engleski

[2.2.2]Propellane Isomerization via Grob Rearrangement - An Ab Initio MR-AQCC Study

The multireference average quadratic couple cluster method is employed in exploring the [2.2.2]propellane 1 isomerism 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 reactant, a value which is in excellent agreement with experiment. However, the system 2 is not a stable species, because it is located in a very shallow energy minimum. Consequently, it undergoes an almost barrierless cage opening leading 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 the photochemical closure of the bridgehead C-C bond yielding 1 has lower probability than the geometry relaxation leading to structure 2 and ultimately to the stable compound 3. Structural features of elusive systems 1 and 2 are discussed in some detail.

Ab initio calculations; Grob rearrangement; Isomerisation; [2.2.2]Propellane; Strained molecules

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