1,3-dipolar cycloaddition reactions 1,3-DPCAs

There is a large class of reactions known as 1,3-dipolar cycloaddition reactions (1,3-DPCA) that are analogous to the Diels-Alder reaction in that they are concerted [tt4,+ tt2J cycloadditions.1,3-DPCA reactions can be represented as shown in the following diagram. The entity a-b-c is called the 1,3-dipole and d-e is the dipolarophile. 

Dipolar cycloadditions are the reactions that involve atoms with formal charges. Usually four-membered rings are constructed by [2 -I- 2] cycloaddition, six-membered rings by [4 + 2] cycloaddition, and the five-membered rings by 1,3-dipolar cycloaddition reactions (1,3-DPCAs). In general, a... 

The cycloaddition reactions of 1,3-dipolar compounds with alkenes or alkynes or heteroatom containing double or triple bonds are known as 1,3-Dipolar cycloaddition reactions, or simply 3-DPCA reactions. These are another type of concerted [7is+7ts]-cycloaddition reactions, analogous to D-A reactions [104]. 1,3-Dipolar compounds are known as 1,3-dipoles and alkenes or alkynes are known as dipo-larophiles. These reactions are represented as... 

The most widely applied interpretation of substituent effects on relative reactivity is based on FMO theory. According to FMO theory, interacting orbitals are most stabilized when they are closest in energy. Substituent effects on dipolar cycloadditions can be interpreted in terms of matching of HOMO and LUMO orbitals of the two reactants.This is the same concept used in applying FMO theory to D-A reactions (see p. 844-848). In the D-A reaction, it is fairly clear which reactant is electrophilic and which is nucleophilic, and the interpretation of substituent effects follows directly. This choice is not always so obvious for 1,3-DPCA reactions. In fact, for several of the 1,3-dipoles both EWGs and ERGs in the dipolarophile enhance reactivity. These 1,3-dipoles are called ambiphilic. Let us look carefully to see why they have this property. 

---