Arylthallium trifluoroacetates carbon-13 and fluorine-19 nuclear magnetic resonance: The substituent effect of the bis(trifluoroacetato)- thallium(III) group

¹³C NMR data for a series of arylthallium trifluoroacetates (ArTlX₂, X = OCOCF₃) are reported and assigned. The range of carbon—thallium couplings to be expected, the dependence on the disposition of coupled nuclei, and chemical shift effects are discussed. The Tl(OCOCF₃)₂ group is shown to be a powerful electron withdrawing group, from both the ¹³C data and ¹⁹F substituent chemical shifts of the p-fluorophenyl derivative.     

Oxymetallation: XIX. Preparation of the first oxymercurials derived from cyclopropane

Three oxymercurials, RO(CH₂)₃HgBr where R = Bu^tO, BrHg(CH₂)₃, and Ac, have been isolated from the reaction of cyclopropane with mercury(II) acetate, t-butyl hydroperoxide, and 60% aqueous perchloric acid (20 mol%) in dichloromethane for 5 d, followed by anion exchange with aqueous potassium bromide.     

The triplet-state lifetime of indole in aqueous and viscous environments: significance to the interpretation of room temperature phosphorescence in proteins

The interpretation of room temperature phosphorescence studies of proteins requires an understanding of the mechanisms governing the tryptophan triplet-state lifetimes of residues fully exposed to solvent and those deeply buried in the hydrophobic core of proteins. Since solvents exposed tryptophans are expected to behave similarly to indole free in solution, it is important to have an accurate measure of the triplet state lifetime of indole in aqueous solution. Using photon counting techniques and low optical fluence (J/cm(2)), we observed the triplet-state lifetime of aqueous, deoxygenated indole and several indole derivatives to be approximately 40 micros, closely matching the previous reports by Bent and Hayon based on flash photolysis (12 micros; Bent, D. V.; Hayon, E. J. Am. Chem. Soc. 1975, 97, 2612-2619) but much shorter than the 1.2 ms lifetime observed more recently (Strambini, G. B.; Gonnelli, M. J. Am. Chem. Soc. 1995, 117, 7646-7651). However, we have now been able to reproduce the long lifetime reported by the latter workers for aqueous indole solutions and show that it likely arises from geminate recombination of the indole radical cation and solvated electron, a conclusion based on studies of the indole radical cation in water (Bent and Hayon, 1975). The evidence for this comes from a fast rise in the phosphorescence emission and measurements of a corresponding enhanced quantum yield in unbuffered solutions. This species can be readily quenched, and the corresponding fast rise disappears, leaving a monoexponential 40 micros decay, which we argue is the true indole triplet lifetime. The work is put in the context of room temperature phosphorescence studies of proteins.     

Lens alpha-crystallin and hypericin: a photophysical mechanism explains observed lens

Determining whether alpha-crystallin (the major lens protein) affects the photophysics of hypericin, a photosensitizing agent found in various plants, such as St. John's Wort, is important. Hypericin shows promise in cancer and human immunodeficiency virus therapy but may harm individuals taking St. John's Wort extracts (for mild to moderate depression). Hypericin causes hypericism, which is characterized by cellular damage in light-exposed areas. Ocular tissues are at risk for photosensitized damage; thus, we investigated the effects on hypericin photophysics by alpha-crystallin. We measured the transient absorption spectra and the 1270 nm luminescence of singlet (1Deltag) oxygen produced from hypericin in the presence of alpha-crystallin. alpha-Crystallin complexes hypericin, extending the lifetime of its triplet excited state; the Stern-Volmer slope is negative, but not linear, after a saturation curve. Damage to the lens protein by hypericin is known to occur via singlet oxygen, which oxidizes methionine, tryptophan and histidine residues. Binding to alpha-crystallin does not inhibit singlet oxygen formation by hypericin. alpha-Crystallin reacts with singlet oxygen with a rate constant of 1.3 x 10(8) M(-1) s(-1). Thus, we anticipate that hypericin will be an effective photosensitizer in the lens.     

The Electronic Structure of Vinyl Ethers and Sulfides with Interrupted Conjugation Examined by Photoelectron Spectroscopy

Photoelectron spectroscopy has been used to examine the consequences of steric inhibition of conjugation in a distorted vinyl sulfide and vinyl ether. Based upon the degree of interaction a deviation from coplanarity of 70°–75° is calculated for 9-oxabicyclo[3.3.1]non-1-ene ( 3 ) and approximately 75°–80° for the sulfur analog 6 .     

Palladium salicylaldimine complexes containing boronate esters

Reactions of salicylaldehydes with boronate ester derivatives of aniline have been examined. Addition of these Schiff base ligands to palladium acetate or Na₂PdCl₄ afforded novel boron-containing trans-bis(N-arylsalicylaldiminato) palladium complexes.Condensation of salicylaldehyde (2-HOC₆H₄C(O)H) with H₂NC₆H₄Bpin (pin=1,2-O₂C₂Me₄) afforded the boron-containing Schiff bases, 2-HOC₆H₄C(H)=NC₆H₄Bpin (1–3a). Similar reactivity with 2-hydroxy-5-nitrobenzaldehyde and 2-hydroxy-1-naphthaldehyde gave the corresponding Schiff bases (1-3b) and (1-3c), respectively. Reaction of Schiff bases (2) and (3) with palladium acetate or Na₂PdCl₄ afforded complexes of the type PdL₂ (4,5), where L=deprotonated Schiff base. The molecular structure of the nitro-salicylaldehyde 4-Bpin palladium complex (5b) was characterized by an X-ray diffraction study. All new palladium compounds have been characterized fully and tested for their antifungal activity against Aspergillus niger and Aspergillus flavus.     

Dimethylzinc-mediated additions of alkenylzirconocenes to aldimines. New methodologies for allylic amine and C-cyclopropylalkylamine syntheses

Hydrozirconation of alkynes with zirconocene hydrochloride followed by in situ transmetalation to dimethylzinc provides access to reactive alkenyl organometallic reagents from readily available precursors. Upon addition of imines, 1,2-attack leads to synthetically useful allylic amine building blocks. In the presence of CH(2)I(2) or CH(2)Cl(2), the N-metalated allylic amide intermediate is cyclopropanated and C-cyclopropylalkylamines are formed in high yield and excellent diastereoselectivities favoring the anti products. The use of enynes as starting materials for this domino reaction provides conjugated biscyclopropanes and thus allows the stereoselective formation of five new carbon-carbon bonds. A transition state that explains the need for both zirconocene complex and alkyl zinc in the cyclopropanation reaction is proposed.     

Substituent effects in the interconversion of phenylcarbene, bicyclo[4.1.0]hepta-2,4,6-triene, and 1,2,4,6-cycloheptatetraene

The effect of aryl substituents on the interconversion of phenylcarbene (PC), bicyclo[4.1.0]hepta-2,4,6-triene (BCT), and 1,2,4,6-cycloheptatetraene (CHTE) has been studied by density functional theory. It is found that substituents have a large effect on both the thermochemistry and activation energy of these rearrangements. For instance, para-substitution yields a range of overall activation energies for the formation of BCT from PC of 20.3 to 11.7 kcal/mol for the NH(2) and NO(2) substituents, respectively. In the syn-meta-substituted cases, all of the rearrangements to the substituted CHTE species are more exothermic than that of the parent PC. The proximity of the substituent to the carbene center can also affect the overall chemistry as in the case of ortho-substituted species. Here, formation of bicyclic structures and ylides, which can then rearrange to stable structures, can compete with the ring-expansion process. Also, as calculated herein, the ortho substituents can, by a combination of mesomeric and steric interactions with the carbene center, affect the overall barrier to reversible ring expansion. Most notably, in the anti-ortho-substituted species, halogens (F and Cl) raise the activation barrier to ring expansion by approximately 5 kcal/mol. This is reminiscent of the effect of fluorine substitution on the chemistry (inter- and intramolecular) of phenylnitrene.     

N‐(4‐Nitrobenzoyl)‐N′‐phenylhydrazine: a three‐dimensional hydrogen‐bonded framework

In the title compound (systematic name: N‐anilino‐4‐nitrobenzamide), C₁₃H₁₁N₃O₃, the molecules are linked into a complex three‐dimensional framework structure by a combination of two‐centre N—H...O and C—H...O hydrogen bonds and a three‐centre N—H...(O,N) hydrogen bond.