Long-range interactions. Part III. A comparison of arene and olefin as neighbouring groups towards an allylic anion

Abstract

Rates of hydrogen-isotope exchange at the 4-position of 6,7-benzobicyclo[3,2,1]octa-2,6-diene (Ia) have been measured and compared with corresponding values for the parent bicyclo[3,2,1]octa-2,6-diene (II) in perdeuteriodimethyl sulphoxide (0·9M in KOBu^t) at 45·0°. By use of 220 MHz n.m.r. spectroscopy for the isotopic analysis, partial rate factors have been determined for exchange at the 2,4-exo- and 4-endo-positions in both hydrocarbons, and show that exchange occurs most rapidly in the 4-exo-position and that both hydrocarbons have a similar profile of exchange rates with (I) exchanging nine times more slowly than (II). Tetra-amminelithium(0) is shown to be a useful dehalogenating reagent.