Transitions state, four-center

The influence of the solvent on the decomposition rate of phenylpentazole (see Table II) supports the one-step four-centered 1,3-flssion of phenylpentazole according to 22. The correlation of the polarity of the transition states of 1,3-additions, which resemble the assumed transition state of the phenylpentazole decomposition (22), andsolvent effects has recently been discussed by Huisgen. ... 

The Diels-Alder reaction is one of the most useful synthetic reactions for the construction of the cyclohexane framework. Four contiguous stereogenic centers are created in a single operation, with the relative stereochemistry being defined by the usually ewdo-favoring transition state. 

In the next step, one of the borane-hydrogens is transferred to a sp -carbon center of the alkene and a carbon-boron bond is formed, via a four-membered cyclic transition state 6. A mono-alkyIborane R-BH2 molecule thus formed can react the same way with two other alkene molecules, to yield a trialkylborane R3B. In case of tri- and tctra-substituted alkenes—e.g. 2-methylbut-2-ene 7 and 2,3-dimethylbut-2-ene 9—which lead to sterically demanding alkyl-substituents at the boron center, borane will react with only two or even only one equivalent of alkene, to yield a alkylborane or mono alky Iborane respectively ... 

Recognition of the fact that tin(IV) enolates exist predominantly as the carbon-bound enolate has led to the alternative suggestion that a four-center transition state, such as L could also rationalize the reversal of diastereoselectivity upon changing the enolate counterion from aluminum to tin26-44. 

When chiral enolates or chiral Michael acceptors are used, for instance, when stereogenic centers are present in the substrate or when X or Y are chiral auxiliaries, both simple and induced diastereoselectivity is observed. This results, in principle, in the formation of four diastereomers 1 -4. The diastereoselectivity in the Michael addition of lithium enolates to enones can be rationalized by consideration of chelated transition states A-D372. 

Thermal rearrangement of trans-l,2-dibromo compounds is known in the literature (refs. 6-10). In all case studies only one pair of bromine in each organic molecular was studied. Bellucci (ref. 10), for example, studied the kinetics of such trans-l,2-cyclo alkanes as cyclopentane, hexane, octane, etc. The intermediates suggested as an explanation for the experimental results are bromonium bromide I in polar solvents and four center transition state II in non-polar solvents. 

It has been suggested that a second mechanism, involving a four-center transition state, is also possible Bellus, D. Schaffner, K. Hoigne, J. Helv. Chim. Acta, 1968, 51, 1980 Sander, M.R. Hedaya, E. Trecker, D.J. J. Am. Chem. Soc., 1968, 90,7249 Bellus, D. Ref. 413. 

The degree of vibrational excitation in a newly formed bond (or vibrational mode) of the products may also increase with increasing difference in bond length (or normal coordinate displacement) between the transition state and the separated products. For example, in the photodissociation of vinyl chloride [9] (reaction 7), the H—Cl bond length at the transition state for four-center elimination is 1.80 A, whereas in the three-center elimination, it is 1.40 A. A Franck-Condon projection of these bond lengths onto that of an HCl molecule at equilibrium (1.275 A) will result in greater product vibrational excitation from the four-center transition state pathway, and provides a metric to distinguish between the two pathways. 

M amines competed effectively with 55.5 M H O and 0.1 M OH for the nitrosating agent and suggested that possibly more reactive isomers of N O and N 0. are generated by the gaseous NO and NO components Here, -nltrosamines result from reaction of the unsymmetrical tautomer (ON-NO ), whereas the symmetrical tautomer (O N-NO ) produces an N-nitramine possibly via a four-center transition state. The results for N O may be xplalned similarly in terms of the corresponding ON-nO.ON-ONO tautomers. This conclusion has a prece-... 

The H—D exchange between triethyltin deuteride and di-isobutylaluminum hydride has been studied by Neumann 84). A four-center transition state has been proposed for this reaction... 

However, when (+)-methylneophylphenyltin deuteride, (+)-(56) ([ot] s + 10.7) is kept in the dark mixed with five equivalents of diethylaluminum hydride for ten hours at room temperature in benzene, optically inactive (72) is formed 44). (In the absence of (Et2AlH)2 less than 3 % of (12) is racemized under these conditions). The four-center transition state is therefore very unlikely. 

In the second step, the n complex becomes the addition product by passing through a four-center transition state in which the boron atom is partially bonded to the less substituted carbon atom of the double bond. 

Addition takes place through the initial formation of a n complex, which changes into a cyclic four-center transition state with the boron atom adding to the less hindered carbon atom. The dashed bonds in the transition state represent bonds... 

However, the rate of substitution of pyrrole is too high and that of benzene too low to be followed by standard techniques, and consequently a kinetic study was limited to furan, thiophene, selenophene, and tellurophene. Activation entropies are constant for all four members of the series, indicating that the arrangement of the atoms around the reaction center is similar, i.e., the transition states of all four rings occur at similar positions along the reaction coordinate. The relative rates for the formylation are thus controlled by the activation enthalpies. At 30UC relative rates are furan (107), thiophene (1), selenophene (3.64), and tellurophene (36.8).68... 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

As a model for the insertion process in the polymerization of ethylene, the reaction of Cp ScMe with 2-butyne was investigated. The reaction was revealed to have a relatively small enthalpy of activation and a very large negative entropy of activation a highly ordered four-centered transition state (117) was proposed [111, 112]. 

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

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