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-dimethylsila analogues is described. They solvolyze under formation of the corresponding isomeric 7,9,11- trihalobicyclo[5.3.l]undec-1(11)-enes which are “anti-Bredt” olefins with a strained trans double bond in a bridged eight-membered 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 (1a, 9a, 15, 17b), the effect of strain and of the substitution pattern on the degree of twisting and pyramidalization of the double bond is discussed.