Asynchronous-concerted pathway

This is in agreement with a transition state in which the C—H bond is at least partially broken. The current transition state hypothesis for the C—H insertion with a rhodium nitrene species involves an asynchronous-concerted pathway, in which the C—H—N angle is smaller than 180° (Figure 5.3). 

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. 

All of the ab initio calcnlations that include electron correlation to some extent clearly favor the concerted pathway for Reaction 4.1. All of these computations also identified a transition state with Q symmetry, indicating perfectly synchronons bond formation. One method for distinguishing a synchronous from an asynchronous transition state is by secondary kinetic isotope effects (KIEs). Isotopic snbstitution alters the frequencies for all vibrations in which that isotope is involved. This leads to a different vibrational partition function for each isotopicaUy labeled species. Bigeleisen and Mayer determined the ratio of partition functions for isotopicaUy labeled species. Incorporating this into the Eyring transition state theory results in the ratio of rates for the isotopicaUy labeled species (Eq. (d. ))." Computation of the vibrational frequencies is thus... 

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... 

In a recent theoretical study of the reaction of 1 -aza-2-azoniaallene salts, with isocyanates it was concluded that these reactions proceed in asynchronous but concerted pathways. The presence of chloro groups in the l-aza-2-azoniaallene salts, as well as methyl groups in the isocyanates, favor the cycloaddition reactions . 

Equation 1.7). A concerted synchronous transition state [15] (the formation of new bonds occurs simultaneously) and a concerted asynchronous transition state [16] (the formation of one reaction depends on the nature of the reagents and the experimental conditions [17]. 

The orbital interaction depicted in Scheme 1.15 shows that the two cr-bonds form at the same time but do not develop to the same extent. The Diels-Alder cycloaddition of unsymmetrical starting materials is therefore concerted but asynchronous. A highly unsymmetrical diene and/or dienophile give rise to a highly unsymmetrical transition state and a stepwise pathway can be followed. 

As can be seen from Fig. 19, the activation energy of the reaction in the presence of the 1-naphthyl substituted TADDOL catalyst was reduced by 10.2 kcal mol, in comparison with the uncatalyzed reaction (20.2 kcal moF ). The reaction proceeds via a concerted but asynchronous pathway, and no zwitterionic intermediate or transition state corresponding to a stepwise Mukaiyama-aldol type pathway could be located. 

Epoxidations of methyl-snbstituted alkenes and o ,/ -unsaturated systems as 1,3-bntadiene and acrylonitrile with peroxyformic acid follow a concerted asynchronous pathway. 

Theoretical calculations (B3LYP) gave predicted isotope effects consistent with experimental effects. Modelled reaction pathway involves complexation of the diazoesters to rhodium, loss of N2 and rhodium carbenoid formation and formation of asynchronous but concerted cyclopropanation transition state. 

The latest proposal by Vedejs (3d) is that the Wittig reaction proceeds via a concerted but asynchronous puckered 4-center cycloaddition pathway in which the stereoselectivity is determined by multiple steric effects and varying degrees of rehybridization at the phosphorus atom in the transition state. At present, there is not definitive evidence to prove that the reaction must proceed in this manner. Recent MNDO-PM3 computations by us (7,8) and somewhat related MNDO computations by Yamataka et al, (9) do not support the puckered 4-center cycloaddition hypothesis for the reactions of unstabilized ylides with aldehydes (3d), Instead, the MNDO-PM3 computations indicate that such Wittig reactions proceed through an essentially planar transition state (TS) with respect to the four central atoms, P-C-C-O. This process is... 

Rhodium Catalysts Rhodium catalysts have been widely used in C—N bond formation via the Rh-nitrene intermediates. Du Bois and co-workers have developed various types of Rh-catalyzed intramolecular C—N bond-forming reactions (Equation 11.18) [44] and intermolecular C—N bond-forming reactions (Equation 11.19) [45]. The mechanism of these C—H amidation reactions proceeds via a concerted asynchronous two-electron oxidation pathway. 

The migration of the phenyl group or the proton was computed to occur in a concerted but asynchronous process. The concreteness of this step implies serious limitations on the relative orientation of the two reactants leaving in each case only two stereochemical possibilities for the formation of the C-C bond between coordinated benzaldehyde and diazoester 2 (Figure 3.14). Hence, totally four reaction pathways were considered in the computational study. 

MO calculations show that the diradical stepwise pathways are preferred over the concerted paths in the Diels-Alder cycloaddition of strained allenes, such as cyclohexa-1,2-diene, with cyclobuta-l,3-diene. An ab initio study of the regio- and stereo-selectivity of the Diels-Alder reaction of vinylallenes and methyl-substituted vinylallenes with acrolein showed the reactions to be asynchronous. Allenamides undergo inverse-electron-demand 4 - - 2-cycloaddition with alkyl vinyl ketones to yield pyranyl ketones. The cycloaddition of (silyloxyvinyl)cyclohexene (147) with the allenecarboxylate (148) produced the exo-cycloadduct (149), which is a key intermediate in the total synthesis of the phosphatase inhibitor dysidiolide (Scheme 57). ... 

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