Metalla-cope rearrangements: bridging organic and inorganic chemistry

Density functional theory calculations are performed to explore both concerted chairlike and boatlike as well as stepwise mechanisms of the Cope rearrangement of two hypothetical metalladienes. An osma-1,5-hexadiene is designed by substituting CH(2) in 1,5-hexadiene by its isolobal analogue, 16-electron Os(PH(3))(4). The energy of activation corresponding to the rearrangement of osma-1,5-hexadiene involving the chairlike saddle point is computed as 37.4 kcal/mol, 3.9 kcal/mol above the energy barrier of the parent 1,5-hexadiene calculated with the same method and basis set, and is 4.5 kcal/mol below that of the boatlike pathway. In another isolobal replacement, the CH in 1,5-hexadiene is substituted by a 15-electron Re(PH(3))(3) fragment. Now the chairlike rearrangement of the rhenia-1,5-hexadiene has an E(a) value of 23.0 kcal/mol, 10.8 kcal/mol less than the energy barrier of the parent 1,5-hexadiene calculated at the same level of theory. The ring inversion of the chair and osma-chair diradical intermediates of the stepwise reaction pathway is also examined and is found in both cases to proceed through a very flat potential energy surface involving twist intermediates.