The Carbene/Bridgehead-Olefin Rearrangement: DFT Calculations on 1-Bicyclo[2.1.1]hexylcarbene and 1-Bicyclo[3.1.1]heptylcarbene; Rearrangement of (5-Bromobicyclo[3.1.1]heptyl)bromocarbene

Abstract

B3LYP/6-31G(d) calculations on bicyclo[2.1.1]hexylcarbene 7ashowed a preferred propensity for enlargement of the four-membered ring to give 1-norbornene 8. Only at high temperature does a rearrangement of bridgehead olefin 8 to 2-norbornylidene 11 seem to be possible; the competing rearrangement to afford 2-bicyclo[3.1.1]heptylidene 10has a higher activation barrier by 20.2 kcal/mol. For 1-bicyclo[3.1.1]heptylcarbene 12, the preferred stabilization is again enlargement of the four-membered ring to give bicyclo[3.2.1]oct-1(7)-ene 13. The further rearrangement of this alkene to 7-bicyclo[3.2.1]octylidene 16is endothermic by 32.2 kcal/mol and should not take place under moderate conditions. (5-Bromobicyclo[3.1.1]heptyl)bromocarbene 32, a derivative of 12 generated from the tetrabromide 23 with MeLi, rearranged by enlargement of the four-membered ring to give alkene 31, which reacted to give the head-to-head 2π+2π dimer 26, whose structure was established by X-ray crystallography.