Phenylimidorhenium(V) Complexes with 1,3‐Diethyl‐4,5‐dimethylimidazole‐2‐ ylidene Ligands

The phenylimidorhenium(V) complexes [Re(NPh)X₃(PPh₃)₂] (X = Cl, Br) react with the N‐heterocyclic carbene (NHC) 1,3‐diethyl‐4,5‐dimethylimidazole‐2‐ylidene (L^Et) under formation of the stable rhenium(V) complex cations [Re(NPh)X(L^Et)₄]²⁺ (X = Cl, Br), which can be isolated as their chloride or [PF₆]^? salts. The compounds are remarkably stable against air, moisture and ligand exchange. The hydroxo species [Re(NPh)(OH)(L^Et)₄]²⁺ is formed when moist solvents are used during the synthesis. The rhenium atoms in all three complexes are coordinated in a distorted octahedral fashion with the four NHC ligands in equatorial planes of the molecules. The Re–C(carbene) bond lengths between 2.171(8) and 2.221(3) Å indicate mainly σ‐bonding between the NHC ligand and the electron deficient d² metal atoms. Attempts to prepare analogous phenylimido complexes from [Re(NPh)Cl₃(PPh₃)₂] and 1,3‐diisopropyl‐4,5‐dimethylimidazole‐2‐ylidene (L^i?Pr) led to a cleavage of the rhenium‐nitrogen multiple bond and the formation of the dioxo complex [ReO₂(L^i?Pr)₄]⁺.     

Probing Reaction Mechanism of [1,5]‐Migration in Pyrrolium and Pyrrole Derivatives: Activation of a Stronger Bond in Electropositive Groups Becomes Easier

The [1,5]‐migration reaction has attracted considerable attention from experimentalists and theoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, a theoretical study on the reaction mechanism of [1,5]‐migration in both pyrrolium and pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]‐migration of electropositive groups (AuPMe₃ and SnH₃) in pyrrolium derivatives, although the bond dissociation energies of the Au?N bond (98.8 kcal mol^?1) and Sn?N bond (81.7 kcal mol^?1) are larger than that of the N?F bond (57.6 kcal mol^?1). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol^?1 for AuPMe₃ and SnH₃, respectively) for [1,5]‐migration of electropositive groups, in comparison with the [1,5]‐fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus‐independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also play some roles in [1,5]‐migration of pyrrolium derivatives.     

Iridicycle‐Catalysed Imine Reduction: An Experimental and Computational Study of the Mechanism

The mechanism of imine reduction by formic acid with a single‐site iridicycle catalyst has been investigated by density functional theory (DFT), NMR spectroscopy, and kinetic measurements. The NMR and kinetic studies suggest that the transfer hydrogenation is turnover‐limited by the hydride formation step. The calculations reveal that, amongst a number of possibilities, hydride formation from the iridicycle and formate probably proceeds by an ion‐pair mechanism, whereas the hydride transfer to the imino bond occurs in an outer‐sphere manner. In the gas phase, in the most favourable pathway, the activation energies in the hydride formation and transfer steps are 26–28 and 7–8 kcal mol^?1, respectively. Introducing one explicit methanol molecule into the modelling alters the energy barrier significantly, reducing the energies to around 18 and 2 kcal mol^?1 for the two steps, respectively. The DFT investigation further shows that methanol participates in the transition state of the turnover‐limiting hydride formation step by hydrogen‐bonding to the formate anion and thereby stabilising the ion pair.     

Conversion of CO2 and C2H6 to Propanoic Acid over a Au‐Exchanged MCM‐22 Zeolite

Finding novel catalysts for the direct conversion of CO₂ to fuels and chemicals is a primary goal in energy and environmental research. In this work, density functional theory (DFT) is used to study possible reaction mechanisms for the conversion of CO₂ and C₂H₆ to propanoic acid over a gold‐exchanged MCM‐22 zeolite catalyst. The reaction begins with the activation of ethane to produce a gold ethyl hydride intermediate. Hydrogen transfers to the framework oxygen leads then to gold ethyl adsorbed on the Brønsted‐acid site. The energy barriers for these steps of ethane activation are 9.3 and 16.3 kcal mol^?1, respectively. Two mechanisms of propanoic acid formation are investigated. In the first one, the insertion of CO₂ into the Au?H bond of the first intermediate yields gold carboxyl ethyl as subsequent intermediate. This is then converted to propanoic acid by forming the relevant C?C bond. The activation energy of the rate‐determining step of this pathway is 48.2 kcal mol^?1. In the second mechanism, CO₂ interacts with gold ethyl adsorbed on the Brønsted‐acid site. Propanoic acid is formed via protonation of CO₂ by the Brønsted acid and the simultaneous formation of a bond between CO₂ and the ethyl group. The activation energy there is 44.2 kcal mol^?1, favoring this second pathway at least at low temperatures. Gold‐exchanged MCM‐22 zeolite can therefore, at least in principle, be used as the catalyst for producing propanoic acid from CO₂ and ethane.     

Mechanism of the Tyrosine Ammonia Lyase Reaction—Tandem Nucleophilic and Electrophilic Enhancement by a Proton Transfer

Quantum mechanics/molecular mechanics calculations in tyrosine ammonia lyase (TAL) ruled out the hypothetical Friedel–Crafts (FC) route for ammonia elimination from L ‐tyrosine due to the high energy of FC intermediates. The calculated pathway from the zwitterionic L ‐tyrosine‐binding state (0.0 kcal mol^?1) to the product‐binding state ((E)‐coumarate+H₂N? MIO; ?24.0 kcal mol^?1; MIO=3,5‐dihydro‐5‐methylidene‐4H‐imidazol‐4‐one) involves an intermediate (IS, ?19.9 kcal mol^?1), which has a covalent bond between the N atom of the substrate and MIO, as well as two transition states (TS1 and TS2). TS1 (14.4 kcal mol^?1) corresponds to a proton transfer from the substrate to the N1 atom of MIO by Tyr300? OH. Thus, a tandem nucleophilic activation of the substrate and electrophilic activation of MIO happens. TS2 (5.2 kcal mol^?1) indicates a concerted C? N bond breaking of the N‐MIO intermediate and deprotonation of the pro‐S β position by Tyr60. Calculations elucidate the role of enzymic bases (Tyr60 and Tyr300) and other catalytically relevant residues (Asn203, Arg303, and Asn333, Asn435), which are fully conserved in the amino acid sequences and in 3D structures of all known MIO‐containing ammonia lyases and 2,3‐aminomutases.     

σ‐Insertive Mechanism versus Concerted Non‐insertive Mechanism in the Intramolecular Hydroamination of Aminoalkenes Catalyzed by Phenoxyamine Magnesium Complexes: A Synthetic and Computational Study

The phenoxyamine magnesium complexes [{ONN}MgCH₂Ph] ( 4 a : {ONN}=2,4‐tBu₂‐6‐(CH₂NMeCH₂CH₂NMe₂)C₆H₂O^?; 4 b : {ONN}=4‐tBu‐2‐(CH₂NMeCH₂CH₂NMe₂)‐6‐(SiPh₃)C₆H₂O^?) have been prepared and investigated with respect to their catalytic activity in the intramolecular hydroamination of aminoalkenes. The sterically more shielded triphenylsilyl‐substituted complex 4 b exhibits better thermal stability and higher catalytic activity. Kinetic investigations using complex 4 b in the cyclisation of 1‐allylcyclohexyl)methylamine ( 5 b ), respectively, 2,2‐dimethylpent‐4‐en‐1‐amine ( 5 c ), reveal a first‐order rate dependence on substrate and catalyst concentration. A significant primary kinetic isotope effect of 3.9±0.2 in the cyclisation of 5 b suggests significant N?H bond disruption in the rate‐determining transition state. The stoichiometric reaction of 4 b with 5 c revealed that at least two substrate molecules are required per magnesium centre to facilitate cyclisation. The reaction mechanism was further scrutinized computationally by examination of two rivalling mechanistic pathways. One scenario involves a coordinated amine molecule assisting in a concerted non‐insertive N?C ring closure with concurrent amino proton transfer from the amine onto the olefin, effectively combining the insertion and protonolysis step to a single step. The alternative mechanistic scenario involves a reversible olefin insertion step followed by rate‐determining protonolysis. DFT reveals that a proton‐assisted concerted N?C/C?H bond‐forming pathway is energetically prohibitive in comparison to the kinetically less demanding σ‐insertive pathway (ΔΔG^≠=5.6 kcal mol^?1). Thus, the σ‐insertive pathway is likely traversed exclusively. The DFT predicted total barrier of 23.1 kcal mol^?1 (relative to the {ONN}Mg pyrrolide catalyst resting state) for magnesium?alkyl bond aminolysis matches the experimentally determined Eyring parameter (ΔG^≠=24.1(±0.6) kcal mol^?1 (298 K)) gratifyingly well.     

Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism

We report that 2,6‐lutidine?trichloroborane (Lut?BCl₃) reacts with H₂ in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl₂. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl₃ was calculated to react with H₂ and form the ion pair, [LutH⁺][HBCl₃^?], with a barrier of ΔH^≠=24.7 kcal mol^?1 (ΔG^≠=29.8 kcal mol^?1). Metathesis with a second molecule of Lut?BCl₃ produced Lut?BHCl₂ and [LutH⁺][BCl₄^?]. The overall reaction is exothermic by 6.0 kcal mol^?1 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})BCl₂]. Barriers for addition of H₂ across the Lut/LutBCl₂⁺ pair and the boracycle B?C bond are substantially higher (ΔG^≠=42.1 and 49.4 kcal mol^?1, respectively), such that these pathways are excluded. The barrier for addition of H₂ to the boracycle B?N bond is comparable (ΔH^≠=28.5 and ΔG^≠=32 kcal mol^?1). Conversion of the intermediate 2‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) to Lut?BHCl₂ may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl₃. Intramolecular protodeboronation, which could form Lut?BHCl₂ directly, is prohibited by a high barrier (ΔH^≠=52, ΔG^≠=51 kcal mol^?1).     

Hydrosilylation Induced by N→Si Intramolecular Coordination: Spontaneous Transformation of Organosilanes into 1‐Aza‐Silole‐Type Molecules in the Absence of a Catalyst

Our attempts to synthesize the N→Si intramolecularly coordinated organosilanes Ph₂L¹SiH ( 1 a ), PhL¹SiH₂ ( 2 a ), Ph₂L²SiH ( 3 a ), and PhL²SiH₂ ( 4 a ) containing a CH?N imine group (in which L¹ is the C,N‐chelating ligand {2‐[CH?N(C₆H₃‐2,6‐iPr₂)]C₆H₄}^? and L² is {2‐[CH?N(tBu)]C₆H₄}^?) yielded 1‐[2,6‐bis(diisopropyl)phenyl]‐2,2‐diphenyl‐1‐aza‐silole ( 1 ), 1‐[2,6‐bis(diisopropyl)phenyl]‐2‐phenyl‐2‐hydrido‐1‐aza‐silole ( 2 ), 1‐tert‐butyl‐2,2‐diphenyl‐1‐aza‐silole ( 3 ), and 1‐tert‐butyl‐2‐phenyl‐2‐hydrido‐1‐aza‐silole ( 4 ), respectively. Isolated organosilicon amides 1 – 4 are an outcome of the spontaneous hydrosilylation of the CH?N imine moiety induced by N→Si intramolecular coordination. Compounds 1–4 were characterized by NMR spectroscopy and X‐ray diffraction analysis. The geometries of organosilanes 1 a – 4 a and their corresponding hydrosilylated products 1 – 4 were optimized and fully characterized at the B3LYP/6‐31++G(d,p) level of theory. The molecular structure determination of 1 – 3 suggested the presence of a Si?N double bond. Natural bond orbital (NBO) analysis, however, shows a very strong donor–acceptor interaction between the lone pair of the nitrogen atom and the formal empty p orbital on the silicon and therefore, the calculations show that the Si?N bond is highly polarized pointing to a predominantly zwitterionic Si⁺N^? bond in 1 – 4 . Since compounds 1 – 4 are hydrosilylated products of 1 a – 4 a , the free energies (ΔG₂₉₈), enthalpies (ΔH₂₉₈), and entropies (ΔH₂₉₈) were computed for the hydrosilylation reaction of 1 a – 4 a with both B3LYP and B3LYP‐D methods. On the basis of the very negative ΔG₂₉₈ values, the hydrosilylation reaction is highly exergonic and compounds 1 a – 4 a are spontaneously transformed into 1 – 4 in the absence of a catalyst.     

Anionic CH⋅⋅⋅X− Hydrogen Bonds: Origin of Their Strength,Geometry, and Other Properties

CF₃H as a proton donor was paired with a variety of anions, and its properties were assessed by MP2/aug‐cc‐pVDZ calculations. The binding energy of monoanions halide, NO₃^?, formate, acetate, HSO₄^?, and H₂PO₄^? lie in the 12–17 kcal mol^?1 range, although F^? is more strongly bound, by 26 kcal mol^?1. Dianions SO₄^2? and HPO₄^2? are bound by 27 kcal mol^?1, and trianion PO₄^3? by 45 kcal mol^?1. When two O atoms are available on the anion, the CH???O^? H‐bond (HB) is usually bifurcated, although asymmetrically. The CH bond is elongated and its stretching frequency redshifted in these ionic HBs, but the shift is reduced in the bifurcated structures. Slightly more than half of the binding energy is attributed to Coulombic attraction, with smaller contributions from induction and dispersion. The amount of charge transfer from the anions to the σ*(CH) orbital correlates with many of the other indicators of bond strength, such as binding energy, CH bond stretch, CH redshift, downfield NMR spectroscopic chemical shift of the bridging proton, and density at bond critical points.     

A Mechanistic Study of the Spontaneous Hydrolysis of Glycylserine as the Simplest Model for Protein Self‐Cleavage

A common feature of several classes of intrinsically reactive proteins with diverse biological functions is that they undergo self‐catalyzed reactions initiated by an N→O or N→S acyl shift of a peptide bond adjacent to a serine, threonine, or cysteine residue. In this study, we examine the N→O acyl shift initiated peptide‐bond hydrolysis at the serine residue on a model compound, glycylserine (GlySer), by means of DFT and ab initio methods. In the most favorable rate‐determining transition state, the serine ?COO^? group acts as a general base to accept a proton from the attacking ?OH function, which results in oxyoxazolidine ring closure. The calculated activation energy (29.4 kcal mol^?1) is in excellent agreement with the experimental value, 29.4 kcal mol^?1, determined by ¹H NMR measurements. A reaction mechanism for the entire process of GlySer dipeptide hydrolysis is also proposed. In the case of proteins, we found that when no other groups that may act as a general base are available, the N→O acyl shift mechanism might instead involve a water‐assisted proton transfer from the attacking serine ?OH group to the amide oxygen. However, the calculated energy barrier for this process is relatively high (33.6 kcal mol^?1), thus indicating that in absence of catalytic factors the peptide bond adjacent to serine is no longer a weak point in the protein backbone. An analogous rearrangement involving the amide N‐protonated form, rather than the principle zwitterion form of GlySer, was also considered as a model for the previously proposed mechanism of sea‐urchin sperm protein, enterokinase, and agrin (SEA) domain autoproteolysis. The calculated activation energy (14.3 kcal mol^?1) is significantly lower than the experimental value reported for SEA (≈21 kcal mol^?1), but is still in better agreement as compared to earlier theoretical attempts.     

Strong N−X⋅⋅⋅O−N Halogen Bonds: A Comprehensive Study on N‐Halosaccharin Pyridine N‐Oxide Complexes

A study of the strong N?X???^?O?N⁺ (X=I, Br) halogen bonding interactions reports 2×27 donor×acceptor complexes of N‐halosaccharins and pyridine N‐oxides (PyNO). DFT calculations were used to investigate the X???O halogen bond (XB) interaction energies in 54 complexes. A simplified computationally fast electrostatic model was developed for predicting the X???O XBs. The XB interaction energies vary from ?47.5 to ?120.3 kJ mol^?1; the strongest N?I???^?O?N⁺ XBs approaching those of 3‐center‐4‐electron [N?I?N]⁺ halogen‐bonded systems (ca. 160 kJ mol^?1). ¹H NMR association constants (K_XB) determined in CDCl₃ and [D₆]acetone vary from 2.0×10⁰ to >10⁸ m ^?1 and correlate well with the calculated donor×acceptor complexation enthalpies found between ?38.4 and ?77.5 kJ mol^?1. In X‐ray crystal structures, the N‐iodosaccharin‐PyNO complexes manifest short interaction ratios (R_XB) between 0.65–0.67 for the N?I???^?O?N⁺ halogen bond.     

Atomistic understanding of the C·T mismatched DNA base pair tautomerization via the DPT: QM and QTAIM computational approaches

It was established that the cytosine·thymine (C·T) mismatched DNA base pair with cis‐oriented N1H glycosidic bonds has propeller‐like structure (|N3C4C4N3| = 38.4°), which is stabilized by three specific intermolecular interactions–two antiparallel N4H…O4 (5.19 kcal mol^?1) and N3H…N3 (6.33 kcal mol^?1) H‐bonds and a van der Waals (vdW) contact O2…O2 (0.32 kcal mol^?1). The C·T base mispair is thermodynamically stable structure (ΔG_int = ?1.54 kcal mol^?1) and even slightly more stable than the A·T Watson–Crick DNA base pair (ΔG_int = ?1.43 kcal mol^?1) at the room temperature. It was shown that the C·T ? C*·T* tautomerization via the double proton transfer (DPT) is assisted by the O2…O2 vdW contact along the entire range of the intrinsic reaction coordinate (IRC). The positive value of the Grunenberg's compliance constants (31.186, 30.265, and 22.166 Å/mdyn for the C·T, C*·T*, and TS_{C·T ? C*·T*}, respectively) proves that the O2…O2 vdW contact is a stabilizing interaction. Based on the sweeps of the H‐bond energies, it was found that the N4H…O4/O4H…N4, and N3H…N3 H‐bonds in the C·T and C*·T* base pairs are anticooperative and weaken each other, whereas the middle N3H…N3 H‐bond and the O2…O2 vdW contact are cooperative and mutually reinforce each other. It was found that the tautomerization of the C·T base mispair through the DPT is concerted and asynchronous reaction that proceeds via the TS_{C·T ? C*·T*} stabilized by the loosened N4? H? O4 covalent bridge, N3H…N3 H‐bond (9.67 kcal mol^?1) and O2…O2 vdW contact (0.41 kcal mol^?1). The nine key points, describing the evolution of the C·T ? C*·T* tautomerization via the DPT, were detected and completely investigated along the IRC. The C*·T* mispair was revealed to be the dynamically unstable structure with a lifetime 2.13·× 10^?13 s. In this case, as for the A·T Watson–Crick DNA base pair, activates the mechanism of the quantum protection of the C·T DNA base mispair from its spontaneous mutagenic tautomerization through the DPT. © 2013 Wiley Periodicals, Inc.     

Mechanism of the Pd‐catalyzed Decarboxylative Allylation of α‐Imino Esters: Decarboxylation via Free Carboxylate Ion

The Pd‐catalyzed decarboxylative allylation of α‐(diphenylmethylene)imino esters ( 1 ) or allyl diphenylglycinate imines ( 2 ) is an efficient method to construct new C(sp³)? C(sp³) bonds. The detailed mechanism of this reaction was studied by theoretical calculations [ONIOM(B3LYP/LANL2DZ+p:PM6)] combined with experimental observations. The overall catalytic cycle was found to consist of three steps: oxidative addition, decarboxylation, and reductive allylation. The oxidative addition of 1 to [(dba)Pd(PPh₃)₂] (dba=dibenzylideneacetone) produces an allylpalladium cation and a carboxylate anion with a low activation barrier of +9.1 kcal mol^?1. The following rate‐determining decarboxylation proceeds via a solvent‐exposed α‐imino carboxylate anion rather than an O‐ligated allylpalladium carboxylate with an activation barrier of +22.7 kcal mol^?1. The 2‐azaallyl anion generated by this decarboxylation attacks the face of the allyl ligand opposite to the Pd center in an outer‐sphere process to produce major product 3 , with a lower activation barrier than that of the minor product 4 . A positive linear Hammett correlation [ρ=1.10 for the PPh₃ ligand] with the observed regioselectivity ( 3 versus 4 ) supports an outer‐sphere pathway for the allylation step. When Pd combined with the bis(diphenylphosphino)butane (dppb) ligand is employed as a catalyst, the decarboxylation still proceeds via the free carboxylate anion without direct assistance of the cationic Pd center. Consistent with experimental observations, electron‐withdrawing substituents on 2 were calculated to have lower activation barriers for decarboxylation and, thus, accelerate the overall reaction rates.     

Interconversion of Molybdenum Imido and Amido Complexes by Proton‐Coupled Electron Transfer

Interconversion of the molybdenum amido [(^PhTpy)(PPh₂Me)₂Mo(NHtBuAr)][BArF²⁴] (^PhTpy=4′‐Ph‐2,2′,6′,2“‐terpyridine; tBuAr=4‐tert‐butyl‐C₆H₄; ArF²⁴=(C₆H₃‐3,5‐(CF₃)₂)₄) and imido [(^PhTpy)(PPh₂Me)₂Mo(NtBuAr)][BArF²⁴] complexes has been accomplished by proton‐coupled electron transfer. The 2,4,6‐tri‐tert‐butylphenoxyl radical was used as an oxidant and the non‐classical ammine complex [(^PhTpy)(PPh₂Me)₂Mo(NH₃)][BArF²⁴] as the reductant. The N?H bond dissociation free energy (BDFE) of the amido N?H bond formed and cleaved in the sequence was experimentally bracketed between 45.8 and 52.3 kcal mol^?1, in agreement with a DFT‐computed value of 48 kcal mol^?1. The N?H BDFE in combination with electrochemical data eliminate proton transfer as the first step in the N?H bond‐forming sequence and favor initial electron transfer or concerted pathways.     

Cooperativity between the Halogen Bond and the Hydrogen Bond in H3N⋅⋅⋅XY⋅⋅⋅HF Complexes (X,Y=F,Cl, Br)

Ab initio calculations are used to provide information on H₃N???XY???HF triads (X, Y=F, Cl, Br) each having a halogen bond and a hydrogen bond. The investigated triads include H₃N???Br₂‐HF, H₃N???Cl₂???HF, H₃N???BrCI???HF, H₃N???BrF???HF, and H₃N???ClF???HF. To understand the properties of the systems better, the corresponding dyads are also investigated. Molecular geometries, binding energies, and infrared spectra of monomers, dyads, and triads are studied at the MP2 level of theory with the 6‐311++G(d,p) basis set. Because the primary aim of this study is to examine cooperative effects, particular attention is given to parameters such as cooperative energies, many‐body interaction energies, and cooperativity factors. The cooperative energy ranges from ?1.45 to ?4.64 kcal mol^?1, the three‐body interaction energy from ?2.17 to ?6.71 kcal mol^?1, and the cooperativity factor from 1.27 to 4.35. These results indicate significant cooperativity between the halogen and hydrogen bonds in these complexes. This cooperativity is much greater than that between hydrogen bonds. The effect of a halogen bond on a hydrogen bond is more pronounced than that of a hydrogen bond on a halogen bond.     

Different catalysis role of in‐loop and out‐of‐loop waters in assisting HNS/HSN proton transfer isomerizations: Bridging vs. surrounding effect

In this work, a density function theory (DFT) study is presented for the HNS/HSN isomerization assisted by 1–4 water molecules on the singlet state potential energy surface (PES). Two modes are considered to model the catalytic effect of these water molecules: (i) water molecule(s) participate directly in forming a proton transfer loop with HNS/HSN species, and (ii) water molecules are out of loop (referred to as out‐of‐loop waters) to assist the proton transfer. In the first mode, for the monohydration mechanism, the heat of reaction is 21.55 kcal · mol^?1 at the B3LYP/6‐311++G** level. The corresponding forward/backward barrier lowerings are obtained as 24.41/24.32 kcal · mol^?1 compared with the no‐water‐assisting isomerization barrier T (65.52/43.87 kcal · mol^?1). But when adding one water molecule on the HNS, there is another special proton‐transfer isomerization pathway with a transition state 10T′ in which the water is out of the proton transfer loop. The corresponding forward/backward barriers are 65.89/65.89 kcal · mol^?1. Clearly, this process is more difficult to follow than the R–T–P process. For the two‐water‐assisting mechanism, the heat of reaction is 19.61 kcal · mol^?1, and the forward/backward barriers are 32.27/12.66 kcal · mol^?1, decreased by 33.25/31.21 kcal · mol^?1 compared with T. For trihydration and tetrahydration, the forward/backward barriers decrease as 32.00/12.60 (30T) and 37.38/17.26 (40T) kcal · mol^?1, and the heat of reaction decreases by 19.39 and 19.23 kcal · mol^?1, compared with T, respectively. But, when four water molecules are involved in the reactant loop, the corresponding energy aspects increase compared with those of the trihydration. The forward/backward barriers are increased by 5.38 and 4.66 kcal · mol^?1 than the trihydration situation. In the second mode, the outer‐sphere water effect from the other water molecules directly H‐bonded to the loop is considered. When one to three water molecules attach to the looped water in one‐water in‐loop‐assisting proton transfer isomerization, their effects on the three energies are small, and the deviations are not more than 3 kcal · mol^?1 compared with the original monohydration‐assisting case. When adding one or two water molecules on the dihydration‐assisting mechanism, and increasing one water molecule on the trihydration, the corresponding energies also are not obviously changed. The results indicate that the forward/backward barriers for the three in‐loop water‐assisting case are the lowest, and the surrounding water molecules (out‐of‐loop) yield only a small effect. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

C-H bond activation of methane in aqueous solution: a hybrid quantum mechanical/effective fragment potential study

In this study, we investigated the C? H bond activation of methane catalyzed by the complex [PtCl₄]^2?, using the hybrid quantum mechanical/effective fragment potential (EFP) approach. We analyzed the structures, energetic properties, and reaction mechanism involved in the elementary steps that compose the catalytic cycle of the Shilov reaction. Our B3LYP/SBKJC/cc‐pVDZ/EFP results show that the methane activation may proceed through two pathways: (i) electrophilic addition or (ii) direct oxidative addition of the C? H bond of the alkane. The electrophilic addition pathway proceeds in two steps with formation of a σ‐methane complex, with a Gibbs free energy barrier of 24.6 kcal mol^?1, followed by the cleavage of the C? H bond, with an energy barrier of 4.3 kcal mol^?1. The activation Gibbs free energy, calculated for the methane uptake step was 24.6 kcal mol^?1, which is in good agreement with experimental value of 23.1 kcal mol^?1 obtained for a related system. The results shows that the activation of the C? H bond promoted by the [PtCl₄]^2? catalyst in aqueous solution occurs through a direct oxidative addition of the C? H bond, in a single step, with an activation free energy of 25.2 kcal mol^?1, as the electrophilic addition pathway leads to the formation of a σ‐methane intermediate that rapidly undergoes decomposition. The inclusion of long‐range solvent effects with polarizable continuum model does not change the activation energies computed at the B3LYP/SBKJC/cc‐pVDZ/EFP level of theory significantly, indicating that the large EFP water cluster used, obtained from Monte Carlo simulations and analysis of the center‐of‐mass radial pair distribution function, captures the most important solvent effects. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011     

Catalytic Electrophilic CH Silylation of Pyridines Enabled by Temporary Dearomatization

A C? H silylation of pyridines that seemingly proceeds through electrophilic aromatic substitution (S_EAr) is reported. Reactions of 2‐ and 3‐substituted pyridines with hydrosilanes in the presence of a catalyst that splits the Si? H bond into a hydride and a silicon electrophile yield the corresponding 5‐silylated pyridines. This formal silylation of an aromatic C? H bond is the result of a three‐step sequence, consisting of a pyridine hydrosilylation, a dehydrogenative C? H silylation of the intermediate enamine, and a 1,4‐dihydropyridine retro‐hydrosilylation. The key intermediates were detected by ¹H NMR spectroscopy and prepared through the individual steps. This complex interplay of electrophilic silylation, hydride transfer, and proton abstraction is promoted by a single catalyst.     

Solvent effects on the activation parameters of the reaction between an α‐tocopherol analogue and dpph•: The role of H‐bonded complexes

The analysis of the activation parameters for the formal H‐atom transfer reaction between 2,2,5,7,8‐pentamethyl‐6‐chromanol (ChrOH) and 2,2‐diphenyl‐1‐picrylhydrazyl (dpph^?) reveals that these parameters are effective probes of the actual reaction mechanism. Indeed, the A factors measured in various polar and apolar solvents are localized in three distinct domains according to whether the reaction occurs via outer‐sphere electron transfer (ET) from the anion ChrO^? or hydrogen atom transfer (HAT). For instance, A = 5.9 × 10⁵ M^?1 s^?1 and E_a = 2.5 kcal mol^?1 in cyclohexane where the reaction proceeds by HAT, whereas in methanol, ethanol, and their mixtures with water where there is a substantial ET contribution A > 10⁹ M^?1s^?1 and E_a > 7 kcal mol^?1. Interestingly, in nonhydroxylic polar solvents, A～ 10⁷ M^?1s^?1 and the E_a values reflect the H‐bond accepting ability of the solvent in agreement with the “standard” kinetic solvent effects on HAT reactions. Addition of small quantities of pyridine accelerates the reaction rates in these solvents. This suggests that the H‐bonded complex (ChrOH···Py) is able to react via intermolecular ET with dpph^?. It is known, in fact, that pyridine lowers the oxidation potential of phenols by ～0.5 V and the ΔG_ET of ChrOH + dpph^? consequently decreases by about 10 kcal mol^?1. © 2012 Wiley Periodicals, Inc. Int J Chem Kinet 44: 524–531, 2012     

Heck‐Type Reactions of Imine Derivatives: A DFT Study

The mechanism of a recently discovered intramolecular Heck‐type coupling of oximes with aryl halides (Angew. Chem. Int. Ed. 2007 , 46, 6325) was systematically studied by using density functional methods enhanced with a polarized continuum solvation model. The overall catalytic cycle of the reaction was found to consist of four steps: oxidative addition, migratory insertion, β‐H elimination, and catalyst regeneration, whereas an alternative base‐promoted C? H activation pathway was determined to be less favorable. Migratory insertion was found to be the rate determining step in the catalytic cycle. The apparent activation barrier of migratory insertion of the (E)‐oxime was +20.5 kcal mol^?1, whereas the barrier of (Z)‐oxime was as high as +32.7 kcal mol^?1. However, (Z)‐oxime could isomerize to form the more active (E)‐oxime with the assistance of K₂CO₃, so that both the (E)‐ and (Z)‐oxime substrates could be transformed to the desired product. Our calculations also indicated that the Z product was predominant in the equilibrium of the isomerization of the imine double bond, which constituted the reason for the good Z‐selectivity observed for the reaction. Furthermore, we examined the difference between the intermolecular Heck‐type reactions of imines and of olefins. It was found that in the intermolecular Heck‐type coupling of imines, the apparent activation barrier of migratory insertion was as high as +35 kcal mol^?1, which should be the main obstacle of the reaction. The analysis also revealed the main problem for the intermolecular Heck‐type reactions of imines, which was that the breaking of a C?N π bond was much more difficult than the breaking of a C?C π bond. After systematic examination of a series of substituted imines, (Z)‐N‐amino imine and N‐acetyl imine were found to have relatively low barriers of migratory insertion, so that they might be possible substrates for intermolecular Heck‐type coupling.