Catalytic asymmetric synthesis of allylic thiol derivatives

The palladium(II) complex [(Rp,S)-COP-Cl]2 and its enantiomer catalyze the rearrangement of linear prochiral O-allyl carbamothioates under mild conditions to provide branched S-allyl carbamothioates in high yield and high enantiomeric purity.     

Generation and cyclization of unsaturated carbamoyl radicals derived from S-4-pentynyl carbamothioates under tin-free conditions

The radical reaction of benzenethiol with S-4-pentynyl carbamothioates provides a valuable protocol for the tin-free generation of carbamoyl radicals, which arise from intramolecular substitution at sulfur by the initial sulfanylvinyl radicals. This procedure can be usefully employed to achieve N-benzylcarbamoyl radical 5-exo and 4-exo cyclizations leading, respectively, to pyrrolidinones and azetidinones, although, for the latter, it seems of lesser utility. Novel evidence is presented that N-tosyl-substituted carbamoyl radicals display a peculiar tendency to yield the corresponding isocyanate by beta-elimination of the tosyl radical.     

Synthesis of sulfur containing analogues of the soluble guanylate cyclase inhibitor 8-bromo-4H-[1,2,4]oxadiazolo[3,4-c][1,4]benzoxazin-1-one NS2028

Sulfur analogues of the soluble guanylate cyclase (sGC) inhibitor NS2028 1a are synthesized. Treating 8-bromo-2H-benzo[b][1,4]oxazin-3(4H)-one oxime (6) with 1,1′-thiocarbonyldiimidazole (1.1 equiv) gave the carbamothioate 8-bromo-4H-[1,2,4]oxadiazolo[3,4-c][1,4]benzoxazine-1-thione (3a) in 83% yield. Alternatively reacting NS2028 1a with P₂S₅ (0.5 equiv) affords the carbamothioate 3a in 80% yield. Similar treatment of 8-aryl substituted NS2028 analogues 1b-d with P₂S₅ gave the carbamothioates 3b-d in 64-91% yields. Although quite stable, the carbamothioates 3a-d could be thermally isomerized in the presence of Cu (10 mol %) to afford the thiocarbamates 4a-d in high yields. Interestingly, in the case of carbamothioate 3a Pd and In metals also facilitated the isomerization. Furthermore, treatment of the thiocarbamates 4a-d with P₂S₅ (0.5 equiv) affords the carbamodithioates 5a-d in 72-89% yields. All new compounds are fully characterized including single crystal X-ray data for carbamothioate 3a and thiocarbamate 4a. Finally, a mechanism is proposed for the carbamothioate to thiocarbamate isomerization.     

A multi-technique surface study of the mercury(II) chalcogenide ion-selective electrode in saline media

X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), rotating disc electrode-electrochemical impedance spectroscopy (RDE-EIS) and synchrotron radiation-grazing incidence X-ray diffraction (SR-GIXRD) have been used to study the response mechanism of the mercury(II) chalcogenide ion-selective electrode (ISE) in saline media. XPS and SIMS have shown that the chalcogenide surface is poisoned by silver chloride, or a mixture of silver halides, on continuous exposure to synthetic and real seawater. Significantly, the in-situ SR-GIXRD study demonstrated that electrode fouling in synthetic seawater is linked to the formation of poorly crystalline or amorphous silver chloride, and that the low level of free mercury(II) in a calibration buffer (i.e., 10(-14) M) is able to undergo metathesis with silver(II) sulfide in the membrane generating mercury(II) sulfide. Significantly, the results of this detailed surface study have shown that silver chloride fouling of the electrode is ameliorated in real seawater comprising natural organic ligands, and this has been attributed to the peptization of silver chloride by the surfactant-like nature of seawater ligands at pH 8. RDE-EIS aging studies have revealed that the chalcogenide membrane experiences a sluggish charge transfer reaction in seawater, and contrary to a previous report for a static electrode, the seawater matrix does not passivate the RDE. The results of this XPS, SIMS, RDE-EIS and SR-GIXRD study have elucidated the response mechanism of the mercury(II) ISE in saline media.     

Optimization of catalyst enantioselectivity and activity using achiral and meso ligands

The optimization of asymmetric catalysts for enantioselective synthesis has conventionally revolved around the synthesis and screening of enantiopure ligands. In contrast, we have optimized an asymmetric reaction by modification of a series of achiral ligands. Thus, employing (S)-3,3'-diphenyl BINOL [(S)-Ph(2)-BINOL] and a series of achiral diimine and diamine activators in the asymmetric addition of alkyl groups to benzaldehyde, we have observed enantiomeric excesses between 96% (R) and 75% (S) of 1-phenyl-1-propanol. Some of the ligands examined have low-energy chiral conformations that can contribute to the chiral environment of the catalyst. These include achiral diimine ligands with meso backbones that adopt chiral conformations, achiral diimine ligands with backbones that become axially chiral on coordination to metal centers, achiral diamine ligands that form stereocenters on coordination to metal centers, and achiral diamine ligands with pendant groups that have axially chiral conformations. Additionally, we have structurally characterized (Ph(2)-BINOLate)Zn(diimine) and (Ph(2)-BINOLate)Zn(diamine) complexes and studied their solution behavior.     

Orientational relaxation dynamics in aqueous ionic solution: polarization-selective two-dimensional infrared study of angular jump-exchange dynamics in aqueous 6M NaClO4

The dynamics of hydrogen bond (H-bond) formation and dissociation depend intimately on the dynamics of water rotation. We have used polarization resolved ultrafast two-dimensional infrared (2DIR) spectroscopy to investigate the rotational dynamics of deuterated hydroxyl groups (OD) in a solution of 6M NaClO(4) dissolved in isotopically mixed water. Aqueous 6M NaClO(4) has two peaks in the OD stretching region, one associated with hydroxyl groups that donate a H-bond to another water molecule (OD(W)) and one associated with hydroxyl groups that donate a H-bond to a perchlorate anion (OD(P)). Two-dimensional IR spectroscopy temporally resolves the equilibrium inter conversion of these spectrally distinct H-bond configurations, while polarization-selective 2DIR allows us to access the orientational motions associated with this chemical exchange. We have developed a general jump-exchange kinetic theory to model angular jumps associated with chemical exchange events. We use this to model polarization-selective 2DIR spectra and pump-probe anisotropy measurements. We determine the H-bond exchange induced jump angle to be 49 ± 5° and the H-bond exchange rate to be 6 ± 1 ps. Additionally, the separation of the 2DIR signal into contributions that have or have not undergone H-bond exchange allows us to directly determine the orientational dynamics of the OD(W) and the OD(P) configurations without contributions from the exchanged population. This proves to be important because the orientational relaxation dynamics of the populations that have undergone a H-bond exchange differ significantly from the populations that remain in one H-bond configuration. We have determined the slow orientational relaxation time constant to be 6.0 ± 1 ps for the OD(W) configuration and 8.3 ± 1 ps for the OD(P) configuration. We conclude from these measurements that the orientational dynamics of hydroxyl groups in distinct H-bond configurations do differ, but not significantly.     

Geometry of the Fluorides, Oxofluorides, Hydrides, and Methanides of Vanadium(V), Chromium(VI), and Molybdenum(VI): Understanding the Geometry of Non-VSEPR Molecules in Terms of Core Distortion

This paper describes a study of the topology of the electron density and its Laplacian for the molecules VF(5), VMe(5), VH(5), CrF(6), CrMe(6), CrOF(4), MoOF(4), CrO(2)F(2,) CrO(2)F(4)(2)(-) and CrOF(5)(-) all of which, except VF(5,) CrF(6), and CrOF(5)(-) have a non-VSEPR geometry. It is shown that in each case the interaction of the ligands with the metal atom core causes it to distort to a nonspherical shape. In particular, the Laplacian of the electron density reveals the formation of local concentrations of electron density in the outer shell of the core, which have a definite geometrical arrangement such as four in a tetrahedral arrangement or five in a square pyramidal or trigonal bipyramidal and six in an octahedral arrangement. Ligands that are predominately covalently bonded are found opposite regions of charge depletion between these core charge concentrations. In VH(5), VMe(5), CrOF(4), and MoOF(4), these core charge concentrations have a square pyramidal arrangement, and the regions of charge depletions have the corresponding inverse square pyramidal arrangement so that these molecules have a square pyramidal geometry rather than a trigonal prism geometry. In CrMe(6), there are five core charge concentrations with a trigonal bipyramidal arrangement so that the regions of charge depletion have a trigonal prismatic arrangement and the molecule has the corresponding trigonal prism geometry rather than an octahedral geometry. In contrast, molecules in which the only ligand is the more ionically bound fluorine are less affected by core distortion and have VSEPR-predicted structures. The unexpected bond angles in CrO(2)F(2) and the preference of CrO(2)F(4)(2)(-) for a cis structure are also discussed in terms of the pattern of core charge concentrations.     

Macromolecular chirality induction on optically inactive poly(4-carboxyphenyl isocyanide) with chiral amines: a dynamic conformational transition of poly(phenyl isocyanide) derivatives

Optically active polyisocyanides (poly(iminomethylenes)) have been prepared with much interest in developing new functional materials. Polyisocyanides have been considered to have a stable 4(1) helical conformation even in solution when they have a bulky side group. However, the conformational characteristics of poly(phenyl isocyanide) (PPI) derivatives are still under debate. We now report that an optically inactive PPI derivative, poly(4-carboxyphenyl isocyanide) (poly-1), shows optical activity in the polymer backbone induced by external, chiral stimuli through acid-base interactions under thermodynamic control and exhibits induced circular dichroism (ICD) in the UV-visible region in DMSO. The ICD intensities of the poly-1-chiral amine complexes in DMSO gradually increased with time, and, in one case, the value reached 3 times that of the original value after 2 months at 30 degrees C. The conformational changes also occurred very slowly for poly-1 alone and its ethyl ester with time on the basis of (1)H NMR spectroscopic analysis. These results indicate that PPIs bearing a less bulky substituent may not have a 4(1) helical conformation but have a different type of prochiral conformation, for instance, an s-trans (zigzag) structure which may transform to a dynamic, one-handed helical conformation when the PPIs have a functional group capable of interacting with chiral compounds. The mechanism of helicity induction on poly-1 through a dynamic conformational transition is discussed on the basis of the above results together with molecular dynamic simulation results for PPI.     

Thermodynamic properties of Ga27Si3 cluster using density functional molecular dynamics

Density functional molecular dynamical calculations have been carried out to explore the effect of silicon impurities on thermodynamic properties of Ga(30). We have obtained 500 distinct low energy equilibrium geometries of Ga(27)Si(3) in order to obtain reliable ground state geometry. The specific heat has been calculated using multiple histogram techniques and compared with that of Ga(30). We demonstrate that silicon impurities have a dramatic effect on the thermodynamic properties of the host cluster. In contrast to Ga(30), the specific heat of Ga(27)Si(3) shows a clear melting peak at ≈500 K, changing the character of Ga(30) from a nonmelter to a melter.     

Molecular Dynamics Insights for Screening the Ability of Polymers to Remove Pesticides from Water

The use of pesticides in agriculture is known to have environmental impacts, namely it leads to underground and spring water contamination. Thus, it turns out that nowadays general-endeavor towards the sustainability of farmer production requires novel strategies to capture pesticides from water and soils. We propose a methodology based on molecular dynamics simulations to identify polymers that are potentially featured to be applied for pesticide remediation in water and soils. We have employed cymoxanil (CYM), glufosinate ammonium (GLF), imidacloprid (IMI) and mancozeb (MAN) as pesticides, and have tested polymers with different characteristics as removing agents. Specifically, we have investigated oligomers of polypropylene (PP), poly(acrylic acid) protonated (PAAH) and deprotonated (PAA), and chitosan protonated (CTH) and deprotonated (CT). It has been found that all oligomers show a certain degree of selectivity concerning the interaction with the tested pesticides.     

NMR studies on how the binding complex of polyisoprenol recognition sequence peptides and polyisoprenols can modulate membrane structure

The glycosyl carrier lipids, dolichylphosphate (C(95)-P) and undecapreylphosphate (C(55)-P) are key molecular players in the synthesis and translocation of complex glycoconjugates across cell membranes. The molecular mechanism of how these processes occur remains a mystery. Failure to completely catalyze C(95)-P-mediated N-linked protein glycosylation is lethal, as are defects in the C(55)-P-mediated synthesis of bacterial cell surface polymers. Our recent NMR studies have sought to understand the role these "super-lipids" play in biosynthetic and translocation pathways, which are of critical importance to problems in human biology and molecular medicine. The PIs can alter membrane structure by inducing in the lamellar phospholipids (PL) bilayer a non-lamellar or hexagonal (Hex(II)) structure. Membrane proteins that bind PIs contain a transmembrane binding motif, designated a PI recognition sequence (PIRS). Herein we review our recent combination of (1)H- and (31)P NMR spectroscopy and energy minimized molecular modeling studies that have determined the preferred orientation of PIs in model phospholipids membranes. They also show that the addition of a PIRS peptide to nonlamellar membranes induced by the PIs can reverse the Hex(II) phase back to a lamellar structure. Our molecular modeling calculations have also shown that as many as five PIRS peptides can bind to a single PI molecule. These findings lead to the hypothesis that the PI-induced Hex(II) structure may have the potential of forming a membrane channel that could facilitate glycoconjugate translocation processes. This is an alternate hypothesis to the possible existence of hypothetical "flippases" to accomplish movement of hydrophilic sugar chains across hydrophobic membranes.     

Is the band gap of pristine TiO(2) narrowed by anion- and cation-doping of titanium dioxide in second-generation photocatalysts?

Second-generation TiO(2)-(x)D(x) photocatalysts doped with either anions (N, C, and S mostly) or cations have recently been shown to have their absorption edge red-shifted to lower energies (longer wavelengths), thus enhancing photonic efficiencies of photoassisted surface redox reactions. Some of the studies have proposed that this red-shift is caused by a narrowing of the band gap of pristine TiO(2) (e.g., anatase, E(bg) = 3.2 eV; absorption edge ca. 387 nm), while others have suggested the appearance of intragap localized states of the dopants. By contrast, a recent study by Kuznetsov and Serpone (J. Phys. Chem. B, in press) has proposed that the commonality in all these doped titanias rests with formation of oxygen vacancies and the advent of color centers (e.g., F, F(+), F(++), and Ti(3+)) that absorb the visible light radiation. This article reexamines the various claims and argues that the red-shift of the absorption edge is in fact due to formation of the color centers, and that while band gap narrowing is not an unknown occurrence in semiconductor physics it does necessitate heavy doping of the metal oxide semiconductor, thereby producing materials that may have completely different chemical compositions from that of TiO(2) with totally different band gap electronic structures.     

On the role of tetramethylammonium cation and effects of solvent dynamics on the stability of the cage-like silicates Si6O156- and Si8O208- in aqueous solution. A molecular dynamics study

We have undertaken explicit solvent molecular dynamics simulations to investigate the preferential stabilization of the silicate octamer Si(8)O(20)(8-) over the hexamer Si(6)O(15)(6-) in relation with the ability of tetramethylammonium (TMA) to form an adsorption layer around these cage-like polyions. We have found that the hexamer cannot support such a layer and as a result is vulnerable to hydrolysis. The dynamics of TMA desorption off the surface of the hexamer is investigated in connection with the solvent dynamics. We have studied the energetics of this preferential stabilization by calculating the relative change in the free energies of formation between the complexes Si(8)O(20)(8-).8TMA and Si(6)O(15)(6-).6TMA and found the former to be more stable by 70 kcal/mol. We also find that the energetics are consistent with experimental data, suggesting that the hexamer is a long-lived metastable species. Furthermore, we have studied the solvent structure and dynamics in the vicinity of both the bare polyions and their complexes with TMA. We have found that, as anticipated, both the octamer and the hexamer participate in hydrogen bonds with the water molecules, regardless of whether a TMA adsorption layer exists or not. In fact, we find that the presence of a TMA adsorption layer has a rather profound effect on the stability of these hydrogen bonds-it increases their lifetime by at least a factor of 2 relative to that of the hydrogen bonds between water and the bare polyions.     

Crystal structure and microstructure of some La(2/3-x)Li 3xTiO3 oxides: an example of the complementary use of electron diffraction and microscopy and synchrotron X-ray diffraction to study complex materials

Three representative oxides of the La(2/3)(-)(x)()Li(3)(x)()TiO(3) system have been studied by selected area electron diffraction (SAED), high-resolution transmission electron microscopy (HRTEM), and powder synchrotron X-ray diffraction. HRTEM showed that the materials have a complex microstructure. The SAED and HRTEM results have allowed us to propose a model to refine the crystal structure of these oxides that also accounts for their microstructure. The materials have a perovskite-related structure with a diagonal unit cell ( radical 2a(p) x radical 2a(p) x 2a(p)) as a consequence of the tilting of the TiO(6) octahedra. Ordering of lanthanum and lithium ions and vacancies along the 2a(p)-axis, as well as displacements of titanium ions from the center of the octahedra, have been determined. The size and shape of the domains have been obtained from the synchrotron X-ray diffraction data; in addition, other extended defects such as strains and compositional fluctuations have been detected.     

Oxygen photoevolution on a tantalum oxynitride photocatalyst under visible-light irradiation: how does water photooxidation proceed on a metal-oxynitride surface?

The mechanism of water photooxidation (oxygen photoevolution) on a TaON photocatalyst was studied on the basis of our previous studies on the mechanism of this reaction on TiO(2) and N-doped TiO(2). We have confirmed that photocatalytic O(2) evolution occurs from an aqueous TaON suspension in the presence of Fe(3+), as reported. In-situ MIR-IR experiments have indicated that the TaON surface is slightly oxidized under visible-light irradiation, indicating that the oxygen photoevolution on TaON actually occurs on a thin Ta-oxide overlayer. The in-situ MIR-IR experiments have also shown that a certain surface peroxo species, tentatively assigned to adsorbed HOOH, is formed as an intermediate of the O(2) photoevolution reaction. Studies on the effect of addition of reductants to the electrolyte on the IPCE have shown that photogenerated holes at the TaON surface cannot oxidize reductants such as SCN(-), Br(-), methanol, ethanol, 2-propanol, and acetic acid, though they can oxidize H(2)O into O(2). Detailed considerations of these results have strongly suggested that the water photooxidation reaction on TaON proceeds by our recently proposed new mechanism, that is, the reaction is initiated by a nucleophilic attack of a water molecule (Lewis base) on a surface-trapped hole (Lewis acid).     

Bifunctional chelators for copper radiopharmaceuticals: the synthesis of [Cu(ATSM)-amino acid] and [Cu(ATSM)-octreotide] conjugates

Two new bifunctional chelators that are derivatives of the bis(thiosemicarbazone) ATSMH(2) proligand have been prepared, one with two phenyl carboxylate substituents on the exocyclic nitrogens (L(1)H(2)) and one with a single phenyl carboxylate (L(2)H(2)). The new ligands have been characterised by NMR spectroscopy, mass spectrometry and in the case of L(1)H(2) by X-ray crystallography. The copper, nickel and zinc complexes of the new ligands have been synthesised and characterised. Electrochemical measurements show that the copper(II) complexes undergo a reversible reduction attributable to a Cu(II)/Cu(I) process. The new proligands have been tethered to the N-alpha-Boc-protected amino acids lysine and ornithine using solution and solid phase methods. The new amino acid conjugates form copper complexes and the complexes have been characterised by mass spectrometry and electronic spectroscopy. The bifunctional chelator L(2)H(2) has been conjugated to the tumour targeting peptide octreotide and the new ATSMH(2)-octreotide conjugate and its copper complex have been characterized by mass spectrometry. These new systems have the potential to be used for new targeted copper radiopharmaceuticals for imaging and therapy.     

Substituent electronic effects on the persistence and absorption spectra of (Z)-o-xylylenols. A nanosecond laser flash photolysis study

[reaction: see text] A systematic investigation on a broad set of aldehydes reveals that the lifetimes of (Z)-photoenols can be modulated by variation of the substituents. We have found that the lifetimes of (Z)-enols (in benzene) can be varied by more than 1 order of magnitude with a judicious choice of the substituents that exert mesomeric and inductive effects as, for example, in the case of pentamethylbenzaldehyde (tau = 35 ns) and dicyanomesitaldehyde (tau = 760 ns). This study thus points to the fact that the electronic factors in conjunction with hydrogen bonding stabilization can considerably broaden the uni- as well as bimolecular chemistry based on photoenolization. Further, we have shown that the photoenols exhibit dramatic shifts in their absorption properties with variation of the substituents; although the photoenols have long been considered to be colored, their absorption properties have not been heretofore comprehensively examined.     

Novel binding of beryllium to dicarboxyimidazole-based model compounds and polymers

The ligand 4,5-dicarboxyimidazole (H(2)DCI) and its methyl derivative 1-methyl-4,5-dicarboxyimidazole (H(2)MDCI) have been shown to bind to Be(II) forming a zwitterionic species that has been structurally characterized. A new dicarboxyimidazole-based polymer has been prepared and its Be-binding properties have been studied using NMR ((1)H and (9)Be) and fluorescence spectroscopy; it represents a rare example of beryllium binding to a polymer. Models of the mononuclear and polymeric Be(II)-binding sites have been studied using density functional theory (DFT), and the (9)Be NMR chemical shifts of these model materials have been calculated for the purpose of direct comparison to experimentally observed values. Differences in the binding modes of the mononuclear and polymeric species are discussed.