Two-photon fluorescent probe for hydrogen sulfide based on a red-emitting benzocoumarin dye

Hydrogen sulfide (H₂S) is an endogenous gasotransmitter and plays intriguing biological roles. To study the biological role of H₂S, efficient fluorescent probes are in great demand. For imaging of H₂S in deep-tissue, a two-photon probe that emits in the red wavelength region is of choice to avoid the autofluorescence from intrinsic biomolecules. Here, we disclose such a probe, which, developed based on an acetyl benzocoumarin fluorophore, can be excited at 900?nm under two-photon excitation and emit in the red region. The probe shows high reactivity, selectivity, and sensitivity in in vitro assays. Two-photon microscopic imaging of H₂S in HeLa cells aided by the probe demonstrates that it is potentially useful to study H₂S level changes in cells and tissues influenced by external stimuli.     

Rhenium chemistry of azooximes: oxygen atom transfer, azoimine chelation, and imine-oxime contrast

The concerned azooximes (L1OH, 1) are of type p-X-C6H4C(N2Ph)(NOH) (X = H, Me, Cl). The reaction of [Re(MeCN)Cl3(PPh3)2] with [Ag(L1OH)(L1O)] in cold dichloromethane-acetonitrile solvent has furnished the green colored ionized azoimine complex [ReV(O)Cl(PPh3)2(L1)](PF6), 2. In effect L1O- has undergone oxidative addition, the oxygen atom being transferred to the metal site. Upon treatment of [ReV(NPh)Cl3(PPh3)2] with L1OH in solution, the neutral azoimine complex [ReV(NPh)Cl3(L1H)], 3, resulted due to the spontaneous transfer of the oxime oxygen atom to a PPh3 ligand, which is eliminated as OPPh3. In contrast, the oxime of 2-acetylpyridine (L2OH, 4) did not undergo oxygen atom transfer and simply afforded the imine-oxime complex [ReV(NC6H4Y)Cl2(PPh3)(L2O)], 5, upon reacting with [ReV(NC6H4Y)Cl3(PPh3)2] (Y = H, Me, Cl). The spectral and electrochemical properties of 2, 3, and 5 and the structures of three representative compounds are reported. In the cation of 2 (X = H) the two PPh3 ligands lie trans to each other and the equatorial plane is defined by the five-membered azoimine chelate ring and the oxo and chloro ligands. The oxo ligand which forms a model triple bond (Re-O length 1.616(6) A) lies cis to the imine-N atom. In 3 (X = Cl) the ReCl3 fragment has meridional geometry and the imido nitrogen lies trans to the imine nitrogen of the planar azoimine chelate ring. In 5 x H2O (Y = Me), the Cl, oximato-N, and P atoms define an equatorial plane and the pyridine-N lies trans to the imido-N. The water of crystallization is hydrogen bonded to the oximato oxygen atom (O...O, 2.829(5) A). Reaction models in which chelation of the azooxime precedes oxygen atom transfer are proposed on the basis of oxophilicity of trivalent rhenium, Lewis acid activity of pentavalent rhenium, electron withdrawal by the azo group, and observed relative disposition of ligands in products.     

Synthesis and structure of the 2-(chlorophenylazo)pyridine chelated tricarbonylrhenium(I) complex and its anion radical derivatives <Emphasis Type="Italic">via</Emphasis> electrochemical reduction

Reacting Re(CO)₅Cl with the azopyridine ligand (1) (L) in boiling benzene afford the complex Re(CO)₃Cl(L), (2) in excellent yield [L=2-(p-Cl-C₆H₄NN)C₅H₄N]. The chelation of the azopyridine ligand accompanied by displacement of the two carbon monoxide ligands furnish a five-membered chelate ring. Structure determination of complex (2) has revealed a distorted octahedral ReC₃N₂Cl coordination sphere. The Re–N(pyridine) and, Re–N(azo) distances are 2.158(3) and 2.153(6) Å respectively, and the N–N length [1.273(4) Å], implicate relatively weak Re-azo(π*) back–bonding. The Re(CO)₃Cl(L) lattice consists of C–H...Cl hydrogen bonding and Cl...O non-bonded interactions constituting a supramolecular network. Extended Hückel calculations reveal that the LUMO of Re(CO)₃Cl(L) is Ca. 57% azo in character. One-electron quasireversible electrochemical reduction of the complex occurs near −0.3 V versus Saturated Calomel electrode(s.c.e.) The redox orbital is believed to belong to the above noted LUMO. Electrogenerated Re(CO)₃Cl(L^•–) underwent spontaneous solvolytic chloride displacement in MeCN, resulting in the isolation of Re(CO)₃(MeCN)(L^•–). The latter complex in turn reacted with imidazole and triphenylphosphine, furnishing Re(CO)₃(C₃H₄N₂)(L^•–) and Re(CO)₃(PPh₃)(L^•–), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)₃Cl(L) but with shifts to lower frequencies by 10–20 cm⁻¹. All three radical anion systems are one-electron paramagnetic (1.7–1.8 μ_B). The unpaired electron is primarily localized on the azoheterocycle ligand in a predominantly azo-π* orbital, but a small metal contribution (^{185, 187}Re, I=5/2) is also present. Thus Re(CO)₃(MeCN)(L^•–) and Re(CO)₃(C₃H₄N₂)(L^•–) display six-line e.p.r. spectra (A ˜ 28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)₃(PPh₃)(L^•–) seven nearly equispaced lines are observed due to virtually equal coupling between the metal and ³¹P (I=&frac;) nuclei. The g-values of the radical species are slightly higher than the free-electron value of 2.0023.     

Complexes of cobalt(II), nickel(II), copper(II), cadmium(II), and mercury(II) with tetradentate Schiff base ligands

Summary A series of metal complexes with three new tetradentate Schiff bases derived from benzoin and benzil withc-toluidine and benzil with diaminoethane have been prepared and characterised by physical and chemical methods. The modes of bonding of the ligands with the metal ions have been proposed. Electronic spectra and room temperature magnetic moment values suggest octahedral geometry for the Co^II and Ni^II complexes, whereas the Hg^II and Cd^II complexes have tetrahedral geometry. The Cu^II complexes are square planar. Apart from the complexes of the Schiff bases derived from benzoin, all the other complexes have high molar conductance values suggesting them to be electrolytes. The complexes have been screened against some fungal pathogens.     

Computer-aided design of chiral ligands. Part 2. Functionality mapping as a method to identify stereocontrol elements for asymmetric reactions

A computational method to determine the energetically favorable positions of functional groups with respect to the transition states of stereoselective reactions based on force field energy minimization is presented. The parameters of this functionality mapping, the characteristics of the target transition states, and the features of the probe structures are outlined. Our method was found to reproduce the positions of the stereodiscriminating fragments for some known chiral ligands including the Masamune dimethylborolane, dimenthylborane, the Corey stien reagent, the Roush allylboronate tartrates, and the secondary amine Diels-Alder catalysts described by MacMillan. Functionality mapping can be used to better understand the specific interactions in the transition states leading to the products by providing a quantitative measure of the stabilization/destabilization afforded by the different ligand components via nonbonded interactions. The method can determine if a chiral ligand imparts the observed selectivity by stabilizing one reaction pathway, by destabilizing a reaction pathway, or by a combination of both. Orientational as well as positional information about potential functional groups is readily obtained. In addition to its utility as an analytical tool, functionality mapping can be used to explore starting points for the design of new chiral ligands.     

In situ 1,3-dipolar azide cycloaddition reaction: synthesis of functionalized d-glucose based chiral piperidine and oxazepine analogues

Functionalized furanose-fused piperidines 4-6 and oxazepines 15-17, useful precursors for structurally unique bioactive nucleosides as well as for potential glycosidase inhibitors, have been synthesized by the application of 1,3-dipolar azide cycloaddition (DAC) reaction on d-glucose based substrates. The strategy works well even with the nucleoside analogue 8, affording the bicyclic nucleoside analogues 11 and 12.     

Quantum mechanical models correlating structure with selectivity: predicting the enantioselectivity of beta-amino alcohol catalysts in aldehyde alkylation

Quantitative structure selectivity relationship (QSSR) models are described that provide consistently reliable predictions for the asymmetric addition of Et2Zn to PhCHO catalyzed by beta-amino alcohols. Statistically valid two-variable linear regression models that correlate the structures of the chiral catalysts with their enantioselectivities are obtained from three-dimensional physical property grids. The strength of the present method is that statistical models obtained from a small set of experimentally determined selectivities and relatively simple theoretical calculations yield selectivity predictions that are as accurate as those derived from higher-level calculations of transition-structure energies. Only minutes of computing time are required. Simple models are obtained which permit straightforward physical interpretation and generate realistic predictions.     

Structurally characterized acylruthenium organometallics bearing a pendant aldehyde function

By reacting [Ru(RL¹)(PPh₃)₂(CO)Cl], (1) (R = C₆H₄Me, Et) with an excess of CNBu^t in the presence of NH₄PF₆, organometallics of the type [Ru(RL²)(PPh₃)₂(CNBu^t)₂]PF₆, (2) have been isolated in excellent yield [RL¹ = C₆H₂O-2-CHNHR(p)-3-Me-5, RL² = C₆H₂(CO)-O-2-CHNHR(p)-3-Me-5]. These organometallics, on controlled hydrolysis, produce [Ru(L³)(PPh₃)₂(CNBu^t)₂], (3) in very good yield (L³ = C₆H₂(CO)-O-CHO-3-Me-5). In both ([2(C₆H₄Me)] · 2H₂O) and ([3] · 2CH₂Cl₂) the two phosphine ligands lie in trans positions. In ([2(C₆H₄Me)] · 2H₂O) the Ru(C₆H₄MeL²) fragment, excluding the pendant tolyl ring, is a near perfect plane (mean dev ~ 0.02Å) which makes a dihedral angle of 5.2° with the tolyl plane. The acyl chelate ring in ([2(C₆H₄Me)] · 2H₂O) is excellently planar with a mean deviation of 0.006°. In ([3] · 2CH₂Cl₂) the Ru(L³) fragment defines a crystallographic plane of symmetry, the coordinates of the atom being of the type x, 1/4z. The complex ([2C₆H₄Me)] · 2H₂O) displays N–HȮFO (iminium-phenolato) hydrogen bonding while in ([3] · 2CH₂Cl₂) C–H...O hydrogen bonding is present. Characteristic spectral data (u.v.–vis, i.r. and ¹H n.m.r.) of the complexes are reported. A notable feature is that an allowed band near 500 nm due to the t_2g → π^*(azomethine) charge transfer transition, which is diagnostic of the coordinated iminium-phenolato function, is present in (3) but this band is absent in the aldehydic acyl complex (2). In the ¹H n.m.r. spectrum the N⁺–H signal in (2) (near 13.5 ppm) is split into a doublet due to transcoupling with the azomethine proton. The aldehydic proton of (3) resonates as a sharp singlet near 10 ppm. In CH₂Cl₂ solution (2) and (3) display quasireversible a Ru^III/Ru^IIcyclic voltammetric response with E_1/2 near 0.9 and 0.5 V versuss.c.e. The conversion (2) → (3) is accompanied by the nucleophilic attack of water. The complex (3) is also obtained directly from (1) by reaction with CNBu^t in the presence of H₂O. The aldehyde function in (3) is deactivated by the existing acyl moiety; as a result further decarbonylation does not take place.