Stepwise Reaction Path Research Articles

Exploring Adjacent Pentagons in Non-IPR and SW Defective Si60 and Si70 Silicon Fullerenes: a Computational Study

We have applied DFT calculations to investigate the effect of the adjacent pentagons (APs) on the geometries, stabilities, and electronic structures of the non-IPR isomers of Si60 and Si70 fullerenes containing three adjacent pentagon pairs, Si60(D3) and Si70(C2v), and the SW defective Si60 and Si70 fullerenes with four AP pairs. These non-IPR isomers of Si60 and Si70 cages are more stable than their IPR ones. Natural bond orbital analyses and electrostatic potential surfaces indicate the charge densities are more localized at the pentagon-pentagon edges of the non-IPR fullerenes, which increase by going to the charged ones. Based on our results, the SW rearrangement process in the Si60 and Si70 silicon fullerenes is exothermic. A silylene-like transition state along a stepwise reaction path is characterized at the B3LYP/6-311 + G* level of theory. The barrier for the SW rearrangement of Si60 fullerene is obtained to be 5.36 eV which is smaller than that reported for SW rearrangement of C60 fullerene.

Mechanistical Insights into the Bioconjugation Reaction of Triazolinediones with Tyrosine.

The bioconjugation at tyrosine residues using cyclic diazodicarboxamides, especially 4-substituted 3 H-1,2,4-triazole-3,5(4 H)-dione (PTAD), is a highly enabling synthetic reaction because it can be employed for orthogonal and site-selective (multi)functionalizations of native peptides and proteins. Despite its importance, the underlying mechanisms have not been thoroughly investigated. The reaction can proceed along four distinctive pathways: (i) the SEAr path, (ii) along a pericyclic group transfer pathway (a classical ene reaction), (iii) along a stepwise reaction path, or (iv) along an unusual higher order concerted pericyclic mechanism. The product mixtures obtained from reactions of PTAD with 2,4-unsubstituted phenolate support the SEAr mechanism, but it remains unclear if other mechanisms also take place. In the present work, the various mechanisms are compared using high-level quantum chemistry approaches for the model reaction of 4 H,3 H-1,2,4-triazole-3,5(4 H)-dione (HTAD) with p-cresol and p-cresolate. In a protic solvent (water), the barriers of the SEAr mechanism and the ene reaction are similar but still too high to explain the available experimental observations. This is only possible if the SEAr reaction of cresolate with HTAD is taken into account for which nearly vanishing barriers are computed. This satisfactorily explains measured conversion rates in buffered aqueous solutions and the strong activation effects observed upon addition of bases.

Alkaline-Earth-Catalysed Cross-Dehydrocoupling of Amines and Hydrosilanes: Reactivity Trends, Scope and Mechanism.

Alkaline-earth (Ae=Ca, Sr, Ba) complexes are shown to catalyse the chemoselective cross-dehydrocoupling (CDC) of amines and hydrosilanes. Key trends were delineated in the benchmark couplings of Ph3 SiH with pyrrolidine or tBuNH2 . Ae{E(SiMe3)2}2 ⋅(THF)x (E=N, CH; x=2-3) are more efficient than {N^N}Ae{E(SiMe3)2}⋅(THF)n (E=N, CH; n=1-2) complexes (where {N^N}(-) ={ArN(o-C6H4)C(H)=NAr}(-) with Ar=2,6-iPr2 -C6H3) bearing an iminoanilide ligand, and alkyl precatalysts are better than amido analogues. Turnover frequencies (TOFs) increase in the order Ca<Sr<Ba. Ba{CH(SiMe3)2}2 ⋅(THF)3 displays the best performance (TOF up to 3600 h(-1)). The substrate scope (>30 products) includes diamines and di(hydrosilane)s. Kinetic analysis of the Ba-promoted CDC of pyrrolidine and Ph3SiH shows that 1) the kinetic law is rate=k[Ba](1) [amine](0) [hydrosilane](1), 2) electron-withdrawing p-substituents on the arylhydrosilane improve the reaction rate and 3) a maximal kinetic isotopic effect (kSiH/kSiD =4.7) is seen for Ph3SiX (X=H, D). DFT calculations identified the prevailing mechanism; instead of an inaccessible σ-bond-breaking metathesis pathway, the CDC appears to follow a stepwise reaction path with N-Si bond-forming nucleophilic attack of the catalytically competent Ba pyrrolide onto the incoming silane, followed by rate limiting hydrogen-atom transfer to barium. The participation of a Ba silyl species is prevented energetically. The reactivity trend Ca<Sr<Ba results from greater accessibility of the metal centre and decreasing Ae-Namide bond strength upon descending Group 2.

Double-proton transfer mechanism in 1,8-dihydroxydibenzo[a,c]phenazine: a TDDFT and ab initio study

The double-proton transfer mechanism in the newly synthesized molecule 1,8-dihydroxydibenzo[a,c]phenazine containing two hydrogen bonds is investigated by means of the time-dependent density functional theory and resolution-of-identity second-order approximate coupled-cluster methods. Both reaction paths of the concerted and the stepwise mechanisms are examined, and the results show that the double-proton transfer is likely to occur in the excited state following the stepwise mechanism in the gas phase at the lowest excitation energy. Based on the calculated potential energy scheme, the single-proton transfer and the double-proton transfer are likely to coexist, with the former is expected to occupy in a significant number. The concerted reaction path of C 2 symmetry is found to be less preferred compared with the stepwise reaction path, and also could be dynamically open due to the small activation energy.

NO migration from N‐methyl‐N‐nitrosobenzene‐sulfonamide to 3,6‐dibromocarbazole: Concerted or stepwise reaction path?

AbstractThe NO migration from N‐methyl‐N‐nitrosobenzene‐sulfonamide to 3,6‐dibromocarbazole was proposed in a recent literature to follow a stepwise reaction path. However, the present density functional theory calculations at the MP2/6–31G(d,p)//B3LYP/6–31G(d,p) level show that this reaction exclusively proceeds via a concerted mechanism involving a four‐membered ring transition state. The calculated barrier is in good agreement with the experimental finding. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007

Computational Study of the Stone−Wales Transformation in C36

The transition of the D6h neutral and charged isomers to D2d isomers of C36 via Stone-Wales transformation has been studied by means of the hybrid density functional method (B3LYP). The results show that the transition state (TS) and reaction pathway could be identified for the rearrangement from C36-D6h to C36-D2d on the potential energy surface. We found that the neutral and charged transition states all have C2 molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other kind of possible TS with a carbene-like structure along the stepwise reaction path does not exist as a stationary point with the density functionals utilized here. The classical barriers are 6.23 eV through the neutral TS, 6.37 eV through the anionic TS, and 6.29 eV through the cationic TS at the B3LYP/6-31G level of theory.

Can the nitroso ene reaction proceed concertedly?

All nitroso ene reactions so far reported follow exclusively stepwise reaction paths. Herein, we report the first concerted nitroso ene reaction that occurs between o-isotoluene (or its naphthalenic analogues) and nitroso compounds (e.g., nitrosomethane and 4-nitronitrosobenzene). [reaction: see text]

Theoretical and Physical Aspects of Stepwise Mechanisms in Acyl-Transfer Reactions

Recent applications of theoretical and physical approaches to the elucidation of acyl-transfer mechanisms have led to further clarifications of structures of intermediates and transition states involved in the stepwise reaction paths. Gas-phase acyl-transfer reactions, both experimental and theoretical, are surveyed, in which factors conductive to the stepwise path are discussed. Aminolysis reactions of carbonyl and thiocarbonyl esters in solution are reviewed with special emphasis of transition state variation with substituents in the nucleophile, acyl and leaving groups applying Marcus theory, Bell-Evans-Palanyi principle, principle of microscopic reversibility, DFTFukui function, reactivity-selectivity principle, natural bond orbital analysis and More OFerrall-Jencks diagram etc. The use of cross-interaction constants as a mechanistic tool is stressed.

Theoretical analysis of concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde

The concerted and stepwise mechanisms of the hetero-Diels–Alder reaction of butadiene with formaldehyde and thioformaldehyde were studied by a CASSCF molecular orbital method. The energy barrier of the concerted reaction of butadiene with formaldehyde is about 21 kcal/mol higher than that of butadiene with thioformaldehyde at the CAS-MP2 calculation level. For the stepwise reaction paths, the energy barrier for the first step process of the reaction of butadiene with formaldehyde is about 17 kcal/mol above that of butadiene with thioformaldehyde. The concerted pathways for both systems are more favorable by 9–12 kcal/mol than the stepwise pathways. The electronic mechanisms for the concerted reactions of both reaction systems are also discussed by a CiLC analysis.

Scratching the surface of buckminsterfullerene: the barriers for Stone-Wales transformation through symmetric and asymmetric transition states.

General-gradient approximation (PBE) and hybrid Hartree-Fock density functional theories (B3LYP) in conjunction with basis sets of up to polarized triple-zeta quality have been applied to study the Stone-Wales transformation of buckminsterfullerene (BF) to yield a C(60) isomer of C(2)(v) symmetry with two adjacent pentagons (#1809). In agreement with earlier investigations, two different transition states and reaction pathways could be identified for the rearrangement from BF to C(60)-C(2)(v) on the C(60) potential energy surface (PES). One has C(2) molecular point group symmetry with the two migrating carbon atoms remaining close to the fullerene surface. The other one has a high-energy carbene-like (sp(3)) structure where a single carbon atom is significantly moved away from the C(60) surface. The carbene intermediate and the second transition state along the stepwise reaction path characterized previously at lower levels of theory do not exist as stationary points with the density functionals utilized here. The classical barriers of both mechanisms are essentially identical, 6.9 eV using PBE and 7.3 eV with B3LYP.

On the Hydronium Ion Catalyzed Mechanism in Vinyl Alcohol−Acetaldehyde Isomerization: Ab initio Molecular Orbital Theory and Monte Carlo Simulation

The hydronium ion catalyzed vinyl alcohol−acetaldehyde isomerization was investigated via ab initio molecular orbital (MO) calculations with/without a dielectric continuum (DC) method and Monte Carlo (MC) simulations with the statistical perturbation theory. The cluster models, composed of CH2CHOH, H3O+, and H2O molecules, were considered, and the potential energy profiles of double proton transfers both in the gas phase and in the aqueous solution were obtained by ab initio MO calculations. In order to clarify the mechanism suitable for the reaction, two possible mechanisms, concerted and stepwise, have been proposed. Our results indicated that the double-proton transfers prefer to occur separately not only in the gas phase but also in the aqueous solution, to support the preference for the stepwise mechanism. The solvent effect difference between the DC method and MC simulations was also examined along the stepwise reaction path by plotting the free energy profiles. It was found that MC simulations improve considerably the energy barrier of activation by the DC method (5.0 kcal/mol) to present 13.5 kcal/mol for the free energy barrier of activation. The present value shows good accordance with the experimental value, i.e., 15.2 kcal/mol.

A stepwise reaction path for the homolytic chemisorption of H 2 onto the MgO (100) surface

The potential energy surface for the homolytic chemisorption of H 2 at the MgO (100) surface using both a cluster model employing a gaussian basis, and a periodic model employing a plane wave pseudopotential description, predicts a stepwise reaction path rather than a concerted one.

Stepwise Oxidation of Benzylamine Coordinated to the [Tp‘W(CO)(PhC2Me)]+ Moiety

Coordination of benzylamine to the [Tp‘W(CO)(PhC2Me)]+ moiety activates the amine for stepwise oxidation by sequential hydride/proton removal steps. Each metal complex along the stepwise reaction path (amine, amido, imine, azavinylidene, and nitrile) has been isolated and characterized. The structure of the imine complex [Tp‘W(CO)(PhC2Me)(NHCHPh)][BF4] (4) has been confirmed by an X-ray diffraction study. Complex 4 crystallizes in the space group P21/c (Z = 4, a = 13.790(3) Å, b = 13.225(3) Å, c = 19.150(4) Å, β = 108.769(1)°, R = 3.5%, Rw = 4.4%), and key geometric features include a W−N dative bond length of 2.135(6) Å to the imine nitrogen and a W−N−C angle of 129.7(5)°.

Theoretical studies on the substituent effect of the thermal cycloaddition of ethylene and formaldehyde

Ab initio HF molecular orbital calculations using 3-21G and 6-31G ∗ basis sets and including electron correlation (MP2 to MP4) have been applied to investigate the concerted supra-antara [2 s + 2 a] thermal cycloaddition reaction of some substituted ethylenes and formaldehyde. The stepwise reaction paths have also been studied for one of the reactions by using UHF/3-21G and MP2/3-21G. The calculated results indicate that although the presence of some particular substituents can decrease the activation barriers obviously, the title reaction is difficult to carry out via the supra-antara [2 s + 2 a] thermal cycloaddition pathway under moderate conditions. However, the reactions are probably carried out via stepwise pathways when there are strong electron-withdrawing groups in formaldehyde and/or electron-releasing groups in ethylene.

Studies of the borderline between stepwise and concerted mechanisms of elimination: leaving group effects in elimination of (fluoren-9-ylmethyl)-ammonium ions

By comparison with related substrates β-elimination of (fluoren-9-ylmethyl)trimethylammonium ion to form dibenzofulvene in methanolic sodium methoxide is unexpectedly rapid. The reaction has been investigated as a possible example of an E2 mechanism at the borderline between concerted and stepwise reaction paths. Evidence from buffer saturation in dimethylaminoacetonitrile buffers and comparisons with other amine leaving groups suggests, however, that the mechanism is probably ElcB, with ionization to a carbanion intermediate rate-determining. As measured by σ* for the trimethylammonium group the rate of elimination is 105 times faster than predicted from a Taft correlation of rates of carbanion formation. Probably this reflects the sensitivity of substituent constants for monopoles to solvent and reaction, and a rate-enhancing effect from N-methylation of (fluoren-9-ylmethy1)amine. Comparison of the trimethylammonium leaving group with eliminations of neutral amines also confirms an earlier conclusion that intramolecular proton transfer is not an important reaction path in eliminations of amines activated by a β-fluorenyl group.

Radikalische reaktionswege bei thermisch induzierten umsetzungen von zirconocenkomplexen

The formation of products 2, 3 and 4 ( a–l) from both the reaction of η 2-benzophenone zirconocene 1 with alkyl halides and the thermolysis of the α-(Cp 2ZrCl)-substituted benzhydrylmethylether 9, the latter proceeding with anchimeric assistance of the metal, can be understood assuming a stepwise reaction path through radical intermediates. The proposed intermediate transition-metal benzophenone ketyl 12 exhibits a reaction pattern differing from analogous main-group-metal ketyls.

Relationships between E2 and E1cB mechanisms of β-elimination

It is shown that for base-catalysed β-elimination of 9-fluorenylmethyl derivatives, the stability of the 9-fluorenylcarbanion is too great to be consistent with a continuity of transition states between E2 and E1cB mechanisms. A model is formulated in which the reactants, products, and transition states for concerted elimination are represented on a common potential-energy surface with the carbonium ion and carbanion intermediates of stepwise reaction paths. The effect of energy changes in each stable species upon the energy and structure of the E2 transition state is considered within the framework of the Hammond postulate. The model provides an interpretation of the effects of substituents and reaction conditions upon β-eliminations. It is concluded that a more stable carbanion causes the E2 transition state to approach the carbanion in structure, leading to an apparent relation between E2 and E1cB mechanisms. At the borderline between the mechanisms a continuity of transition states, or operational indistinguishability between them, can be expected only when the reaction of the carbanion approaches diffusion control.