Theoretical study of C-H and N-H sigma-bond activation reactions by titinium(IV)-imido complex. Good understanding based on orbital interaction and theoretical proposal for N-H sigma-bond activation of ammonia

The C-H sigma-bond activation of methane and the N-H sigma-bond activation of ammonia by (Me3SiO)2Ti(=NSiMe3) 1 were theoretically investigated with DFT, MP2 to MP4(SDQ), and CCSD(T) methods. The C-H sigma-bond activation of methane takes place with an activation barrier (Ea) of 14.6 (21.5) kcal/mol and a reaction energy (DeltaE) of -22.7 (-16.5) kcal/mol to afford (Me3SiO)2Ti(Me)[NH(SiMe3)], where DFT- and MP4(SDQ)-calculated values are given without and in parentheses, respectively, hereafter. The electron population of the CH3 group increases, but the H atomic population decreases upon going to the transition state from the precursor complex, which indicates that the C-H sigma-bond activation occurs in heterolytic manner unlike the oxidative addition. The Ti atomic population considerably increases upon going to the transition state from the precursor complex, which indicates that the charge transfer (CT) occurs from methane to Ti. These population changes are induced by the orbital interactions among the d(pi)-p(pi) bonding orbital of the Ti=NSiMe3 moiety, the Ti d(z2) orbital and the C-H sigma-bonding and sigma*-antibonding orbitals of methane. The reverse regioselective C-H sigma-bond activation which leads to formation of (Me3SiO)2Ti(H)[NMe(SiMe3)] takes place with a larger Ea value and smaller exothermicity. The reasons are discussed in terms of Ti-H, Ti-CH3, Ti-NH3, N-H, and N-CH3 bond energies and orbital interactions in the transition state. The N-H sigma-bond activation of ammonia takes place in a heterolytic manner with a larger Ea value of 19.0 (27.9) kcal/mol and considerably larger exothermicity of -45.0 (-39.4) kcal/mol than those of the C-H sigma-bond activation. The N-H sigma-bond activation of ammonia by a Ti-alkylidyne complex, [(PNP)Ti(CSiMe3)] 3 (PNP = N-[2-(PH2)2-phenyl]2-]) ,was also investigated. This reaction takes place with a smaller E(a) value of 7.5 (15.3) kcal/mol and larger exothermicity of -60.2 (-56.1) kcal/mol. These results lead us to predict that the N-H sigma-bond activation of ammonia can be achieved by these complexes.     

Titanium-catalyzed hydrosilylation of olefins: A comparison study on Cp2TiCl2/Sm and Cp2TiCl2/LiAlH4 catalyst system

Hydrosilylation of olefins catalyzed by Cp₂TiCl₂/Sm (Cp?=?cyclopentadienyl) under solvent free conditions have been investigated. By using Cp₂TiCl₂/Sm as catalyst system, β-adducts and hydrogenation products were detected. Hydrosilylation of olefins catalyzed by Cp₂TiCl₂/LiAlH₄ under room temperature has also been studied. The influence of TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) on Cp₂TiCl₂/Sm and Cp₂TiCl₂/LiAlH₄, respectively, indicated that hydrosilylation of olefins catalyzed with Cp₂TiCl₂/Sm went through a free radical reaction pathway while a coordination mechanism was applied for Cp₂TiCl₂/LiAlH₄ catalyst system.     

Catalytic hydrosilylation of alkenes by a ruthenium silylene complex. Evidence for a new hydrosilylation mechanism

Novel ruthenium and osmium silylene species containing Si-H bonds have been synthesized and characterized by NMR spectroscopy. The ruthenium complex [Cp*(iPr3P)(H)2Ru=Si(H)Ph.Et2O][B(C6F5)4] catalyzes the hydrosilyation of alkenes with excellent substrate selectivity for primary silanes and exclusive anti-Markovnikov regiochemistry. Evidence for a novel mechanism involving direct addition of an alkene to the silylene Si-H bond is presented.     

Density functional study of the insertion and ring-opening mechanism of MCP over Cp2LaH and Cp2LuH catalysts

The insertion and ring opening of methylenecyclopropane (MCP) catalyzed by Cp2LnH (Ln = La and Lu) was investigated using DFT method. Geometries and energies of the reactants, minima, and transition states for this reaction were obtained. The present study indicates the formation of Cp2LaH-MCP complex followed by 1,2-insertion through a tetrahedral transition state with subsequent ring opening at the proximal bond via hydrogen transfer transition state resulting in the formation of the final product.     

ESR Study of the Reaction Between Cp2Ti(CO)2 and Cp2TiBr2

Introduction Floriani and Fachinetti have proposed that (Cp_2TiCl)_2 can be prepared by the reaction between Cp_2TiCl_2 and Cp_2Ti(CO)_2. We extended the method to the preparation of (Cp_2TiBr)_2 and characterized an active intermediate by ESR method.     

Reaction of Cp2ZrCl2 with Inorganic Amides

The reactions of [η⁵‐Cp₂ZrCl₂] (Cp = η⁵‐C₅H₅) with [K(THF)_n][N(PPh₂)₂] (n = 1.25—1.5) and K[CH(PPh₂NSiMe₃)₂] are reported. The first reaction led to the monoamido complex [η⁵‐Cp₂Zr(Cl)N(PPh₂)₂] in which the {(Ph₂P)₂N}^— ligand — via a phosphorous and the nitrogen atom — is coordinated to the zirconium atom in a chelating (η²) fashion. Reaction of the potassium methanide compound, K{CH(PPh₂NSiMe₃)₂} with zirconocene dichloride yield the carbene‐like mono cyclopentadienyl complex [η⁵‐CpZr(Cl){C(PPh₂NSiMe₃)₂}]. The complex is formed by a salt metathesis and concomitant a cyclopentadiene extrusion.     

Reaction mechanism of naphthyl radicals with molecular oxygen. 1. Theoretical study of the potential energy surface

Potential energy surfaces (PESs) of the reactions of 1- and 2-naphthyl radicals with molecular oxygen have been investigated at the G3(MP2,CC)//B3LYP/6-311G** level of theory. Both reactions are shown to be initiated by barrierless addition of O(2) to the respective radical sites of C(10)H(7). The end-on O(2) addition leading to 1- and 2-naphthylperoxy radicals exothermic by 45-46 kcal/mol is found to be more preferable thermodynamically than the side-on addition. At the subsequent reaction step, the chemically activated 1- and 2-C(10)H(7)OO adducts can eliminate an oxygen atom leading to the formation of 1- and 2-naphthoxy radical products, respectively, which in turn can undergo unimolecular decomposition producing indenyl radical + CO via the barriers of 57.8 and 48.3 kcal/mol and with total reaction endothermicities of 14.5 and 10.2 kcal/mol, respectively. Alternatively, the initial reaction adducts can feature an oxygen atom insertion into the attacked C(6) ring leading to bicyclic intermediates a10 and a10' (from 1-naphthyl + O(2)) or b10 and b10' (from 2-naphthyl + O(2)) composed from two fused six-member C(6) and seven-member C(6)O rings. Next, a10 and a10' are predicted to decompose to C(9)H(7) (indenyl) + CO(2), 1,2-C(10)H(6)O(2) (1,2-naphthoquinone) + H, and 1-C(9)H(7)O (1-benzopyranyl) + CO, whereas b10 and b10' would dissociate to C(9)H(7) (indenyl) + CO(2), 2-C(9)H(7)O (2-benzopyranyl) + CO, and 1,2-C(10)H(6)O(2) (1,2-naphthoquinone) + H. On the basis of this, the 1-naphthyl + O(2) reaction is concluded to form the following products (with the overall reaction energies given in parentheses): 1-naphthoxy + O (-15.5 kcal/mol), indenyl + CO(2) (-123.9 kcal/mol), 1-benzopyranyl + CO (-97.2 kcal/mol), and 1,2-naphthoquinone + H (-63.5 kcal/mol). The 2-naphthyl + O(2) reaction is predicted to produce 2-naphthoxy + O (-10.9 kcal/mol), indenyl + CO(2) (-123.7 kcal/mol), 2-benzopyranyl + CO (-90.7 kcal/mol), and 1,2-naphthoquinone + H (-63.2 kcal/mol). Simplified kinetic calculations using transition-state theory computed rate constants at the high-pressure limit indicate that the C(10)H(7)O + O product channels are favored at high temperatures, while the irreversible oxygen atom insertion first leading to the a10 and a10' or b10 and b10' intermediates and then to their various decomposition products is preferable at lower temperatures. Among the decomposition products, indenyl + CO(2) are always most favorable at lower temperatures, but the others, 1,2-C(10)H(6)O(2) (1,2-naphthoquinone) + H (from a10 and b10'), 1-C(9)H(7)O (1-benzopyranyl) + CO (from a10'), and 2-C(10)H(7)O (2-benzopyranyl) + O (from b10 and minor from b10'), may notably contribute or even become major products at higher temperatures.     

Theoretical study on the mechanism of the cycloaddition reaction between stannylene and ethylene or formaldehyde

The mechanism of the cycloaddition reaction of singlet stannylene and ethylene or formaldehyde has been studied by using density functional theory. The geometrical parameters, harmonic vibrational frequencies and energies of stationary points for potential energy surface are calculated by RB3LYP/3–21G* method. The results show that the two reaction processes are both two steps: (1) stannylene and ethylene or formaldehyde form an energy‐rich intermediate complex respectively, which is an exothermal reaction with no barrier; (2) two intermediate complexes isomerize to the product, respectively, with the barriers of these two reactions being 52.97 and 45.15 kJ/mol at RB3LYP/3–21G* level.     

Electrophilic C-H activation at Cp*Ir: ancillary-ligand control of the mechanism of C-H activation

Density functional calculations on the low-temperature cyclometalation of dimethylbenzylamine with [IrCl2Cp*]2/NaOAc have characterized a novel electrophilic activation pathway for C-H bond activation. C-H activation occurs from [Ir(DMBA-H)(kappa2-OAc)Cp*]+, and OAc plays a central role in determining the barrier for reaction. Dissociation of the proximal OAc arm sets up a facile intramolecular deprotonation via a geometrically convenient six-membered transition state. Dissociation of the distal OAc arm, however, leads to a higher energy four-membered (sigma-bond metathesis) transition state, while oxidative addition is even higher in energy. For this Ir3+ system, these three mechanisms appear to lie within a continuum in which the participation of the metal center and an H-accepting ancillary ligand are inversely related. The ability of the ancillary ligand to act as a proton acceptor is the key factor in determining which mechanism pertains.     

DFT study of H--H activation by Cp(2) LnH d(0) complexes.

The energy required to activate the H--H bond in the entire series of Cp(2)LnH complexes has been calculated by DFT (B3PW91) methods. The activation energies have been calculated to vary from 0.5 to 8.0 kcal x mol(-1), indicating an overall facile reaction. The electronegativity of the lanthanide in its most stable oxidation state is suggested to be a leading factor for interpreting the trends in activation energy. The geometry of the transition state is best viewed as an almost linear H(3)(-) ligand with short H--H distances and strong M--H interaction, through the wingtip H centers, with Ln. The exchange reaction is thus established to be a sigma bond metathesis reaction.     

Reaction of oxazolidone-2 with ethylene oxide. XIV

The reaction of ethylene oxide with oxazolidone-2 under various conditions is investigated. A method of preparing 3--hydroxyethyloxazolidone-2 from ethylene oxide and oxazolidone-2 under pressure in an autoclave, at 72–75° in aqueous medium, is described.For Part XIII see [8].     

Theoretical study on the mechanism of the oxygen activation process in cysteine dioxygenase enzymes

Cysteine dioxygenase (CDO) is a vital enzyme for human health involved in the biodegradation of toxic cysteine and thereby regulation of the cysteine concentration in the body. The enzyme belongs to the group of nonheme iron dioxygenases and utilizes molecular oxygen to transfer two oxygen atoms to cysteinate to form cysteine sulfinic acid products. The mechanism for this reaction is currently disputed, with crystallographic studies implicating a persulfenate intermediate in the catalytic cycle. To resolve the dispute we have performed quantum mechanics/molecular mechanics (QM/MM) calculations on substrate activation by CDO enzymes using an enzyme monomer and a large QM active region. We find a stepwise mechanism, whereby the distal oxygen atom of the iron(II)-superoxo complex attacks the sulfur atom of cysteinate to form a ring structure, followed by dioxygen bond breaking and the formation of a sulfoxide bound to an iron(IV)-oxo complex. A sulfoxide rotation precedes the second oxygen atom transfer to the substrate to give cysteine sulfinic acid products. The reaction takes place on several low-lying spin-state surfaces via multistate reactivity patterns. It starts in the singlet ground state of the iron(II)-superoxo reactant and then proceeds mainly on the quintet and triplet surfaces. The initial and rate-determining attack of the superoxo group on the cysteinate sulfur atom involves a spin-state crossing from singlet to quintet. We have also investigated an alternative mechanism via a persulfenate intermediate, with a realignment of hydrogen bonding interactions in the substrate binding pocket. However, this alternative mechanism of proximal oxygen atom attack on the sulfur atom of cysteinate is computed to be a high-energy pathway, and therefore, the persulfenate intermediate is unlikely to participate in the catalytic cycle of CDO enzymes.     

Reaction between quinone and thiazolidine. A study on the formation mechanism of new antiproliferative quinolindiones

Reaction between quinolinquinone and thiazolidine in basic medium was investigated. 2-Arylthiazolidine-4-carboxylic acid ethyl esters undergo two different cleavages in basic medium, yielding the 1-aryl-2-azadiene and a thiolic species. In the presence of quinolinquinone, the isomeric 1-aryl-3-ethoxycarbonyl-pyridoisoquinolin-5,10-diones and 3-amino-3-ethoxycarbonyl-dihydrothienoquinolin-4,9-diones are formed by a hetero-Diels-Alder reaction and 1,4-Michael addition reaction, respectively. A mechanism for the formation of the reaction products is presented.     

Theoretical study of the prohibited mechanism for ethylene/vinyl acetate co-polymers to the wax crystal growth

To understand the effect of pour point depressants (PPD) on the wax growth is important for designing PPD additives for use with different oils with high efficiency and good economics. In our current study, molecular mechanics, molecular dynamics, and quantum mechanics calculations were performed to investigate the prohibited mechanism of ethylene/vinyl acetate (EVA) additives on the paraffin deposition in oils. On the wax surface, a single C18 molecule and clusters were preferably deposited on the wax surface (010) in a parallel conformation, which resulted in the formation of large blocks of wax crystal. MD simulation indicated that the linear conformation of EVA was more favorable to be adsorbed onto the carbon backbone of the wax surface (010) with the polar fragments of vinyl acetate staying upside of the surface. Furthermore, four EVA molecules can efficiently optimize the inhibition effect for the deposition of the solute C18 molecules over 10x8 size wax surface (010). According to the simulation results, a simplified rational model was established to estimate the minimum dosage of EVA-type PPD for fuels with different paraffin contents. In a certain degree, this simplified model has provided an effective route to correlate microstructures and the properties of polymer-involving systems, which will shed light on the application of theoretical studies in industries.     

A DFT study of SiH(4) activation by Cp(2)LnH

A theoretical study of SiH(4) activation by Cp(2)LnH complexes for the entire series of lanthanides has been carried out at the DFT-B3PW91 level of theory. The reaction paths corresponding to H/H exchange and silylation, formation of Cp(2)Ln(SiH(3)), have been computed. They both occur via a single-step sigma-bond metathesis mechanism. For the athermal H/H exchange reaction, the calculated activation barrier averages 1.8 kcal.mol(-)(1) relative to the precursor adduct Cp(2)LnH(eta(2)-SiH(4)) for all lanthanide elements. The silylation path is slightly exogenic (DeltaE approximately -6.5 kcal.mol(-1)) with an activation barrier averaging 5.2 kcal.mol(-1) relative to the precursor adduct where SiH(4) is bonded by two Si-H bonds. Both pathways are therefore thermally accessible. The H/H exchange path is calculated to be kinetically more favorable whereas the silylation reaction is thermodynamically preferred. The reactivity of this familly of lanthanide complexes with SiH(4) contrasts strongly with that obtained previously with CH(4). The considerably lower activation barrier for silylation relative to methylation is attributed to the ability of Si to become hypervalent.     

Theoretical study of the Au–ethylene interaction

The basis‐set dependence and quasirelativistic and nonrelativistic effects on the Au C₂H₄ interaction are examined at the ab initio level. The effects on the interaction energies are modulated by f‐type polarization orbitals, using 19‐VE quasirelativistic pseudopotentials. Oscillation in the equilibrium Au C distance as well as in the interaction energy are sensitive to the electron correlation potential. These effects are evaluated at several levels of theory, ranging from MP2 to CCSD(T). The nature of the Au C₂H₄ interaction is related to a simple dispersion expression involving the individual properties of each component and its long‐distance behavior. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 73: 317–324, 1999     

CH2与HNCO反应机理的量子化学研究

异氰酸(HNCO)分解引发的一系列自由基反应是氮氧化物快速消除机理所研究的领域,由于该反应在燃烧化学中讨论氮氧化物NOx的消除过程十分重要,所以获得这些反应准确的位垒就成为实验化学和理论化学所要解决的问题,本文采用量子化学方法,研究了CH2与HNCO体系的反应机理,力求从理论角度给出合理的解释。     

Benzothiazoline derivatives. I. Reaction of 2-benzothiazolinethione with ethylene oxide

Treatment of 2-benzothiazolinethione with excess ethylene oxide in acetic acid resulted in N-hydroxyethylation and thiono-oxo replacement to give 3-(2-hydroxyethyl)-2-benzothiazolinone. This product was also obtained when S-alkylthiobenzothiazoles were treated in this way. Similar treatment of the N-substituted compounds 3-methyl-2-benzothiazolinethione and 3-(2-hydroxyethyl)-2-benzothiazolinethione gave simple thiono-oxo replacement.