Unimolecular Thermal Elimination Reactions

There is an important group of thermal elimination reactions which find use in synthesis. Some of these are concerted processes. When these processes are 

Just as cycloaddition reactions can be interpreted in terms of orbital-symmetry considerations, elimination reactions which are concerted also require appropriate alignment of orbitals for continuous bonding. The principles of orbital symmetry can then specify what processes can occur in concerted fashion and identify the stereochemical restrictions which are imposed by the concerted process. The number of elimination reactions that have been studied in detail is not large, but there is sufficient information to establish that orbital-symmetry controls are indeed operating. Cheletropic processes are defined as reactions in which two bonds are broken (or formed) to a single atom. 

We are interested here in the elimination process. In these reactions the atom X is bound to o her atoms in such a way that it is part of a small stable molecule. The most common examples involve five-membered cyclic transition states, although the term cheletropic is not restricted with respect to ring size so long as the process can be concerted. 

A good example of a concerted elimination is the reaction that takes place on treatment of 3-pyrrolines with iV-nitrohydroxylamine. The reactive intermediate B is capable of elimination of molecular nitrogen. The reaction has been shown to 

CHAPTER 7 consistent with predictions based on conservation of orbital symmetry. 

This section describes reactions in which elimination to form a double bond or a new ring occurs as a result of thermal activation. There are several such thermal elimination reactions that are used syntheses, some of which are concerted processes. The 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry analysis in the same way as cycloadditions and sigmatropic processes. In the elimination processes of interest here, the atom X is normally bound to other atoms in such a way that elimination gives rise to a stable molecule. In particular, elimination of S02, N2, or CO from five-membered 3,4-unsaturated rings can be a facile process. 

A good example of a concerted cheletropic elimination is the reaction of 3-pyrroline with IV-nitrohydroxylamine, which gives rise the the diazene 21, which then undergoes elimination of nitrogen. 

Use of substituted systems has shown that the reaction is stereospecific.300 The groups on C(2) and C(5) of the pyrroline ring rotate in the disrotatory mode on going to product. This stereochemistry is consistent with conservation of orbital symmetry. 

The most synthetically useful cheletropic elimination involves 2,5-dihydrothiophene-1,1-dioxides (sulfolene dioxides). At moderate temperatures they fragment to give dienes and sulfur dioxide.301 The reaction is stereospecific. For example, the dimethyl derivatives 22 and 23 give the E,E- and Z,E-isomers of 2,4-hexadiene, respectively, at temperatures of 100°-150°C.302 This stereospecificity corresponds to disrotatory elimination. 

There are several thermal elimination reactions that find use in synthesis. Some of these are concerted processes. The transition state energy requirements and stereochemistry of concerted elimination processes can be analyzed in terms of orbital symmetry considerations. We will also consider an important group of unimolecular -elimination reactions in Section 6.8.3. 

Cheletropic processes are defined as reactions in which two bonds are broken at a single atom. Concerted cheletropic reactions are subject to orbital symmetry restrictions in the same way that cycloadditions and sigmatropic processes are. 

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