Six-center reactions

In accord with the previously reported results, indeed no KIE was found for allylmagnesum bromide reacting with benzophenone. Quite opposite earlier conclusions, this indicates that the reaction is not dissociative and that there is no ketyl formation. A concerted six-center reaction, nor an 8 2 mechanism must be imagined with C-C bond formation taking place simultaneously with the breaking of the n C=0 bond. 

No carbonyl carbon kinetic isotope effect was found for the reaction of allylmagnesium bromide with benzophenone [13,57]. The rate of the homolytic mechanism—which could be predicted on the basis of the oxidation potential of allylmagnesium bromide [46]—is slower than the concerted six-center reaction. 

Similar equilibria and cis complexes have been proposed in related systems (i, 27, 28, 48). The next step in the reaction pathway has been thought to be a two-carbon insertion reaction. Usually this insertion has been formulated as a four-center reaction. However, for acetate system reactions it is possible to write a more satifying six-center reaction intermediate (Reaction II). 

As shown in the example of pericyclic six centered reactions, the dynamic graph D consists of graphs in all cases and the graph S provides for the classification of reactions (see Figure 3.1) (18,24 26). 

Six-Center Reactions. Apart from the [2 + 4]cycloadditions used for trapping highly strained olefins, systematic studies for probing the nature of distorted double bonds and strain-related features are scarce. The relative rates... 

The authors suggested a concerted addition of hydrogen via a six-centered transition state that would produce 12 directly from the olefin (11) (61). Addition of pyridine or quinoline alters the reaction, but it remains complex. 

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... 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

A normal Diels-Alder reaction is a (n4s + n2s) cycloaddtion and the stereochemistry of both the diene and alkene is retained in the cyclization process. The diene and alkene approach each other in parallel planes. The bonding ineraction involves between Cj and C4 of the diene and carbon atoms of the dienophilic double bond are in a six center arrangement as illustrated below ... 

O Neil, H. E., and Benson, S. W, A method for estimating Arrhenius A factors for four- and six-center unimolecular reactions, J. Phys. Chem. 71, 2903 (1967). 

This is consistent with DFT calculations on [CH3MgCl2], which reveal that the six-centered transition state for the enolisation reaction 66 is entropically favored over the four-centered transition state for the Grignard reaction 67 (Scheme 14). 

Decarboxylations can be regarded as reversals of the addition of carbanions to carbon dioxide (6-32), but free carbanions are not always involved.471 When the carboxylate ion is decarboxylated, the mechanism can be either SeI or Se2. In the case of the SeI mechanism, the reaction is of course aided by the presence of electron-withdrawing groups, which stabilize the carbanion.472 Decarboxylations of carboxylate ions can be accelerated by the addition of a suitable crown ether, which in effect removes the metallic ion.473 The reaction without the metallic ion has also been performed in the gas phase.474 But some acids can also be decarboxylated directly and, in most of these cases, there is a cyclic, six-center mechanism ... 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus cis-1,2-divinylcyclobutane (p. 1131) rearranges smoothly to 1,5-cyclooctadiene, since the geometry is favorable. The trans isomer also gives this product, but the main product is 4-vinylcyclohexene (resulting from 8-33). This reaction can be rationalized as proceeding by... 

There are no detectable intermediate stages or rearrangements in diimide hydrogenation. The reaction is visualized as a six-center (perlcyclic) process in which the bonds are broken and made in a concerted fashion ... 

It must be admitted that 0-0 and a-n switch processes are of limited stereochemical interest. Given the prototypes (92) or (97), the reactions must be syw-stereospecific any other stereochemical result would in fact be used as a criterion of a multi-step mechanism. This is also true of the mixed types, (101)-(104). But these reactions are exceedingly useful as models for orbital-based calculations or estimates of free energies of activation, with the use of extended HMO theory. The four- or six-center process (92)-(104) does not appear to be more complex than the Diels-Alder reaction, which has been investigated theoretically (Herndon and Hall, 1967). 

Mechanisms of these reactions have been discussed in detail (4). Interestingly, the phenyl substituted 2-butenylsilane failed to react with HFA even at 140°C. These observations have been explained by a mechanism involving a six-center intermediate with significant polar contribution to the transition state. 

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