Asynchronous reaction

The activation energies were computed to 3.0 (toward 183), 0.3 (toward 182), and 21.8 kcal/mol (toward 184) at the B3-LYP/6-31G level, and thus the mechanism leading to 182 is the preferred one. The transition states of all three reactions belong to concerted but asynchronous reaction paths. The transacetalization of 177 with acylium cations results in the formation of the thermodynamically stabilized 187 (Scheme 121) [97JCS(P2)2105]. 186 is less stable than 187, and 185 is destabilized by 32.5 kcal/mol. Moreover, transacetalization of 177 with sulfinyl cations is not a general reaction. Further computational studies on dioxanes cover electrophilic additions to methylenedioxanes [98JCS(P2)1129] and the influence... 

This is the case for the quadricyclane - to norbornadiene" reaction. Although the C2K reaction path provides an attractive interpretational tool for understanding the progress of this reaction, its highest point represents a conical intersection at which the two relevant states have the same energy at the same geometry. This point cannot be a transition state, so that lowering the symmetry in any direction leads to a stabilization. The result is an asynchronous reaction path in which one of the two cyclopropane bonds is broken first to form the biradical-like transition state la. The second bond can then break to form the norbornadiene radical cation 2. 

The kinetic data for cycloadditions of 2,4,6-trimethylbenzonitrile oxide to benzonitriles are consistent with a concerted although asynchronous reaction with a transition state, in which the formation of the O—C bond is more advanced than the making of the C—N bond <90JCS(P2)245>. 

Two mechanistic variations can be envisioned to be operative in some Wittig reactions. In some cases (Z-stereoselective reactions) a very asynchronous reaction with a 2-center "anti" pseudo betaine transition state is involved. The other mechanism involves a more synchronous concerted reaction with a 4-center "syn" transition state. These two mechanisms may compete, e.g., in the reactions of semistabilized ylides with aldehydes where mixtures of Z- and E-alkenes are obtained. Further details of these studies will be provided in future publications, as well as additional data about solvent effects. 

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... 

G calculations. Their (6/6)CASSCE/6-31G PES showed two C2 stationary points—one the TS for a concerted Cope rearrangement, with / = 2.189 A, and the other an intermediate in a highly asynchronous reaction, with R = 1.641 A. The latter C711 stationary point was calculated to be 1.9 kcal/mol lower in energy than the former. Thus, the results of Davidson s then state-of-the-art ab initio calculations actually validated Dewar s AMI results [15,16] ... 

On the other hand, MINDO/3 calculations, which often favor biradical formation, suggest that the reaction occurs via a biradical.But subsequent calculations at various ab initio levels revealed a concerted asynchronous reaction pathway. In the latest effort using large basis sets and 10/10 MCSCF approaches, the pathway of Scheme 5.4 was found to be of lowest energy by 14 kcal/mol. The transition state has one bond stretched to 2.25 A while the cross-ring bond is stretched by only 0.05 A from starting material, and the methylene group is twisted by approximately... 

This is generally a concerted synchronous process with both new bonds forming simultaneously. Combinations of some highly polar substrates undergo concerted asynchronous reactions in which the two new o bonds are formed at different rates. In some rare cases, the reaction proceeds in a stepwise fashion with the absence of expected stereoselectivity. Orfanopoulos reported an example of a stepwise hetero Diels-Alder reaction in 2009. ... 

The site-, regio-, and stereochemistry of some of these DA reactions has been investigated by the density functional theory, including solvent effects. Generally, these cycloadditions proceed by a concerted but asynchronous reaction mechanism. The endo stereochemistry is in most case preferred. 

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