitated by deformations of the tube.