Formation of Anti-Bredt Olefins from Bridgehead Carbene Precursors:  A Computational Study

Density functional theory (DFT) calculations are employed to examine the energetics of the rearrangement of bridgehead carbenes to anti-Bredt olefins. These C-migrations are found to have very small activation barriers. For instance, rearrangement of bicyclo[2.2.2]octylcarbene has a transition-state energy that lies 3.8 kcal/mol above the carbene. These rearrangements are also quite exothermic, despite the instability of the bridgehead alkene. Rearrangement of bicyclo[1.1.1]pentylcarbene to bicyclo[2.1.1]hex-1-ene is exothermic by 18.4 kcal/mol. The greater thermodynamic stability of the anti-Bredt olefin products, as well as the low reaction barrier, demonstrates that bridgehead carbenes are a viable source of these strained alkenes.