Concerted asynchronous mechanism

In one study, the mechanisms of the reaction of methyl cinnamate and cyclopen-tadiene with BF3, A1C13, and catecholborane bromide as catalysts were compared.29 According to these computations (B3LYP/6-31G ), the uncatalyzed and BF3- and AlCl3-catalyzed reactions proceed by asynchronous concerted mechanisms, but a... 

Unimolecular peroxide decomposition chemiluminescence, 1227-31 asynchronous concerted mechanism, 1230 biradical mechanism, 1181-2, 1227-31 concerted mechanism, 1227, 1228-9, 1230... 

The n-jr state selectivity observed in the decomposition of 1,2-dioxetanes, even in cases in which the it-it excited state of the carbonyl product possesses lower energy105-108, appears to be best explained by the concerted biradicaloid decomposition of the dioxetanes, or by the intermediacy of an extremely short-lived biradical60. These studies provide strong evidence for the asynchronous, concerted mechanism because a... 

The molecular mechanism of the Hoffmann elimination involving (iV-Cl)-N-methyl-ethanolamine has been theoretically characterized by using DFT at the B3LYP/ 6-31++G computing level.49 The role of water as a solvent has been analysed by using both discrete and hybrid discrete-continuum models. The rearrangement proceeds by a water-assisted asynchronous concerted mechanism. 

The mechanism and kinetic aspects of the retro-ene reaction of the allyl n-propyl sulfide and its deuterated derivatives have been studied using four different types of DFT methods with eight different levels of the basis sets.7 The mechanistic studies revealed that the reaction proceeds through an asynchronous concerted mechanism. Theoretical calculations have indicated that the reaction displays a kinetic isotope effect of 2.86 at 550.65 K. 

Calculations on the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism. The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged at a highly asynchronous concerted transition structure. 

The selection between concerted and stepwise mechanism on the basis of qualitative analysis of 13C KIEs (Table 2) do not allow to select dominating mechanism. The large carbon isotope effect on Cl and small effects on other carbons suggest the stepwise mechanism or highly asynchronous concerted mechanism. The deuterium KIEs are consistent with a concerted mechanism. The inverse KIEs in the H4out and H4in positions are indicative for the bond formation to C4 in the transition state and are not expected for the stepwise mechanism. 

The [3+2] cycloaddition reactions of isocyanates with 1 -aza-2-azoniaallene salts proceed by an asynchronous concerted mechanism . 

In this paper, we have shown the theoretical validity of the suggested catalytic cycle for the hydrosilylation of ketones using N-heterocyclic diaminocarbenes Cu(l) hydride catalysts. The activation of the catalyst from a copper fluoride complex, as well as both steps of the catalytic cycle, involves a 4-center a metathesis transition state. The reactants are guided toward these transition states, through the formation of energetically favored van der Waals complexes, formed through favorable electrostatic interactions. Analysis of transition states shows that the a metathesis reactions studied here can be described as asynchronous concerted mechanisms, with the transfer from Cu-linked atoms occurring prior to the transfer toward the metal center. 

Honk et al. concluded that this FMO model imply increased asynchronicity in the bond-making processes, and if first-order effects (electrostatic interactions) were also considered, a two-step mechanisms, with cationic intermediates become possible in some cases. It was stated that the model proposed here shows that the phenomena generally observed on catalysis can be explained by the concerted mechanism, and allows predictions of the effect of Lewis acid on the rates, regioselectivity, and stereoselectivity of all concerted cycloadditions, including those of ketenes, 1,3-dipoles, and Diels-Alder reactions with inverse electron-demand [2],... 

One mechanistic matter that has caused quite a bit of general consternation about a decade ago concerns the experimental evidence for the involvement of diradical intermediates (proposed as sources for the observed radical products) in dioxirane epoxidations, which were thought to be formed through induced peroxide-bond homolysis by the alkene. Nonetheless, rigorous experimental and high-level theoretical work disposed such radical chemistry in the epoxidation of alkenic substrates. The latter computations unequivocally confirm the established concerted mechanism, in which both CO single bonds in the incipient epoxide are concurrently formed by way of an asynchronous, spiro-structured transition state for the oxygen transfer. 

In view of the above exposed facts, to date there is no direct experimental evidence of the intermediary 1,4-dioxy biradical in the decomposition of 1,2-dioxetanes. Therefore, it appears that the asynchronous (biradicaloid or biradical-like) concerted mechanism (merged mechanism) is the one consistent with aU the experimental and theoretical data currently available. 

W-C4H9) [106, 107]. Ab initio MO calculations have shown that the [2 - - 2] cycloaddition between cis- or Jraw-alkenes and isocyanates to yield y9-lactams takes place by a concerted mechanism in the gas phase, with retention of the alkene configuration in the product. However, increasing solvent polarity contributes to the asynchronicity of the reaction to such an extent that the mechanism changes from a concerted to a two-step process involving a zwitterionic intermediate, with a consequent loss of alkene stereospecificity, as observed in the reaction between chlorosulfonyl isocyanates and vinyl ethers [794]. 

The Unuc and Utg values indicate that there is a positive imbalance of +0.67 units of normalised effective charge on the SO3 group of atoms, needed to supply electrons to support the charge during bond fission (ttLg = 0.80), in conjunction with a bond formation of Qnuc = 0.13. The displacement mechanism is thus a highly asynchronous concerted process with a transition state structure possessing substantial, neutral monomeric, sulfur tri oxide character (Scheme 15). 

Dirhodium tetracarboxylate complexes are among the most successful and well-studied catalysts for C-H amination. Early work by Miiller provided support for a concerted asynchronous reaction mechanism for intermolecular amination reactions using Rh2(OAc)4 and NsN=IPh [22-24]. Du Bois and coworkers have shown that carbamate and sulfamate esters can engage in oxidative cyclization reactions promoted by these same types of Rh complexes using PhI(OAc)2 as the terminal oxidant [93-96]. Mechanistic studies, which include Hammett analysis (p = 0.55... 

KIEs are frequently measured to determine whether PCET reactions proceed via stepwise or concerted mechanisms. Small KIEs are frequently cited as evidence for an asynchronous PCET or stepwise ET/PT reaction whereas large KIEs often... 

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