[2.2.2]Propellane isomerization by Grob rearrangement: An Ab initio MR-AQCC study

The multireference average quadratic couple cluster (MR-AQCC) method has been employed to explore isomerism in the [2.2.2]propellane 1 as well as the accompanying Grob-type fragmentation. It is shown that the open form 2 is more stable than the conventional closed propellane 1 by 3.6 kcal mol^-1. These two isomers are separated by a barrier as high as 22.7 kcal mol ^-1, taking 1 as a reactant, a value that is in excellent agreement with experiment. However, system 2 is not a stable species because it is located in a very shallow energy minimum. Consequently, it undergoes almost barrierless cage-opening to the thermodynamically most stable 1,4-dimethylenecyclohexane (3). The singlet-triplet gap along the bond-stretch reaction coordinate has been calculated too. It is shown that intersystem crossing can be expected in the vicinity of the saddle point. It is concluded that photochemical closure of the bridgehead C-C bond yielding 1 has a lower probability than geometry relaxation leading to structure 2 and ultimately to the stable compound 3. The structural features of the elusive systems 1 and 2 are discussed in some detail.