Concerted Reaction Pathway Research Articles

Direct Methane−Methanol and Benzene−Phenol Conversions on Fe−ZSM-5 Zeolite: Theoretical Predictions on the Reaction Pathways and Energetics

The reaction pathways and the energetics for the direct methane−methanol and benzene−phenol conversions that occur on the surface of Fe−ZSM-5 zeolite are analyzed from B3LYP DFT computations. We propose a reasonable model for “α-oxygen”, a surface oxygen species responsible for the catalytic reactivities of Fe−ZSM-5 zeolite. Our model involves an iron−oxo species on the AlO4 surface site of the zeolite as a catalytic active center and as a source of oxygen. The essential features of the reaction pathways for the methane−methanol and benzene−phenol conversions are identical, especially in bonding characters. In the initial stages of each reaction, methane or benzene comes into contact with the active iron site of the “α-oxygen” model, leading to the reactant (methane or benzene) complex. After the initial complex is formed, each reaction takes place in a two-step concerted manner, via neither radical species nor ionic intermediates. The concerted reaction pathway for the methane (benzene) hydroxylation inv...

Mechanism of intramolecular mixed anhydride formation as intermediates in the reactions of N-phosphorylamino-acids —An MNDO research

N-phosphoryl-α-amino acids can react to form esters, peptides, phosphoryl ester-exchanged products and phosphoryl group migration products under mild conditions, which are very important in researches on arohebiont. It has been proposed that intramolecular mixed carboxylic-phosphoric anhydrides (IMCPAs) might be the common intermediates of the reactions, which are difficult to be experimentally observed because of their high activity. In this note, it has been revealed, by means of MNDO, that the IMCPAs arc situated at the relatively minimal points of the potential surface and can form in a non-synchronous concerted reaction pathway that passes through a transition state with a hydrogen-bond-bridge structure. The results give a successful explanation to the structure and formation of IMCPAs and their ability to promote the various reactions mentioned above.

A Theoretical Study of Unimolecular Reactions of Dimethyl Persulfoxide

Unimolecular reactions on the (CH3)2SOO potential energy surface have been calculated in an attempt to rationalize the reaction of singlet oxygen with dimethyl sulfide. Geometries were optimized with B3LYP/6-31+G(d) except for (CH3)2SOO which is not correctly described at that level of theory. Single-point calculations were made with a computational scheme similar to G2(MP2,SVP). A second intermediate, CH3S(CH2)OOH, formed by intramolecular hydrogen abstraction (ΔHa = 4.8 kcal/mol), is found to play a pivotal role in the concerted reaction pathway. From this intermediate, dimethyl sulfone can be formed in two steps. First, a hydroxyl group migrates to the sulfur (ΔHa = 8.3 kcal/mol) followed by intramolecular hydrogen migration to the methylene group (ΔHa = 14.4 kcal/mol). Alternatively, the CH3S(CH2)(O)OH intermediate can be protonated by trace water and deprotonated to form dimethyl sulfone, which can explain the H/D exchange process observed experimentally. A sulfurane, formed by the addition of water to (CH3)2SOO, is calculated to be bound by 13.5 kcal/mol.

1,3-Dipolar cycloaddition of diazomethane to chiral azlactones. Experimental and theoretical studies

Abstract The reaction of (E)-2-phenyl-4-[(S)-2,2-dimethyl-1,3-dioxolan-4-ylmethylen]-5(4H)-oxazolone with diazomethane has been studied under a variety of reaction conditions, and the results compared with those obtained with the corresponding (Z)-isomer. The origin of the high diastereofacial selectivity in the cyclopropane products, observed with both oxazolones, is discussed in the light of theoretical calculations of the first reaction step, i.e., the 1,3-dipolar cycloaddition of diazomethane, carried out at the semiempirical (AM1) and ab initio (RHF/3-21G) levels. Both theoretical levels correctly predict the direction of the asymmetric induction in the concerted reaction pathway leading to the Δ1-pyrazoline. AM1 calculations predict also the existence of a stepwise reaction pathway leading to an open-chain reaction intermediate, which is not supported by the ab initio results.

On the real nature of aliphatic carbenium ions as active intermediates of homogeneous and heterogeneous acid catalysis

The structure and properties of active intermediates of acid-catalyzed homogeneous and heterogeneous transformations of olefins and alcohols are discussed, taking into account the hydration in homogeneous systems or the interaction with the surface of heterogeneous zeolite catalysts on the basis of nonempirical quantum-chemical calculations. In both cases carbenium ions do not represent the actual existing active intermediates, but rather the excited transition states. They could be formed from the more covalent and more stable surface esters in heterogeneous acid catalysis or from hydrated alkoxonium ions in acid catalysis in aqueous solutions. This results in concerted reaction pathways with low activation energies due to almost complete compensation of the energy required for the rupture of the reacting chemical bonds in the initial compounds by the energy released by the formation of the new bonds in the final products.

Thermal and photochemical decomposition of 9-arylxanthen-9-yl hydroperoxides and peroxides

The thermolysis and photolysis of a series of 9-arylxanthenyl hydroperoxides and peroxides have been investigated by means of differential scanning calorimetry, electron paramagnetic spectroscopy and product studies. These relatively stable peroxidic compounds were found to rearrange smoothly on being heated at temperatures > 120°C and on photolysis, respectively, through concerted reaction pathways. Photolysis of 9-hydroperoxy-9-phenylxanthene and di(9-phenylxanthen-9-yl) peroxide, respectively, have been shown by EPR spectroscopy to generate 9-phenylxanthenyl radicals which dimerise at low temperatures and reform the free radicals at ambient temperature.

Formal 1,6-insertion of an alkylidenecarbene into a carbon-hydrogen bond. Unveiling of a stepwise reaction mechanism.

The formation of 1,3-dimethylquinol-2-one ( 6a) by reaction of N-methyl- N-phenyl-2-oxopropanamide ( 1a) with diethyl (diazomethyl)phosphonate ( 2), an apparent CH insertion by an alkylidenecarbene, is shown to involve a stepwise rather than a concerted reaction pathway.

Free-radical and concerted reaction pathways in dibenzyl ether thermolysis

Free-radical and concerted reaction pathways in dibenzyl ether thermolysis

Organocadmium reagents : VII. Competition between displacement of alkoxy and acyloxy in 3-substituted phthalides

The reaction of organocadmium reagents with 3-alkoxyphthalides to give o-substituted benzoic acids by an acyloxy displacement is found to be general and highly favored over the displacement of an alkoxy substituent. In the case of 3-acyloxyphthalides, the “exocyclic” group is displaced preferentially. Two concerted reaction pathways, consistent with the stereochemical results, could account for this behavior.