Basicity of some aliphatic amines in mixed H2O?CH3CN solvents

The values of pK_a^ms (K_a^ms represents ionization constant of conjugate acid of amine base in mixed water–acetonitrile solvent) for all amines, except for charged amine bases, show a mild decrease (ca. 0.1–0.4 pK units) with the increase in CH₃CN content from 2 to ∼60% v/v. However, the pK_a^ms values at 70% v/v CH₃CN become nearly equal or slightly larger (by ≤0.7 pK units) than the corresponding pK_a^ms at 2% v/v CH₃CN for all neutral and charged amines. The values of pK_a^ms for phenol increase from 10.17 to 13.38 with the increase in the content of CH₃CN from 2 to 70% v/v in mixed aqueous solvent. Taft reaction constants, ρ*, obtained from the plots of pK_a^ms against ∑σ* for primary and secondary amines decrease by ca. 0.8 ρ* units with the increase in the CH₃CN content from 2 to 70% v/v. The values of pK_a^ms show an empirical linear relationship with the corresponding values of pK_a^w (where pK_a^w represents the pK_a obtained in aqueous solvent containing 2% v/v CH₃CN), which allows the estimation of a pK_a in mixed H₂O CH₃CN solvents from that in water. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 146–152, 2000     

The effect of methyl group on the cooperativity between three types of hydrogen bond: O?H···O,C?H···O,and O?H···π

The effect of the methyl group on the cooperativity between three types of hydrogen bond (O H···O, C H···O, and O H···π) in cyclic complex involving an acetylene and two waters has been studied on the basis of high-level ab initio calculations. The total interaction energy of three hydrogen bonds increases as the number of methyl group in the complex increases. The binding distances of O H···π and O H···O hydrogen bonds shorten, while that of C H···O hydrogen bond elongates with increasing methyl group. This indicates that addition of methyl group leads to enhancement of O H···π and O H···O hydrogen bonds, and weakening of C H···O hydrogen bond, as also shown in frequency shift, chemical shifts, charge populations, and stabilization energies of orbital interactions. Although the presence of methyl group has a complicated effect on different type of hydrogen bond, the cooperativity of three hydrogen bonds increases in general with the addition of methyl group. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

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.     

Stereoselective Synthesis of Highly Substituted Tetrahydrofurans through Diverted Carbene O?H Insertion Reaction

Copper‐ or rhodium‐catalyzed reactions of diazocarbonyl compounds with β‐hydroxyketones give highly substituted tetrahydrofurans with excellent diastereoselectivity. Under mild conditions, the single‐step process starts as a carbene O H insertion reaction, but is diverted by an intramolecular aldol reaction.     

An ab initio interpretation in gas phase and aqueous solution of the generalized anomeric effect in R?O?CR2?NR2 (R = H,CH3)

The conformational stability of aminomethanol and its methylated derivatives has been investigated by means of ab initio methods in the gas phase and aqueous solution. Among the computational levels employed, HF/6‐31G**//HF/6‐31G** calculations correctly describe the conformational features of this series of compounds, and agree well with the results obtained using larger basis sets and including ZPE or electron correlation corrections. Calculated energies and geometries follow the known trends associated to the generalized anomeric effect. Thus, the most stable conformers exhibit preferences for the trans orientations of the Lp N C O and Lp O C N moieties. However, reverse anomeric effects are observed when a methyl group is bonded to the oxygen, because the Lp O C N unit prefers a gauche orientation (that is, trans Me O C N). The natural bond orbital (NBO) method was employed to explain the cited conformational preferences. According to the NBO results, trans arrangements are preferred because the stabilization due to charge delocalization is more important than electrostatic and steric contributions. This explanation agrees with the conclusions obtained by other independent procedures based on energy decomposition schemes. The NBO method was also used to explain the origin of the rotational barriers around the C O and C N bonds in terms of the balance between unfavorable hyperconjugation and electrostatic and steric effects. Changes in conformational stability caused by methylations in different molecular positions were also explained by the influence of the methyl groups on lone‐pair delocalization and on steric effects. Finally, the effect of solvation was studied by means of the ab initio PCM method, and the significant changes on relative energies found were analyzed. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 462–477, 2000     

O?O bond homolysis in hydrogen peroxide

O O bond homolysis in hydrogen peroxide (H₂O₂) has been studied using theoretical methods of four conceptually different types: hybrid DFT (B3LYP, M06‐2X), double‐hybrid DFT (B2‐PLYP), coupled‐cluster (CCSD(T)), and multiconfigurational (CASPT2). In addition, the effects of basis set size have also been analyzed. For all of these methods, the O O bond homolysis in hydrogen peroxide has been found to proceed through hydrogen bonded radical pair complexes. Reaction barriers for collapse of the radical pairs to hydrogen peroxide are minute, leading to an overall very flat potential energy surface. However, hydrogen bonding energies in the radical pair complex expressed as the energy difference to two separate hydroxyl radicals are sizeable and exceed 10 kJ/mol for all theoretical methods considered in this study. © 2017 Wiley Periodicals, Inc.     

Singlet methylene insertion into polar O?H and N?H bonds of water and ammonia—Ab initio and DFT study

Correlated ab initio molecular orbital, DFT, QCISD, G3MP2, and QCISD(T) calculations have been used to investigate the geometries, energetics, and mechanisms governing the insertion reactions of ¹CH₂ into O H and N H bonds of water and ammonia, respectively, in gas phase adopting 6‐311++g(d, p) basis set. It is found that ¹CH₂ reacts with water and ammonia to produce the ylide‐like intermediates H₂C OH₂ and H₂C NH₃, which in turn undergo 1,2‐hydrogen shift to produce methanol and methylamine, respectively. Results obtained indicate that in the gas phase, the ylides and the transition states are located below the reactants' energy levels. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

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).

     

Role of O?H bonding in catalytic activity of Nb2O5 during the course of dehydrogenation of MgH2

The hydrogen desorption reaction of MgH₂, MgH₂ → Mg + H₂, is accelerated by the addition of metal oxide catalysts (e.g., Nb₂O₅). From our theoretical calculation of electronic structure, it was predicted that the catalytic activities of metal oxides are related closely to the O? H interaction operating at the interface between oxide catalyst and MgH₂. In this study, the O? H vibration on the Nb₂O₅‐catalyzed MgH₂ was investigated experimentally using FTIR spectroscopy. The broad absorption band due to the O? H stretching mode was observed in the region of 2,800–3,600 cm^?1 in the FTIR spectra of the specimens when hydrogen desorption reaction was in progress. The absorbance of the band decreased monotonously with decreasing hydrogen content in the specimen during the course of dehydrogenation of MgH₂. This experimental result was in agreement with our prediction for the existence of O? H interaction in the hydrogen desorption process. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

A Convenient Photocatalytic Fluorination of Unactivated C?H Bonds

Fluorination reactions are essential to modern medicinal chemistry, thus providing a means to block site‐selective metabolic degradation of drugs and access radiotracers for positron emission tomography imaging. Despite current sophistication in fluorination reagents and processes, the fluorination of unactivated C H bonds remains a significant challenge. Reported herein is a convenient and economic process for direct fluorination of unactivated C H bonds that exploits the hydrogen abstracting ability of a decatungstate photocatalyst in combination with the mild fluorine atom transfer reagent N‐fluorobenzenesulfonimide. This operationally straightforward reaction provides direct access to a wide range of fluorinated organic molecules, including structurally complex natural products, acyl fluorides, and fluorinated amino acid derivatives.     

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     

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.     

Catalytic Methylation of C?H Bonds Using CO2 and H2

Formation of C C bonds from CO₂ is a much sought after reaction in organic synthesis. To date, other than C H carboxylations using stoichiometric amounts of metals, base, or organometallic reagents, little is known about C C bond formation. In fact, to the best of our knowledge no catalytic methylation of C H bonds using CO₂ and H₂ has been reported. Described herein is the combination of CO₂ and H₂ for efficient methylation of carbon nucleophiles such as indoles, pyrroles, and electron‐rich arenes. Comparison experiments which employ paraformaldehyde show similar reactivity for the CO₂/H₂ system.     

Evaluation of a Combined Quantum Chemical Method Used in Calculating O?H Bond Dissociation Enthalpy

O? H bond dissociation enthalpies (BDE) for a variety of substituted phenols were calculated using a combined quantum chemical method. It is found that the calculated O? H BDE correlated well with the recommended values, except for ortho‐tert‐butyl substituted phenols. For the electron‐donating group substituted phenols the calculated O? H BDE are slightly higher than the recommended values, however, for the electron‐withdrawing group substituted phenols the calculated O? H BDE are slightly lower than the recommended values.     

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.     

Resolving the enigma of prebiotic C?O?P bond formation: Prebiotic hydrothermal synthesis of important biological phosphate esters

Important biological phosphate esters such as sn‐glycerol‐3‐phosphate, glycerol‐2‐phosphate, and phosphoethanolamine were synthesized under hydrothermal conditions. Phosphorus was incorporated into the biomolecules, leading to the formation of C O P type compounds hydrothermally. Only perlite‐catalyzed reaction at 180°C could result in the formation of sn‐glycerol‐3‐phosphate, whereas glycerol‐2‐phosphate could be easily synthesized at 100°C with or without minerals and phosphoethanolamine was obtained within a temperature range of 100 to 120°C. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 21:161–167, 2010; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20591     

Studies on the Synthesis and Characterization of Pd?TiO2?SiO2 Nanocomposite for Electroanalytical Applications

A nanocomposite of Pd? TiO₂? SiO₂ is developed through a sol‐gel process from the reaction products of titanium isopropoxide followed by mixing the same with palladium linked 3‐glycidoxypropyltrimethoxysilane. The reaction product is sonicated and calcinated to obtain the nanocomposite of Pd? TiO₂? SiO₂. The calcination at 600 °C yielded an amorphous structure whereas at 900 °C it resulted into a nanocrystalline structure. The nanocomposite of palladium was further characterized by TEM, XRD, IR and EDS. The material acts as an efficient electrocatalyst. Electrocatalysis of ascorbic acid is observed at 0.1 V vs. Ag/AgCl, shows linearity between 1 µM and 1 mM in 0.1 M phosphate buffer (pH 7.0).