Enhancing Flavins Photochemical Activity in Hydrogen Atom Abstraction and Triplet Sensitization through Ring-Contraction.

The isoalloxazine heterocycle of flavin cofactors reacts with various nucleophiles to form covalent adducts with important functions in enzymes. Molecular flavin models allow for the characterization of such adducts and the study of their properties. A fascinating set of reactions occurs when flavins react with hydroxide base, which leads to imidazolonequinoxalines, ring-contracted flavins, with so far unexplored activity. We report a systematic study of the photophysical properties of this new chromophore by absorption and emission spectroscopy as well as cyclic voltammetry. Excited, ring-contracted flavins are significantly stronger hydrogen atom abstractors when compared to the parent flavins, which allowed the direct trifluoromethylthiolation of aliphatic methine positions (bond dissociation energy (BDE) of 400.8 kJ mol-1). In an orthogonal activity, their increased triplet energy (E(S0←T1)=244 kJ mol-1) made sensitized reactions possible which exceeded the power of standard flavins. Combining both properties, ring-contracted flavin catalysts enabled the one-pot, five-step transformation of α-tropolone into trans-3,4-disubstituted cyclopentanones. We envision this new class of flavin-derived chromophores to open up new modes of reactivity that are currently impossible with unmodified flavins.

A Copper-Catalyzed Interrupted Domino Reaction for the Synthesis of Fused Triazolyl Benzothiadiazine-1-oxides.

Copper(I)-catalyzed domino reactions of 2-azido sulfoximines with 1-iodoalkynes yield fused triazolyl-containing benzothiadiazine-1-oxides. The protocol features the synthesis of two fused heterocyclic rings in one step with good to excellent yields and a broad functional group tolerance. Detailed mechanistic investigations indicate that a copper π-complex initiates a cycloaddition and oxidative C-N coupling reaction sequence. The results suggest an interrupted domino process involving an iodinated triazole as a key intermediate.

The Structure of Maneb, An Important Manganese-Containing Bis(dithiocarbamate) Fungicide.

Maneb is a manganese(II)-containing fungicide with a multi-site effect and no resistance, therefore it is widely applied in many parts of the world. There is, however, mounting evidence for neurotoxic effects with Parkinson-like symptoms (manganism) related to usage of Maneb. Due to its insolubility in most solvents and its paramagnetism, structural elucidation is not trivial, and thus its exact molecular structure remains unknown. We report herein a synthesis procedure to prepare Maneb reproducibly in pure form and the use of various analytical techniques including X-ray diffraction, X-ray absorption spectroscopy and electron diffraction to determine the molecular structure of Maneb in the solid state and also in solution.

The Difficult Marriage of Triarylcorroles with Zinc and Nickel Ions.

The coordination chemistry of corrole has witnessed a great improvement in the past few years and its Periodic Table has been widened to be so large that it is compared with that of porphyrins. However, Ni and Zn ions, commonly used with porphyrins for both synthetic and theoretical purposes, are sparsely reported in the case of corroles. Here, we report synthetic protocols for preparing Ni and Zn triarylcorrole complexes. In the case of Zn, the preliminary oxidation of the free base corrole in DMSO to the neutral corrole radical is a necessary step to obtain the coordination of the metal ion, because the direct reaction led to the formation of an open-chain tetrapyrrole. The Ni complex could be directly obtained by heating the free base corrole and Ni(II) salt to 100 °C in a DMSO solution containing FeCl3. The non-innocent nature of the corrole ligand for both complexes has been elucidated by EPR, and in the case of the Zn derivative the first spectroelectrochemical characterization is presented.

Iodine(I) and Silver(I) Complexes of Benzoimidazole and Pyridylcarbazole Derivatives.

The synthesis of iodine(I) complexes with either benzoimidazole or carbazole-derived sp2 N-containing Lewis bases is described, as well as their corresponding silver(I) complexes. The addition of elemental iodine to the linear two-coordinate Ag(I) complexes produces iodine(I) complexes with a three-center four-electron (3c-4e) [N-I-N]+ bond. The 1 H and 1 H-15 N HMBC NMR studies unambiguously confirm the formation of the complexes in all cases via the [N-Ag-N]+ →[N-I-N]+ cation exchange, with the 15 N NMR chemical shift change between 94 to 111 ppm when compared to the free ligand. The single crystal X-ray crystallographic studies on eight I+ complexes revealed highly symmetrical [N-I-N]+ bonds with I-N bond distances of 2.21-2.26 Šand N-I-N angles of 177-180°, whilst some of the corresponding Ag+ complexes showed a clear deviation from linearity with N-Ag-N angles of ca. 150° and Ag-N bond distances of 2.09-2.18 Å.

Mechanochemical Syntheses of N-Containing Heterocycles with TosMIC.

A mechanochemical van Leusen pyrrole synthesis with a base leads to 3,4-disubstitued pyrroles in moderate to excellent yields. The developed protocol is compatible with a range of electron-withdrawing groups and can also be applied to the synthesis of oxazoles. Attempts to mechanochemically convert the resulting pyrroles into porphyrins proved to be difficult.

Studies of Nature of Uncommon Bifurcated I-I···(I-M) Metal-Involving Noncovalent Interaction in Palladium(II) and Platinum(II) Isocyanide Cocrystals.

Two isostructural trans-[MI2(CNXyl)2]·I2 (M = Pd or Pt; CNXyl = 2,6-dimethylphenyl isocyanide) metallopolymeric cocrystals containing uncommon bifurcated iodine···(metal-iodide) contact were obtained. In addition to classical halogen bonding, single-crystal X-ray diffraction analysis revealed a rare type of metal-involved stabilizing contact in both cocrystals. The nature of the noncovalent contact was studied computationally (via DFT, electrostatic surface potential, electron localization function, quantum theory of atoms in molecules, and noncovalent interactions plot methods). Studies confirmed that the I···I halogen bond is the strongest noncovalent interaction in the systems, followed by weaker I···M interaction. The electrophilic and nucleophilic nature of atoms participating in I···M interaction was studied with ED/ESP minima analysis. In trans-[PtI2(CNXyl)2]·I2 cocrystal, Pt atoms act as weak nucleophiles in I···Pt interaction. In the case of trans-[PdI2(CNXyl)2]·I2 cocrystal, electrophilic/nucleophilic roles of Pd and I are not clear, and thus the quasimetallophilic nature of the I···Pd interaction was suggested.

Synthesis of trifluoromethyl-substituted 1,2,6-thiadiazine 1-oxides from sulfonimidamides under mechanochemical conditions.

TBS-protected or NH-sulfonimidamides react with ß-alkoxyvinyl trifluoromethylketones under solvent-free mechanochemical conditions to give 3-trifluoromethyl-substituted three-dimensional 1,2,6-thiadiazine 1-oxides. C4-Functionalized products can be obtained by starting from cyclic enones and brominations of the initially formed heterocycles. The stability of the products was investigated by varying the pH value and storage under aerobic conditions.

Carbonyl Hypoiodites as Extremely Strong Halogen Bond Donors.

Neutral halogen-bonded O-I-N complexes were prepared from in situ formed carbonyl hypoiodites and aromatic organic bases. The carbonyl hypoiodites have a strongly polarized iodine atom with larger σ-holes than any known uncharged halogen bond donor. Modulating the Lewis basicity of the selected pyridine derivatives and carboxylates leads to halogen-bonded complexes where the classical O-I⋅⋅⋅N halogen bond transforms more into a halogen-bonded COO- ⋅⋅⋅I-N+ ion-pair (salt) with an asymmetric O-I-N moiety. X-ray analyses, NMR studies, and calculations reveal the halogen bonding geometries of the carbonyl hypoiodite-based O-I-N complexes, confirming that in the solid-state the iodine atom is much closer to the N-atom of the pyridine derivatives than its original position at the carboxylate O-atom.

Iron(III) Chloride as a Mild Catalyst for the Dearomatizing Cyclization of N-Acylindoles.

A catalytic approach for the preparation of indolines by dearomatizing cyclization is presented. FeCl3 acts as a catalyst to afford tetracyclic 5a,6-dihydro-12H-indolo[2,1-b][1,3]benzoxazin-12-ones in good yields. The cyclization also proceeds with tosylamides forming C-N bonds in 53% yield.

Room-Temperature Phosphorescence and Efficient Singlet Oxygen Production by Cyclometalated Pt(II) Complexes with Aromatic Alkynyl Ligands.

The synthesis of five novel cyclometalated platinum(II) compounds containing five different alkynyl-chromophores was achieved by the reaction of the previously synthesized Pt-Cl cyclometalated compound (1) with the corresponding RC≡CH by a Sonogashira reaction. It was observed that the spectral and photophysical characteristics of the cyclometalated platinum(II) complexes (Pt-Ar) are essentially associated with the platinum-cyclometalated unit. Room-temperature emission of the Pt-Ar complexes was attributed to phosphorescence in agreement with DFT calculations. Broad nanosecond (ns)-transient absorption spectra were observed with decays approximately identical to those obtained from the emission of the triplet state. From the femtosecond-transient absorption (fs-TA) data, two main excited-state decay components were identified: one in the order of a few picoseconds was assigned to fast intersystem crossing to populate the triplet excited-state and the second (hundreds of ns) was associated with the decay of the transient triplet state. In general, efficient singlet oxygen photosensitization quantum yields were observed from the triplet state of these complexes.

Titanium Carbene Complexes Stabilized by Alkali Metal Amides.

Facile α-H elimination from tetrakis(trimethylsilylmethyl)titanium precursors to give adducts of (alkylidene)bis(alkyl)titanium complexes is induced by light alkali metal amides of the NNNN-type macrocyclic anionic ligand Me3 TACD [(Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane]. In the crystal, the alkali metal interacts with the carbene carbon atom or with the CH2 group of the trimethylsilymethyl ligand. The nucleophilic character of the carbene carbon atom was shown by the reaction with benzophenone and terminal acetylenes.

From Monodisperse Thienyl- and Furylborane Oligomers to Polymers: Modulating the Optical Properties through the Hetarene Ratio.

The application of our newly developed B-C coupling method by catalytic Si/B exchange is demonstrated for the synthesis of a series of triarylboranes (1), monodisperse thienyl- and furylborane dimers (2) and trimers (9), extended oligomers (3) and polymers (3'), as well as mixed (oligo)thienyl-/furylboranes. The structures of 1 aaTip , 1 bbTip , and 2 bbbMes* , determined by X-ray crystallography, reveal largely coplanar hetarene rings and BR3 environments, which are most pronounced in the furylborane species. Photophysical investigations, supported by TD-DFT calculations, revealed pronounced π-electron delocalization over the hetarene backbones including the boron centers. With an extended series of derivatives of varying chain lengths available, we were able to determine the effective conjugation lengths (ECL) of poly(thienylborane)s and poly(furylborane)s, which have been reached with the highest-molecular-weight derivatives of our study. Through variation of the furan-to-thiophene ratio, the photophysical properties of these materials are effectively modulated. Significantly, higher furan contents lead to considerably increased fluorescence intensities. Compounds 1 aaTip , 1 bbTip , and 3 aTip showed the ability to bind fluoride anions. The binding process is signaled by a distinct change in their optical absorption characteristics, thus rendering these materials attractive targets for sensory applications.

Interplay Between Microwave Spectroscopy and X-ray Diffraction: The Molecular Structure and Large Amplitude Motions of 2,3-Dimethylanisole.

To determine the structural properties of 2,3-dimethylanisole, a multidisciplinary approach was carried out where gas phase rotational spectroscopy recording a spectrum from 2 to 26.5 GHz using a pulsed molecular jet Fourier transform microwave spectrometer was combined with solid-state X-ray diffraction. Both methods revealed that only one conformer with a planar heavy-atom structure exists. In the solid state, the packing in the monoclinic space group is P21 /n with Z=4. In the gas phase spectrum, torsional splittings due to the internal rotations of two methyl groups attached to the phenyl ring were resolved and analyzed, providing an estimate of the barriers to methyl internal rotation of V3 =26.9047(5) and 518.7(12) cm-1 for the methyl groups at the ortho- and meta-position, respectively. The coupling between the two internal rotations is modeled on a two-dimensional potential energy surface, which was obtained by quantum chemical calculations at the B3LYP/6-311++G(d,p) level of theory.

Mononuclear Alkali Metal Organoperoxides Stabilized by an NNNN-Macrocycle and Short Hydrogen Bonds from ROOH Molecules.

A series of light alkali metal organoperoxides [MOOR] (M=Li, Na, K; R=tBu, CMe2 Ph) were isolated in mononuclear form, aided by the tetradentate and neutral NNNN-type macrocycle Me4 TACD (L; Me4 [12]aneN4 =1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane) and strong hydrogen bonds from additional ROOH molecules. The Na and K compounds are characterized by short O-H⋅⋅⋅O contacts, in the case of the Na derivative as short as 2.41 Å.

Favored Conformations of Carbonyl Compounds: A Structural Study of n-Octanal.

We report on the molecular structures of the two most abundant conformers of n-octanal observed by molecular beam Fourier transform microwave spectroscopy. Next to limonene, which is the main component of citrus-oil, octanal and other n-alkyl aldehydes strongly enhance the typical fresh smell of lemon-oil. Due to the high flexibility of its n-alkyl chain and the high number of possible conformers, different semi-empirical methods (AM1, PM3, MMFF94) were used to sample the conformational space of octanal before performing more sophisticated quantum chemical calculations at the MP2 level of theory. This technique has previously been shown to be an ideal tool to characterize relevant odorant structures in fragrance chemistry. The structure of octanal and structurally related molecules is discussed in the context of the most abundant chain conformations and the potential use of the microwave validated structures for further studies in biological media.

Divergent Reactivity of a Dinuclear (NHC)Nickel(I) Catalyst versus Nickel(0) Enables Chemoselective Trifluoromethylselenolation.

We herein showcase the ability of NHC-coordinated dinuclear NiI -NiI complexes to override fundamental reactivity limits of mononuclear (NHC)Ni0 catalysts in cross-couplings. This is demonstrated with the development of a chemoselective trifluoromethylselenolation of aryl iodides catalyzed by a NiI dimer. A novel SeCF3 -bridged NiI dimer was isolated and shown to selectively react with Ar-I bonds. Our computational and experimental reactivity data suggest dinuclear NiI catalysis to be operative. The corresponding Ni0 species, on the other hand, suffers from preferred reaction with the product, ArSeCF3 , over productive cross-coupling and is hence inactive.

Base-pairing of uracil and 2,6-diaminopurine: from cocrystals to photoreactivity.

We show that the non-canonical nucleobase 2,6-diaminopurine (D) spontaneously base pairs with uracil (U) in water and the solid state without the need to be attached to the ribose-phosphate backbone. Depending on the reaction conditions, D and U assemble in thermodynamically stable hydrated and anhydrated D-U base-paired cocrystals. Under UV irradiation, an aqueous solution of D-U base-pair undergoes photochemical degradation, while a pure aqueous solution of U does not. Our simulations suggest that D may trigger the U photodimerization and show that complementary base-pairing modifies the photochemical properties of nucleobases, which might have implications for prebiotic chemistry.

A handy and accessible tool for identification of Sn(II) in toothpaste.

An easily accessible colorimetric probe, a carbazole-naphthaldehyde conjugate (CNP), was successfully prepared for the selective and sensitive recognition of Sn(II) in different commercially-available toothpaste and mouth wash samples. The binding mechanism of CNP for Sn2+ was confirmed by UV-Vis, 1H, and 13C NMR titrations. The proposed sensing mechanism was supported by quantum chemical calculations. Selective detection of Sn(II) in the nanomolar range (85 nM), among other interfering metal ions, makes it exclusive. Moreover, Sn2+ can be detected with a simple paper strip from toothpaste, which makes this method handy and easily accessible. The potential application of this system for monitoring Sn2+ can be used as an expedient tool for environmental and industrial purposes.

The Preparation of Diaryl Sulfoxinium Triflates and Their Application in Palladium-Catalyzed Cross-Coupling Reactions.

Treatment of N-methyl-S,S-diaryl sulfoximines with methyl trifluoromethanesulfonate provides bench-stable sulfoxinium salts in excellent yields. Applying them in Sonogashira-, Heck- and Suzuki-type cross-coupling reactions leads to the corresponding products by sequential C-S bond cleavage and C-C bond formation. Electronic factors induced by substituents on the S-aryl groups govern the coupling efficiency.