Crystal and molecular structure of tricyclo[5,2,1,02,6]deca-4,8-dienyl p-bromobenzoate and its reactivity in the Cope rearrangement

Abstract

The crystal and molecular structure of tricyclo[5,2,1,0^2,6]deca-4,8-dienyl p-bromobenzoate, C₁₇H₁₅O₂Br, has been determined by X-ray diffraction. Refinement was carried out by block-diagonal least squares to a final R value of 0·068. The compound is particularly interesting because of the remarkable ease with which it undergoes the Cope rearrangement. The molecular geometry indicates that in addition to the favourable activation entropy, a state of internal strain facilitates the reaction. This strain is reflected in the value (1·59 Å) of the C(6)–C(7) bond which is broken during the rearrangement. The favourable pre-orientation of the diallylic system seems to be of minor importance.