X-ray crystal structures and conformational analysis of bicyclo[5.3.1]undec-1(11)-enes: Twisting versus pyramidalization in anti-Bredt olefins

The synthesis of several 9,11,11-trihalo[5.3.1]propellanes and their 4-dime thylsila analogues is described. They solvolyze under formation of the corr esponding isomeric 7,9,11-trihalobicyclo[5.3.1]undec-1(11)-enes which are " anti-Bredt" olefins with a strained trans double bond in a bridged eight-me mbered ring; in the presence of nucleophiles such as water or ethanol, the corresponding 7-hydroxy or 7-ethoxy derivatives, respectively, are obtained . On the basis of the X-ray crystal structures of four of these compounds ( la, 9a, 15, 17b), the effect of strain and of the substitution pattern on t he degree of twisting and pyramidalization of the double bond is discussed.