Cyclic concerted transition state mechanisms

Mechanistically, the exchange process may involve a four-membered or other cyclic concerted transition state, or may possibly proceed via an electron-transfer sequence, however direct nucleophilic attack, at least on iodine, has been demonstrated in the case of iodobenzene, and cannot therefore be dismissed as a more general mechanism. 

The mechanism of the Chugaev elimination involves a cyclic, concerted transition state as originally proposed by Barton11 and Cram.12... 

Pericyclic reactions are a class of reactions that include some of the most powerful synthetically useful reactions such as the Diels-Alder reaction. Pericyclic reactions often proceed with simultaneous reorganization of bonding electron pairs and involve a cyclic delocalized transition state. They differ from ionic or free radical reactions as there are no ionic or free radical intermediates formed during the course of the reaction. They proceed by one-step concerted mechanisms and have certain characteristic properties (although there are some exceptions to all these rules). 

The [2 + 2] cycloaddition represents the most general and direct pathway for the formation of a cyclobutane structure from two alkene moieties, as outlined in Scheme 2.126. This process may occur as a concerted reaction via a cyclic transition state (mechanism a), as a stepwise reaction involving the formation of an acyclic biradical (mechanism b), or through bipolar (mechanism c) intermediates. Depending upon the structure of the reactants, cycloaddition may occur by any of these mechanisms. 

Like the Diels-Alder reaction discussed in Sections 14.4 and 14.5, the Claisen rearrangement reaction takes place through a pericyclic mechanism in which a concerted reorganization of bonding electrons occurs through a six-membered, cyclic transition state. The 6-allyl-2,4-cyclohexadienone intermediate then isomerizes to o-allylpbenol (Figure 18.1). 

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

The evidence presented so far excludes the formation of dissociated ions as the principal precursor to sulfone, since such a mechanism would yield a mixture of two isomeric sulfones. Similarly, in the case of optically active ester a racemic product should be formed. The observed data are consistent with either an ion-pair mechanism or a more concerted cyclic intramolecular mechanism involving little change between the polarity of the ground state and transition state. Support for the second alternative was found from measurements of the substituent and solvent effects on the rate of reaction. 

There are, broadly speaking, three possible mechanisms that have been considered for the uncatalyzed Diels-Alder reaction. In mechanism a there is a cyclic six-centered transition state and no intermediate. The reaction is concerted and occurs in one step. In mechanism b, one end of the diene fastens to one end of the dienophile first to give a diradical, and then, in a second step, the other ends become fastened. A diradical formed in this manner must be a singlet that is, the... 

Harger has studied the rearrangement of A-substituted N-phosphinoylhydroxylamines in the presence of base . He proposed a concerted mechanism based on the observed retention of the configuration at the phosphorous center during the transposition , and on studies with 0-labelled compounds . Similar cyclic transition states 572 were proposed in the base-induced rearrangement of A,0-bis(diphenylphosphinoyl)hydroxylamines (571) (equation 254). However, in the rearrangement of O-benzoyl-A-(diphenylphosphino-thiol)hydroxylamine where a transposition of O and S atoms occurs, the proposed cyclic transition state has sulfur participation . 

Another important family of elimination reactions has as the common mechanistic feature cyclic transition states in which an intramolecular proton transfer accompanies elimination to form a new carbon-carbon double bond. Scheme 6.16 depicts examples of the most important of these reaction types. These reactions are thermally activated unimolecular reactions that normally do not involve acidic or basic catalysts. There is, however, a wide variation in the temperature at which elimination proceeds at a convenient rate. The cyclic transition states dictate that elimination occurs with syn stereochemistry. At least in a formal sense, all the reactions can proceed by a concerted mechanism. The reactions, as a group, are referred to as thermal syn eliminations. 

Irradiation of pyruvic acid (26) in the vapor phase produces acetaldehyde and carbon dioxide.85 The most reasonable mechanism for the reaction appears to be a concerted decarboxylation via a cyclic four- or five-membered transition state. Although the four-membered transition state has an analogy... 

Pericyclic reactions are unimolecular, concerted, uncatalyzed transformations. They take place in a highly stereoselective manner governed by symmetry proper-ties of interacting orbitals. - Characteristic of all these rearrangements is that they are reversible and may be effected thermally or photochemically. The compounds in equilibrium are usually interconverted through a cyclic transition state,224 although biradical mechanisms may also be operative. A few characteristic examples of pericyclic rearrangements relevant to hydrocarbon isomerizations are presented here. 

H-Cl plus intimate ion-pairs, H+C1-). A mechanism involving concerted attack by the proton and by the chloride ion was proposed. Since the deuterium isotope effect of DC1 was very small, it was suggested28 that the H-Cl bond in the transition state was only slightly weakened. In the present terminology, the mechanism would be regarded as SE2(cyclic), perhaps verging towards SE2(co-ord), viz. 

The gas-phase unimolecular elimination reactions of 2-substituted ethyl N,N-dimethylcarbamates23,24 and several heterocyclic carbamates25 have been studied using the Moller-Plesset MP2/6-31G method. On the basis of these calculations, the mechanism appears to be concerted, asynchronous, through a six-membered cyclic transition-state structure. 

A concerted mechanism in which the characteristic bond shifts take place through the cyclic transition state has been favored for some time [29,30], The lack of substituent and solvent effects on product distribution [31] and the lack of direct evidence of any intermediates were given as support for the concerted ene mechanism. 

The kinetics of the gas-phase elimination of 3-hydroxy-3-methylbutan-2-one have been investigated in a static system, seasoned with allyl bromide, and in the presence of the free chain radical inhibitor toluene.14 The reaction was found to be homogeneous, unimolecular and to follow a first-order rate law. The products of elimination are acetone and acetaldehyde. Theoretical estimations suggest a molecular mechanism involving a concerted non-synchronous four-membered cyclic transition state process. 

Prior to 1953, few kinetic works on the homogeneous, unimolecular gas-phase pyrolysis or elimination of simple alkyl halides were reported. According to these experimental data the commonly accepted mechanism consisted of a concerted four-membered cyclic transition state yielding the corresponding olefin and hydrogen halide as shown in equation 1. For molecular dehydrohalogenation, the presence of a /i-hydrogen adjacent to the C—X bond is necessary. 

The mechanism of the a,a-elimination (path a) implies substantial freezing of rotational modes of the transition state by the concerted transfer of the F atom and a three-mem-bered cyclic transition state for elimination of HC1. These geometric factors should be... 

When the /3-ketoacid is heated, carbon dioxide is lost. This step, a decarboxylation, occurs by a mechanism that is quite different from any other that we have encountered so far. Three bonds are broken and three bonds are formed in a concerted reaction that proceeds through a cyclic, six-membered transition state. The product of this step is an enol. which tautomerizes to the final product, a ketone ... 

The mechanism of the Diels-Alder reaction is a concerted cyclic movement of six electrons four in the diene and two in the dienophile. For the three pairs of electrons to move simultaneously, the transition state must have a geometry that allows overlap of the two end p orbitals of the diene with those of the dienophile. Figure 15-15 shows the required geometry of the transition state. The geometry of the Diels-Alder transition state explains why some isomers react differently from others, and it enables us to predict the stereochemistry of the products. 

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