Alkenes theoretical explanation

It is well known that alkynes are less reactive than alkenes in electrophilic reactions 1117). The theoretical explanation is not obvious [118]. Although some theoretical calculations were also reported on the reactions of acetylene with [119], Si [120] and rare metal ions (Sc", , ... 

For the three vinyl halides and CHCl=CCl2 negative temperature dependencies similar to those for the alkenes are observed, and the theoretical explanation must be considered... 

A secondary orbital interaction has been used to explain other puzzling features of selectivity, but, like frontier orbital theory itself, it has not stood the test of higher levels of theoretical investigation. Although still much cited, it does not appear to be the whole story, yet it remains the only simple explanation. It works for several other cycloadditions too, with the cyclopentadiene+tropone reaction favouring the extended transition structure 2.106 because the frontier orbitals have a repulsive interaction (wavy lines) between C-3, C-4, C-5 and C-6 on the tropone and C-2 and C-3 on the diene in the compressed transition structure 3.55. Similarly, the allyl anion+alkene interaction 3.56 is a model for a 1,3-dipolar cycloaddition, which has no secondary orbital interaction between the HOMO of the anion, with a node on C-2, and the LUMO of the dipolarophile, and only has a favourable interaction between the LUMO of the anion and the HOMO of the dipolarophile 3.57, which might explain the low level or absence of endo selectivity that dipolar cycloadditions show. 

It is well established that ultrasmall metal clusters on supports have catalytic properties distinct from those properties of large bulk-like particles, as illustrated by the selective oxidation of propylene to propylene oxide by gold, alkene and arene hydrogenation catalysis,and CO oxidation. In these examples, the catalytic properties improve as the clusters become smaller. On the other hand, a reduction in size of the metal cluster can lead to less desirable catalytic properties as seen for ammonia synthesis on iron. Various explanations have been offered to account for the unique properties of nanoscaled metal catalysts, however, much remains to be understood. Clearly, experimental and theoretical studies will be required to develop an in-depth under-... 

Our initial explanation of the negative activation energies postulated that the carbene-alkene additions involved the intermediate and reversible formation of loose carbene/alkene change-transfer complexes. The partitioning of these intermediates between cyclopropane formation and reversion to PhCCl and alkene would determine the observed rate constants and their temperature dependence. [95] However, although carbene-arene Jt complexes do appear to modulate the chemistry of some carbenes in solution, carbene-alkene complexes have not been supported by theoretical studies. [96]... 

---