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.     

Revealing Unexpected Mechanisms for Nucleophilic Attack on S?S and Se?Se Bridges

The reactivity of disulfide and diselenide derivatives towards F^? and CN^? nucleophiles has been investigated by means of B3PW91/6‐311+G(2df,p) calculations. This theoretical survey shows that these processes, in contrast with the generally accepted view of disulfide and diselenide linkages, do not always lead to S? S or Se? Se bond cleavage. In fact, S? S or Se? Se bond fission is the most favorable process only when the substituents attached to the S or the Se atoms are not very electronegative. Highly electronegative substituents (X) strongly favor S? X bond fission. This significant difference in the observed reactivity patterns is directly related to the change in the nature of the LUMO orbital of the disulfide or diselenide derivative as the electronegativity of the substituents increases. For weakly electronegative substituents, the LUMO is a σ‐type S? S (or Se? Se) antibonding orbital, but as the electronegativity of the substituents increases the π‐type S? X antibonding orbital stabilizes and becomes the LUMO. The observed reactivity also changes with the nature of the nucleophile and with the S or Se atom that undergoes the nucleophilic attack in asymmetric disulfides and diselenides. The activation strain model provides interesting insights into these processes. There are significant similarities between the reactivity of disulfides and diselenides, although some dissimilarities are also observed, usually related to the different interaction energies between the fragments produced in the fragmentation process.     

Stannic chloride‐induced unsymmetrical C?Se bond cleavage of bis(N,N‐dimethylcarbamoylseleno)methanes: Novel generation of selenoaldehydes

Treatment of bis(N,N‐dimethylcar‐ bamoylseleno)methanes with SnCl₄ afforded β‐1,3,5‐triselenanes in moderate to high yields, and the key intermediates of the reactions, i.e., acylselonium ions and selenoaldehydes, were successfully trapped by using allyltrimethylsilane or 2,3‐dimethyl‐1,3‐butadiene to obtain the allylation products or the cycloadducts, respectively. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:125–135, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20190     

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.     

Autoxidation of acridanyl and benzhydryl carbanions adjacent to sulfur groups leading to C?S bond cleavage with weak chemiluminescence

Autoxidation of acridanyl and benzhydryl carbanions stabilized by sulfur groups gave the corresponding ketones via radical intermediates, being accompanied by a weak light emission. While oxidation of 9‐sulfenylacridanes ( 1a and 1b ) in the presence of tert‐BuOK showed direct chemiluminescence (CL) due to excited 10‐methylacridone, that of benzhydryl phenyl sulfide ( 2 ) and benzhydryldimethylsulfomium bromide ( 3 ) displayed CL in the presence of a fluorescer, 9,10‐dibromoanthracene (DBA), due to excited benzophenone. The yields of excited species in the present oxidation are low but comparable to acyclic peroxide systems. The bimolecular energy transfer from triplet benzophenone to DBA was established in CL of the sulfonium ylide from 3 . © 2002 Wiley Periodicals, Inc. Heteroatom Chem 13:252–257, 2002; Published online in Wiley Interscience (www.interscience.wiley.com). DOI 10.1002/hc.10025     

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.     

Palladium(II)?NHC complexes containing benzimidazole ligand as a catalyst for C?N bond formation

The reaction of 2‐(2‐bromoethyl)‐1,3‐dioxane with 1‐alkylbenzimidazole derivatives results in the formation of the new benzimidazolium salts (1). The reaction of Pd(OAc)₂ with 1,3‐dialkylbenzimidazolium salts (1a–c) yields palladium N‐heterocyclic carbene (NHC) complexes (2a–c). All synthesized compounds were characterized by ¹H NMR, ¹³ C NMR, IR and elemental analysis techniques which support the proposed structures. As catalysts, these new palladium complexes offer a simple and efficient methodology for the synthesis of triarylamines and secondary amines from anilines and amines and in a single step with potassium tertiary butoxide as a base. Copyright © 2010 John Wiley & Sons, Ltd.     

Correlated ab initio calculations of one-bond 31P?77Se and 31P?125Te spin–spin coupling constants in a series of PSe and PTe systems accounting for relativistic effects (part 2)

Synthetic chalcogen–phosphorus chemistry permanently makes new challenges to computational Nuclear Magnetic Resonance (NMR) spectroscopy, which has proven to be a powerful tool of structural analysis of chalcogen–phosphorus compounds. This paper reports on the calculations of one-bond ³¹P ⁷⁷Se and ³¹P ¹²⁵Te NMR spin–spin coupling constants (SSCCs) in the series of phosphine selenides and tellurides. The applicability of the combined computational approach to the one-bond ³¹P ⁷⁷Se and ³¹P ¹²⁵Te SSCCs, incorporating the composite nonrelativistic scheme, built of high-accuracy correlated SOPPA (CC2) and Coupled Cluster Single and Double (CCSD) methods and the Density Functional Theory (DFT) relativistic corrections (four-component level), was examined against the experiment and another scheme based on the four-component relativistic DFT method. A special J-oriented basis set (acv3z-J) for selenium and tellurium atoms, developed previously by the authors, was used throughout the NMR calculations in this work at the first time. The proposed computational methodologies (combined and ‘pure’) provided a reasonable accuracy for ³¹P ⁷⁷Se and ³¹P ¹²⁵Te SSCCs against experimental data, characterizing by the mean absolute percentage errors of about 4% and 1%, and 12% and 8% for selenium and tellurium species, respectively. The present study reports typical relativistic corrections to ⁷⁷Se ³¹P and ¹²⁵Te ³¹P SSCCs, calculated within the four-component DFT formalism for a broad series of tertiary phosphine selenides and tellurides with different substituents at phosphorus.     

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     

Intramolecular H?Ar Ligand Exchange between Silicon and Boron: Functionality Transfer of Si?H to B?H

o‐C₆H₄(SiR_3?nH_n)(BMes₂) ( 1 ; R=Me, Ph; n=1, 2) undergo Mes? H (Mes=mesityl) ligand exchange between the silicon atom and the boron atom to form o‐C₆H₄(SiMesR_3?nH_n?1)(BMesH) ( 6 ) upon heating. The resulting hydroborane intermediates ( 6 ) immediately react with benzaldehyde to afford their corresponding benzyloxyboranes ( 5 ). A DFT study of model compounds reveals the transition states of the ligand exchange. A hydride abstraction from the silicon atom by the boron center is key to reaching the transition states, which include the tricoordinate silyl‐cation moiety and the tetracoordinate hydridoborate moiety.     

A computational study of C?S bond dissociation enthalpies in petroleum chemistry

High‐level theoretical methods (BMK, B3LYP, B98, B3P86, B3PW91, PBE1PBE, PBE1KIS, MPWPW91, MPW1KCIS, TPSS1KCIS, G3, G3//BMK, and CBS‐Q) were utilized to study the carbon–sulfur bond dissociation enthalpies (BDEs) of hydrocarbons in petroleum chemistry. The performance of these methods was evaluated on the basis of a training set including the available experimental BDEs, and it was found that the BMK (Boese‐Martin for Kinetics) method had the best agreement with experimental values. By using the BMK method to calculate C S BDEs of saturated hydrocarbon, the main factors, which determine the changing trend of BDE values, were discussed. Results revealed that the repulsive energies played an important role in determining a change in the trend of BDEs as well as the radical effect. Good agreements were obtained between further calculated BDEs and the experimental ones for C S and C O bonds. Moreover, the same calculation method was applied to predict C S BDEs for which the experimental values were still unavailable. A range of predicted bond dissociation enthalpy values were provided according to the calculations. © 2010 Wiley Periodicals, Inc. Heteroatom Chem 22:97–105, 2011; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.20662     

C?H bond functionalization of aromatic heterocycles with chelating dicarbene palladium(II) and platinum(II) complexes

Chelating dicarbene complexes of palladium(II) and platinum(II) catalyse at room temperature with 1% catalyst loading the reaction of ethyl phenylpropiolate with aromatic heterocycles to yield synthetically useful intermediates for fine chemicals without the need to use prefunctionalized substrates. The reaction outcome was found to be strongly dependent on the nature of the anionic ligands at the metal complex. Addition of silver salts to replace halide ligands with more weakly coordinating anions improves the reaction yield and changes the product distributions: heterocycle? alkyne 2:1 adducts are obtained together with the usual hydroarylation products, which potentially broadens the scope of the reaction. The nature of the employed heterocycle, in particular its steric characteristics, is also found to strongly influence the outcome of the reaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Activation of Homolytic Si?Zn and Si?Hg Bond Cleavage,Mediated by a Pt0 Complex,via Novel Pt?Zn and Pt?Hg Compounds

The thermally stable [(tBuMe₂Si)₂M] (M=Zn, Hg) generate R₃Si^. radicals in the presence of [(dmpe)Pt(PEt₃)₂] at 60–80 °C. The reaction proceeds via hexacoordinate Pt complexes, (M=Zn ( 2 a and 2 b ), M=Hg ( 3 a and 3 b )) which were isolated and characterized. Mild warming or photolysis of 2 or 3 lead to homolytic dissociation of the Pt MSiR₃ bond generating silyl radicals and novel unstable pentacoordinate platinum paramagnetic complexes (M=Zn ( 5 ), Hg ( 6 )) whose structures were determined by EPR spectroscopy and DFT calculations.     

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.     

Interplay of Magnetic Exchange Interactions and Ni?S?Ni Bond Angles in Polynuclear Nickel(II) Complexes

The ability of bridging thiophenolate groups (RS^?) to transmit magnetic exchange interactions between paramagnetic Ni^II ions is examined. Specific attention is paid to complexes with large Ni? SR? Ni angles. For this purpose, dinuclear [Ni₂L¹(μ‐OAc)?I₂][I₅] ( 2 ) and trinuclear [Ni₃L²(OAc)₂][BPh₄]₂ ( 3 ), where H₂L¹ and H₂L² represent 24‐membered macrocyclic amino‐thiophenol ligands, are prepared and fully characterized by IR‐ and UV/Vis spectroscopy, X‐ray crystallography, static magnetization M measurements and high‐field electron spin resonance (HF‐ESR). The dinuclear complex 2 has a central N₃Ni₂(μ‐S)₂(μ‐OAc)Ni₂N₃ core with a mean Ni? S? Ni angle of 92°. The macrocycle L² supports a trinuclear complex 3 , with distorted octahedral N₂O₂S₂ and N₂O₃S coordination environments for one central and two terminal Ni^II ions, respectively. The Ni? S? Ni angles are at 132.8° and 133.5°. We find that the variation of the bond angles has a very strong impact on the magnetic properties of the Ni complexes. In the case of the Ni₂‐complex, temperature T and magnetic field B dependencies of M reveal a ferromagnetic coupling J=?29 cm^?1 between two Ni^II ions (H=JS₁S₂). HF‐ESR measurements yield a negative axial magnetic anisotropy (D<0) which implies a bistable (easy axis) magnetic ground state. In contrast, for the Ni₃‐complex we find an appreciable antiferromagnetic coupling J′=97 cm^?1 between the Ni^II ions and a positive axial magnetic anisotropy (D>0) which implies an easy plane situation.     

A New Dynamic Covalent Bond of Se?N: Towards Controlled Self‐Assembly and Disassembly

A new kind of Se? N dynamic covalent bond has been found that can form between the Se atom of a phenylselenyl halogen species and the N atom of a pyridine derivative, such as polystyrene‐b‐poly(4‐vinylpyridine). This Se? N dynamic covalent bond can be reversibly and rapidly formed or cleaved under acidic or basic conditions, respectively. Furthermore, the bond can be dynamically cleaved by heating or treatment with stronger electron‐donating pyridine derivatives. The multiple responses of Se? N bond to external stimuli has enriched the existing family of dynamic covalent bonds. It can be used for controlled and reversible self‐assembly and disassembly, which may find potential applications in a number of areas, including self‐healing materials and responsive assemblies.     

Reaction of diphenylphosphine oxide with a 10-methylacridinium salt. Reversible formation of a P?C covalent bond

Diphenylphosphine oxide ( 1 ′) reacts reversibly with a 10-methylacridinium salt ( 2 ) in acetonitrile at 20°C to equilibrate with a new salt 3 formed by the addition of 1 ′ to the cation of 2 . The forward reaction in this equilibrium proceeds via nucleophilic attack by the phosphorus atom of diphenylphosphinous acid ( 1 ), the trivalent tautomer of 1 ′, upon the 9-carbon atom of the cation of 2 , forming a phosphorus–carbon covalent bond. The equilibrium constant has been determined by UV-vis and ¹H NMR spectroscopy as well as by HPLC analysis. The reaction has been analyzed kinetically, and the results have been compared with those obtained in the similar reaction of an alkyl ester of 1 , the diphenylphosphinite ( 5 ), with 2 that gives the phosphonium salt 6 . It is suggested that 6 is much more stable than 3 . The equilibrium constant for the tautomerism between 1 ′ and 1 has also been estimated. © 1995 John Wiley & Sons, Inc.