Cheletropic and Other Cycloaddition Reactions

Cheletropic and other cydoaddition reactions conserve orbital symmetry and can occur both intramolecularly as well as tmermolecularly. Both versions are peri-cyclic reactions. The imermolecular set is discussed here (since reactions in that set may be viewed as addition reactions between two molecules). The m/ramolecular reactions, which can be considered rearrangements, will be treated in that section of this chapter. 

In all pericydic processes, the bonds are presumed to form simultaneously (or nearly so) but not necessarily to the same extent at the same time. Thus, they are occurring in concert (and the process is concerted ). However, the process need not be synchronous. 

The ring forming reactions described in this section, with the exception of the reaction of triplet methylene (carbene, CH2), the second example in Table 6.5, follow the symmetry rules outlined in Chapter 4. Thus, the simplest example of an allowed (2 -H 2) addition is shown the first item in Table 6.5. 

Although halogenated carbenes such as dichlorocarbene ( CCl2) are common and can be generated (among other ways) by base treatment of the corresponding trihalomethane, for example, chloroform (trichloromethane, HCCI3), with base, the discussion of that and related processes and those compounds is reserved until 

Alkene Example Electrons from Addend Electrons from Alkene 

---

The table contains examples of the set of cheletropic and other cycloaddition reactions to be considered here. 

---