Evaluation of helicene-derived 2,2'-bipyridine monoxide catalyst for the enantioselective propargylation of -acylhydrazones with allenyltrichlorosilane.

Helicene-derived 2,2'-bipyridine -monoxide was evaluated as a Lewis base catalyst for the enantioselective propargylation of -acylhydrazones with allenyltrichlorosilane. The helicene-derived catalyst provided moderate-to-good reactivity and enantioselectivity for a range of acylhydrazones. This study represents the first example of the catalytic asymmetric propargylation of non-activated acylhydrazones.

Binding of Nitriles and Isonitriles to V(III) and Mo(III) Complexes: Ligand vs Metal Controlled Mechanism.

The synthesis and structures of nitrile complexes of V(N[Bu]Ar), (Ar = 3,5-MeCH), are described. Thermochemical and kinetic data for their formation were determined by variable temperature Fourier transform infrared (FTIR), calorimetry, and stopped-flow techniques. The extent of back-bonding from metal to coordinated nitrile indicates that electron donation from the metal to the nitrile plays a less prominent role for than for the related complex Mo(N[Bu]Ar), .

Design and synthesis of 3,3'-triazolyl biisoquinoline ,-dioxides via Hiyama cross-coupling of 4-trimethylsilyl-1,2,3-triazoles.

A new strategy to effectively lock the conformation of substituents at the 3,3'-positions of axial-chiral biisoquinoline ,-dioxides was developed based on the strong dipole-dipole interaction between 1,2,3-triazole and pyridine -oxide rings. The crystal structure and the DFT calculations of 3,3'-bis(1-benzyl-1-1,2,3-triazole-4-yl)-1,1'-biisoquinoline ,-dioxide () provided strong support for this strategy. Furthermore, we successfully demonstrated that readily available 4-trimethylsilyl-1,2,3-triazoles are viable nucleophiles for Hiyama cross-coupling.

Asymmetric Catalytic Ketimine Mannich Reactions and Related Transformations.

The catalytic enantioselective ketimine Mannich and its related reactions provide direct access to chiral building blocks bearing an α-tertiary amine stereogenic center, a ubiquitous structural motif in nature. Although ketimines are often viewed as challenging electrophiles, various approaches/strategies to circumvent or overcome the adverse properties of ketimines have been developed for these transformations. This review showcases the selected examples that highlight the benefits and utilities of various ketimines and remaining challenges associated with them in the context of Mannich, allylation, and aza-Morita-Baylis-Hillman reactions as well as their variants.

Mechanistic Pathways for NO Elimination from -RSn-O-N═N-O-SnR and for Reversible Binding of CO to RSn-O-SnR (R = Ph, Cy).

The rate and mechanism of the elimination of NO from -RSn-O-N═N-O-SnR (R = Ph () and R = Cy ()) to form RSn-O-SnR (R = Ph () and R = Cy ()) have been studied using both NMR and IR techniques to monitor the reactions in the temperature range of 39-79 °C in CD. Activation parameters for this reaction are Δ = 15.8 ± 2.

Production of -NaNO and NaNO by Ball Milling NaO and NO in Alkali Metal Halide Salts.

Ball milling of sodium oxides and alkali metal halide salts under a pressure of 2 atm nitrous oxide at temperatures of 38 ± 4 °C is reported. After 2.5 h of ball milling, FTIR data for both NO and NO additions show conclusively that -NaNO is formed based on excellent agreement with data reported earlier by Jansen and Feldmann who prepared pure crystalline -NaNO by reaction of sodium oxide and nitrous oxide for 2 h at 360 °C in a tube furnace.

Visualization and quantitation of electronic communication pathways in a series of redox-active pillar[6]arene-based macrocycles.

While oxidized pillar[5]arenes with 1-5 benzoquinone units are known, very few examples of oxidized pillar[6]arenes have been reported. We describe here the synthesis, characterization and electrochemical behavior of a series of macrocyclic hosts prepared by the stepwise oxidation of 1,4-diethoxypillar[6]arene, resulting in high-yield and high-purity isolation of two constitutional isomers for each macrocycle, in which two, three or four 1,4-diethoxybenzene units are replaced by benzoquinone residues. A careful structural comparison with their counterparts in the pillar[5]arene framework indicates that the geometries of the macrocycles are better described as non-Euclidean hyperbolic hexagons and elliptic pentagons, respectively.

High-Throughput Single-Molecule Spectroscopy Resolves the Conformational Isomers of BODIPY Chromophores.

A borondipyrromethene (BODIPY) chromophore is connected to a benzoxazole, benzothiazole, or nitrobenzothiazole heterocycle through an olefinic bridge with configuration. Rotation about the two [C-C] bonds flanking the olefinic bridge occurs with fast kinetics in solution, leading to the equilibration of four conformational isomers for each compound. Ensemble spectroscopic measurements in solutions fail to distinguish the coexisting isomers.

Photopotentiation of the GABA receptor with caged diazepam.

As the inhibitory γ-aminobutyric acid-ergic (GABAergic) transmission has a pivotal role in the central nervous system (CNS) and defective forms of its synapses are associated with serious neurological disorders, numerous versions of caged GABA and, more recently, photoswitchable ligands have been developed to investigate such transmission. While the complementary nature of these probes is evident, the mechanisms by which the GABA receptors can be photocontrolled have not been fully exploited. In fact, the ultimate need for specificity is critical for the proper synaptic exploration.

A Photoactivatable Far-Red/Near-Infrared BODIPY To Monitor Cellular Dynamics in Vivo.

A mechanism to photoactivate far-red/near-infrared fluorescence with infinite contrast and under mild visible illumination was designed around the photophysical properties of borondipyrromethene (BODIPY) dyes and the photochemical behavior of oxazine heterocycles. Specifically, the photoinduced and irreversible cleavage of an oxazine ring with a laser line at 405 nm extends the electronic conjugation of a BODIPY chromophore over a 3 H-indole auxochrome with a 2-(4-methoxyphenyl)ethenyl substituent in position 5. This structural transformation shifts bathochromically the main absorption band of the BODIPY component to allow the selective excitation of the photochemical product with a laser line of 633 nm and produce fluorescence between 600 and 850 nm.

Ligand-Directed Reactivity in Dioxygen and Water Binding to cis-[Pd(NHC)(η-O)].

Reaction of [Pd(IPr)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) and O leads to the surprising discovery that at low temperature the initial reaction product is a highly labile peroxide complex cis-[Pd(IPr)(η-O)]. At temperatures ≳ -40 °C, cis-[Pd(IPr)(η-O)] adds a second O to form trans-[Pd(IPr)(η-O)]. Squid magnetometry and EPR studies yield data that are consistent with a singlet diradical ground state with a thermally accessible triplet state for this unique bis-superoxide complex.

Structural implications on the excitation dynamics of fluorescent 3H-indolium cations.

Fluorescent 3H-indolium cations are valuable components for the realization of activatable fluorophores for bioimaging applications. Their relatively poor fluorescent quantum yields in organic solvents, however, appear to be in contradiction to their good performance in analytical methods based on single-molecule detection. The elucidation of the structural factors governing the excitation dynamics of these compounds is, therefore, essential to rationalize these effects and possibly guide the future design of activatable probes with improved performance.

Thermodynamic, Kinetic, Structural, and Computational Studies of the PhSn-H, PhSn-SnPh, and PhSn-Cr(CO)CMe Bond Dissociation Enthalpies.

The kinetics of the reaction of PhSnH with excess •Cr(CO)CMe = •Cr, producing HCr and PhSn-Cr, was studied in toluene solution under 2-3 atm CO pressure in the temperature range of 17-43.5 °C. It was found to obey the rate equation d[PhSn-Cr]/dt = k[PhSnH][•Cr] and exhibit a normal kinetic isotope effect (k/k = 1.

Tuning the Activation Wavelength of Photochromic Oxazines.

The activation wavelength of a photochromic oxazine can be shifted bathochromically with the introduction of a methoxy substituent on the chromophore responsible for initiating the photochemical transformation. This structural modification permits switching under mild illumination conditions, enhances the photoisomerization quantum yield and ensures outstanding fatigue resistance. Thus, these results can guide the design of new members of this family of photoresponsive molecular switches with improved photochemical and photophysical properties.

Reversible Inter- and Intramolecular Carbon-Hydrogen Activation, Hydrogen Addition, and Catalysis by the Unsaturated Complex Pt(IPr)(SnBu(t)3)(H).

The complex Pt(IPr)(SnBu(t)3)(H) (1) was obtained from the reaction of Pt(COD)2 with Bu(t)3SnH and IPr [IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]. Complex 1 undergoes exchange reactions with deuterated solvents (C6D6, toluene-d8, and CD2Cl2), where the hydride ligand and the methyl hydrogen atoms on the isopropyl group of the IPr ligand have been replaced by deuterium atoms. Complex 1 reacts with H2 gas reversibly at room temperature to yield the complex Pt(IPr)(SnBu(t)3)(H)3 (2).

Synthesis of [Pt(SnBu(t)3)(IBu(t))(μ-H)]2, a Coordinatively Unsaturated Dinuclear Compound which Fragments upon Addition of Small Molecules to Form Mononuclear Pt-Sn Complexes.

The reaction of Pt(COD)2 with one equivalent of tri-tert-butylstannane, Bu(t)3SnH, at room temperature yields Pt(SnBu(t)3)(COD)(H)(3) in quantitative yield. In the presence of excess Bu(t)3SnH, the reaction goes further, yielding the dinuclear bridging stannylene complex [Pt(SnBu(t)3)(μ-SnBu(t)2)(H)2]2 (4). The dinuclear complex 4 reacts rapidly and reversibly with CO to furnish [Pt(SnBu(t)3)(μ-SnBu(t)2)(CO)(H)2]2 (5).

Supramolecular nanoreactors for intracellular singlet-oxygen sensitization.

An amphiphilic polymer with multiple decyl and oligo(ethylene glycol) chains attached to a common poly(methacrylate) backbone assembles into nanoscaled particles in aqueous environments. Hydrophobic anthracene and borondipyrromethene (BODIPY) chromophores can be co-encapsulated within the self-assembling nanoparticles and transported across hydrophilic media. The reversible character of the noncovalent bonds, holding the supramolecular containers together, permits the exchange of their components with fast kinetics in aqueous solution.

Photoactivatable BODIPYs designed to monitor the dynamics of supramolecular nanocarriers.

Self-assembling nanoparticles of amphiphilic polymers can transport hydrophobic molecules across hydrophilic media and, as a result, can be valuable delivery vehicles for a diversity of biomedical applications. Strategies to monitor their dynamics noninvasively and in real time are, therefore, essential to investigate their translocation within soft matrices and, possibly, rationalize the mechanisms responsible for their diffusion in biological media. In this context, we designed molecular guests with photoactivatable fluorescence for these supramolecular hosts and demonstrated that the activation of the fluorescent cargo, under optical control, permits the tracking of the nanocarrier translocation across hydrogel matrices with the sequential acquisition of fluorescence images.

Pendant alkyl and aryl groups on tin control complex geometry and reactivity with H2/D2 in Pt(SnR3)2(CNBu(t))2 (R = Bu(t), Pr(i), Ph, mesityl).

The complex Pt(SnBu(t)3)2(CNBu(t))2(H)2, 1, was obtained from the reaction of Pt(COD)2 and Bu(t)3SnH, followed by addition of CNBu(t). The two hydride ligands in 1 can be eliminated, both in solution and in the solid state, to yield Pt(SnBu(t)3)2(CNBu(t))2, 2. Addition of hydrogen to 2 at room temperature in solution and in the solid state regenerates 1.

Role of axial base coordination in isonitrile binding and chalcogen atom transfer to vanadium(III) complexes.

The enthalpy of oxygen atom transfer (OAT) to V[(Me3SiNCH2CH2)3N], 1, forming OV[(Me3SiNCH2CH2)3N], 1-O, and the enthalpies of sulfur atom transfer (SAT) to 1 and V(N[t-Bu]Ar)3, 2 (Ar = 3,5-C6H3Me2), forming the corresponding sulfides SV[(Me3SiNCH2CH2)3N], 1-S, and SV(N[t-Bu]Ar)3, 2-S, have been measured by solution calorimetry in toluene solution using dbabhNO (dbabhNO = 7-nitroso-2,3:5,6-dibenzo-7-azabicyclo[2.2.1]hepta-2,5-diene) and Ph3SbS as chalcogen atom transfer reagents.

Comparison of templating abilities of urea and thioruea during photodimerization of bipyridylethyelene and stilbazole crystals.

Photodimerization of cocrystals of four bispyridylethylenes and two stilbazoles with urea as a template in the solid state has been investigated following our success with thiourea. Four investigated olefins photodimerized quantitatively to a single dimer in the crystalline state only. The reactivity of urea-olefin crystals is understood on the basis of their packing arrangements in the crystalline state.

Autocatalytic fluorescence photoactivation.

We designed an autocatalytic photochemical reaction based on the photoinduced cleavage of an α-diketone bridge from the central phenylene ring of a fluorescent anthracene derivative. The product of this photochemical transformation sensitizes its own formation from the reactant, under illumination at a wavelength capable of exciting both species. Specifically, the initial and direct excitation of the reactant generates the product in the ground state.

Role of hydrogen bonds in molecular packing of photoreactive crystals: templating photodimerization of protonated stilbazoles in crystalline state with a combination of water molecules and chloride ions.

A difference in photobehavior and molecular packing between hydrated and anhydrous crystals of protonated trans-stilbazoles has been identified. While stilbazoles are not photoreactive in the crystalline state, upon protonation with HCl in the solid state they dimerized to a single dimer (anti-head-tail) when exposed to UV light. In these photoreactive crystals the protonated stilbazole molecules are arranged in a ladder-like format with the rungs made up of water molecules and chloride ions.

Photoactivatable anthracenes.

Fifteen substituted maleimide cycloadducts of anthracene derivatives were synthesized in one or two steps from available precursors in yields ranging from 32 to 63%. They differ in the nature of the group on the maleimide nitrogen atom and of the substituents on the anthracene platform. In all instances, the introduction of a maleimide bridge across positions 9 and 10 of the anthracene skeleton isolates electronically its peripheral phenylene rings and suppresses its characteristic fluorescence.

Synthesis and structural characterization of ruthenium carbonyl cluster complexes containing platinum with a bulky N-heterocyclic carbene ligand.

The reaction of Ru3(CO)12 with Pt(IMes)2 in benzene solvent at room temperature afforded the monoplatinum-triruthenium cluster complex Ru3Pt(IMes)2(CO)11, 1, in 21% yield and the trigonal bipyramidal cluster complex Ru3Pt2(IMes)2(CO)12, 2, in 26% yield. The reaction of Ru(CO)5 with Pt(IMes)2 in benzene solvent at 0 °C yielded two trinuclear cluster complexes, the monoplatinum-diruthenium Ru2Pt(IMes)(CO)9, 3, and the monoruthenium-diplatinum cluster complex RuPt2(IMes)2(CO)6, 4. The reaction of 2 with hydrogen at 80 °C afforded the tetrahydrido-tetraruthenium complex Ru4(IMes)(CO)11(μ-H)4, 5, and the dihydrido-diruthenium-diplatinum complex Ru2Pt2(IMes)2(CO)8(μ-H)2, 6.