The Paternò-Büchi reaction: importance of triplet states in the excited-state reaction pathway.

The Paternò-Büchi (PB) reaction between an excited carbonyl compound and an alkene has been widely studied, but so far little is known about the excited-state dynamics of the reaction. In this investigation, we used a compound in which a formyl and a vinyl group are attached to a [2.2]paracyclophane in order to obtain a model system in pre-reactive conformation for the PB reaction. We studied the excited-state dynamics of the isolated molecule in a molecular beam using femtosecond time-resolved photoelectron spectroscopy and ab initio calculations. The results show that inter-system crossing within two picoseconds competes efficiently with the reaction in the singlet manifold. Thus, the PB reaction in this model system takes place in the triplet state on a time scale of nanoseconds. This result stresses the importance of triplet states in the excited-state pathway of the PB reaction involving aromatic carbonyl compounds, even in situations in which the reacting moieties are in immediate vicinity.

Pseudo-bimolecular [2+2] cycloaddition studied by time-resolved photoelectron spectroscopy.

The first study of pseudo-bimolecular cycloaddition reaction dynamics in the gas phase is presented. We used femtosecond time-resolved photoelectron spectroscopy (TRPES) to study the [2+2] photocycloaddition in the model system pseudo-gem-divinyl[2.2]paracyclophane. From X-ray crystal diffraction measurements we found that the ground-state molecule can exist in two conformers; a reactive one in which the vinyl groups are immediately situated for [2+2] cycloaddition and a nonreactive conformer in which they point in opposite directions. From the measured S(1) lifetimes we assigned a clear relation between the conformation and the excited-state reactivity; the reactive conformer has a lifetime of 13 ps, populating the ground state through a conical intersection leading to [2+2] cycloaddition, whereas the nonreactive conformer has a lifetime of 400 ps. Ab initio calculations were performed to locate the relevant conical intersection (CI) and calculate an excited-state [2+2] cycloaddition reaction path. The interpretation of the results is supported by experimental results on the similar but nonreactive pseudo-para-divinyl[2.2]paracyclophane, which has a lifetime of more than 500 ps in the S(1) state.

Mechanical gas capture and release in a network solid via multiple single-crystalline transformations.

Metal-organic frameworks have demonstrated functionality stemming from both robustness and pliancy and as such, offer promise for a broad range of new materials. The flexible aspect of some of these solids is intriguing for so-called 'smart' materials in that they could structurally respond to an external stimulus. Herein, we present an open-channel metal-organic framework that, on dehydration, shifts structure to form closed pores in the solid. This occurs through multiple single-crystal-to-single-crystal transformations such that snapshots of the mechanism of solid-state conversion can be obtained. Notably, the gas composing the atmosphere during dehydration becomes trapped in the closed pores. On rehydration, the pores open to release the trapped gas. Thus, this new material represents a thermally robust and porous material that is also capable of dynamically capturing and releasing gas in a controlled manner.

(13)C CP MAS NMR of halogenated (Cl, Br, I) pharmaceuticals at ultrahigh magnetic fields.

This work reports significantly improved spectral resolution of (13)C CP MAS NMR spectra of chlorinated, brominated and iodinated solid organic compounds when such spectra are recorded at ultrahigh magnetic field strengths. The cause of this is the residual dipolar coupling between carbon atoms and quadrupolar halogen nuclides (chlorine-35/37, bromine-79/81 or iodine-127), an effect inversely proportional to the magnetic field strength which declines in importance markedly at 21.1 T as compared to lower fields. In favorable cases, the fine structure observed can be used for spectral assignment, e.g. for Cl-substituted aromatics where the substituted carbon as well as the ortho-carbons show distinct doublets. The experimental results presented are supported by theoretical modeling and calculations. The improved spectral resolution in the studied systems and similar halogenated materials will be of particular interest and importance for polymorph identification, drug discovery and quality control in the pharmaceutical industry.

Probing local structure in zeolite frameworks: ultrahigh-field NMR measurements and accurate first-principles calculations of zeolite 29Si magnetic shielding tensors.

The principal components of zeolite 29Si magnetic shielding tensors have been accurately measured and calculated for the first time. The experiments were performed at an ultrahigh magnetic field of 21.1 T in order to observe the small anisotropies of the 29Si shielding interactions that arise for Si atoms in near-tetrahedral geometries. A robust two-dimensional (2D) chemical shift anisotropy (CSA) recoupling pulse sequence was employed that enables quasi-static powder patterns to be resolved according to the isotropic chemical shifts. For the zeolites Sigma-2 and ZSM-12, it is demonstrated that the 29Si chemical shift (CS) tensor components measured by the recoupling experiment are in excellent agreement with those determined from spinning sidebands in slow magic-angle spinning (MAS) experiments. For the zeolite ZSM-5, the principal components of the 29Si CS tensors of 15 of the 24 Si sites were measured using the 2D CSA recoupling experiment, a feat that would not be possible with a slow MAS experiment due to the complexity of the spectrum. A simple empirical relationship between the 29Si CS tensors and local structural parameters could not be established. However, the 29Si magnetic shielding tensors calculated using Hartree-Fock ab initio calculations on clusters derived from the crystal structures are in excellent agreement with the experimental results. The accuracy of the calculations is strongly dependent on the quality of the crystal structure used in the calculation, indicating that the 29Si magnetic shielding interaction is extremely sensitive to the local structure around each Si atom. It is anticipated that the measurement and calculation of 29Si shielding tensors could be incorporated into the "NMR crystallography" of zeolites and other related silicate materials, possibly being used for structure refinements that may lead to crystal structures with very accurate Si and O atomic coordinates.

Benzofuran-derived cyclic beta-amino acid scaffold for building a diverse set of flavonoid-like probes and the discovery of a cell motility inhibitor.

We report here a practical, enantioselective synthesis of benzofuran-derived, cyclic trans-beta-amino acid scaffold. In two cases, tricyclic derivatives having six- and eight-membered unsaturated lactams were obtained from this versatile scaffold. To explore the biological applications, these compounds were subjected to cell-based assays, using NIH3T3 mouse cells to examine their potency as cell motility inhibitors and identified 18 as a potent cell motility inhibitor (IC50 approximately 40 microM in chamber cell migration assay).

Reagent-based, modular, tandem Michael approach for obtaining different indoline alkaloid-inspired polycyclic architectures.

A modular, reagent-based approach to obtain different indoline alkaloid-inspired, tetracyclic architectures is developed. With the use of TBSOTf as a Lewis acid, we report here a tandem Michael-based approach that led to the synthesis of a diastereomeric mixture of tetracyclic derivatives with two additional six-membered rings. By simply changing the Lewis acid to TMSOTf, we were able to obtain a different tetracyclic compound having additional functionalized 5- and 7-membered rings with complete stereocontrol.

Vinigrol: a compact, diene-transmissive Diels-Alder strategy to the tricyclic core.

A short (six steps from 17), versatile route to the tricyclic core of Vinigrol is described. A Lewis acid catalyzed and self-assembled Diels-Alder (LACASA-DA) reaction from the homoallylic cross-conjugated trienol 15 with N-methylmaleimide afforded the monoadduct 17 with diastereo-, regio-, and chemoselective control. Oxidation and installation of an acetylene (Ohira's reagent) followed by further manipulations afforded trienone 24. The second intramolecular Diels-Alder (at 45 degrees C) reaction assembled the tricyclic skeleton 25 directly. The configuration of 25 was confirmed by X-ray analysis.

Oxidation, deformation, and destruction of carbon nanotubes in aqueous ceric sulfate.

A simple wet chemical method involving only ultrasonic processing in dilute ceric sulfate (CS) was used to functionalize carbon nanotubes (CNTs). Unexpectedly, single-walled and multiwalled carbon nanotubes (SWCNTs and MWCNTs) were cut, oxidized, and disintegrated by sonication in 0.1 N CS for 2-5 h. Transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction spectroscopy (XRD), Raman scattering, and photoacoustic Fourier transform infrared spectroscopy (FTIR) were used to probe wall damage during the chemical processing. Cyclic voltammetry and impedance spectroscopy were used to evaluate the conductivity of the CS-treated CNTs. This one-step process resulted in the destruction of SWCNTs to produce nonconducting amorphous carbon. MWCNTs were oxidized and converted to graphitic materials and amorphous carbon with retained conductivity.

Extending the chemistry of p-tert-butylcalix[4]arene with H-bonding and secondary coordination.

Through hydrogen bonding and secondary coordination, aliphatic amines, together with water and metals, form novel compounds with p-tert-butylcalix[4]arene containing complex clusters and layers.

The first example of a crystalline guest-free form of the tris(5-acetyl-3-thienyl)methane (TATM) host material.

The first example of a guest-free crystalline form of tris(5-acetyl-3-thienyl)methane (TATM), in both powder and single crystal form, was obtained by leaching water-soluble guests out of the guest-host inclusion compounds with acetone, ethanol and methanol.

Synthesis of a novel radical trapping and carbonyl group trapping anti-AGE agent: a pyridoxamine analogue for inhibiting advanced glycation (AGE) and lipoxidation (ALE) end products.

[structure: see text] Pyridoxamine is known to be an effective inhibitor of both advanced glycation (AGE) and advanced lipoxidation (ALE) end products. The synthesis of a novel multifunctional AGE and ALE inhibitor, 6-dimethylaminopyridoxamine (dmaPM, 11) is described. The 6-dimethylamino substituent increases the radical trapping ability of pyridoxamine's phenolic group. Results obtained during ribose glycations show that both the new dmaPM and a known strong radical trapping agent, 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox), prevent intermolecular protein cross-linking more effectively than pyridoxamine (PM).

Electrostatic and short-range interactions compete in directing the structure of p-tert-butylcalix[4]arene inclusion compounds of fluorinated benzenes.

p-tert-Butylcalix[4]arene guest-host compounds with fluorinated benzenes show several structural motifs, thus indicating that the guest-host structure can be tuned to produce either a form with included guests or a form where the host self-includes with the guests outside the cavity.

Pseudopolymorphism in the p-tert-butylcalix[4]arene--n-butylamine system: directing the structural motifs.

p-tert-Butylcalix[4]arene forms three compounds with n-butylamine, two weakly interacting guest-host compounds of 1:1 and 1:2 stoichiometry, as well as a hydrogen-bonded 3:1 compound containing some n-butylamine molecules not coordinated to the host.

Photoinduced alkyl group exchange of ethylzinc alkoxides: X-ray crystal structure of an iodomethylzinc methoxide.

Irradiation of a solution of ethyl zinc alkoxides and CH2I2 leads to clean formation of iodomethylzinc alkoxides; these intermediates are important species generated in stereoselective cyclopropanation processes; no alkyl group exchange is observed in the absence of irradiation; the solid-state structure of (MeO)8Zn7(CH2I)6 is also reported.

The complex relationship between guest-free polymorphic products and desolvation of p-tert-butylcalix[4]arene inclusion compounds.

Depending on the details of the technique employed, desolvation of tBC inclusion compounds leads to either a dense, guest-free polymorph, or a guest-free low density polymorph, the latter also having distinct high and low temperature forms.

Ferrocenoyl Amino Acids: A Synthetic and Structural Study.

A series of ester-protected amino acids were coupled to ferrocenecarboxylic acid (1) using the DCC/HOBt protocol to give ferrocenoyl N-amino acids (amino acid = Glu(OBz)(2) (2a), Gly(OEt) (2b), Pro(OBz) (2c), Cys(SBz)OMe (2d), Ala(OBz) (2e), Tyr(OBz) (2f), Phe(OBz) (2g)). All products were fully characterized. The intermediate hydroxybenzotriazole active ester FcCOOBt (3) was isolated and fully characterized. The solid state structures of 2a, 2d, and 3 were determined by single-crystal X-ray diffraction. 2a: monoclinic P2(1) with a = 11.8142(5) Å, b = 9.7560(5) Å, c = 22.9456(10) Å, beta = 90.246(5) degrees, V = 2644.7(2) Å(3), Z = 2, R = 0.046. 2d: orthorhombic P2(1)2(1)2(1) with a = 9.957(2) Å, b = 11.680(2) Å, c = 36.452(2) Å, V = 4239.5(13) Å(3), Z = 4, R = 0.065. The solid state structures of 2a and 2d show extensive C=O.H-N hydrogen bonding. 3: triclinic P&onemacr; with a = 7.0391(5) Å, b = 10.7922(7) Å, c = 11.1690(7) Å, alpha = 108.071(5) degrees, beta = 107.957(5) degrees, gamma = 103.896(5) degrees, V = 712.5(2) Å(3), Z = 2, R = 0.030. The long ester bond distance of 1.427(2) Å provides a rationale for its inherent reactivity toward primary and secondary amines.

Self-Assembly of Lamellar and Expanded Lamellar Coordination Networks.

Structural similarity with clay-type minerals is apparent in the self-assembled layered coordination network [{Ag(1)(MeCN)2 }∞ ][BF4 ]∞ , which is stabilized by a lamellar chelate effect (section of structure shown on the right). The interlayer region is expandable through the use of larger ligands such as PhCN. The void space is occupied by one PhCN molecule per repeat unit.