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.     

Bifunctional transition metal‐based molecular catalysts for asymmetric C?C and C?N bond formation

This paper describes the recent advances in the conceptually new bifunctional Ir and Ru catalysts for asymmetric catalytic reactions. These reactions include the enantioselective Michael addition of 1,3‐dicarbonyl compounds to cyclic enones and nitroalkenes, and the enantioselective direct amination of α‐cyanoacetates with diazoesters. The outcome of these reactions in terms of reactivity and selectivity was delicately influenced by the catalyst structures and the reaction conditions including the solvents used. Even with a 1 : 1 molar ratio of donors to acceptors, the reactions proceeded smoothly to give the corresponding chiral adducts with an excellent yield and enantiomeric excess (ee). Preliminary mechanistic studies showed that the key stage of the catalytic cycle is the interaction of the bifunctional catalyst with a pronucleophilic reagent that leads to stereoselective formation of C‐, O‐, or N‐bound complexes. The resulting protonated catalyst bearing metal‐bound nucleophiles readily reacts with electrophiles to provide C? C and C? N bond formation products in a highly stereoselective manner. © 2009 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 9: 106–123; 2009: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20172     

Cascade C?N and C?O bond constructions for the synthesis of dibenzoimidazo[1,4]oxazepines catalyzed by CuI/o-phen

A novel method for the synthesis of dibenzo[b,f]imidazo[1,2-d][1,4]oxazepine derivatives was described via cascade Csp² N and Csp² O bond constructions. It was a crossed double Ullmann reactions using 4,5-diaryl-2-(2-hydroxylphenyl)-1H-imidazole as the double nucleophilic centers in the presence of Cs₂CO₃, while 1-bromo-2-iodobenzene was used as a substrate catalyzed by CuI and o-phenanthroline in good yields.     

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.     

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.     

The nature of the N?O bond in amine oxides

Despite the ubiquitous presence of amine oxides in chemistry, there is no consensus about the nature of the N O bond in these compounds. In this work, we have used electron density analysis to investigate the nature of this bond in substituted amine oxides, R₃NO, and have compared it with the nature of the N O bond in hydroxylamines, R₂NOR, and model molecules that have well-established chemical bond character. The results showed that the N O bond length and relative stability are proportional to the inductive effect of the substituents. Quantum chemical topology, natural bond orbitals (NBO), and natural resonance theory (NRT) analyses indicated that the N O bond is polar covalent in all the studied amine oxides, but the ionic contribution is different. NBO and NRT analyses revealed that molecules with more electronegative substituents have strongly delocalized N O and N R bonds, whereas molecules with electropositive substituents have localized bonds.     

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     

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.     

PCM study of the solvent and substituent effects on bond dissociation energies of the C?NO bond

Quantum chemical calculations are used to estimate the equilibrium C? NO bond dissociation energies (BDEs) for eight X? NO molecule (X = CCl₃, C₆F₅, CH₃, CH₃CH₂, iC₃H₇, tC₄H₉, CH₂CHCH₂, and C₆H₅CH₂). These compounds are studied by employing the hybrid density functional theory (B3LYP, B3PW91, B3P86) methods together with 6‐31G** and 6‐311G** basis sets and the complete basis set (CBS‐QB3) method. The obtained results are compared with the available experimental results. It is demonstrated that B3P86/6‐31G** and CBS‐QB3 methods are accurate for computing the reliable BDEs for the X? NO molecule. Considering the inevitably computational cost of CBS‐QB3 method and the reliability of the B3P86 calculations, B3P86 method with 6‐31G** basis set may be more suitable to calculate the BDEs of the C? NO bond. The solvent effects on the BDEs of the C? NO bond are analyzed and it is shown that the C? NO BDEs in a vacuum computed by using B3PW91/6‐311G** method are the closest to the computed values in acetontrile and the average solvent effect is 1.48 kcal/mol. Subsequently, the substituent effects of the BDEs of the C? NO bond are further analyzed and it is found that electron denoting group stabilizes the radical and as a result BDE decreases; whereas electron withdrawing group stabilizes the group state of the molecule and thus increases the BDE from the parent molecule. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

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.     

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.     

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     

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     

An efficient construction of C?N bond under catalyst-free and room temperature conditions

An efficient and facile process for the construction of C N bond from 1,6-conjugate addition reaction of 4-arylidene-2,6-di-tert-butylcyclohexa-2,5-dien-1-one (or substituted 3-[3,5-di-tert-butyl-4-oxocyclohexa-2,5-dien-1-ylidene]indolin-2-one) and substituted 1,2,3,4-tetrahydroisoquinoline (or substituted indoline) at room temperature was reported. The advantages of this reaction were catalyst-free, mild condition, broad substrate scope, and environmentally benign process.     

P?N ligands in lanthanide chemistry

The coordination chemistry of inorganic amides in Group 3 and lanthanide chemistry is discussed. Three different ligand systems (phosphino‐amides, bis(phosphino)amides, and bis(phosphinimino)methanides) that consist of one or more P N units were used. In this series the steric demand of the ligands is increased in a stepwise fashion and the negative charge is delocalized over more atoms. These properties were used in the design of new lanthanide complexes. For all three compounds the synthesis of the alkali metal derivatives is reported first, followed by the reaction of the alkali metal salts with various lanthanide trichlorides. Further reactions of the obtained lanthanide complexes as well as their application as catalysts are discussed. Most of the reported complexes show a dynamic behavior in solution. In phosphinoamide and bis(phosphino)amide complexes, in which the phosphorus atom is in oxidation state +3, there is always a weak coordination of the phosphorus atom to the lanthanide atom observed. In bis(phosphinimino)methanide complexes, in which the phosphorus atom is in oxidation state +5, no such interaction is noticeable. Instead a weak coordination of the methine atom to the center metal can be seen in the solid state. © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:514–520, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10096