THEORETICAL-STUDIES ON THE ACID-CATALYZED AROMATIZATION OF BENZENE-CIS-1, 2-DIHYDRODIOLS

Theoretical studies on the acid-catalysed dehydration of 15-substitute d dihydrodiols were carried out using the AM1 method, In agreement wit h the experimental results, the rate-limiting step is dehydration of t he protonated diols, R(+), and the o-phenol-forming pathway (path 1) i s favoured both kinetically and thermodynamically over m-phenol produc t formation, The transition state for path 1 (TS 1) is found to be at an early position (ca 23% progress) along the reaction coordinate and a better correlation of the rate constant is obtained with sigma(+). T hree resonance forms can exist for the benzenonium intermediate but th e contribution of one form is dominant in the later TS, which resemble s the cationic intermediate. The contribution of the other two, howeve r, weakens the sigma(+) effect of +M subsituents. The sigma(p) correla tion of the rate constant in acid solution is therefore suggestive of a later TS resembling the cationic intermediate which is stabilized by solution.