Is the μ‐Oxo‐μ‐Peroxodiiron Intermediate of a Ribonucleotide Reductase Biomimetic a Possible Oxidant of Epoxidation Reactions?

Density functional calculations on a mu-oxo-mu-peroxodiiron complex (1) with a tetrapodal ligand BPP (BPP=N,N-bis(2-pyridylmethyl)-3-aminopropionate) are presented that is a biomimetic of the active site region of ribonucleotide reductase (RNR). We have studied all low-lying electronic states and show that it has close-lying broken-shell singlet and undecaplet (S=0, 5) ground states with essentially two sextet spin iron atoms. In strongly distorted electronic systems in which the two iron atoms have different spin states, the peroxo group moves considerably out of the plane of the mu-oxodiiron group due to orbital rearrangements. The calculated absorption spectra of (1,11)1 are in good agreement with experimental studies on biomimetics and RNR enzyme systems. Moreover, vibrational shifts in the spectrum due to (18)O(2) substitution of the oxygen atoms in the peroxo group follow similar trends as experimental observations. To identify whether the mu-oxo-mu-1,2-peroxodiiron or the mu-oxo-mu-1,1-peroxodiiron complexes are able to epoxidize substrates, we studied the reactivity patterns versus propene. Generally, the reactions are stepwise via radical intermediates and proceed by two-state reactivity patterns on competing singlet and undecaplet spin state surfaces. However, both the mu-oxo-mu-1,2-peroxodiiron and mu-oxo-mu-1,1-peroxodiiron complex are sluggish oxidants with high epoxidation barriers. The epoxidation barriers for the mu-oxo-mu-1,1-peroxodiiron complex are significantly lower than the ones for the mu-oxo-mu-1,2-peroxodiiron complex but still are too high to be considered for catalytic properties. Thus, theory has ruled out two possible peroxodiiron catalysts as oxidants in RNR enzymes and biomimetics and the quest to find the actual oxidant in the enzyme mechanism continues.     

A Straightforward Synthesis of 2‐[1‐Alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic Acid Derivatives via Domino Michael Addition?Cyclization Reaction

A new and efficient synthesis of 2‐[1‐alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic acid derivatives by a one‐pot three‐component reaction between primary amine, dialkyl acetylenedicarboxylate, and itaconic anhydride (=3,4‐dihydro‐3‐methylidenefuran‐2,5‐dione) is reported. The reaction was performed without catalyst and under solvent‐free conditions with excellent yields. Notably, the ready availability of the starting materials, and the high level of practicability of the reaction and workup make this approach an attractive complementary method to access to unknown 2‐[1‐alkyl‐5,6‐bis(alkoxycarbonyl)‐1,2,3,4‐tetrahydro‐2‐oxopyridin‐3‐yl]acetic acid derivatives. The structures were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, and EI‐MS) and by elemental analyses. A plausible mechanism for this type of domino Michael addition? cyclization reaction is proposed (Scheme 2).     

Protonation of a Biologically Relevant CuII μ‐Thiolate Complex: Ligand Dissociation or Formation of a Protonated CuI Disulfide Species?

The proton‐induced electron‐transfer reaction of a Cu^II μ‐thiolate complex to a Cu^I‐containing species has been investigated, both experimentally and computationally. The Cu^II μ‐thiolate complex [Cu^II₂( L^MeS )₂]²⁺ is isolated with the new pyridyl‐containing ligand L^MeSSL^Me , which can form both Cu^II thiolate and Cu^I disulfide complexes, depending on the solvent. Both the Cu^II and the Cu^I complexes show reactivity upon addition of protons. The multivalent tetranuclear complex [Cu^I₂Cu^II₂( LS )₂(CH₃CN)₆]⁴⁺ crystallizes after addition of two equivalents of strong acid to a solution containing the μ‐thiolate complex [Cu^II₂( LS )₂]²⁺ and is further analyzed in solution. This study shows that, upon addition of protons to the Cu^II thiolate compound, the ligand dissociates from the copper centers, in contrast to an earlier report describing redox isomerization to a Cu^I disulfide species that is protonated at the pyridyl moieties. Computational studies of the protonated Cu^II μ‐thiolate and Cu^I disulfide species with LSSL show that already upon addition of two equivalents of protons, ligand dissociation forming [Cu^I(CH₃CN)₄]⁺ and protonated ligand is energetically favored over conversion to a protonated Cu^I disulfide complex.     

A Facile Synthesis of a Wide Variety of Cationic Ruthenium Hydrido‐Arene Complexes of binap (=1,1′‐Binaphthalene‐2,2′‐diylbis(diphenylphosphane)) and MeO?biphep (=6,6′‐Dimethoxybiphenyl‐2,2′‐diylbis(diphenylphosphane))

A straightforward high‐yield synthetic route to the cationic hydrido‐arene complexes [RuH(η⁶‐arene)(binap or MeO biphep)](CF₃SO₃), with a variety of arenes containing both donor and acceptor substituents, is described. ¹³C‐NMR Data for these complexes are reported. Several of these Ru‐complexes have been used as transfer‐hydrogenation catalysts in the reduction of acetophenone.     

C?C Cross‐Coupling Reactions of O6‐Alkyl‐2‐Haloinosine Derivatives and a One‐Pot Cross‐Coupling/O6‐Deprotection Procedure

Reaction conditions for the C? C cross‐coupling of O⁶‐alkyl‐2‐bromo‐ and 2‐chloroinosine derivatives with aryl‐, hetaryl‐, and alkylboronic acids were studied. Optimization experiments with silyl‐protected 2‐bromo‐O⁶‐methylinosine led to the identification of [PdCl₂(dcpf)]/K₃PO₄ in 1,4‐dioxane as the best conditions for these reactions (dcpf=1,1′‐bis(dicyclohexylphosphino)ferrocene). Attempted O⁶‐demethylation, as well as the replacement of the C‐6 methoxy group by amines, was unsuccessful, which led to the consideration of Pd‐cleavable groups such that C? C cross‐coupling and O⁶‐deprotection could be accomplished in a single step. Thus, inosine 2‐chloro‐O⁶‐allylinosine was chosen as the substrate and, after re‐evaluation of the cross‐coupling conditions with 2‐chloro‐O⁶‐methylinosine as a model substrate, one‐step C? C cross‐coupling/deprotection reactions were performed with the O⁶‐allyl analogue. These reactions are the first such examples of a one‐pot procedure for the modification and deprotection of purine nucleosides under C? C cross‐coupling conditions.     

Electrophilic Arene Hydroxylation and Phenol O?H Oxidations Performed by an Unsymmetric μ‐η1:η1‐O2‐Peroxo Dicopper(II) Complex

Reactions of the unsymmetric dicopper(II) peroxide complex [Cu^II₂(μ‐η¹:η¹‐O₂)(m‐XYL^N3N4)]²⁺ ( 1 O₂ , where m‐XYL is a heptadentate N‐based ligand), with phenolates and phenols are described. Complex 1 O₂ reacts with p‐X‐PhONa (X=MeO, Cl, H, or Me) at ?90 °C performing tyrosinase‐like ortho‐hydroxylation of the aromatic ring to afford the corresponding catechol products. Mechanistic studies demonstrate that reactions occur through initial reversible formation of metastable association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(p‐X‐PhO)(m‐XYL^N3N4)]⁺ ( 1 O₂ ?X‐PhO) that then undergo ortho‐hydroxylation of the aromatic ring by the peroxide moiety. Complex 1 O₂ also reacts with 4‐X‐substituted phenols p‐X‐PhOH (X=MeO, Me, F, H, or Cl) and with 2,4‐di‐tert‐butylphenol at ?90 °C causing rapid decay of 1 O₂ and affording biphenol coupling products, which is indicative that reactions occur through formation of phenoxyl radicals that then undergo radical C? C coupling. Spectroscopic UV/Vis monitoring and kinetic analysis show that reactions take place through reversible formation of ground‐state association complexes [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhOH)(m‐XYL^N3N4)]²⁺ ( 1 O₂ ?X‐PhOH) that then evolve through an irreversible rate‐determining step. Mechanistic studies indicate that 1 O₂ reacts with phenols through initial phenol binding to the Cu₂O₂ core, followed by a proton‐coupled electron transfer (PCET) at the rate‐determining step. Results disclosed in this work provide experimental evidence that the unsymmetric 1 O₂ complex can mediate electrophilic arene hydroxylation and PCET reactions commonly associated with electrophilic Cu₂O₂ cores, and strongly suggest that the ability to form substrate?Cu₂O₂ association complexes may provide paths to overcome the inherent reactivity of the O₂‐binding mode. This work provides experimental evidence that the presence of a H⁺ completely determines the fate of the association complex [Cu^II₂(μ‐η¹:η¹‐O₂)(X‐PhO(H))(m‐XYL^N3N4)]ⁿ⁺ between a PCET and an arene hydroxylation reaction, and may provide clues to help understand enzymatic reactions at dicopper sites.     

Removal and Recovery of UO2(??) from Water Samples Using 2,2′‐Diamino‐4,4′‐bithiazole as a New Reagent for Solid Phase Extraction

A novel sensitive and simple method for rapid and selective extraction, preconcentration and determination of uranyl as its 2,2′‐diamino‐4,4′‐bithiazole (DABTZ) complex by using octadecylsilica columns and spectrophotometry is presented. Extraction efficiency and the influence of flow rates of sample solution and eluent, pH, amount of DABTZ, type and least amount of eluent for elution of uranyl complex from columns, break‐through volume and limit of detection were evaluated. Also the effects of various cationic and anionic interferences on percent recovery of uranyl were studied. Average extraction efficiency of ca. 90% was obtained by elution of the column with minimal amount of solvent in the presence of interferences. The average preconcentration factor, 136 and a detection limit 0.32 ng·mL^?1 were obtained. The method was applied to the recovery and determination of uranyl in different water samples.     

C?H/π‐Interaction‐Guided Self‐Assembly in π‐Conjugated Oligomers

We report CH/π hydrogen‐bond‐driven self‐assembly in π‐conjugated skeletons based on oligophenylenevinylenes (OPVs) and trace the origin of interactions at the molecular level by using single‐crystal structures. OPVs were designed with appropriate pendants in the aromatic core and varied by hydrocarbon or fluorocarbon tails along the molecular axis. The roles of aromatic π‐stack, van der Waals forces, fluorophobic effect and CH/π interactions were investigated on the theromotropic liquid crystallinity of OPV molecules. Single‐crystal structures of hydrocarbon OPVs provided direct evidence for the existence of CH/π interactions between the π‐ring (H‐bond acceptor) and alkyl C? H (H‐bond donor). The four important crystallographic parameters, d_c?x=3.79 Å, θ=21.49°, φ=150.25° and d_Hp?x=0.73 Å, matched in accordance with typical CH/π interactions. The CH/π interactions facilitate the close‐packing of mesogens in x–y planes, which were further protruded along the c axis producing a lamellar structure. In the absence of CH/π interactions, van der Waals interactions drove the assembly towards a Schlieren nematic texture. Fluorocarbon OPVs exhibited smectic liquid‐crystalline textures that further underwent Smectic A (SmA) to Smectic C (SmC) phase transitions with shrinkage up to 11 %. The orientation and translational ordering of mesogens in the liquid‐crystalline (LC) phases induced H‐ and J‐type molecular arrangements in fluorocarbon and hydrocarbon OPVs, respectively. Upon photoexcitation, the H‐ and J‐type molecular arrangements were found to emit a blue or yellowish/green colour. Time‐resolved fluorescence decay measurements confirmed longer lifetimes for H‐type smectic OPVs relative to that of loosely packed one‐dimensional nematic hydrocarbon‐tailed OPVs.     

2‐Halogeno‐1,3,2‐diazaarsolenes and 2‐halogeno‐1,3,2‐diazastibolenes: Examples for spontaneous E?X bond heterolysis or not?

2‐X‐1,3,2‐diazaarsolenes and 2‐X‐1,3,2‐ stibolenes (X = Cl, Br) were prepared from appropriate α‐amino‐aldimine precursors via transamination with ClSb(NMe₂)₂ or via base‐induced dehydrohalogenation with EX₃ (E = As, Sb). The products were further converted into 2‐iodo‐derivatives via halide exchange with Me₃SiI, or into 1,3,2‐diazaarsolenium or 1,3,2‐stibolenium salts via halide abstraction using E′X₃ (E′ = Al, Ga, Sb) or Me₃SiOTf. All compounds synthesized were characterized by spectroscopic data and several of them by single‐crystal X‐ray diffraction studies. The results of these investigations confirmed that diazaarsolenium or stibolenium cations are stabilized by similar π‐delocalization effects as the corresponding diazaphospholenium cations. 2‐Halogeno‐1,3,2‐diazaarsolenes and 2‐halogeno‐132‐stibolenes are best addressed as molecular species whose covalent E X bonds are as in 2‐chloro‐diazaphospholenes weakened by intramolecular π(C₂N₂) → σ*(E X) and, in the case of the Sb‐containing heterocycles, inter‐ molecular n(X′) → σ*(E X) hyperconjugation between the σ* (E X) orbital and a lone‐pair of electrons on the halogen atom of a neighboring molecule. Correlation of structural and spectroscopic data and the evaluation of halide transfer reactions allowed to conclude that the extent of E X bond weakening in the 2‐X‐substituted heterocycles decreases and thus the Lewis acidity of the cations increases, with increasing atomic number of the pnicogen atom. © 2005 Wiley Periodicals, Inc. Heteroatom Chem 16:327–338, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20098     

Lamellare Schichtstrukturen von M[N(SO2C6H4‐4‐COOCH3)2] · n H2O (M = Na,n = 1; M = K,n = 0): Gibt es schwache Wasserstoffbrücken CH2–H…O=C zwischen den Schichten?

Structures of Ionic Di(arenesulfonyl)amides. 5. Lamellar Layer Structures of M[N(SO₂C₆H₄‐4‐COOCH₃)₂] · n H₂O (M = Na, n = 1; M = K, n = 0): Are there Weak Hydrogen Bonds CH₂–H…O=C between the Layers? The new lamellar compounds NaN(SO₂C₆H₄‐4‐COOMe)₂ · H₂O ( 1 a ) and KN(SO₂C₆H₄‐4‐COOMe)₂ ( 2 a ), obtained by neutralizing the corresponding strong NH acid with aqueous NaOH or KOH, were characterized by low‐temperature X‐ray diffraction ( 1 a : orthorhombic, space group Pbca, Z = 8, Z′ = 1; 2 a : orthorhombic, P2₁2₁2₁, Z = 8, Z′ = 2). Both crystals are further examples of layered inorgano‐organic solids where the inorganic component is comprised of metal cations, coordinating N(SO₂)₂ groups and water molecules (in 1 a ) and the outer regions are formed by the 4‐substituted phenyl rings of the folded anions, which approximate to mirror symmetry. In the two‐dimensional networks, the cations adopt respectively a distorted octahedral [Na(O–S)₄(OH₂)₂], an irregular [K(1)O₆N₂], or an irregular [K(2)O₉] coordination. In contrast to the previously reported carboxylic congeners, where an extensive series of C–O–H…O=C hydrogen bonds is observed between layers, the esterified materials do not display any O–CH₂–H…O=C interlayer bonding, although such interactions and short C_ar–H…O=C/O=S contacts exist within the lamellae.