Density functional and ab initio molecular orbital calculations have been used to search for the low energy path of the thermal isomerization of [2.1.1]propellane 1. Three reaction modes were considered: ring opening of the bicyclo[1.1.0]butane unit in 1 to give 1,2-dimethylenecyclobutane 21, opening of the four-membered ring of 1 to afford 1,3-dimethylenecyclobutane 20, and breaking of the [2.1.1]propellane central bond and one of the bicyclo[1.1.0]butane side bonds to form carbene 17. At the CAS(12,12)PT2N/6-31G(d) level of theory, the activation barrier of the latter route was lowest in energy. Further investigation of this process at the QCISD(T)/6-311G(d,p)//QCISD/6-31G(d) and B3PW91/6-311G(d,p)// B3PW91/6-311G(d,p) level of theory indicated that the barrier of isomerization of 1 --> 17 amounts to 29 kcal/mol and that 17 is stabilized by hydrogen migration to give dienes 18 and 19.
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