Isomerisierung und Ringinversion von Tribenzaspiro[5.5]undecaphanen

Abstract

For the two possible ring inversions in tribenzaspiro[5.5]undecaphanes—corresponding to the isomerizations7→9 and9→7 resp.—no mutual influence is found. Thus the degenerate isomerizations (“topomerizations”)7→7 and9→9 can proceedvia two consecutive (non-concerted) reversals. A ring inversion path similar to that for [2.2]metacyclophane (1) is postulated. The considerable ease of the isomerization of9 as compared to1 can be understood on the basis of thermodynamic arguments.

Moreover it is confirmed that the free enthalpy difference ΔG ₃₆₀=3.84 kcal/mol of7 and9 at equilibrium originates almost entirely from the different stabilities of boat and chair shaped benzene rings.

These conclusions are drawn from a kinetic study of the thermal isomerization of9 and equilibration experiments in comparison with the corresponding data of1. The activation parameters for the process9→7 are: ΔG ^A≠_360isom =26.52 kcal/mole, ΔH ^A≠_isom =25.35 kcal/mole, ΔS ^A≠_isom =−3.28 cal/deg·mole (isomerization) and ΔG ^A≠_360inv =27.03 kcal/mole, ΔH ^A≠_inv =25.35 kcal/mole, ΔS ^A≠_inv =−4.67 cal/deg·mol (ring reversal); the corresponding data for the degenerate isomerizations of7 and9—identical with those for the ring reversal in7—were deduced to be ΔG ^B≠_360inv =30.87 kcal/mole, ΔH ^B≠_inv =27.50 kcal/mole and ΔS ^B≠_inv =−9.5 cal/deg·mole.