Ab initio study of the mechanisms of intermolecular and intramolecular [4?+?2] cycloaddition reactions of conjugated enynes

Competing nature of intramolecular [4 + 2] and [3 + 2] cycloaddition reactions: a theoretical study

“…The MP3 and MP4(SDQ) energy data will not be discussed here, since it has been shown already that these methods do not accomplish an accuracy level of the full fourth-order Møller-Plesset theory. 10 DE energy surfaces (X1 = O) at the MP4/6-31G*//MP2/6-31G* and MP2/6-31G*//MP2/6-31G* levels are in good agreement with each other except for 8-TS, which is 7.1 kcal mol À1 lower for the former. The MP4/6-31G*// B3LYP/6-31G* and B3LYP/6-31G*//B3LYP/6-31G* calculations are in reasonable agreement including the [3 2] cycloaddition transition state (8-TS).…”

“…29 MP2-calculated potential energy surfaces are helpful for comparing the present results with the theoretical study of hydrocarbon cycloaddition reactions published earlier. 10 For the studied system, full MP4(SDTQ) calculations reproduce well the highly accurate CCSD(T) results 10 and, therefore, can be used to improve MP2 energies. In certain cases the MP2 theory level provides less accurate geometry and energy for carbenes compared with the B3LYP density functional method.…”

“…2 also display the 9-TS→10→11-TS→12 pathway studied earlier. 10 In this case the discussion of particular geometries and energies will not be repeated for X1 = CH 2 structures; rather, the differences from heterocyclic reaction (X1 = O) will be outlined.…”

“…Since enynes have hydrogen atoms at X1 (X1 = CH 2 ), an alternative pathway involving two 1,2-H shifts becomes applicable. 10 According to the energy data, this pathway is more preferable: (12) is energetically more favored than cyclopentadiene (7) by 32.9 kcal mol À1 . Hence the 1→12 conversion is the most likely pathway.…”

“…(I)]. 10 The theoretical study revealed that a cyclic allene intermediate is formed on the reaction potential energy surface after the cycloaddition stage.…”