Synthesis,Characterization and Crystal Structure of （2,2′—Bipyridine）（μ—maleato）copper（Ⅱ）Dihydrate

The title polymeric complex [Cu(C₄H₂O₄)(C₁₀H₈N₂)]_n·2nH₂O was prepared and its crystal structure was determined by X‐ray diffraction methods. The crystal belongs to space group P2₁/c with cell dimensions of a = 1.0104(1), b = 1.9952(2), c = 0.7357(2) nm, β= 98.38(2)° and Z = 4. The complex forms zig‐zag chains along crystallographic axis c via Cu—O (carboxyl) bond in the apical direction. Each repeated unit consists of a square pyramidal Cu(II) centre with one maleate dianion and one 2,2′‐bipyridine forming a basal plane. Adjacent chains link to each other by H‐bonding between carboxyl groups and crystalline water. The distance of 0.3482 nm between parallel bipyridine rings shows a π‐π stacking interaction. The title complex was also characterized by IR, UV and ESR spectra.     

Flash photolytic generation of o-quinone alpha-phenylmethide and o-quinone alpha-(p-anisyl)methide in aqueous solution and investigation of their reactions in that medium. Saturation of acid-catalyzed hydration.

o-quinone alpha-phenylmethide was generated as a short-lived transient species in aqueous solution by flash photolysis of o-hydroxy-alpha-phenylbenzyl alcohol, and its rate of decay was measured in HClO4 and NaOH solutions as well as in CH3CO2H, H2PO4-, and HCO3- buffers. These data show that hydration of this quinone methide back to its benzyl alcohol precursor occurs by acid-, base-, and uncatalyzed routes. The acid-catalyzed reaction gives the solvent isotope effect kH+/kD+ = 0.34, whose inverse nature indicates that this reaction occurs via rapid preequilibrium protonation of the quinone methide on its carbonyl oxygen atom followed by rate-determining capture of the ensuing carbocationic intermediate by water, a conclusion supported by the saturation of acid catalysis in concentrated HClO4 solution. o-quinone alpha-(p-anisyl)methide was also generated by flash photolysis of the corresponding benzyl alcohol and of the p-cyanophenol ether of this alcohol as well, and its rate of decay was measured in HClO4 and NaOH solutions and in HCO2H, CH3CO2H, HN3, CF3CH2NH3+, imidazolium ion, H2PO4-, (CH2OH)3CNH3+, (CH3)3CPO3H-, and HCO3- buffers. Acid-, base-, and uncatalyzed hydration reaction routes were again found, and solvent isotope effects as well as saturation of acid catalysis, this time in dilute HClO4, confirmed a preequilibrium mechanism for the acid-catalyzed reaction. Analysis of the buffer data gave buffer-base rate constants that did not conform to the Br?nsted relation, consistent with the expected nucleophilic nature of the buffer reactions.     

(R)‐Mandelic acid (S)‐alanine hemihydrate

Crystals of the title complex, C₃H₇NO₂·C₈H₈O₃·0.5H₂O, were obtained from an aqueous solution containing racemic mandelic acid and (S)‐alanine. The unit cell includes two independent molecular complexes and one water molecule. The structure formed by (R)‐mandelic acid and (S)‐alanine in a 1:1 molar ratio shows the successful optical separation of racemic mandelic acid. Strong hydrogen bonding, with a rather short O?O separation of 2.494 (3) Å, is observed between the carboxyl and carboxyl­ate groups. A structural comparison suggests that the strong hydrogen bonding affects the neighbouring covalent bond.     

Flash Photolysis of 5-Methyl-1,4-naphthoquinone in Aqueous Solution: Kinetics and mechanism of photoenolization and of enol trapping

5-Methyl-1,4-naphthoquinone ( 1 ) is a remarkable probe to study hydrogen and proton transfer reactions. The photoenol 4-hydroxy-5-methylidene naphthalen-1 (5H)-one ( 2 ) is formed in the ground state within 2 ps of excitation and with a quantum yield of unity, presumably through a conical intersection of the S₀ and S₁ hypersurfaces. In aqueous acid, enol 2 is hydrated to 5-(hydroxymethyl)naphthalene-1,4-diol 3 (X ? OH, Scheme 1). The rate of hydration of 2 increases linearly with acid concentration from ca. 1.5 × 10⁴ s^?1 at pH 6 to reach a maximum value of 9 ×10⁷ s^?1 when the remaining carbonyl function is protonated, pK_a(2⁺) = 1.1. Contrary to an earlier suggestion, the rate-determining step in the acid-catalyzed hydration of 2 is addition of water to the conjugate acid 2 ⁺. Pronounced acceleration of the decay rate of 2 by hydrazoic-acid buffers indicates competitive trapping of 2 ⁺ by the azide ion. In neutral-to-weakly-basic solutions, enol 2 reacts by ionization, pK^a( 2 ) = 6.5, and nearly diffusion-controlled condensation of the carbanionic species 2 ^? with quinone 1 . Proto-nation at the methylidene C-atom does not compete measurably with protonation on carbonyl O-atom, despite a Substsial thermodynamic basic for carbon Portoation of ca. 50 kJ mol^?1 for 2 and 100 mol^?1 for 2^?.     

Structure and topography of molecular assemblies on solid substrates by infrared spectroscopy and scanning tunneling microscopy

Recently we have combined infrared spectroscopy and atomic resolution scanning tunneling microscopy (STM) to probe the local structure and intermolecular arrangement of molecules within thin films. IR spectroscopy provides spatially averaged information about orientation of the molecules with respect to the surface and about intermolecular arrangement within the crystallographic unit cell. STM data yields a local picture of molecular packing within the film. The requirements of an atomically flat (over distances of hundreds of angstroms) conducting substrate for the STM are fulfilled by an epitaxially grown film of gold on a cleaved mica substrate which also provides a good infrared reflective surface, enabling IR and STM measurements on identical samples. Systems investigated include Langmuir-Blodgett films of cadmium arachidate and self-assembled films of octadecyltrichlorosilane.     

The first A-nor-hippuristanol and two novel 4,5-secosuberosanoids from the Gorgonian Isis hippuris

The first A-nor-hippuristanol, A-nor-22-epi-hippurin-2α-carboxylic acid (1), and two 4,5-secosuberosane sesquiterpenoids, isishippuric acids A and B (2 and 3), have been isolated from the gorgonian coral Isis hippuris. Those structures were deduced by extensive 1D and 2D NMR studies. The structure of 1 was further supported by a single crystal X-ray diffraction analysis. Isishippuric acid B has been shown to exhibit potent cytotoxicity toward a limited panel of cancer cells.     

Isomerization of dewar benzenes involving electron donor-acceptor exciplexes

Valence photoisomerization of hexamethyl (Dewar benzene) (HMDB) is sensitized by aromatic singlet photosensitizers 1,4-dicyanobenzene, 1-cyanonaphthalene, 9-cyanoanthracene, and 9,10-dicyanoanthracene with a limiting quantum efficiency of 1.0 in cyclohexane solvent. Quenching of the fluorescence of the aromatic sensitizers leads to exciplex emission which is identical to that obtained by quenching with the isomer, hexamethylbenzene (HMB). The emission is identified as HMB exciplex emission on the basis of relative lifetime and dual quenching experiments. The relative yield of HMDB-derived (“adiabatic”) emission is 20–50% depending on the excitation energy of the HMB exciplex product. Neither biacetyl singlet or triplet nor 1-cyanonaphthalene triplet photosensitization is successful in bringing about isomerization of HMDB. Dimethyl 1,4,5,6-tetramethylbicy-clo[2.20]hexa-2,5-diene-2, 3-dicarboxylate undergoes valence isomerization on quenching electron donor fluorophores, with a quantum efficiency of 0.2. The aromatic valence isomer is not produced in an excited state in this case. Factors which govern the efficiency of adiabatic and diabatic isomerization of the Dewar benzenes are discussed, including sensitizer redox properties, configuration, and multiplicity, the excitation energy and binding characteristics of exciplexes, and the Dewar benzene substituent pattern.     

Peri-bridged naphthalenes. 4. chalcogen-bridged acenaphthylenes

Three new electron donors, acenaphtho[5,6-$\text{cd}$]-1,2-dithiole ($\text{1}$), acenaphtho[5,6-$\text{cd}$]-1,2-diselenole ($\text{2}$), and acenaphtho[5,6-$\text{cd}$]-1,2-ditellurole ($\text{3}$), can be prepared in 28%, 22%, and 14% yields respectively by reaction of the elemental chalcogens (S, Se, Te) with 5,6-dilithioacenaphthylene ($\text{4}$). Compound $\text{4}$ is generated by treatment of 5,6-dibromoacenaphthylen ($\text{5}$), for which a convenient preparation is described, with $\text{n}$-butyllithium (2 equiv.) in THF at ?78°C.     

Intense near-infrared optical absorbing emerald green [60]fullerenes

Synthesis of emerald green fullerenes (EF) C60[C(CH3)(CO2Et)2]6 and C60[C(CH3)(CO2-t-Bu)2]6 was performed by using hexaanionic C60 intermediate (C60-6) as a reagent in one-pot reaction for attaching six alkyl ester addends on one C60 cage. These EF compounds exhibit intense optical absorption over 600-940 nm, the longest optical absorption of the C60 cage among many [60]fullerene derivatives synthesized.     

Efficient epoxidation of alkenes with aqueous hydrogen peroxide catalyzed by methyltrioxorhenium and 3-cyanopyridine

The epoxidation of alkenes with 30% aqueous hydrogen peroxide is catalyzed efficiently by methyltrioxorhenium (MTO) in the presence of pyridine additives. The addition of 1-10 mol % of 3-cyanopyridine increases the system's efficiency for terminal and trans-disubstituted alkenes resulting in high isolated yields of the corresponding epoxides. The system allows for epoxidation of alkenes with various functional groups. Alkenes leading to acid-sensitive products are efficiently epoxidized using a mixture of pyridine and 3-cyanopyridine as additives. This method is operationally very simple and uses an environmentally benign oxidant. The effects of different pyridine additives on the alkene conversion and the catalyst lifetime are discussed.