Laser-spectroscopic laboratory studies of atmospheric aqueous phase free radical chemistry

Nitrate radical (NO(3)) reactions with benzene (R-1), toluene (R-2), p-xylene (R-3), p-cresol (R-4) and mesitylene (R-5) have been studied by laser photolysis/long path laser absorption (LP-LPLA) in aqueous solution. Rate constants of k(1)=(4.0+/-0.6). 10(8), k(2)=(1.2+/-0.3). 10(9), k(3)=(1.6+/-0.1). 10(9), k(4)= (8.4+/-2.3). 10(8) and k(5)=(1.3+/-0.3). 10(9) lmol(-1)s(-1) were obtained at T=298 K. In addition, reaction rate coefficients for SO(-)(5)+Fe(2+)-->prod. (R-6) and SO(-)(5)+Mn(2+)-->prod. (R-7) of k(6)=(4.3+/-2.4). 10(7) lmol(-1)s(-1) and k(7)=(4.6+/-1.0). 10(6) lmol(-1)s(-1) (T=298 K, I-->0) have been obtained by the application of laser photolysis/UV-VIS broadband diode array spectroscopy. A new laser photolysis/UV-long path laser absorption experiment has been applied to study the reaction of the Cl(-)(2) radical anion with dissolved sulfur(IV). For the reactions Cl(-)(2)+HSO(-)(3)-->2Cl(-)+H(+)+SO(-)(3) (R-8) and Cl(-)(2)+SO(2-)(3)-->2Cl(-)+SO(-)(3) (R-9) rate coefficients of k(8)=(1.7+/-0.2). 10(8) lmol(-1)s(-1) (T=298 K, I-->0) and of k(9)=(6.2+/-0.3). 10(7) lmol(-1)s(-1) (T=279 K, I-->0) were obtained.     

Cathodic reduction of SO2 in the presence of organic dihalides

The reaction of the electrolytically generated SO ₂ ^– anion radical with 1,-dihalides is used for the synthesis of sulfur containing heterocycles like oxathiolane-, oxathiane-, thiane- and thiepane-oxides. 1,2-Dihalides are reductively deblocked to give olefines.

---

Kathodische Reduktion von SO₂ in Gegenwart organischer Dihalogenide

Zusammenfassung Die Reaktion des elektrolytisch erzeugten SO ₂ ^– -Anionradikals mit 1,-Dihalogeniden führt zu schwefelhaltigen Heterocyclen wie Oxathiolan-, Oxathian-, Thian- und Thiepanoxiden. 1,2-Dihalogenide geben durch reduktive Eliminierung Olefine.

     

Hierarchical assembly of helicate-type dinuclear titanium(IV) complexes

The ligands 4-7-H(2) were used in coordination studies with titanium(IV) and gallium(III) ions to obtain dimeric complexes Li(4)[(4-7)(6)Ti(2)] and Li(6)[(4/5a)(6)Ga(2)]. The X-ray crystal structures of Li(4)[(4)(6)Ti(2)], Li(4)[(5b)(6)Ti(2)], and Li(4)[(7a)(6)Ti(2)] could be obtained. While these complexes are triply lithium-bridged dimers in the solid state, a monomer/dimer equilibrium is observed in solution by NMR spectroscopy and ESI FT-ICR MS. The stability of the dimer is enhanced by high negative charges (Ti(IV) versus Ga(III)) of the monomers, when the carbonyl units are good donors (aldehydes versus ketones and esters), when the solvent does not efficiently solvate the bridging lithium ions (DMSO versus acetone), and when sterical hindrance is minimized (methyl versus primary and secondary carbon substituents). The dimer is thermodynamically favored by enthalpy as well as entropy. ESI FT-ICR mass spectrometry provides detailed insight into the mechanisms with which monomeric triscatecholate complexes as well as single catechol ligands exchange in the dimers. Tandem mass spectrometric experiments in the gas phase show the dimers to decompose either in a symmetric (Ti) or in an unsymmetric (Ga) fashion when collisionally activated. The differences between the Ti and Ga complexes can be attributed to different electronic properties and a charge-controlled reactivity of the ions in the gas phase. The complexes represent an excellent example for hierarchical self-assembly, in which two different noncovalent interactions of well balanced strengths bring together eleven individual components into one well-defined aggregate.     

Regio- and Enantioselective Substitution of Acyclic Allylic Sulfoximines with Butylcopper in the Presence of Lithium Iodide and Boron Trifluoride

Enantiomerically pure N-methyl-, N-benzyl-, and N-(methoxyethyl)-S-(phenyl)cinnamylsulfoximines as well as the corresponding crotylsulfoximines have been prepared from N-methyl-, N-benzyl-, and N-(methoxyethyl)-S-(lithiomethyl)sulfoximines and carbonyl compounds by an addition-elimination-isomerization reaction sequence. Under basic conditions, complete isomerization of the vinylic sulfoximines, obtained as intermediates, to the corresponding allylic sulfoximines takes place. Chromatographically separable mixtures of (E) and (Z) allylic sulfoximines were isolated in the case of beta,gamma-disubstituted allylic sulfoximines. The (E/Z) ratio depends on the nature of the substituents in the beta- and gamma-positions, and the equilibrium amount of the (Z) isomer varies from 68% to nil. The allylic N-methylsulfoximines do not racemize thermally, and their rearrangement to the corresponding allylic sulfinamides is negligible. Upon prolonged treatment with boron trifluoride at low temperatures allylic N-methylsulfoximines are recovered unchanged. The crystal structure of S-(3,4-dihydronaphthalen-2-ylmethyl)-N-methyl-S-phenylsulfoximine was determined. Reaction of the allylic sulfoximines with butylcopper in the presence of lithium iodide and boron trifluoride leads with very high gamma-selectivities and moderate to high enantioselectivities to the corresponding chiral alkenes. Their configuration was determined by chemical correlation through ozonolysis to the corresponding carbonyl compounds. The asymmetric induction exerted by the chiral N-methyl-S-phenylsulfoximine group strongly depends on the double bond configuration and the substituents in the beta- and gamma-positions. The (E) allylic sulfoximines are substituted with low to moderate enantioselectivities (2-66%), whereas the (Z) allylic sulfoximines react with much higher enantioselectivities (69-92%). Interestingly, substitution of the beta-methyl-gamma-phenyl-substituted (Z) allylic sulfoximine and its beta-phenyl-gamma-methyl isomer proceeded with almost the same degree of asymmetric induction but with the opposite sense. Replacement of the N-methyl group by a benzyl or a methoxyethyl group has no significant influence on the regio- and enantioselectivity of the substitution.     

From Weakly Coordinating to Non‐Coordinating Anions? A Simple Preparation of the Silver Salt of the Least Coordinating Anion and Its Application To Determine the Ground State Structure of the Ag(η2‐P4)2+ Cation

The unexpected but facile preparation of the silver salt of the least coordinating [(RO)₃Al‐F‐Al(OR)₃]^? anion (R=C(CF₃)₃) by reaction of Ag[Al(OR)₄] with one equivalent of PCl₃ is described. The mechanism of the formation of Ag[(RO)₃Al‐F‐Al(OR)₃] is explained based on the available experimental data as well as on quantum chemical calculations with the inclusion of entropy and COSMO solvation enthalpies. The crystal structures of (RO)₃Al←OC₄H₈, Cs⁺[(RO)₂(Me)Al‐F‐Al(Me)(OR)₂]^?, Ag(CH₂Cl₂)₃⁺[(RO)₃Al‐F‐Al(OR)₃]^? and Ag(η²‐P₄)₂⁺[(RO)₃Al‐F‐Al(OR)₃]^? are described. From the collected data it will be shown that the [(RO)₃Al‐F‐Al(OR)₃]^? anion is the least coordinating anion currently known. With respect to the fluoride ion affinity of two parent Lewis acids Al(OR)₃ of 685 kJ mol^?1, the ligand affinity (441 kJ mol^?1), the proton and copper decomposition reactions (?983 and ?297 kJ mol^?1) as well as HOMO level and HOMO–LUMO gap and in comparison with [Sb₄F₂₁]^?, [Sb(OTeF₅)₆]^?, [Al(OR)₄]^? as well as [B(R^F)₄]^? (R^F=CF₃ or C₆F₅) the [(RO)₃Al‐F‐Al(OR)₃]^? anion is among the best weakly coordinating anions (WCAs) according to each value. In contrast to most of the other cited anions, the [(RO)₃Al‐F‐Al(OR)₃] anion is available by a simple preparation in conventional inorganic laboratories. The least coordinating character of this anion was employed to clarify the question of the ground state geometry of the Ag(η²‐P₄)₂⁺ cation (D_2h, D₂ or D_2d?). In agreement with computational data and NMR spectra it could be shown that the rotation along the Ag‐(P‐P‐centroid) vector has no barrier and that the structure adopted in the solid state depends on packing effects which lead to an almost D_2h symmetric Ag(η²‐P₄)₂⁺ cation (0 to 10.6° torsion) for the more symmetrical [Al(OR)₄]^? anion, but to a D₂ symmetric Ag(η²‐P₄)₂⁺ cation with a 44° twist angle of the two AgP₂ planes for the less symmetrical [(RO)₃Al‐F‐Al(OR)₃]^? anion. This implies that silver back bonding, suggested by quantum chemical population analyses to be of importance, is only weak.     

Laser-spectroscopic laboratory studies of atmospheric aqueous phase free radical chemistry

Nitrate radical (NO₃) reactions with benzene (R-1), toluene (R-2), p-xylene (R-3), p-cresol (R-4) and mesitylene (R-5) have been studied by laser photolysis/long path laser absorption (LP-LPLA) in aqueous solution. Rate constants of k₁=(4.0±0.6) 10⁸, k₂=(1.2±0.3)10⁹, k₃=(1.6±0.1)10⁹, k₄= (8.4±2.3)10⁸ and k₅=(1.3±0.3)10⁹ lmol^-1s^-1 were obtained at T=298 K. In addition, reaction rate coefficients for SO^-₅+Fe²⁺prod. (R-6) and SO^-₅+Mn²⁺prod. (R-7) of k₆=(4.3±2.4) 10⁷ lmol^-1s^-1 and k₇=(4.6±1.0)10⁶ lmol^-1s^-1 (T=298 K, I0) have been obtained by the application of laser photolysis/UV-VIS broadband diode array spectroscopy. A new laser photolysis/UV-long path laser absorption experiment has been applied to study the reaction of the Cl^-₂ radical anion with dissolved sulfur(IV). For the reactions Cl^-₂+HSO^-₃2Cl^-+H⁺+SO^-₃ (R-8) and Cl^-₂+SO^2-₃2Cl^-+SO^-₃ (R-9) rate coefficients of k₈=(1.7±0.2)10⁸ lmol^-1s^-1 (T=298 K, I0) and of k₉=(6.2±0.3)10⁷ lmol^-1s^-1 (T=279 K, I0) were obtained.     

Pd@SnO2 and SnO2@Pd Core@Shell Nanocomposite Sensors

Pd@SnO₂ and SnO₂@Pd core@shell nanocomposites are prepared via a microemulsion approach. Both nanocomposites exhibit high‐surface, porous matrices of SnO₂ shells (>150 m² g^?1) with very small SnO₂ crystallites (<10 nm) and palladium (Pd) nanoparticles (<10 nm) that are uniformly distributed in the porous SnO₂ matrix. Although similar by first sight, Pd@SnO₂ and SnO₂@Pd are significantly different in view of their structure with Pd inside or outside the SnO₂ shell and in view of their sensor performance. As SMOX‐based sensors (SMOX: semiconducting metal oxide), both nanocomposites show a very good sensor performance for the detection of CO and H₂. Especially, the Pd@SnO₂ core@shell nanocomposite is unique and shows a fast response time (τ₉₀ < 30 s) and a very good response at low temperature (<250 °C), especially under humid‐air conditions. Extraordinarily high sensor signals are observed when exposing the Pd@SnO₂ nanocomposite to CO in humid air. Under these conditions, even commercial sensors (Figaro TGS 2442, Applied Sensor MLC, E2V MICS 5521) are outperformed.     

A Supramolecular Chiral Auxiliary Approach: “Remote Control” of Stereochemistry at a Hierarchically Assembled Dimeric Helicate

Dimeric hierarchically‐assembled titanium(IV) helicates are in solvent‐dependent equilibrium with the corresponding monomers. Statistically formed mixtures of such complexes bearing chiral stereocontrolling ligands and achiral diene‐substituted ligands show high diastereoselectivity and reasonable enantioselectivity in the Diels–Alder reaction with maleimides if the reaction proceeds with the dimer but not with the monomer. Thus, solvent dependent switching between the monomer and dimer enables on/off switching of the enantioselectivity.