Synthesis of Phospholipid?Ribavirin Conjugates

New conjugates of antiviral nucleoside Ribavirin (=1‐(β‐D ‐ribofuranosyl)‐1H‐1,2,4‐triazole‐3‐carboxamide; 1 ) with 1,2‐ and 1,3‐diacyl glycerophosphates have been synthesized by the phosphoramidite method. A combination of 2′,3′‐phenylboronate protecting group for the sugar moiety of the ribonucleoside 1 and 2‐cyanoethyl protection for the phosphate fragment ensured the preparation of the desired compounds with reasonable yields via a small number of synthetic steps.     

Oxindole Synthesis by Direct Coupling of C?H and C?H Centers

An sp ² /sp ³ get‐together : A novel and efficient method can be used to synthesize 3,3‐disubstitued oxindoles by the direct intramolecular oxidative coupling of an aryl C? H and a C? H center (see scheme; DMF=N,N‐dimethylformamide).

     

A theoretical investigation of Zn?Zn and Be?Be one electron bond

Attachment of one electron to 1,2-diBeX-benzene and 1,2-diZnX-benzene derivatives leads to the formation of stronger Be Be and Zn Zn interaction compared to the neutral one. This is reflected in the dramatic shortening of the Be Be and Zn Zn distance. The formation of these 2-center-1-electron bonds have also been confirmed by topological survey of electron density using quantum theory of atoms in molecules and electron localization function. The formation of these bonds is expected to render stability to these radical anions. These radical anions are stable toward electron detachment and computed bond dissociation energy values are also significant.     

B?H,C?H,and B?C Bond Activation: The Role of Two Adjacent Agostic Interactions

Tuning the nature of the linker in a L∼BHR phosphinoborane compound led to the isolation of a ruthenium complex stabilized by two adjacent, δ‐C H and ε‐B_sp2 H, agostic interactions. Such a unique coordination mode stabilizes a 14‐electron “RuH₂P₂” fragment through connected σ‐bonds of different polarity, and affords selective B H, C H, and B C bond activation as illustrated by reactivity studies with H₂ and boranes.     

Kinetics and modeling of the H2?O2?NOx system

The addition of NO (0 to 400ppm) to mixtures of H₂ (ca. 1%) and O₂ (0.7 to 22%) has been studied over the temperature range 700 to 825 K, in a flow reactor at atmospheric pressure. The overall effect of NO is to promote the oxidation of H₂ but high concentrations of O₂ actually inhibit the NO-promoted oxidation of H₂. A detailed kinetic mechanism has been constructed and found to describe the experimental observations. The promotion of the oxidation of H₂ arises through the catalytic cycle The ability of R.34 to reactivate chains normally terminated by the formation of HO₂ is a key feature of this system. The predictions are highly sensitive to the rate of the reaction R.5 and the rate constants for this reaction is the only adjustable parameter required in the model. The value of k_5,N2 found to describe all the results has an absolute uncertainty <35%. The uncertainty relative to other important rate constants in the H₂? O₂ system is less than 10%. © 1995 John Wiley & Sons, Inc.     

An efficient synthesis of phosphenimidous esters,RO?PN?Ar

Trimethylsilyltrifluoromethane sulfonate is shown to be an efficient catalyst for the elimination of Me₃SiCl from N-trimethylsilyl-N-(2,4,6-tri-tert-butylphenyl)amidochlorophosphites la-f , leading to the phosphenimidous esters 3a–f. The crystal structures of phosphites 1a and 1d provide a stereochemical explanation for the better thermal stability of 1d On the basis of these observations a convenient and general synthesis of phosphenimidous esters 3a–f is presented.     

Amide‐Directed Tandem C?C/C?N Bond Formation through C?H Activation

The transformation of C? H bonds into other chemical bonds is of great significance in synthetic chemistry. C? H bond‐activation processes provide a straightforward and atom‐economic strategy for the construction of complex structures; as such, they have attracted widespread interest over the past decade. As a prevalent directing group in the field of C? H activation, the amide group not only offers excellent regiodirecting ability, but is also a potential C? N bond precursor. As a consequence, a variety of nitrogen‐containing heterocycles have been obtained by using these reactions. This Focus Review addresses the recent research into the amide‐directed tandem C? C/C? N bond‐formation process through C? H activation. The large body of research in this field over the past three years has established it as one of the most‐important topics in organic chemistry.     

Catalytic C?C,C?N,and C?O Ullmann‐Type Coupling Reactions

Copper‐catalyzed Ullmann condensations are key reactions for the formation of carbon–heteroatom and carbon–carbon bonds in organic synthesis. These reactions can lead to structural moieties that are prevalent in building blocks of active molecules in the life sciences and in many material precursors. An increasing number of publications have appeared concerning Ullmann‐type intermolecular reactions for the coupling of aryl and vinyl halides with N, O, and C nucleophiles, and this Minireview highlights recent and major developments in this topic since 2004.     

Modeling the H bond donor strength of ?OH, ?NH,and ?CH sites by local molecular parameters

A quantum chemical model is introduced to predict the H‐bond donor strength of monofunctional organic compounds from their ground‐state electronic properties. The model covers ? OH, ? NH, and ? CH as H‐bond donor sites and was calibrated with experimental values for the Abraham H‐bond donor strength parameter A using the ab initio and density functional theory levels HF/6‐31G** and B3LYP/6‐31G**. Starting with the Morokuma analysis of hydrogen bonding, the electrostatic (ES), polarizability (PL), and charge transfer (CT) components were quantified employing local molecular parameters. With hydrogen net atomic charges calculated from both natural population analysis and the ES potential scheme, the ES term turned out to provide only marginal contributions to the Abraham parameter A, except for weak hydrogen bonds associated with acidic ? CH sites. Accordingly, A is governed by PL and CT contributions. The PL component was characterized through a new measure of the local molecular hardness at hydrogen, η(H), which in turn was quantified through empirically defined site‐specific effective donor and acceptor energies, EE_occ and EE_vac. The latter parameter was also used to address the CT contribution to A. With an initial training set of 77 compounds, HF/6‐31G** yielded a squared correlation coefficient, r², of 0.91. Essentially identical statistics were achieved for a separate test set of 429 compounds and for the recalibrated model when using all 506 compounds. B3LYP/6‐31G** yielded slightly inferior statistics. The discussion includes subset statistics for compounds containing ? OH, ? NH, and active ? CH sites and a nonlinear model extension with slightly improved statistics (r² = 0.92). © 2008 Wiley Periodicals, Inc. J Comput Chem 2009     

Simple modification of the Lee?Yang?Parr correlation functional to satisfy exact nonuniform scaling requirements

In density functional theory (DFT) the exchange-correlation energy has to be approximated. One of the most widely used approximations to the correlation component, the Lee(SINGLEBOND)Yang(SINGLEBOND)Parr (LYP) functional does not obey nonuniform scaling requirements. We propose a modifying function that makes this functional satisfy nonuniform scaling constraints. As a result, two modified LYP functionals are suggested. One of them, optimized in a preliminary way, exhibits some properties better than the original LYP functional. This implies that nonuniform scaling requirements convey very important information and all approximate functionals in DFT should be made to satisfy these requirements. Our modifying factor could be used to improve the nonuniform scaling properties not only of E_c ^LYP [n] but also of most functionals which do not obey these conditions. © 1996 John Wiley & Sons, Inc.     

Visible light-mediated photocatalyzed synthesis of oxazole via intermolecular C?N and C?O bond formation

An efficient visible light-mediated, eosin Y-catalyzed synthesis of oxazole has been developed from benzil with primary amines, that providing a straightforward, green, and environmentally benign access to a wide variety of substituted oxazole-2-amines under mild reaction conditions.     

Ethylenedithio?Tetrathiafulvalene?Helicenes: Electroactive Helical Precursors with Switchable Chiroptical Properties

Electroactive fused ethylenedithio? tetrathiafulvalene? [4]helicene and ‐[6]helicenes have been synthesized through a strategy that involved the preparation of 2,3‐dibromo‐helicene derivatives as intermediates. The dihedral angles between the terminal helicenes, as determined by single‐crystal X‐ray analysis, are 22.7° and 50.7° for the [4]helicene and [6]helicene, respectively. Their solid‐state architectures show interplay between S???S and π???π intermolecular interactions. The chiroptical properties of the enantiopure EDT? TTF? [6]helicene derivatives have been investigated and supported by TDDFT calculations. Remarkable redox switching of the circular dichroism (CD) signal between the neutral and radical‐cation species has been achieved.     

Electronic Structure of Monodithiolated Iron?Oxotungsten Heterometallic Complexes: Integer‐Spin Fe?W Assembly

Fe? W heterometallic complexes, in which an FeX₂ (X=Cl, SPh) moiety is attached to monodithiolene oxotungsten through a sulfide bridge, that is, [Ph₄P]₂[Cl₂Fe(S)₂WOS₂] ( 1 ), [Ph₄P]₂[Cl₂Fe(S)₂WOS₂(DMED)] ( 2 , DMED=dimethylethylenedicarboxylate), [Ph₄P]₂[Cl₂Fe(S)₂WO(tdt)] ( 3 , tdt=toluenedithiolate), [Ph₄P]₂[(SPh)₂Fe(S)₂WO(tdt)] ( 4 ), and [Ph₄P]₂[Cl₂Fe(S)₂WO(edt)] ( 5 , edt=ethanedithiolate), are reported. Mössbauer and EPR spectroscopy, magnetism, electrochemistry, and electronic structural analysis based on DFT and TD‐DFT calculations show the transfer of electron from the iron center to the tungsten center, thus resulting in a ferromagnetically coupled Fe^IIIW^V unit, along with antiferromagnetic intermolecular interactions, from the starting Fe^II and W^VI compounds. A net spin of a S=3 ground state, which arises from ferromagnetically coupled Fe^III and W^V atoms, displays a rare X‐band EPR in normal mode at g≈7 in the solid state.     

On the reactivity of metal and organometallic halides toward R3Sn?O?SnR3 systems

Interaction of metallic salts (M = Hg, Sb, and Te) with bis(triorganotin)oxide, (R₃Sn)₂O, where (R = C₆H₅, p‐CH₃C₆H₄, and cyclo‐C₆H₁₁) at room temperature proceeded with the simultaneous cleavage of the Sn C and Sn O bonds, invariably yielding R₂SnO along with other products. Thus the treatment of HgX₂ (X = Cl, CN, SCN) with (R₃Sn)₂O resulted in the formation of polymeric diorganotin oxide R₂SnO along with R₃SnX and RHgX derivatives. The reaction of SbCl₃ with (R₃Sn)₂O was found to give R₂SnO, R₃SnCl, and RSbCl₂, whereas interaction with SbCl₅ provided R₂SnO, R₂SnCl₂, and R₂SbCl₃. Treatment of TeCl₄ with (R₃Sn)₂O provided R₂SnO, R₃SnCl, and RTeCl₃ at room temperature. At reflux temperature, reaction of PhTeCl₃ with (R₃Sn)₂O yielded R₂SnO, R₃SnCl, and mixed diorganotellurium dichloride, RPhTeCl₂. The course of reaction indicated the instability of Sn O Sn system proceeding via a four‐centered mechanism, providing organometallic compounds in profitable yield. © 2009 Wiley Periodicals, Inc. Heteroatom Chem 20:278–283, 2009; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20547     

Ab Initio energetics of Si?O bond cleavage

A multilevel approach that combines high‐level ab initio quantum chemical methods applied to a molecular model of a single, strain‐free Si O Si bridge has been used to derive accurate energetics for Si O bond cleavage. The calculated Si O bond dissociation energy and the activation energy for water‐assisted Si O bond cleavage of 624 and 163 kJ mol⁻¹, respectively, are in excellent agreement with values derived recently from experimental data. In addition, the activation energy for H₂O‐assisted Si O bond cleavage is found virtually independent of the amount of water molecules in the vicinity of the reaction site. The estimated reaction energy for this process including zero‐point vibrational contribution is in the range of −5 to 19 kJ mol⁻¹. © 2017 Wiley Periodicals, Inc.     

Manganese‐Catalyzed Dehydrogenative [4+2] Annulation of N?H Imines and Alkynes by C?H/N?H Activation

Described herein is a manganese‐catalyzed dehydrogenative [4+2] annulation of N H imines and alkynes, a reaction providing highly atom‐economical access to diverse isoquinolines. This transformation represents the first example of manganese‐catalyzed C H activation of imines; the stoichiometric variant of the cyclomanganation was reported in 1971. The redox neutral reaction produces H₂ as the major byproduct and eliminates the need for any oxidants, external ligands, or additives, thus standing out from known isoquinoline synthesis by transition‐metal‐catalyzed C H activation. Mechanistic studies revealed the five‐membered manganacycle and manganese hydride species as key reaction intermediates in the catalytic cycle.