Sigmatropic transition states

OTHER VIEWS ABOUT SIGMATROPIC TRANSITION STATE 1. Formation of a cyclic transition state structure... 

Other Views about Sigmatropic Transition State... 

OTHER VIEWS ABOUT SIGMATROPIC TRANSITION STATE... 

Sigmatropic rearrangements are normally classified as concerted processes with relatively nonpolar transition states. However, the Fischer cyclization involves rearrangement of a charged intermediate and ring substituents have a significant effect on the rate of the rearrangement. The overall cyclization rate... 

These reactions will be discussed in more detail under the topic of 3,3-sigmatropic rearrangements in Chapter 11. For the present we simply want to focus on the fact that the reaction is stereospecific-, the -isomer gives one diastereomeric product whereas the related Z-isomer gives a different one. The stereochemical relationship between reactants and products can be explained if the reaction occurs through a chairlike transition state in... 

Just like the Diels Alder reaction or the 1,5-sigmatropic hydrogen shift, the ene reaction is believed to proceed via a six-membered aromatic transition state. 

In certain cases the reaction may proceed by a concerted mechanism. With allyl ethers a concerted [2,3]-sigmatropic rearrangement via a five-membered six-electron transition state is possible " ... 

A pericyclic reaction is one that takes place in a single step through a cyclic transition state without intermediates. There are three major classes of peri-cyclic processes electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. The stereochemistry of these reactions is controlled by the symmetry of the orbitals involved in bond reorganization. 

In any given sigmatropic rearrangement, only one of the two pathways is allowed by the orbital-symmetry rules the other is forbidden. To analyze this situation we first use a modified frontier-orbital approach. We will imagine that in the transition state the migrating H atom breaks away from the rest of the system, which we may treat as if it were a free radical. 

The actual reported results bear out this analysis. Thus a thermal [1,3] migration is allowed to take place only antarafacially, but such a transition state would be extremely strained, and thermal [1,3] sigmatropic migrations of hydrogen are unknown." On the other hand, the photochemical pathway allows suprafacial [1,3] shifts, and a few such reactions are known, an example being " ... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

Alcaide, Aknendros and coworkers developed a combination of a 3,3-sigmatropic rearrangement of the methanesulfonate of an a-allenic alcohol to give a 1,3-bu-tadiene which is intercepted by a dienophile present in the molecule to undergo an intramolecular Diels-Alder reaction [83]. Thus, on treatment of 4-236 with CH3S02C1, the methanesulfonate was first formed as intermediate, and at higher temperature this underwent a transposition to give 4-237 (Scheme 4.51). This then led directly to the cycloadduct 4-238 via an exo transition state. 

A particularly instructive example is the thermolysis of (Z)-l,3,8-nonatriene in which an intramolecular Diels-Alder reaction competes with a sigmatropic [1,5] hydrogen shift (Scheme 24). The use of high pressure here enables a reversal of the selectivity. At 150°C and 1 bar the [1,5] hydrogen shift passing through a monocyclic transition state is preferred. At 7.7 kbar the intramolecular Diels-Alder reaction is preferred due to its bicyclic transition state. 

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