reacts with the electron-rich 2-methylpropene about 100,000 times faster than the electron-poor tetrachloroethene (see reactivity scale below). This type resembles the inverse electron-demand Diels-Alder reaction, in which the diene LUMO combines with the dienophile HOMO. Reactivity Concerted processes such as the 1,3-cycloaddition require a highly ordered transition state (high negative entropy of activation) and only moderate enthalpy requirements. Using competition reaction experiments, relative rates of addition for different cycloaddition reactions have been found to offer general findings on factors in reactivity. Conjugation, especially with aromatic groups, increases the rate of reaction by stabilization of the transition state. During the transition, the two sigma bonds are being formed at different rates, which may generate partial charges in the transition state that can be stabilized by charge distribution into conjugated substituents. More polarizable dipolarophiles are more reactive because diffuse electron clouds are better suited to initiate the flow of electrons. Dipolarophiles with high angular strain are more reactive due to increased energy of the ground state. Increased steric hindrance in the transition state as a result of unhindered reactants dramatically lowers the reaction rate. Hetero-dipolarophiles add more slowly, if at all, compared to C,C-diapolarophiles due to a lower gain in sigma bond energy to offset the loss of a pi bond during the transition state. Isomerism of the dipolarophile affects reaction rate due to sterics. trans-isomers are more reactive (trans-stilbene will add diphenyl(nitrile imide) 27 times faster than cis-stilbene) because during the reaction, the 120° bond angle shrinks to 109°, bringing eclipsing cis-substituents towards each other for increased steric clash. Stereospecificity 1,3-dipolar cycloadditions usually result in retention of configuration with respect to both the 1,3-dipole and the dipolarophile. Such high degree of stereospecificity is a strong support for the concerted over the stepwise reaction mechanisms. As mentioned before, many examples show that the reactions were stepwise, thus, presenting partial or no stereospecificity. With respect to dipolarophile cis-Substituents on the dipolarophilic alkene end up cis, and trans-substituents end up trans in the resulting five-membered cyclic compound (see scheme below). With respect to dipole Generally, the stereochemistry of the dipole is not of major concern because only few dipoles could form stereogenic centers, and resonance structures allow bond rotation which scrambles the stereochemistry. However, the study of azomethine ylides has verified that cycloaddition is also stereospecific with respect to the dipole component. Diastereopure azomethine ylides are generated by electrocyclic ring opening of aziridines, and