Hammond postulate 2+2 -cycloaddition

Cycloadditions are less exothermic than [4+2]-cycloadditions, since the former result in a strained cycloadduct while the latter give unstrained rings. Thus, according to the Hammond postulate, the [2+2]-cycloadditions should occur more slowly than the [4+2]-cycloadditions. While these cycloadditions would be expected to be slower, thermochemistry does not explain why one-step cycloadditions between two alkenes or between an alkene and an alkyne—in the absence of light—cannot be achieved at all, whereas other one-step... 

The computed geometries for two isomeric transition states are presented in Table 5. All computational methods predicted a synchronous formation of the two new C-C bonds and a concerted mechanism for the cycloaddition reaction. Due to the lack of an electron correlation in the HF ab initio computational approach, the computed bonds were substantially shorter when compared with experimental data or computational data obtained with correlational computational methods. If the two isomeric transition state structures obtained with DFT (B3LYP and BLYP) methods are taken into consideration, then it can be seen that the exo transition state structure is the closest structure to the reactants. According to the Hammond postulate [38], the exo transition state structure should then have a lower energy than the isomeric endo transition state structure. 

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