Aluminum Hydride Catalyzed Hydroboration of Alkynes

An aluminum‐catalyzed hydroboration of alkynes using either the commercially available aluminum hydride DIBAL‐H or bench‐stable Et₃Al?DABCO as the catalyst and H‐Bpin as both the boron reagent and stoichiometric hydride source has been developed. Mechanistic studies revealed a unique mode of reactivity in which the reaction is proposed to proceed through hydroalumination and σ‐bond metathesis between the resultant alkenyl aluminum species and HBpin, which acts to drive turnover of the catalytic cycle.     

Is PdII‐Promoted σ‐Bond Metathesis Mechanism Operative for the PdPEPPSI Complex‐Catalyzed Amination of Chlorobenzene with Aniline? Experiment and Theory

Reduction of the Pd?PEPPSI precatalyst to a Pd⁰ species is generally thought to be essential to drive Buchwald–Hartwig amination reactions through the well‐ documented Pd⁰/Pd^II catalytic cycle and little attention has been paid to other possible mechanisms. Considered here is the Pd?PEPPSI‐catalyzed aryl amination of chlorobenzene with aniline. A neat reaction system was used in new experiments, from which the potentially reductive roles of the solvent and labile ligand of the PEPPSI complex in leading to Pd⁰ species are ruled out. Computational results demonstrate that anilido‐containing Pd^II intermediates involving σ‐bond metathesis in pathways leading to the diphenylamine product have relatively low barriers. Such pathways are more favorable energetically than the corresponding reductive elimination reactions resulting in Pd⁰ species and other putative routes, such as the Pd^II/Pd^IV mechanism, single electron transfer mechanism, and halide atom transfer mechanism. In some special cases, if reactants/additives are inadequate to reduce a Pd^II precatalyst, a Pd^II‐involved σ‐bond metathesis mechanism might be feasible to drive the Buchwald–Hartwig amination reactions.     

The competition of Y⋯o and X⋯n halogen bonds to enhance the group V σ‐hole interaction in the NCY⋯oPH3⋯NCX and OPH3⋯NCX⋯NCY (X,YF,Cl, and Br) complexes

The positive electrostatic potentials (ESP) outside the σ‐hole along the extension of O? P bond in O?PH₃ and the negative ESP outside the nitrogen atom along the extension of the C? N bond in NCX could form the Group V σ‐hole interaction O?PH₃?NCX. In this work, the complexes NCY?O?PH₃?NCX and O?PH₃?NCX?NCY (X, Y?F, Cl, Br) were designed to investigate the enhancing effects of Y?O and X?N halogen bonds on the P?N Group V σ‐hole interaction. With the addition of Y?O halogen bond, the V _{S, max} values outside the σ‐hole region of O?PH₃ becomes increasingly positive resulting in a stronger and more polarizable P?N interaction. With the addition of X?N halogen bond, the V _{S, min} values outside the nitrogen atom of NCX becomes increasingly negative, also resulting in a stronger and more polarizable P?N interaction. The Y?O halogen bonds affect the σ‐hole region (decreased density region) outside the phosphorus atom more than the P?N internuclear region (increased density region outside the nitrogen atom), while it is contrary for the X?N halogen bonds. © 2015 Wiley Periodicals, Inc.     

New Routes to a Series of σ‐Borane/Borate Complexes of Molybdenum and Ruthenium

A series of agostic σ‐borane/borate complexes have been synthesized and structurally characterized from simple borane adducts. A room‐temperature reaction of [Cp*Mo(CO)₃Me], 1 with Li[BH₃(EPh)] (Cp*=pentamethylcyclopentadienyl, E=S, Se, Te) yielded hydroborate complexes [Cp*Mo(CO)₂(μ‐H)BH₂EPh] in good yields. With 2‐mercapto‐benzothiazole, an N,S‐carbene‐anchored σ‐borate complex [Cp*Mo(CO)₂BH₃(1‐benzothiazol‐2‐ylidene)] ( 5 ) was isolated. Further, a transmetalation of the B‐agostic ruthenium complex [Cp*Ru(μ‐H)BHL₂] ( 6 , L=C₇H₄NS₂) with [Mn₂(CO)₁₀] affords a new B‐agostic complex, [Mn(CO)₃(μ‐H)BHL₂] ( 7 ) with the same structural motif in which the central metal is replaced by an isolobal and isoelectronic [Mn(CO)₃] unit. Natural‐bond‐orbital analyses of 5–7 indicate significant delocalization of the electron density from the filled σ_B?H orbital to the vacant metal orbital.     

Enantioselective Palladium‐Catalyzed Insertion of α‐Aryl‐α‐diazoacetates into the OH Bonds of Phenols

A palladium‐catalyzed asymmetric O? H insertion reaction was developed. Palladium complexes with chiral spiro bisoxazoline ligands promoted the insertion of α‐aryl‐α‐diazoacetates into the O? H bond of phenols with high yield and excellent enantioselectivity under mild reaction conditions. This palladium‐catalyzed asymmetric O? H insertion reaction provided an efficient and highly enantioselective method for the preparation of synthetically useful optically active α‐aryl‐α‐aryloxyacetates.     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary CH Bonds

Achieving selective C? H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C? H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C? H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H? Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C? H bonds.     

Long‐range π‐type hydrogen bond in dimers CH2?HF,CH2O?H2O,and CH2O?NH3

Using four basis bets, (6‐311G(d,p), 6‐31+G(d,p), 6‐31++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for the dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. In contrast with the above three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD (T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Rhodium(I)‐Catalyzed Enantioselective Cyclization of Enynes by Intramolecular Cleavage of the Rh−C Bond by a Tethered Hydroxy Group

Rhodium(I)‐catalyzed enantioselective intramolecular cyclization of enynes having a hydroxy group in the tether was investigated, and various cyclic compounds possessing a chiral quaternary carbon center were obtained in high yields with high ees. In this cyclization, a Rh?C(sp²) bond in the rhodacyclopentene intermediate, which was formed by enantioselective oxidative cycloaddition of enynes to a chiral rhodium(I) complex, was intramolecularly cleaved by σ‐bond metathesis of a tethered O?H bond in the substrate. Furthermore, it was found that the cyclic compounds were obtained with high ees even when the starting materials having a racemic secondary alcohol moiety were used in this reaction.     

Confirmed by X‐ray Crystallography: The B⋅B One‐Electron σ Bond

Is one electron sufficient to bring about significant σ bonding between two atoms? The chemist’s view on the chemical bond is usually tied to the concept of shared electron pairs, and not too much experimental evidence exists to challenge this firm belief. Whilst species with the unusual one‐electron σ‐bonding motif between homonuclear atoms have so far been identified mainly by spectroscopic evidence, we present herein the first crystallographic characterization, augmented by a detailed quantum‐chemical validation, for a radical anion featuring a B?B one‐electron‐two‐center σ bond.     

The Bright Future of Unconventional σ/π‐Hole Interactions

Non‐covalent interactions play a crucial role in (supramolecular) chemistry and much of biology. Supramolecular forces can indeed determine the structure and function of a host–guest system. Many sensors, for example, rely on reversible bonding with the analyte. Natural machineries also often have a significant non‐covalent component (e.g. protein folding, recognition) and rational interference in such ‘living’ devices can have pharmacological implications. For the rational design/tweaking of supramolecular systems it is helpful to know what supramolecular synthons are available and to understand the forces that make these synthons stick to one another. In this review we focus on σ‐hole and π‐hole interactions. A σ‐ or π‐hole can be seen as positive electrostatic potential on unpopulated σ* or π⁽*⁾ orbitals, which are thus capable of interacting with some electron dense region. A σ‐hole is typically located along the vector of a covalent bond such as X?H or X?Hlg (X=any atom, Hlg=halogen), which are respectively known as hydrogen and halogen bond donors. Only recently it has become clear that σ‐holes can also be found along a covalent bond with chalcogen (X?Ch), pnictogen (X?Pn) and tetrel (X?Tr) atoms. Interactions with these synthons are named chalcogen, pnigtogen and tetrel interactions. A π‐hole is typically located perpendicular to the molecular framework of diatomic π‐systems such as carbonyls, or conjugated π‐systems such as hexafluorobenzene. Anion–π and lone‐pair–π interactions are examples of named π‐hole interactions between conjugated π‐systems and anions or lone‐pair electrons respectively. While the above nomenclature indicates the distinct chemical identity of the supramolecular synthon acting as Lewis acid, it is worth stressing that the underlying physics is very similar. This implies that interactions that are now not so well‐established might turn out to be equally useful as conventional hydrogen and halogen bonds. In summary, we describe the physical nature of σ‐ and π‐hole interactions, present a selection of inquiries that utilise σ‐ and π‐holes, and give an overview of analyses of structural databases (CSD/PDB) that demonstrate how prevalent these interactions already are in solid‐state structures.     

Correlated one‐electron wave functions

Using four basis sets, 6‐311G(d,p), 6‐31+G(d,p), 6‐311++G(2d,2p), and 6‐311++G(3df,3pd), the optimized structures with all real frequencies were obtained at the MP2 level for dimers CH₂O? HF, CH₂O? H₂O, CH₂O? NH₃, and CH₂O? CH₄. The structures of CH₂O? HF, CH₂O? H₂O, and CH₂O? NH₃ are cycle‐shaped, which result from the larger bend of σ‐type hydrogen bonds. The bend of σ‐type H‐bond O…H? Y (Y?F, O, N) is illustrated and interpreted by an attractive interaction of a chemically intuitive π‐type hydrogen bond. The π‐type hydrogen bond is the interaction between one of the acidic H atoms of CH₂O and lone pair(s) on the F atom in HF, the O atom in H₂O, or the N atom in NH₃. By contrast with above the three dimers, for CH₂O? CH₄, because there is not a π‐type hydrogen‐bond to bend its linear hydrogen bond, the structure of CH₂O? CH₄ is a noncyclic shaped. The interaction energy of hydrogen bonds and the π‐type H‐bond are calculated and discussed at the CCSD(T)/6‐311++G(3df,3pd) level. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Thermal Ammonia Activation by Cationic Transition‐Metal Hydrides of the First Row – Small but Mighty

The thermal reactions of cationic 3d transition‐metal hydrides MH⁺ (M=Sc–Zn, except V and Cu) with ammonia have been studied by gas‐phase experiments and computational methods. There are three primary reaction channels: 1) H₂ elimination by N? H bond activation, 2) ligand exchange under the formation of M(NH₃)⁺, and 3) proton transfer to yield NH₄⁺. Computational studies of these three reaction channels have been performed for the couples MH⁺/NH₃ (M=Sc–Zn) to elucidate mechanistic aspects and characteristic reaction patterns of the first row. For N? H activation, σ‐bond metathesis was found to be operative.     

Construction of Hexahydrophenanthrenes By Rhodium(I)‐Catalyzed Cycloisomerization of Benzylallene‐Substituted Internal Alkynes through C−H Activation

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)₂]₂ effects a novel cycloisomerization by C(sp²)?H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp²)?H bond on the benzene ring and the C(sp²)?Rh^III bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates.     

Oxygen‐chelated indenylidene ruthenium catalysts for olefin metathesis

Olefin metathesis is a transition metal‐mediated transformation that rearranges the carbon atoms of the carbon–carbon double bond of olefins. This reaction has become one of the most important and powerful reactions. Therefore development of new, well‐defined, highly active and selective catalysts is very desirable and a valuable goal. This mini‐review mainly introduces the development of ruthenium catalysts in olefin metathesis highlighting oxygen‐chelated indenylidene ruthenium catalysts. Applying an alkoxyl group on the indenylidene ligand fragment can generate the Ru ? O chelating bond. Additionally, various modifications of the ligand as well as the catalytic activity for ring‐closing metathesis reaction and selectivity of cross metathesis reaction are overviewed. Finally, the perspectives on the development of new catalysts are summarized. Copyright © 2015 John Wiley & Sons, Ltd.     

α‐Halogenoacetanilides as Hydrogen‐Bonding Organocatalysts that Activate Carbonyl Bonds: Fluorine versus Chlorine and Bromine

α‐Halogenoacetanilides (X=F, Cl, Br) were examined as H‐bonding organocatalysts designed for the double activation of C?O bonds through NH and CH donor groups. Depending on the halide substituents, the double H‐bond involved a nonconventional C?H???O interaction with either a H?CX_n (n=1–2, X=Cl, Br) or a H?C_Ar bond (X=F), as shown in the solid‐state crystal structures and by molecular modeling. In addition, the catalytic properties of α‐halogenoacetanilides were evaluated in the ring‐opening polymerization of lactide, in the presence of a tertiary amine as cocatalyst. The α‐dichloro‐ and α‐dibromoacetanilides containing electron‐deficient aromatic groups afforded the most attractive double H‐bonding properties towards C?O bonds, with a N?H???O???H?CX₂ interaction.     

σ‐Noninnocence: Masked Phenyl‐Cation Transfer at Formal NiIV

Reductive elimination is an elementary organometallic reaction step involving a formal oxidation state change of ?2 at a transition‐metal center. For a series of formal high‐valent Ni^IV complexes, aryl–CF₃ bond‐forming reductive elimination was reported to occur readily (Bour et al. J. Am. Chem. Soc. 2015 , 137, 8034–8037). We report a computational analysis of this reaction and find that, unexpectedly, the formal Ni^IV centers are better described as approaching a +II oxidation state, originating from highly covalent metal–ligand bonds, a phenomenon attributable to σ‐noninnocence. A direct consequence is that the elimination of aryl–CF₃ products occurs in an essentially redox‐neutral fashion, as opposed to a reductive elimination. This is supported by an electron flow analysis which shows that an anionic CF₃ group is transferred to an electrophilic aryl group. The uncovered role of σ‐noninnocence in metal–ligand bonding, and of an essentially redox‐neutral elimination as an elementary organometallic reaction step, may constitute concepts of broad relevance to organometallic chemistry.     

Synthesis of Functionalized Vinylgermanes through a New Ruthenium‐Catalyzed Coupling Reaction

Vinyl‐substituted germanes react stereo‐ and regioselectively with olefins in the presence of complexes containing Ru? H and Ru? Ge bonds with the formation of functionalized vinylgermanes that cannot be synthesized by olefin cross‐metathesis procedures. The reaction opens a new catalytic route for preparation of a class of organogermanes that are potent organometallic reagents for organic synthesis because they show very low toxicity and could replace organotin compounds. The mechanism of this new catalytic route was proven to involve an interesting insertion of the vinylgermane into the Ru? H bond and β‐Ge transfer to the metal with elimination of ethylene and generation of an Ru? Ge bond, followed by insertion of the alkene into the Ru? Ge bond and β‐H transfer to the metal to eliminate the substituted vinylgermane.     

DFT Mechanistic Study on Diene Metathesis Catalyzed by Ru‐Based Grubbs–Hoveyda‐Type Carbenes: The Key Role of π‐Electron Density Delocalization in the Hoveyda Ligand

The catalytic activity and catalyst recovery of two heterogenized ruthenium‐based precatalysts ( H and NO₂(4) ) in diene ring‐closing metathesis have been studied by means of density functional calculations at the B3LYP level of theory. For comparison and rationalization of the key factors that lead to higher activities and higher catalyst recoveries, four other Grubbs–Hoveyda complexes have also been investigated. The full catalytic cycle (catalyst formation, propagation, and precatalyst regeneration) has been considered. DFT calculations suggest that either for the homogeneous and heterogenized systems the activity of the catalysts mainly depends on the ability of the precursor to generate the propagating carbene. This ability does not correlate with the traditionally identified key factor, the Ru???O interaction strength. In contrast, precatalysts with lower alkoxy‐dissociation energy barriers and lower stabilities compared with the propagating carbene also present larger C1? C2 bond length (i.e., lower π character of the C? C bond that exists between the metal–carbene (Ru?C) and the phenyl ring of the Hoveyda ligand). Catalyst recovery, regardless of whether a release–return mechanism occurs or not, is also mainly determined by the π delocalization. Therefore, future Grubbs–Hoveyda‐type catalyst development should be based on fine‐tuning the π‐electron density of the phenyl moiety, with the subsequent effect on the metalloaromaticity of the ruthenafurane ring, rather than considering the modification of the Ru???O interaction.     

Adsorption of polyfunctional 5‐fluorouracil and 2,4‐dithio‐5‐fluorouracil on Au(111) surface: Structure,energy, and electronic transmission

Adsorption of 5‐fluorouracil (5‐FU) and 2,4‐dithio‐5‐fluorouracil (2,4‐DT‐5‐FU) on Au(111) surface at low coverage is studied by using periodic‐slab‐density functional theory calculation. Isolated 5‐FU molecule adsorbs preferentially at bridge site in a vertical configuration via N? H group by forming the N? H···Au nonconventional H‐bond. The formation of the anchor Au? O bond is not observed. Substitution of oxygen atoms of 5‐FU with sulfur strongly influences the nature of adsorption and leads to the Au? S anchor bond and the N? H···Au nonconventional H‐bond of single 2,4‐DT‐5‐FU molecule on Au(111) surface. The adsorption site and orientation of 2,4‐DT‐5‐FU molecule on the surface are similar to those of 5‐FU. The metal–molecule coupling effects at asymmetric Au/S(N? H)S/mol/C? H/Au and Au/N? H/mol/O/Au transport junctions and symmetric Au/S(N? H)S/mol/mol/S(N? H)S/Au and Au/O/mol/mol/O/Au transport junctions are also investigated. The electronic structure is analyzed in detail, and the obtained results are used for illustrating the electron transmission in metal–molecule–metal systems. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Competition of Nucleophilic Aromatic Substitution, σ‐Bond Metathesis,and syn Hydrometalation in Titanium(III)‐Catalyzed Hydrodefluorination of Arenes

Several functionalized and non‐functionalized perfluoroarenes were catalytically transformed into their para‐hydrodefluorinated products by using catalytic amounts of titanocene difluoride and stoichiometric amounts diphenylsilane. Turnover numbers of up to 93 were observed. Solution density functional theory calculations at the M06‐2X/TZ(PCM)//M06‐2X/TZ(PCM) level of theory provided insight into the mechanism of Ti^III‐catalyzed aromatic hydrodefluorination. Two different substrate approaches, with a Ti–F interaction (pathway A) and without a Ti–F interaction (pathway B), are possible. Pathway A leads to a σ‐bond metathesis transition state, whereas pathway B proceeds by means of a two‐step mechanism through a syn‐hydrometalation intermediate or through a Meisenheimer intermediate. Both pathways are competitive over a broad range of substrates.