REACTION MECHANISM STUDIES: 3. SOLVENT EFFECTS AND THE NATURE OF THE TRANSITION STATE IN THE DIAXIAL → DIEQUATORIAL REARRANGEMENT

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Abstract
The rates of the diaxial → diequatorial rearrangement of 2β,3α-dibromocholestane (Ia), and the analogous bromohydrin p-toluenesulfonates (Id and Ie) and anisates (If and Ig) have been measured in various solvents. The change in rate with variation in solvent was found to correlate with the change in the ionizing power of the solvents. The sensitivity of the rate of rearrangement to changes in solvent ionizing power, as measured by the ratio of the rate of rearrangement in nitromethane to that in decalin, was found to be smaller for the dibromide (Ia) than for the esters (Id to Ig). A detailed discussion of the mechanism of the reaction is presented. It is proposed that the following factors (either singly or simultaneously) could lead to the smaller sensitivity to solvent change shown by the dibromide (Ia) as compared with the esters (Id to Ig): (1) a difference in a postulated change of the axial: equatorial opening ratio with change in solvent, and (2) the development of a smaller charge in the transition state for the rearrangement of the dibromide (Ia) as compared with that in the rearrangement of the esters, when the reactions are carried out in the less polar solvents. It is argued that the operation of the latter factor would be most simply interpreted in terms of a merged ion-pair, cyclic-concerted mechanism, as initially suggested by Grob and Winstein.

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